C-QUAM: Difference between revisions

'''C-QUAM''' (Compatible QUadrature Amplitude Modulation) is the method of [[AM stereo]] broadcasting used in [[Canada]], the [[United States]] and most other countries. It was invented in 1977 by Norman Parker, Francis Hilbert, and Yoshio Sakaie, and [[publish]]ed in an [[IEEE]] [[Academic journal|journal]].

Using [[circuitry]] developed by [[Motorola]], C-QUAM uses [[quadrature amplitude modulation]] (QAM) to [[Code|encode]] the stereo separation [[signal (computing)|signal]]. This extra signal is then stripped down in such a way that it is compatible with the [[envelope detector]] of older receivers, (hence the name C-QUAM, i.e.for Compatible QUadrature Amplitude Modulation). A 25 [[Hertz|Hz]] [[pilot tone]] is added to trigger [[receiver (radio)|receivers]]; itunlike its counterpart in [[FM radio]], this carrier is not necessary for the reconstruction of the original audio sources.

== How it worksDescription ==

Stage 2 provides the stereo mixedmultiplexed (muxed) audio input and replaces the conventional crystal oscillator stage of otherwise AM-Mono transmitters. So as to not create interference with 'envelope detector' receivers, the stage 2 signal takes the mixedmultiplexed (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]] extracts 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-multiplexed (de-muxed) back to 'stereo', a.k.a. Left and Right channels.

{{As of|2014|03}}, there are still a number of AM radio stations in North America broadcasting in C-QUAM stereo. Among those stations are [[WXYG]]/540: Sauk Rapids, MN; [[CFCB]]/570: Corner Brook, NL; [[CFCO]]/630: Chatham, Ontario (covering SW Ontario, Eastern Michigan and Northern Ohio); [[WLS (AM)|WLS]]/890 (now during both day and night hours): [[Chicago|Chicago, Illinois]]; [[WNMB]]/900: [[North Myrtle Beach|North Myrtle Beach, South Carolina]]; [[WBLQ (AM)|WBLQ]]/1230: [[Westerly, Rhode Island|Westerly]], [[Rhode Island]]; [[WIRY (AM)|WIRY]]/1340: Plattsburgh, New York; [[WAXB]]/850: Ridgefield, Connecticut; and [[WLADWYLD-AM]]/800940: DanburyNew Orleans, ConnecticutLouisiana. In addition to FCC-Licensed C-QUAM AM broadcast stations, low-powered (<100&nbsp;mW) [[Part 15]] C-QUAM stereo transmitters are available for sale for use in the United States. In Rome, Italy, there is Broadcastitalia on 1485&nbsp;kHz.

While C-QUAM is an accepted international standard for AM Radio broadcasting, it is [[:wikt:incompatible|incompatible]] with the [[IBOC]] (In-band on-channel) "HD" (Hybrid Digital) radio system, so a broadcaster must choose what system they will use. The IBOC system allows transmission of an audio frequency range extending to approximately 15&nbsp;kHz, 2-ch Stereo on the AM band, but with significant digital artifact and aliasing due to substantial codec inadequacy{{whom|date=August 2023}}.

Very few {{Cn|date=May 2013}} AM radio stations that broadcast with IBOC [[HD Radio]] during the day switch to C-QUAM AM Stereo during nighttime operation to reduce sideband digital (hash) interference and to provide long-range stereo reception. ManyA number of HD radio tuners have athe limited ability to decode C-Quam stereo transmissions, (typically with lower bandwidth), and as a result, reduced audio quality than what could be expected from a specifically designed AMAX/C-QUAM only tuner. C-QUAM AM Stereo transmissions have the same range as AM Monural transmission, a key benefit.

[https://web.archive.org/web/20160305111427/http://djrock.easycgi.com/DigiModSys2013/library/CQuam%20Intro.pdf]