AM STEREO Modulation Systems
Stereo broadcasting on the commercial AM band was originally tested in the 50's and 60's on New York stations, WABC and WCBS, utilizing a quadrature modulation system being developed by Philco.
In the 70's, AM Stereo was tested on various radio stations in Canada, Mexico and the USA, utilizing different modulation systems being developed by competitors Kahn-Hazeltine, Magnavox, Belar, Harris and Motorola.
In the end, the Motorla C-QuAM AM Stereo modulation system was adopted as the defacto standard in most countries.
AM Stereo Technical Issues
The primary difficulty implementing AM Stereo was maintaining mono compatibility with the millions of existing radio receivers already in use, which for the most part, relied upon a simple 'envelope detector' design.
While most of the competitors attempted various angular modulation approaches to superimpose a 2nd information signal on the AM carrier, Kahn-Hazeltine sought to pursue a novel Independent Side Band (ISB) method.
Motorola's "Compatible Quadrature Amplitude Modulation" became the adopted standard in several countries in the 1980's and the USA in 1993.
The C-QuAM signal is comprised of two distinct modulation stages. A conventional AM version and a compatible quadrature Phase Modulation version.
Stage 1 provides the transmitter with a summed L+R mono audio input. This input is precisely the same as conventional AM-Mono transmission methods and ensures 100% compatibility with conventional 'envelope detector' receivers.
Stage 2 provides the stereo multiplexed audio input and replaces the conventional crystal oscillator stage of otherwise AM-Mono transmitters. So as to minimize interference with 'envelope detector' receivers, the stage 2 signal takes the muxed audio signals and phase modulates both, using a divide-by-4 Johnson counter and two balanced modulators operating 90 degrees out of phase with each other. Stage 2 is not amplitude modulated, it is phase modulated, and is made up of both a L+R input and a L-R input.
To recover the 'stereo' audio signals, a 'synchronous detector' is utilized to extract the L-R audio from the phase modulated quadrature portion of the signal created in stage 2. The L+R audio can be extracted from either the AM (stage 1) or the PM (stage 2) modulation component. From there, the audio can be readily de-muxed back to 'stereo', providing the Left and Right channels.
Beyond ISB and C-QuAM with AM Stereo by DJROCK
Independent Side Band and Superimposed Angular Modulation techniques were the state of the art for AM Stereo up until recently. With the evolution of Digital Signal Processing (DSP) and Software Defined Radios (SDR) this is about to change.
DJROCK has invented a new analog AM Stereo modulation system which provides 100% compatibility with all existing commercial band AM radios and provides significantly improved channel separation for the next generation of AM Stereo receivers. This new technology is also, ISB, C-QuAM and AMAX compatible. In fact it's even compatible with iBiquity HD-AM and the Digital Radio Mondiale (DRM) system.