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C-QUAM AM Stereo

- overview, tutorial or information about C-QUAM, Compatible Quardature Amplitude Modulation system to provide AM Stereo facility for medium wave broadcast transmissions.

With the migration of many listeners from the medium wave bands to the higher quality transmissions on VHF FM many AM broadcasters sought ways of improving the transmissions used on the medium wave bands. One way was to introduce a stereo capability, a feature which is now standard on VHF FM. A number of systems were developed to provide this on AM, but the one which has gained the most widespread acceptance is called C-QUAM (Compatible QUadrature Amplitude Modulation). This system is most widespread in North America, and is used in a number of other countries outside this area, although there is little use within Europe.

How C-QUAM AM Stereo works

In order to see how C-QUAM works the first stage is to look at a basic quadrature system works. Here two audio channels are placed onto the same radio frequency carrier, by placing the sidebands for one at 90 degrees to the other. This can be achieved relatively simply. A standard AM transmitter is used as normal to carry the L + R signal. The sidebands for the difference (L - R) signal are generated by a second transmitter. This one only generates the sidebands as no carrier is required. To obtain the quadrature phasing between the two sets of sidebands a 90 degree phase shift is added to the master oscillator signal for the second transmitter. Finally the outputs from the two transmitters are added together to give a signal which has a carrier with one set of sidebands carrying the normal mono or sum signal, and then a second set of sidebands carrying the stereo difference signal at 90 degrees.

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