RF Circuits

The 19 KHz pilot must be generated by encoder. The L-R (difference) signal must be amplitude modulated onto a suppressed 38 KHz carried. The Phase relationship between the pilot and L-R signals is critical since the pilot is used as the reference for demodulating the difference signal.

Both signals are generated digitally. The 19 KHz pilot is derived from a 19 KHz square wave. The frequency spectrum of a 19 KHz square wave would show the main 19 KHz fundamental frequency with decreasing harmonics at every multiple of 19 KHz above that. These harmonics must be removed. One way would be to use a low pass filter. However a very sharp filter would be required to reduce the harmonics to an acceptable level, the effect of this on the phase of the pilot would cause serious problems because of the reasons mentioned above.

A far better approach is to cancel each harmonic using a sine wave of the same frequency and amplitude but opposite phase. Even simpler still a square wave could be used to cancel each harmonic. Taking as an example, the pilots first harmonic at 38 KHz, a 38 HKz square wave can be used to cancel that harmonic. The first harmonic of 38 KHz will occur at 76 KHz which will mix with the third harmonic of the pilot at that frequency. The resultant can then be cancelled with another square wave. The same approach is adopted for the second harmonic of the pilot. By mixing various square wave using some logic IC's the relative levels of the harmonics can be adjusted. This enables the reduction of some higher order harmonics. Once the lower harmonics have been removed the highest one can be removed by a simple filter.

Switching between the left and right audio channels at 38 KHz will create the amplitude modulated, suppressed carried diffrence signal. It also produces harmonics, first at 76 KHz and so on. This harmonic signal can be cancelled by adding difference signal to it that has been switched at 76 KHz. This will leave further harmonics starting at 114 KHz. Again the square wave can be mixed with other square waves modifying the relative harmonic levels to cancel some higher harmonics. Those remaining are of sufficiently high order to be filtered out using a low pass filter, the some one used to filter out the higher order harmonics of the pilot.

All of the digital waveforms required can be obtained from single crystal controlled clock by dividing and an assortment of logic chips.

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