What is Quasi-Sync?
Quasi-sync is basically TWO repeaters running on exactly the same frequency with exactly the same audio coming from each.
Normally with FM having huge amounts of overlap between repeaters creates nasty "hetrodyning" and unwanted burbling noises in the "overlap zone" (areas in which both signals are received at more or less the same signal). However once you get close to one or the other repeater, capture effect takes over and you would hear one or the other clearly.
Quasi-sync is a technique whereby the audio coming from each station is absolutely identical (frequency response, deviation, group delay, etc, etc), and the audio is timed such that there is less than 300 microseconds delay between the two sources. Thereby at any instantaneous period in time then the actual carrier frequency of both stations (remember that with FM the carrier frequency changes depending upon modulation) is always within around 20hz.
To achieve quasi-sync traditionally used complicated and unreliable delay lines, audio filters, graphic equalisers, group delay compensation and suchlike. Circuits between transmitter sites needed to be very carefully controlled - if using BT landlines then even an engineer re-patching the circuit or moving cables around in a green cabinet could mean complete re-alignment of the entire system.
Today with digital radios (DDS synthesizers), digital voice CODECs that allow the audio to be perfectly GPS time-synced upon playout and other such advances, quasi-sync is a possibility within the grasp of radio amateurs.
Receive is simple - both sites receive audio, time-stamp it from GPS, then a central server makes a decision based upon the best receive signal level. This is effectively "diversity receive" and gives around 6dB improvement in receive performance within the overlap zone. Similar gains are also seen on transmit within the overlap zone. Outside of the "overlap zone", where only one repeater is receivable, then signal levels degrade as per normal FM.
In order to avoid standing waves causing deep nulls, it is normal that each transmitter is slightly offset from each other in terms of frequency, typically 5-25hz, depending upon preference of the implementer. This means that in an area of equi-signal you can get "mobile flutter" effects, like you are mobile, even though you are stationary. This is normal and unfortunately an unavoidable side-effect of the system. This is most pronounced in areas of exact equi-signal, where deep fades of up to 20dB can sometimes be observed.
An excellent document for those who wish to read more about this subject in great detail can be found at http://quasi-sync.atackscomputers.co.uk/SimulcastingWithoutTooManyTears.pdf
