AVR FM Stereo Transmitter

Posted on Sep 1, 2012

A composite stereo signal, as transmitted by FM radio stations, is composed of at least three parts: A base band mono signal, a double sideband channel difference signal, and a pilot carrier. The signal composition is somewhat analogous to an NTSC composite color television signal. I said `at least three parts` because some stations transmit other things such as data and background music that our normal FM receivers do not decode. The base band signal falls between 30 Hz and 15 kHz. This is the part of the audio signal that comes out of the speaker on a mono receiver, In the classical encoder, it is made by simply adding the Left and Right channel audio signals together, and is often referred to as `L+R.`

AVR FM Stereo Transmitter
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AVR FM Stereo Transmitter - image 1
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A double sideband channel difference signal, often referred to as "L-R" is also transmitter. The information in this signal is the difference between the signals in the Left and Right channels. In the classical encoder, the channel difference signal is made by subtracting the Right channel's audio signal from the Left channel's audio signal. The channel difference signal is then combined with a 38 kHz carrier in a balanced modulator to form a double sideband signal centered at 38 kHz. The third signal, a pilot carrier at 19 kHz, exactly half the frequency of the carrier used to generate the 38 kHz double sideband signal. The 19 kHz signal is used to regenerate the missing 38 kHz carrier in the receiver and this 38 kHz carrier is used to demodulate the double sideband signal. The three separate signals are not intended to affect each other. Careful filtering can minimize undesirable interactions, most of which would be some kind of beat between the 19 kHz pilot signal and the Left and Right channels and their products. I fiddled with the classical encoder on paper a number of times over the years. The designs always had all sorts of neat blocks - an oscillator and frequency divider, a balanced modulator, one or more summing amplifiers and a few of filters. In all, something that would not result in a home project that would be easy to duplicate. The 38 kHz oscillator could be made with a 74HCT60...

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