C-QUAM (Compatible QUadrature Amplitude Modulation) is the method of

AM stereo AM stereo is a term given to a series of mutually incompatible techniques for radio broadcasting stereo audio in the AM band in a manner that is compatible with standard AM receivers. There are two main classes of systems: independent sideban ...

broadcasting used in

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

, the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

and most other countries. It was invented in 1977 by Norman Parker, Francis Hilbert, and Yoshio Sakaie, and

publish Publishing is the activities of making information, literature, music, software, and other content, physical or digital, available to the public for sale or free of charge. Traditionally, the term publishing refers to the creation and distribu ...

ed in an

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journal. Using circuitry developed by

Motorola Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois. It was founded by brothers Paul and Joseph Galvin in 1928 and had been named Motorola since 1947. Many of Motorola's products had been ...

, C-QUAM uses

quadrature amplitude modulation Quadrature amplitude modulation (QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. It conveys two analog message signa ...

(QAM) to encode the stereo separation

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

. This extra signal is then stripped down in such a way that it is compatible with the

envelope detector An envelope detector (sometimes called a peak detector) is an electronic circuit that takes a (relatively) high-frequency signal as input and outputs the '' envelope'' of the original signal. Diode detector A simple form of envelope detect ...

of older receivers, hence the name C-QUAM for Compatible. A 25 Hz pilot tone is added to trigger receivers; unlike its counterpart in

FM radio FM broadcasting is a method of radio broadcasting that uses frequency modulation (FM) of the radio broadcast carrier wave. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to transmit high fidelity, high-f ...

, this carrier is not necessary for the reconstruction of the original audio sources.

Description

The C-QUAM signal is composed of two distinct modulation stages: a conventional AM version and a compatible quadrature PM 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 (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 multiplexed (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. For additional information, see the attached PDF: "Introduction to the Motorola C-QUAM AM Stereo System".

Known problems

C-QUAM is not perfect, however, in large part because pre- AMAX it exhibited platform motion, with the audio "center" rocking back and forth as if changing the balance knob. This effect is potentially bothersome, especially in a moving vehicle where the received signal changes rapidly, and occupants (particularly the driver) would be more prone to its effects (this was an effect that happened primarily with skywave signals. Groundwave or local coverage usually did not suffer from this issue). This has been alleviated in subsequent revisions. Also, since some stereo information is contained in the

sideband In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands c ...

s, adjacent channel interference can cause problems. Finally, when only part of a sideband is attenuated (as often happens to

skywave In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvatur ...

signals reflecting off the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

), an effect known as selective fading, very unpleasant effects result; hence, the C-QUAM system is not often if ever used for

shortwave Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (app ...

broadcasting, nor by stations which receive a great deal of skywave interference.

User base

Nowadays, this standard faces fierce

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...

with some other stereophonic standards on AM. , 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 CFCB (570 kHz) is an AM radio station in Corner Brook, Newfoundland and Labrador, Canada. It broadcasts at a power of 10,000 watts by day and 1,000 watts at night. Owned by Stingray Group, CFCB first went on the air on October 3, 1960. The sta ...

/570: Corner Brook, NL; CFCO/630: Chatham, Ontario (covering SW Ontario, Eastern Michigan and Northern Ohio); WNMB/900: North Myrtle Beach, South Carolina; WBLQ/1230: Westerly,

Rhode Island Rhode Island ( ) is a state in the New England region of the Northeastern United States. It borders Connecticut to its west; Massachusetts to its north and east; and the Atlantic Ocean to its south via Rhode Island Sound and Block Is ...

; WIRY/1340: Plattsburgh, New York; WAXB/850: Ridgefield, Connecticut and WYLD-AM/940: New Orleans, Louisiana. In addition to FCC-Licensed C-QUAM AM broadcast stations, low-powered (<100 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 kHz. Also see: * AM Stereo radio stations in the United States * AM Stereo radio stations worldwide

Competition from IBOC Hybrid Digital Systems

While C-QUAM is an accepted international standard for AM Radio broadcasting, it is incompatible with the

IBOC In-band on-channel (IBOC) is a hybrid method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency. The name refers to the new digital signals being broadcast in the same AM or FM band (in-band) ...

(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 kHz, 2-ch Stereo on the AM band, but with significant digital artifact and aliasing due to substantial codec inadequacy. In addition, C-QUAM patents have expired. iBiquity still controls IBOC intellectual property through patents, through licensing fees for both the use of the technology, and any modifications to be made, even if the broadcaster in question has purchased the equipment outright and made costly modifications to their transmitter plant in order to implement it. Very few AM radio stations that broadcast with IBOC

HD Radio HD Radio (HDR) is a trademark for in-band on-channel (IBOC) digital radio broadcast technology. HD radio generally simulcast, simulcasts an existing analog radio station in digital format with less noise and with additional text information. HD R ...

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. A number of HD radio tuners have the 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. Whereas many stations in the late 2000s changed from C-QUAM to HD Radio, in the 2010s the trend reversed with many HD Radio stations shutting off their digital equipment. However, few of these stations returned to C-QUAM broadcasts. There has been a move to bring back C-QUAM in the last few years, due to the poor sound quality of digital audio encoding at low bit rates. Where AM stereo receivers use a dual IF bandwidth setup, for an extended audio frequency response over mono receivers. Providing for a full, rich stereo sound is simply not possible with digital audio encoding. The down side of analog broadcasting is the amount of unwanted noise.

References

Introduction to the Motorola C-QUAM AM Stereo System

Introduction to the Motorola C-QUAM AM Stereo System

External links