The circuit below shows a fairly conventional switching type stereo coder. The left and right inputs emerge as a single output to connect straight into your mono fm transmitter. Pre-emphasis is fitted right at the input stages and the 47k resistor paralleled with 1 nF capacitors (47 microseconds)give a time constant very close to the required 50 micro seconds. For the 75 microsecond standard in the USA, the capacitors could be changed to 1.5 nF value. Long term stability is guaranteed by using a 4.864 MHz crystal to control the oscillator within the 4060 oscillator/divider IC. This high frequency is divided down to provide high accuracy pilot tone and subcarrier signals for further processing.

The heart of the circuit is the 4066 IC which is effectively two high speed switches operated by two 38 KHz switching waveforms, 180 degrees out of phase with each other. Thus left and right inputs are alternately switched on and off, the two switched outputs being summed through the 4k7 resistors at pins 2 and 3. So at any given instant in time, either the left or right channel will be transmitted. This changes 38 thousand times per second. At the listener's receiver, a similar set of electronic switches will be synchronised (using the 19 KHz 'pilot tone') and will separate the composite signal back into 2 separate signals, left and right!

If you want to build this nice circuit, the crystal can be obtained from NRG Kits and all the other parts are readily available. Remember, turn your Transmitter pre-emphasis off when using any stereo coder!