C-QUAM (Compatible QUadrature Amplitude Modulation) is the method of AM stereo broadcasting used in

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...

, the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

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

publish Publishing is the activity of making information, literature, music, software and other content available to the public for sale or for free. Traditionally, the term refers to the creation and distribution of printed works, such as books, newsp ...

ed in an

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journal A journal, from the Old French ''journal'' (meaning "daily"), may refer to: *Bullet journal, a method of personal organization *Diary, a record of what happened over the course of a day or other period *Daybook, also known as a general journal, a ...

. Using

circuitry An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electrical ...

developed by

Motorola Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorola ...

, 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 Encyclopedia of DNA Elements (ENCODE) is a public research project which aims to identify functional elements in the human genome. ENCODE also supports further biomedical research by "generating community resources of genomics data, software ...

the stereo separation

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

. 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 amplitude modulated signal as input and provides an output, which is the demodulated ''envelope'' of the original signal. ...

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 using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is cap ...

, 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 In electronics, a synchronous detector is a device that recovers information from a modulated signal by mixing the signal with a replica of the unmodulated carrier. This can be locally generated at the receiver using a phase-locked loop or othe ...

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 AMAX is a certification program for AM radio broadcasting standards, created in the United States beginning in 1991 by the Electronic Industries Association (EIA) and the National Association of Broadcasters (NAB). It was developed with the intent ...

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 ...

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 curvature o ...

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 In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is ...

, very unpleasant effects result; hence, the C-QUAM system is not often if ever used for

shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...

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

User base

, 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 statio ...

/570: Corner Brook, NL;

CFCO CFCO (630 AM) is a news, sports, and country music radio station located in Chatham–Kent, Ontario. The station, owned by London, Ontario-based Blackburn Radio, features a heavy local news commitment. CFCO is one of the few dedicated country ...

/630: Chatham, Ontario (covering SW Ontario, Eastern Michigan and Northern Ohio);

WNMB WNMB (900 AM, "El Gallo") is a radio station licensed to North Myrtle Beach, South Carolina, United States, and serves the greater Myrtle Beach area. The station is licensed by the Federal Communications Commission (FCC) to broadcast at 900&nbs ...

/900:

North Myrtle Beach, South Carolina North Myrtle Beach is a city in Horry County, South Carolina, United States. It was created in 1968 from four existing municipalities, and is located about northeast of Myrtle Beach. It serves as one of the primary tourist destinations along ...

; WBLQ/1230: Westerly,

Rhode Island Rhode Island (, like ''road'') is a state in the New England region of the Northeastern United States. It is the smallest U.S. state by area and the seventh-least populous, with slightly fewer than 1.1 million residents as of 2020, but it ...

; WIRY/1340: Plattsburgh, New York;

WAXB WAXB (850 AM), is a radio station licensed to Ridgefield, Connecticut, United States, one of three area stations owned by The Berkshire Broadcasting Corporation. The others are News/Talk 800 AM WLAD and Hot Adult Contemporary 98Q/98.3 FM W ...

/850: Ridgefield, Connecticut; WYLD-AM/940: New Orleans, Louisiana; and

WLAD WLAD (800 AM) is a commercial radio station licensed to Danbury, Connecticut. It broadcasts a talk format. WLAD is owned by Berkshire Broadcasting. The studios and offices are located on Mill Plain Road in Danbury and the transmitter is off ...

/800: Danbury, Connecticut. In addition to FCC-Licensed C-QUAM AM broadcast stations, low-powered (<100 mW)

Part 15 Code of Federal Regulations, 'Title 47, Part 15(47 CFR 15) is an oft-quoted part of Federal Communications Commission (FCC) rules and regulations regarding unlicensed transmissions. It is a part of Title 47 of the Code of Federal Regulations ( ...

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 iBiquity Digital Corporation is a company formed by the merger of USA Digital Radio and Lucent Digital Radio. Based in Columbia, Maryland, with additional offices in Basking Ridge, New Jersey, and Auburn Hills, Michigan, iBiquity is a privately ...

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 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