In radio broadcasting, quasi-synchronous transmission is a method of achieving wider area coverage using multiple transmitters but without needing multiple frequencies. It became technically feasible in the mid 1970s, but was rapidly superseded by
cellular networks in the early 1980s, so it is rarely found today. It was invented by engineer J. T. Murasko of UK
radiotelephone manufacturer Dymar Electronics.
The principle of operation is to precisely control the transmission frequency so accurately that interference between adjacent transmitters is kept under control, with a
beat frequency
In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, ''perceived'' as a periodic variation in volume whose rate is the difference of the two frequencies.
With tuning instruments that can produce ...
of about 10-15
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...
only. While this creates some audible affects in the receiver in the overlap region, it is usually tolerable and doesn't affect the intelligibility of the signal.
Narrowband
Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, narrowband sounds are sounds that occupy a narrow range of frequencies. In telephony, narrowband is usua ...
Frequency modulation
Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing.
In analog fre ...
is used exclusively, so that the
capture effect will select the stronger of two transmitters wherever one predominates. While frequency control down to 1-2 hertz accuracy is achievable, this is undesirable, since the cancellation of signals would wipe out the signal at the receiver for up to a second. The deliberate offset prevents this at the expense of a "motorboating" effect.
Transmitters use highly stable
crystal oven
A crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic crystal oscillators at a constant temperature, in order to prevent changes in the frequency due to variations in ambient temperature. An oscill ...
s as their primary oscillator and
frequency synthesisers to generate multiple channels from this.
{{DEFAULTSORT:Quasi-Synchronous Transmission
Radio