The superhet radio or to give it its full name the superheterodyne receiver is one of the most popular forms of receiver in use today. Virtually all broadcast radios, televisions and many more types of receiver use the superhet or superheterodyne principle. First developed at the end of the First World War, with its invention credited to the American Edwin Armstrong, the use of the superhet has grown ever since the concept was first discovered.

Mixing
The idea of the superhet revolves around the process of mixing. Here RF mixers are used to multiply two signals together. (This is not the same as mixers used in audio desks where the signals are added together). When two signals are multiplied together the output is the product of the instantaneous level of the signal at one input and the instantaneous level of the signal at the other input. It is found that the output contains signals at frequencies other than the two input frequencies. New signals are seen at frequencies that are the sum and difference of the two input signals, i.e. if the two input frequencies are f1 and f2, then new signals are seen at frequencies of (f1+f2) and (f1-f2). To take an example, if two signals, one at a frequency of 5 MHz and another at a frequency of 6 MHz are mixed together then new signals at frequencies of 11 MHz and 1 MHz are generated.

The signals generated by mixing or multiplying two signals together

Concept of the superheterodyne receiver
In the superhet or superheterodyne radio, the received signal enters one input of the mixed. A locally generated signal (local oscillator signal) is fed into the other. The result is that new signals are generated. These are applied to a fixed frequency intermediate frequency (IF) amplifier and filter. Any signals that are converted down and then fall within the passband of the IF amplifier will be amplified and passed on to the next stages. Those that fall outside the passband of the IF are rejected. Tuning is accomplished very simply by varying the frequency of the local oscillator. The advantage of this process is that very selective fixed frequency filters can be used and these far out perform any variable frequency ones. They are also normally at a lower frequency than the incoming signal and again this enables their performance to be better and less costly.

To see how this operates in reality take the example of two signals, one at 6 MHz and another at 6.1 MHz. Also take the example of an IF situated at 1 MHz. If the local oscillator is set to 5 MHz, then the two signals generated by the mixer as a result of the 6 MHz signal fall at 1 MHz and 11 MHz. Naturally the 11 MHz signal is rejected, but the one at 1 MHz passes through the IF stages. The signal at 6.1 MHz produces a signal at 1.1 MHz (and 11.1 MHz) and this falls outside bandwidth of the IF so the only signal to pass through the IF is that from the signal on 6 MHz.

The basic concept of the superhet radio

If the local oscillator frequency is moved up by 0.1 MHz to 5.1 MHz then the signal at 6.1 MHz will give rise to a signal at 1 MHz and this will pass through the IF. The signal at 6 MHz will give rise to a signal of 0.9 MHz at the IF and will be rejected. In this way the receiver acts as a variable frequency filter, and tuning is accomplished.

Images
The basic concept of the superheterodyne receiver appears to be fine, but there is a problem. There are two signals that can enter the IF. With the local oscillator set to 5 MHz and with an IF it has already been seen that a signal at 6 MHz mixes with the local oscillator to produce a signal at 1 MHz that will pass through the IF filter. However if a signal at 4 MHz enters the mixer it produces two mix products, namely one at the sum frequency which is 10 MHz, whilst the difference frequency appears at 1 MHz. This would prove to be a problem because it is perfectly possible for two signals on completely different frequencies to enter the IF. The unwanted frequency is known as the image. Fortunately it is possible to place a tuned circuit before the mixer to prevent the signal entering the mixer, or more correctly reduce its level to acceptable value.

Fortunately this tuned circuit does not need to be very sharp. It does not need to reject signals on adjacent channels, but instead it needs to reject signals on the image frequency. These will be separated from the wanted channel by a frequency equal to twice the IF. In other words with an IG at 1 MHz, the image will be 2 MHz away from the wanted frequency.

Using a tuned circuit to remove the image signal

Complete receiver
Having looked at the concepts behind the superheterodyne receiver it is helpful to look at a block diagram of a basic superhet. Signals enter the front end circuitry from the antenna. This contains the front end tuning for the superhet to remove the image signal and often includes an RF amplifier to amplify the signals before they enter the mixer. The level of this amplification is carefully calculated so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved.

The tuned and amplified signal then enters one port of the mixer. The local oscillator signal enters the other port. The local oscillator may consist of a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor. Alternatively it may be a frequency synthesizer that will enable greater levels of stability and setting accuracy.

Once the signals leave the mixer they enter the IF stages. These stages contain most of the amplification in the receiver as well as the filtering that enables signals on one frequency to be separated from those on the next. Filters may consist simply of LC tuned transformers providing inter-stage coupling, or they may be much higher performance ceramic or even crystal filters, dependent upon what is required.

Once the signals have passed through the IF stages of the superheterodyne receiver, they need to be demodulated. Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered. The output from the demodulator is the recovered audio. This is passed into the audio stages where they are amplified and presented to the headphones or loudspeaker.

Block diagram of a basic superheterodyne receiver

The diagram above shows a very basic version of the superhet or superheterodyne receiver. Many sets these days are far more complicated. Some superhet radios have more than one frequency conversion, and other areas of additional circuitry to provide the required levels of performance. However the basic superheterodyne concept remains the same, using the idea of mixing the incoming signal with a locally generated oscillation to convert the signals to a new frequency. Read more....!

Digital Radio Mondiale (DRM) is set to revolutionise broadcasting on the long, medium and short wave bands. Since the very earliest days of broadcasting these wavebands have been filled with signals that are amplitude modulated. These transmissions are of low audio quality and particularly in recent years there has been a move away from these bands to find higher quality transmissions. Broadcasts in the VHF FM band have received far more listeners with the result that audience figures are dropping for AM broadcasting. Now DAB Digital Radio is available in many countries and this has set new standards in broadcasting. The next stage is to improve the transmissions on the long medium and short wave bands. As the requirements are very different to those experienced on the higher frequencies the DAB standard is not applicable and as a result a totally new system has been developed. Known as DRM it provides many of the improvements that are badly needed along with the flexibility to allow for future developments.

What is DRM?
DRM itself is a consortium of broadcasters, network operators, equipment manufacturers, broadcasting unions, regulatory bodies and other organisations representing 29 countries. It was founded in Guangzhou, China in 1998 and now has its headquarters in Geneva. Now with 82 members, the wide base of its membership has been part of the reason for its success. It has been able to draw on the experience of the membership to ensure that the resulting standard met the requirements, and it has also drawn on the experience gained by the Eureka project that was set up to develop DAB Digital Radio. As a result the new system has come to fruition remarkably swiftly. A preliminary system was designed and tested within a laboratory and this was later extended to include field trials on air to ensure that the new system would successfully meet all the requirements.

The system
When the specification for DRM was being drawn up there were a number of key requirements that needed to meet. The main thrust of the development was to ensure that far greater audio quality could be achieved, but this needed to be achieved whilst keeping the transmissions in a form where they could operate alongside the existing AM transmissions. This meant having the ability for the transmissions to occupy a variety of different bandwidths dependent up the location and frequencies in use. In the Americas a 10 kHz channel spacing is used on the medium wave band whilst in Europe there is a 9 kHz spacing. On the short wave bands a 5 KHz channel spacing has been adopted. It is necessary for the new standard to be able to be compatible with these whilst offering the possibility of other bandwidth options for the future.

Data can also transmitted. Not only does this supply information required for decoding the signal but it also allows data to be transmitted in support of the programme. One particularly useful feature for the short wave bands is that a list of alternative frequencies is transmitted so that listeners can be transferred to better channels very easily as conditions change.

Another advantage of the new system is that it can support what is termed a single frequency network (SFN). This allows a single frequency to be re-used even within the coverage area of the first transmitter without mutual interference. Currently frequencies can only be re-used used outside the coverage area of the first transmitter to avoid interference problems. By using an SFN, far more efficient use can be made of the available channels. With spectrum bandwidth always in short supply, this is another important feature.

DRM transmissions
There are two main elements to the new transmission system. These are the audio coding and the RF modulation used.

The main audio encoding system employs two main techniques. The first is called Advanced Audio Coding (AAC). It is found that the ear does not perceive all the sounds that are heard. A strong sound on one frequency will mask out others close in frequency that may be weaker. AAC, therefore, analyses each section of the spectrum and only encodes those sounds that will be perceived.

However AAC on its own does not provide sufficient compression of the data to enable the transmissions to be contained within the narrow transmission bandwidths required. To provide the additional data compression required a scheme known as Spectral Band Replication (SBR) is employed. This analyses the sounds in the highest octave which are normally from sounds such as percussion instruments of those that are harmonically related to other sounds lower in frequency. It analyses them and sends data to the receiver that will enable them to be reconstituted later.

Data channels
Data to provide the different functions on the transmission is organised into a number of channels that are then applied to the overall modulating signal. The main payload for the signal is known as the Main Service Channel (MSC) and this includes the audio signal data. Two subsidiary channels are also available. These are known as the Fast Access Channel (FAC) that provides the essential data required to fully decode the signal and the Service Description Channel (SDC).

RF Signal
The transmitted signal uses a form of modulation known as Coded Orthogonal Frequency Division Multiplex (COFDM). This form of modulation is being used more frequently is very resilient to many common forms of interference and fading. Its main drawback has been that it requires a significant level of signal processing to extract the data from the carriers and reassemble it in the correct fashion. However signal processing ICs are now sufficiently powerful and at a reasonable cost to make the use of this form of modulation viable. Interestingly COFDM is also used by DAB Digital Radio.

The signal consists of several carriers, across which the data is spread equally. The carriers are spaced equally apart where the spacing is equal to the inverse of the symbol period of the data applied to the carrier. With this spacing it is found that the energy density in the sidebands has nulls or minimum points that correspond with the position of the next carrier. In this way the interference between the nearby carriers is eliminated and they are said to be orthogonal. Read more....!

Digital technology is being applied to many areas of radio communication including radio broadcasting as it offers some significant advantages. While DAB digital radio is becoming established in some areas of the globe, the system that has been chosen for use in the USA is known as HD, or High Definition, Radio. Using HD Radio, will enable high quality audio to be received along with the ability to incorporate many new features and facilities.

The HD Radio system has been developed by iBiquity, and has now been selected by the FCC in the USA. It will take the place of both the existing AM and FM transmissions, and offers many advantages for both listeners and broadcasters alike:

* Improved audio quality - it is claimed that HD Radio broadcasts on the AM bands will be as good as current FM services and those on the FM band will offer CD quality audio.


* Reduced levels of interference. AM transmissions in particular are prone to static pops and bangs as well as high levels of background noise. HD Radio will almost eliminate this.


* Opportunity to use additional data services. By using digital technology, HD Radio provides the opportunity to add data services such as scrolling programme information, song titles, artist names, and much more.


* There is also the possibility of adding more advanced services such as surround sound, multiple audio sources, on-demand audio services, etc.


* Easy transition for broadcasters and listeners. Although new HD Radio receivers are required to receive the new transmissions in their digital format there is considerable re-use of infrastructure and spectrum.


HD Radio basics
HD Radio uses a variety of technologies to enable it to carry digital audio in an acceptable bandwidth and with the new high quality that is required. The transmission uses COFDM combined with specialised codec to compress the audio.

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period.

This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed.

Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented. Further information on OFDM can be found on this site under the Cellular telecoms section or by using the Search facility.

One of the requirements for HD Radio was that it would maintain compatibility with existing stations. To achieve this there are two versions; one HD Radio system for AM, and the other for FM.

In what is termed hybrid mode, the AM version has a data rate of 36 kbps for the main audio channel and the version of HD radio for the FM bands carries 96 kbps. In addition to this HD radio can also be used to carry multiple audio channels, and in addition to this secondary channels for services such as weather, traffic and the like may be added. However adding additional channels will reduce the available bandwidth for the primary channel and audio quality may be impaired.

In hybrid mode a radio receiver will first lock onto an analogue signal. If this is possible, then it will try to find a stereo component (FM only) and finally it will endeavour to decode a digital signal. If the digital signal is lost then it will fall back to the analogue signal. The success of this process depends upon the transmitting station being able to synchronise the digital and analogue signals. Often the digitisation process takes a noticeable amount of time and the digital and analogue signals may not be transmitted in time with each other.

Once HD Radio is fully established, the hybrid mode may be removed and at this point no analogue information will be transmitted. However it is envisaged that this will take some time as this can only be viable when very few analogue radios are in use. Read more....!

DAB digital radio can be broadcast on a wide number of frequencies. There are both terrestrial and satellite allocations for Digital Audio Broadcasting (DAB). Currently the main frequencies where it is being deployed are within the Band III (Band 3) frequencies. Here a number of channels have been allocated. A complete table of the channels is given below, although in many countries the full number of channels is not available. Within the UK, the DAB multiplexes are being broadcast on channels 11B through to 12D inclusive.

Although it may appear that comparatively few channels are available, each multiplex is able to carry many stations. If high quality audio is required then fewer stations can be accommodated. However it is often possible to accommodate around four or five high quality broadcasts along with several lower quality ones. In addition to this data can also be carried.

Channel Frequency
MHz
5A 174.928
5B 176.640
5C 178.352
5D 180.064
6A 181.936
6B 183.648
6C 185.360
6D 187.072
7A 188.928
7B 190.640
7C 192.352
7D 194.064
8A 195.936
8B 197.648
8C 199.360
8D 201.072
9A 202.928
9B 204.640
9C 206.352
9D 208.064
10A 209.936
10B 211.648
10C 213.360
10D 215.072
11A 216.928
11B 218.640
11C 220.352
11D 222.064
12A 223.936
12B 225.648
12C 227.360
12D 229.072
13A 230.748
13B 232.496
13C 234.208
13D 235.776
13E 237.448
13F 239.200
Read more....!

DAB Digital Radio, which is also known as digital audio broadcasting, is an entirely new system for broadcasting and receiving radio stations. As the name indicates signals are broadcast in a digital format to enable CD quality to be achieved. People who have heard DAB digital radio have commented on the significantly better sound quality and "presence" of the new radio system. Also it does not suffer from the multipath effects often experienced on FM transmissions and as the system uses what it known as a single frequency network (SFN) there is no retuning required when moving from one coverage area to the next.

In addition to this many new services can be carried on these digital radio transmissions enabling the new system to be compatible with the 21st century. The digital radio signal carries data alongside the audio, and this enables text and images to be transmitted alongside the audio to enhance the listening experience. In this way it is possible to transmit the title of a track, and a picture of the artist whilst the some music is being transmitted. It is also possible to have news scrolling across the bottom of the screen used on the radio.

DAB digital radio is now well established in many countries around the world from the UK and Europe to Canada, Australia and many other countries. Wit the facilities that digital radio offers it is now being accepted and listeners are switching to these new digital radio transmissions in the areas where they are available.


To produce a digital system that operates satisfactorily under the conditions required for digital radio a large amount of work was undertaken in the development stages. Some existing digital techniques were investigated but it was realised these had significant limitations for this application. One of the major problems was that many receivers would use non-directional antennas and as a result they would pick up reflected signals. These would be delayed sufficiently for the data to become corrupted. Additionally the bandwidth required to accommodate a full stereo signal would need to be reduced to ensure efficient use of the spectrum. The technical standards for digital radio were developed under the auspices of the European Eureka Project 147. This consortium consisted of manufacturers, broadcasters research bodies and network operators.

There are two main areas of the system that are of interest in digital radio: namely the modulation system and the audio digital encoding and compression system.

The encoding and compression system is of paramount importance. For the system to be viable the data rate has to be considerably reduced from that of a standard CD. The digital radio system adopted reduces the data rate down to 128 kbits / sec, a sixth of the bit rate for a similar quality linearly encoded signal. To achieve these reductions the incoming audio signal is carefully analysed. It is found that the ear has a certain threshold of hearing. Below this the signals are not heard. Additionally if a strong sound is present on one frequency then weaker sounds close to it may not be heard because the threshold of hearing is modified. By analysing the incoming audio and only encoding those constituents that the ear will hear the significant reductions can be made. Further reductions in data rate can be achieved by reducing the audio bandwidth. This is implemented on some channels such as those used only for speech.

The other key to the operation of digital radio is the modulation system. Called Coded Orthogonal Frequency Division Multiplex (COFDM) it is a form of spread spectrum modulation that provides the robustness required to prevent reflections and other forms of interference from disrupting reception.

The system uses about 1500 individual carriers that fill around 1.5 MHz of spectrum. The carriers are spaced very close to one another. Interference between the carriers is prevented by making the individual signals orthogonal to each other. This is done by spacing each one by a frequency equal to the data rate being carried. In this way the nulls in the modulation sidebands fall at the position where the next carrier is located. The audio data is then spread across the carriers so that each carrier takes only a small proportion of the data rate. This has the advantage that if interference is encountered in one area then sufficient data is received to reconstitute the required signal. Guard bands are also introduced at the beginning of each symbol, and the combined effect is such that the system is immune to delays consistent with signals 60 km further away than the primary source.

With this level of immunity, the system can operate with other digital radio transmitters operating on the same frequency without any ill effects. This means that it is possible to set up a system where all the transmitters for a network operate on the same frequency. This means that it is possible to set up single frequency networks throughout an area in which a common "multiplex" is used. Even though it may appear that this is a recipe for poorer reception caused by several transmitters using the same frequency, the opposite is actually true. It is found that out of area signals tend to augment the required signal. It also means that small areas of poor coverage can have a small transmitter on exactly the same frequency filling in the hole and further improving reception in adjacent areas.

A further advantage of this digital radio system is that it requires less power than the more traditional transmitters. For example those that carry the main BBC FM networks from the main transmitting sites like Wrotham in the South East of England run at powers of around 100 kW for each of the four main services that are transmitted. The cost of the electricity alone is a significant factor in the BBC's running costs and the power reductions will bring huge savings, not to mention the environmental benefits.

Digital radio band allocations
In the UK a spectrum allocation between 217.5 and 230 MHz has been reserved for digital radio transmissions. This gives a total of seven blocks of 1.55 MHz, each able to carry a multiplex of services. In other countries as well spectrum is being made available. Within Europe spectrum is being made available either in Band III as in the UK or in L band between 1452 and 1467 MHz. The upper part of the band between 1467 and 1492 will be reserved for satellite delivery of digital radio.

Digital radio equipment
One of the main problems with the initial launch of digital radio was the availability of the equipment. A large investment had been required from the equipment manufacturers. The heavy reliance on digital signal processing techniques meant large development programmes were needed to develop the equipment. There were also problems with the fact that the early implementations required high current levels. These solutions would not have been suitable for portable receivers, and for in car and home applications heat dissipation was a problem. Furthermore the multi-chip solutions made the equipment large and bulky as well as making the manufacturing costs high.

Manufacturers soon solved the problem. Specific chip sets for DAB were developed and these enabled costs to be reduced dramatically from the initial ones that were seen so that DAB is no where near as high as it was when compared to FM receivers.

Many people now comment on the significant enhancements that DAB digital radio brings. One typical example was when a friend walked into a shop and noticed the music playing had an increased presence. He assumed it must be DAB, and this was confirmed when he asked. Others have noticed the seamless performance when in a car. None of the intermittent hissing when travelling through a marginal area between the two transmitters. Read more....!

RDS or Radio Data System is standard on most car radios and hi-fi tuners today. RDS is used on VHF FM radio broadcast transmissions and provides a number of facilities that are of great use to all radio listeners, but particularly to those radio listeners in cars. RDS enables traffic reports to be received more easily, and provides many facilities including enabling the radio station name to be displayed on the radio display.

The system has gained a considerable amount of popularity and is widely used in Europe where it has been established for a number of years.

How RDS Works
RDS operates by adding data to the baseband signal that is used to modulate the radio frequency carrier. The baseband signal consists of a number of components. Firstly there is the mono audio consisting of the left plus right (L+R) component that is transmitted at the normal audio frequencies up to 15 kHz. The stereo difference signal is then amplitude modulated as a double sideband suppressed carrier signal at 38 kHz. A pilot tone at 19 kHz (half the frequency of the stereo difference signal subcarrier) is also transmitted and this is used to enable the receiver demodulator to exactly recreate the 38 kHz subcarrier to decode the stereo difference signal.

The stereo difference signal is above the audio hearing range and as a result it does not detract from the normal mono signal. When adding anything new to a transmission, compatibility must be maintained with existing radios.

The RDS information is placed above the stereo difference signal on a 57 kHz subcarrier as shown. This happens to be three times the stereo pilot tone frequency. For stereo transmissions the RDS subcarrier is locked onto the pilot tone. It can either be in-phase with the third harmonic of the tone, or as in the case of the BBC it can be in quadrature.

The actual subcarrier that is used to carry the information is phase modulated to carry the data. It uses a form of modulation called Quadrature Phase Shift Keying (QPSK). This gives good immunity to data errors caused by noise whilst still allowing the data to be transmitted at a suitable rate. Combined with the fact that the subcarrier operates at a harmonic of the pilot tone, these facts minimise the possibility of interference to the audio signals.

Baseband Coding
The rate at which data is transmitted is 1187.5 bits per second. This is equal to the frequency of the RDS subcarrier divided by 48. By adopting this data rate the decoding circuits to operate synchronously. This reduces problems with spurious signals in the decoding circuits.

Data is transmitted in groups consisting of four blocks. Each block contains a 16 bit information word and a 10 bit check word as shown. This means that with the data rate of 1187.5 bit per second approximately 11.4 groups can be transmitted each second.

A 10 bit check word may seem to be long. However it is very important in view of the poor signal conditions which can exist. This can be particularly true for car or portable radios. The check word enables the radio decoder to detect and correct errors. It also provides a method for synchronisation.

The data groups are structured so that data can be transmitted as efficiently as possible. Different stations will want to transmit different types of data at different times. To cater for this there are a there are a total of 16 different group structures. Their applications are outlined in Figure 3.

Mixing of different types of data within groups is kept to a minimum. However the coding structure is such that messages which need repeating most frequently normally occupy the same position within groups. For example the first block in a group always contains the PI code and PTY and TP are to be found in block 2.

In order that a radio knows how to decode the data correctly, each type of group has to be identified. This function is performed by a four bit code occupying the first four bits in the second block.

Once generated the data is coded onto the subcarrier in a differential format. This allows the data to be decoded correctly whether the signal is inverted or not. When the input data level is "0" the output remains unchanged but when a "1" appears at the input the output changes its state.

With the basic signal generated the spectrum has to be carefully limited. This has to be done to avoid any cross talk in phase locked loop decoders. The power density close to 57 kHz is limited by the encoding each bit as a biphase signal. In addition to this the coded data is passed through a low pass filter.

Facilities
The RDS system offers a wide range of very useful facilities. The most widely publicised one is that of being able to provide travel news. This is available on most local radio stations. All of these stations transmit the TP code to identify that travel messages are flagged by RDS. When the radio is set for travel news it will only tune to stations which carry the TP indication. As the station is about to broadcast a travel announcement the TA code is transmitted. If a CD or cassette is being played then most sets will actually pause the CD or tape and then allow the travel announcement to be heard. In addition to this the volume may also be set slightly higher to allow the announcement to be heard more easily.

Autotuning
RDS brings intelligence into the tuning of a radio. The autotuning facility comes into its own on long journeys when the car moves from the service area of one transmitter to the next. Without RDS the radio has to be manually tuned to the next station. This is not always easy because it is difficult to reliably detect which is the strongest station.

An RDS set will look for the Programme Identification or PI code. A national network will be broadcast from a large number of different transmitters around the country. The station or network eg Radio 4 will have its own PI code. When the radio moves out of the range of one transmitter the radio will seek the strongest signal which has the same PI code, allowing the radio to remain tuned to the same programme.

When radios fitted with RDS store a station frequency, they also store the PI code along side it. This has the advantage that when the radio is turned on in a place outside the coverage area for the transmitter frequency which is stored then the radio will seek the strongest signal which has the correct PI code.

Local radio stations also have a PI code. In view of the local nature of these stations the PI code works slightly differently.

If the station has two or more transmitters then the PI code will operate in the normal way when it is range of these transmitters. However when the radio moves outside this coverage area it will retune to the strongest signal of the same type of station.

The PI code consists of four characters. The first indicates the country of origin and for the UK this is C. The next one indicates the type of coverage. The figure "2" indicates a national station, and the final two characters are the programme reference. For example Radio 3 has the PI code C203 and BBC GLR has C311.

Instant Tuning
It takes a number of seconds for the radio to search for the strongest signal with the correct PI code. During this time the radio would mute itself and the listener would have an annoying gap in listening. To enable the set to tune itself very quickly from one transmission to the next each transmitter broadcasts a short list of frequencies of adjacent transmitters. This vastly reduces the amount of seeking which the radio set has to perform. In addition to this a second front end is often employed to constantly detect the strength of the alternative frequency transmissions. This results in much faster changes in setting - to the extent that the listener should not be able to detect when the radio changes from one transmitter to another.

Another facility associated with tuning is called the Programme Service Name (PS). This enables the set to display the station name. This normally takes a second or two to come up on the display after the station has been tuned in. However it is a most useful facility with the ever-increasing number of stations on the air

New Facilities
A new feature which has been added to RDS is called Enhanced Other Networks (EON). This allows the set to listen to one station like a national network, but still be interrupted by travel news from a local radio station. This feature even allows announcements to be heard whilst travelling in silence or listening to a tape.

EON requires a large amount of co-ordination between the different stations. To achieve this, the BBC have a central computer specifically for this purpose. When a local radio station is about to transmit a traffic message the fact is flagged to the computer. In turn this directs the relevant national radio transmitters to indicate this fact, thereby enabling the radios to change frequency to the local radio station to receive the message. Once the message is complete the radio will return to its original station.

EON is relatively new and the first sets to have it included only appeared in 1991. Although it is being introduced on more sets, the majority still does not have it. However with manufacturers constantly bringing new sets onto the market EON should be included on far more sets in a year or two. Read more....!

When broadcasting first started in the 1920s amplitude modulation was used because it was the obvious and the easiest way to transmit sound. However as radio technology developed its shortcomings became more obvious and the quest for higher quality transmissions lead to the introduction of wideband frequency modulation. Although the first commercial stations were set up in the USA around 1939, it was not until the 1950s that FM started to become really accepted. It was in 1954 that the BBC announced their intention to start FM broadcasting. Now VHF FM is the accepted medium for high quality transmissions, and stations that use AM on the medium and long wave bands have to work hard to retain listeners who prefer the higher quality of VHF FM.

What is FM?
Amplitude modulation, which is the simplest and most obvious form of modulation varies the amplitude of the carrier so that it carries the sound information. Frequency modulation is slightly more subtle and as the name indicates it varies the frequency of the carrier in line with the variations in the modulating audio signal. This as the modulating waveform increases in voltage, so the carrier will swing in one direction and as it decreases it will move in the other direction.

One of the important factors of FM is the degree by which the carrier changes. This deviation is usually expressed in kilohertz variation either side of the centre (no modulation) frequency. Typically a signal may have a deviation of +/- 3kHz if it varies up and down by 3 kHz. There are two main categories on FM. The first is called narrow band FM, and this is where the deviation is relatively small, possibly 5 kHz. This type of transmission is used mainly by VHF / UHF point to point mobile communications. To appreciate the full benefits of FM, wideband FM is used having a greater level of deviation. The standard for broadcasting is +/- 75 kHz. To fully accommodate these transmissions a bandwidth of 200 kHz is used.

The advantage of FM is that as the modulation is carried solely as frequency variations, much noise, which appears mainly as amplitude variations can be discarded in the receiver. Accordingly it is possible to achieve much better noise performance using FM. The upper audio frequency limit is generally taken as 15 kHz for these transmissions. This is quite adequate for most high quality transmissions.

Pre-emphasis and de-emphasis
One of the problems with the high quality VHF FM transmissions is that the increased audio bandwidth means that background noise can often be perceived. Even then it is considerably better than that obtained using and AM system. It is particularly noticeable towards the treble end of the audio spectrum, where it can be heard as a background hiss. To overcome this it is possible to increase the level of the treble frequencies at the transmitter. At the receiver they are correspondingly attenuated to restore the balance. This also has the effect of reducing the treble background hiss which is generated in the receiver. The process of increasing the treble signals is called pre-emphasis, and reducing the in the receiver is called de-emphasis. The rate of pre-emphasis and de-emphasis is expressed as a time constant. It is the time constant of the capacitor-resistor network used to give the required level of change. In the UK, Europe and Australia the time constant is 50 uS whereas in North America it is 75 uS.

Stereo
In recent years stereo transmission has become an accepted part of VHF FM transmissions. The system that is used maintains compatibility with mono only receivers without any noticeable degradation in performance. The system that is used is quite straightforward.

A stereo signal consists of two channels that can be labelled L and R, (Left and Right), providing one channel for each of the two speakers that are needed. An ordinary mono signal consists of the summation of the two channels, i.e. L + R, and this can be transmitted in the normal way. If a signal containing the difference between the left and right channels, i.e. L - R is transmitted then it is possible to reconstitute the left only and right only signals. Adding the sum and difference signals, i.e. (L + R) + (L - R) gives 2L, i.e. the left signal, and subtracting the two signal, i.e. (L + R) - (L - R) gives 2R, i.e. the right signal. This can be achieved relatively simply by adding and subtracting the two signals electronically. It only remains to find a method of transmitting the stereo difference signal in a way that does not affect any mono receivers.

This is achieved by transmitting the difference signal above the audio range. It is amplitude modulated onto a 38 kHz subcarrier. Both the upper and lower sidebands are retained, but the 38 kHz subcarrier itself is suppressed to give a double sideband signal above the normal audio bandwidth as shown below. This whole of the baseband is used to frequency modulate the final radio frequency carrier. It is the baseband signal that is regenerated after the signal is demodulated in the receiver.

To regenerate the 38 kHz subcarrier, a 19 kHz pilot tone is transmitted. The frequency of this is doubled in the receiver to give the required 38 kHz signal to demodulate the double sideband stereo difference signal.

The presence of the pilot tone is also used to detect whether a stereo signal is being transmitted. If it is not present the stereo reconstituting circuitry is turned off. However when it is present the stereo signal can be reconstituted.

To generate the stereo signal, a system similar to that shown in Fig. 8.5 is used. The left and right signals enter the encoder where they are passed through a circuit to add the required pre-emphasis. After this they are passed into a matrix circuit. This adds and subtracts the two signals to provide the L + R and L - R signals. The L + R signal is passed straight into the final summation circuit to be transmitted as the ordinary mono audio. The difference L - R signal is passed into a balanced modulator to give the double sideband suppressed carrier signal centred on 38 kHz. This is passed into the final summation circuit as the stereo difference signal. The other signal entering the balanced modulator is a 38 kHz signal which has been obtained by doubling the frequency of the 19 kHz pilot tone. The pilot tone itself is also passed into the final summation circuit. The final modulating signal consisting of the L + R mono signal, 19 kHz pilot tone, and the L - R difference signal based around 38 kHz is then used to frequency modulate the radio frequency carrier before being transmitted.

Reception of a stereo signal is very much the reverse of the transmission. A mono radio receiving a stereo transmission will only respond to the L + R signal. The other components being above 15 kHz are above the audio range, and in any case they will be suppressed by the de-emphasis circuitry.

For stereo receivers the baseband signal consisting of the stereo sum signal (L+R) and the difference signal (L-R) centred around 38 kHz and the pilot 19kHz tone are obtained directly from the FM demodulator. The decoder then extracts the Left only and Right only signals.

The block diagram of one type of decoder is shown below. Although this is not the only method which can be used it shows the basic processes that are required. The signal is first separated into its three constituents. The L + R mono signal between 0 and 15 kHz, the pilot tone at 19 kHz, and the stereo difference signal situated between 23 and 53 kHz. First the pilot tone at 19 kHz is doubled in frequency to 38 kHz. It is then fed into a mixer with the stereo difference signal to give the L - R signal at audio frequencies. Once the L + R and L - R signals are available they enter a matrix where they are added and subtracted to regenerate the L and R signals. At this point both signals are amplified separately in the normal way in a stereo amplifier before being converted into sound by loudspeakers or headphones. Read more....!

Radio broadcasting is an established use of radio technology. The first organised broadcasts taking place in the 1920s. Now there are many radio stations broadcasting all over the world using a variety of differernt types of transmission. Today, radio broadcast equipment from transmitters and receivers to antennas, studios and relay links are widely available, although with the new standards for transmission including DAB Digital Radio and DRM, new equipment is required. Nevertheless AM as well as FM with its RDS capability are still the most widely used.

VHF FM broadcasting
VHF FM is the most widely used form of broadcasting in areas of the world where the population is relatively high. Its bandwidth enables it to carry high quality transmissions, stereo, and other services such as RDS.
Broadcast VHF FM
RDS - Radio Data Service

Digital Audio Broadcasting (DAB)
DAB digital radio is now widely deployed in many countries around the globe, and now that the cost of radios has fallen, listener figures are rising. Although not available in many countries, it is certainly making a significant impact where it has been deployed, adding more flexibility and the possibility of near CD quality.
DAB digital radio
DAB digital radio Band III channel numbers and frequencies
HD Radio - the new digital radio system for the USA.

Digital Radio Mondiale
While DAB digital radio is focussed at bands at VHF and above there have been developments for the lower frequencies. Digital Radio Mondiale (DRM) is now being deployed and is a replacement for the amplitude modulation (AM) transmissions that have been on the airwaves for over 100 years. The new technology has now been extended for use up to 100 MHz and with many developments under way radios should soon be available.
Digital Radio Mondiale (DRM) - the new standard to replace AM broadcasting Read more....!