Quasi-synchronous transmission
Encyclopedia
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 It became technically feasible in the mid 1970s, but was rapidly superseded by cellular network
s 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 of about 10-15 hertz
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
Frequency modulation
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
s as their primary oscillator and frequency synthesisers to generate multiple channels from this.
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...
s in the early 1980s, so it is rarely found today. It was invented by engineer J. T. Murasko of UK radiotelephone
Radiotelephone
A radiotelephone is a communications system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the...
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 of about 10-15 hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
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
In radio, narrowband describes a channel in which the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.The opposite of...
Frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
is used exclusively, so that the capture effect
Capture effect
In telecommunication, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency will be demodulated....
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
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...
s as their primary oscillator and frequency synthesisers to generate multiple channels from this.