FM broadcasting: Encyclopedia II - FM broadcasting - Technical characteristics

FM broadcasting - Technical characteristics

FM broadcasting - Pre-emphasis and de-emphasis

Random noise has a 'triangular' spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Reducing the high frequencies in the receiver also reduces the high-frequency noise. These processes of boosting and then reducing certain frequencies are known as pre-emphasis and de-emphasis respectively.

The amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. In most of the world a 50 µs time constant is used. In North America, 75 µs is used. This applies to both mono and stereo transmissions.

The amount of pre-emphasis that can be applied is limited by the fact that many forms of contemporary music contain more high-frequency energy than the musical styles which prevailed at the birth of FM broadcasting. They cannot be pre-emphasized as much because it would cause excessive deviation of the FM carrier. (Systems more modern than FM broadcasting tend to use either program-dependent variable pre-emphasis (e.g. J.17) or none at all.)

FM broadcasting - FM stereo

The Zenith-GE pilot tone multiplex system was added to FM radio in the early 1960s to allow FM stereo.

It is important that stereo broadcasts should be compatible with mono receivers. For this reason, the left (L) and right (R) channels are matrixed into sum (M) and difference (S) signals, i.e. M=(L+R)/2 and S=(L−R)/2. A mono receiver will just use the M signal. A stereo receiver will matrix the M and S signals to recover L and R: L=M+S and R=M−S.

The M signal is transmitted as baseband audio in the range 50Hz to 15kHz. The S signal is amplitude-modulated onto a 38kHz suppressed carrier to produce a double-sideband suppressed carrier (DSBSC) signal in the range 23 to 53 kHz.

A 19 kHz pilot tone, at exactly half the 38 kHz subcarrier frequency and with a precisely defined phase relationship to it, is also generated. This is transmitted at 10% of overall modulation level and used by the receiver to regenerate the 38 kHz subcarrier with the correct phase. It is also in phase with the RDS subcarrier at 57kHz (19×3). A 4kHz-wide guard band exists between each of these, to prevent interference.

The final multiplex signal from the stereo generator consists of the baseband audio (M), the pilot tone, and the DSBSC subcarrier (S). This multiplex, along with any other subcarriers, modulates the FM transmitter.

Converting the multiplex signal back to left and right is performed by a stereo decoder, which is built into stereo receivers.

It is normal practice to apply pre-emphasis to the left and right channels before matrixing, and to apply de-emphasis at the receiver after matrixing.

FM broadcasting - Other services

The subcarrier system has been further extended to add other services. Initially these were private analog audio channels which could be used internally or rented out. Radio reading services for the blind are also still common, and there were experiments with quadraphonic sound. If there is no stereo on a station, everything from 23kHz on up can be used for other services. The guard band around 19kHz (±4kHz) must still be maintained, so as not to trigger stereo decoders on receivers.

Digital services are now also available. A 57kHz subcarrier is used to carry a low-bandwidth digital Radio Data System signal, providing extra features such as Alternate Frequency (AF) and Network (NN). This narrowband signal runs at only 1187.5 bits per second, thus is only suitable for text. A few proprietary systems are used for private communications.

The United States is the only country attempting to put digital radio onto FM rather than using EUREKA 147 like most other countries (including Canada), or ISDB like Japan. This in-band on-channel approach results in highly-compressed audio, and blocks any opportunity for new stations to broadcast. The proprietary iBiquity system, branded as "HD Radio", uses subcarriers and extends out somewhat into the sidebands. The hybrid digital (hence "HD") system can later take the bandwidth used by the current analog stereo system, and eventually go all-digital, though this would shut out every existing analog radio.

Adapted from the Wikipedia article "Technical characteristics", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki