AVR FM Stereo Transmitter

Description: 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. 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." A double sideband channel difference signal, often referred to as "L-R," is also transmitted. 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, is 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.

The designs of a classical encoder typically include an oscillator and frequency divider, a balanced modulator, one or more summing amplifiers, and a few filters. The 38 kHz oscillator could be implemented using a 74HCT60 oscillator/counter chip and a 38 kHz crystal. The balanced modulator can be constructed using a Gilbert cell multiplier or a set of transmission gates that switch complementary audio signals. The L+R function can be achieved with a pair of resistors, while the L-R signal can be generated using a simple operational amplifier (op-amp) circuit. If an op-amp is used to sum all the signals together, it must be capable of passing the 38 kHz and its sidebands while maintaining phase with respect to the baseband L+R signal.

A notable implementation of a stereo encoder can be found in the work of Harry Lythall, whose circuit is characterized by simplicity and elegance. This encoder utilizes a microcontroller to replace traditional components such as the oscillator/counter and analog transmission gates. The use of a low-cost microcontroller, an NPN transistor, and a few passive components enables the construction of a straightforward stereo FM transmitter. The design emphasizes efficiency and reliability in signal processing, ensuring the effective transmission of the composite stereo signal while minimizing interference and maintaining audio fidelity.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." 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 oscillator/counter chip and a 38 kHz crystal. The balanced modulator could be made with a Giblert cell multiplier for something nice, or a set of transmission gates switching complimentary audio signals, or ever with resistor networks being switched by micro controller I/O pins. The L+R function could be achieved with a pair or resistors and the L-R signal by a simple op amp circuit.

If an op amp were to be used to sum all the signals together, it would have to be quite good - passing the 38 kHz and its sidebands and maintaining phase with respect to the baseband L+R signal. I had bought some pretty nice op amps in anticipation of using them, until I finally understood the simpler method, described below.

I came across a very simple and robust stereo encoder project on Harry Lythall's website. Harry's amateur radio call sign is SM0VPO, and he can be readily Goggled. The circuit was simple, it was elegant, and I didn't have clue as to how it worked. After coming back to his project several times, I realized that I had seen a write-up of this technique, but I had not understood it well enough to appreciate what I was looking at. Harry Lythall's projects can be found at: http://www.sm0vpo.com/audio/stereo_enc.htm. The technique that lends itself to such an elegant implementation is explained at: http://web.archive.org/web/20060214011755/http://transmitters.tripod.com/stereo.htm <== This will open a new window.

The original link, http://transmitters.tripod.com/stereo.html, stopped working within days of the first publication of this web page, and I am grateful to Internet Archive for making it available to us. What I added to Harry's encoder was a slight twist. That of using an AVR micro controller to replace the oscillator/counter and the analog transmission gates.

And that is what this circuit does. A cheap micro controller, an NPN transistor and a handful of passive components is all that it takes to make a simple stereo FM transmitter.