Stereo Multiplexer

From Electronics Wiki

Overview

 Most of us are familiar with the stereo multiplexer chips that are on the market like the
ROHM BA1404 and so I am posting this so that we don't lose sight of the working internals of a
stereo multiplexer.
Indeed if you have a hand full of transistors and a few other parts, you can build one with
discreet components.

Circuit Description

 At the top left of schematic we see the two audio inputs Left and Right. These are fed to
differential amplifiers so that we can achieve the desired phase from the left & right channels.
With these amplifiers we can produce the desired (L+R) and (L-R). Notice there is only one output
on the left side. We only need one phase for the left channel. The (L-R) are combined and sent to
the balanced modulator and are superimposed on a 38 KHz carrier signal to produce a 38KHz double
sideband signal. The (L+R) are combined into a composite pure audio signal.

 Chip U1 is a '4046 13 stage divider/counter. It has a built in square-wave crystal oscillator. By
dividing down the crystal's frequency of 4.864Mhz we can get both the 38KHz carrier frequency and
the 19KHz pilot frequency. The 19KHz pilot frequency is filtered and fed to the composite output.
The output of the balanced modulator is also fed to the composite output along with the audio (L+R)
monaural signal. The 19Khz should be about 10% to 20% of the total signal strength. The 38Khz
double sideband signal and the monaural audio signal comprise 50% and 50% of the remaining 100%
since they are used to cancel each other out creating the speaker shifting effect of the stereo
signal in the receiver.

 You can feed the composite signal directly to the oscillator or VCO of an FM transmitter for
stereo transmission.