Compatible four channel FM system

DETAILED DESCRIPTION A four channel audio system is provided which is fully compatible with standard FM stereo and mono equipment.
According to the present invention, a switching or sampling system is employed so that four audio channels are transmitted by the FM station.
These four channels are designated left front, right front, left rear and right rear and are sometimes hereinafter abbreviated LF, RF, LR and RR, respectively.
In accordance with one embodiment of the invention, these four channels are superimposed on the 38 kHz subcarrier and the usual 19 kHz pilot signal is used as a switching signal.
During the first half cycle of the 19 kHz signal, the left front and right front information is transmitted while during the second half cycle, the left rear and right rear information is transmitted.
The 19 kHz pilot is then used as a switching signal between the front and rear information as is hereinafter described in detail while the 38 kHz signal switches between left and right in the usual manner.
This system has the advantage of not requiring an increase in bandwidth, not requiring any additional pilot or subcarrier frequencies and permits the radio station to continue to use its normal 67 kHz subcarrier for SCA purposes.
However, this embodiment does not ordinarily permit the use of the full frequency spectrum for front and back information so that it is sometimes preferable to provide another subcarrier to carry the front and rear information.
When this is done, the 19 kHz pilot signal is quadrupled and this quadrupled signal is used for switching between front and back information.
Both embodiments of the present invention are completely compatible with present mono and stereo equipment.
The main channel carriers all four aduio channels so that on a mono receiver the four audio signals are combined.
On a stereo receiver, the left and the right information is extracted in the usual manner and it is only with the receiver equipped for four channel reception that the signal produced by the system of the present invention is broken into its four components