C-QUAM
Encyclopedia
C-QUAM 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 published in an IEEE journal
.
Using circuitry developed by Motorola
, C-QUAM uses quadrature amplitude modulation
(QAM) to encode
the stereo separation 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. Compatible QUadrature Amplitude Modulation). A 25 Hz
pilot tone is added to trigger receivers
; it is not necessary for the reconstruction of the original audio sources.
used for FM stereo, the audio in the C-QUAM signal is the stereo difference — the left channel "minus" the right channel (L − R). (This "subtraction" is accomplished by simply reversing the polarity
of the right channel before mixing it with the left.) The main audio is the stereo sum, or left channel plus right channel (L + R). Once fully demodulated
at the receiver, adding the two together yields the left channel again (L+R + (L−R) = 2L), and subtracting the difference then gives the right (L+R − (L−R) = 2R). This method of multiplexing
audio is common to all analogue stereo systems.
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 has been alleviated in subsequent revisions. Also, since some stereo information is contained in the sideband
s, adjacent channel interference can cause problems. Finally, when only part of a sideband is attenuated (as often happens to skywave
signals reflecting off the ionosphere
), an effect known as selective fading
, very unpleasant effects result; hence, the C-QUAM system is not often if ever used for shortwave
broadcasting, nor by stations which receive a great deal of skywave interference.
1330 kHz, Bartow, Florida; CFCO 630 kHz Chatham, Ontario (covering SW Ontario, Eastern Michigan and Northern Ohio); KCJJ
1630 kHz, Iowa City, Iowa (covering Iowa and several states at night); WLS
890 kHz during night hours, Chicago, Illinois
; WNMB
900 kHz, North Myrtle Beach, South Carolina; WXNI
1230 kHz, Westerly, Rhode Island
; WREF, Ridgefield, Connecticut; and WLAD
, Danbury, Connecticut. In addition to FCC-Licensed C-QUAM AM broadcast stations, low-powered (<100mW) 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.
. The IBOC (In Band On Channel) 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, which is further complicated due to proprietary nature of the company, iBiquity
, which not only sells and licenses the associated equipment to broadcasters, but also maintains control over its deployment, use, and distribution. This includes 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.
A 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. Many HD radio tuners have a limited ability to decode C-Quam stereo transmissions, i.e. at lower bandwidth, and as a result, reduced audio quality than what could be expected from a specifically designed C-Quam only tuner.
AM stereo
AM stereo is a term given to a series of mutually incompatible techniques for wireless radio broadcasting stereo audio in the AM band in a manner that is compatible with standard AM receivers...
broadcasting used in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and most other countries. It was invented in 1977 by Norman Parker, Francis Hilbert, and Yoshio Sakaie, and published in an IEEE journal
Academic journal
An academic journal is a peer-reviewed periodical in which scholarship relating to a particular academic discipline is published. Academic journals serve as forums for the introduction and presentation for scrutiny of new research, and the critique of existing research...
.
Using circuitry developed by Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
, C-QUAM uses quadrature amplitude modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
(QAM) to encode
Encode
Encode may refer to:* Can be related to "Code"* Encode ApS, a Danish software company* Encode SA, a Greek information security company* ENCODE, the ENCyclopedia Of DNA Elements...
the stereo separation signal
Signal (computing)
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
. This extra signal is then stripped down in such a way that it is compatible with the envelope detector
Envelope detector
An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls...
of older receivers (hence the name C-QUAM, i.e. Compatible QUadrature Amplitude Modulation). A 25 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
pilot tone is added to trigger receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
; it is not necessary for the reconstruction of the original audio sources.
How it works
As with the subcarrierSubcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
used for FM stereo, the audio in the C-QUAM signal is the stereo difference — the left channel "minus" the right channel (L − R). (This "subtraction" is accomplished by simply reversing the polarity
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...
of the right channel before mixing it with the left.) The main audio is the stereo sum, or left channel plus right channel (L + R). Once fully demodulated
Demodulation
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...
at the receiver, adding the two together yields the left channel again (L+R + (L−R) = 2L), and subtracting the difference then gives the right (L+R − (L−R) = 2R). This method of multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
audio is common to all analogue stereo systems.
Known problems
C-QUAM is not perfect, however, in large part because pre-AMAXAMAX
AMAX is a United States certification program developed by the Electronic Industries Association and the National Association of Broadcasters in 1993. This quality control program addressed both consumer receiver developments and air chains of broadcast AM transmission stations...
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 has been alleviated in subsequent revisions. Also, since some stereo information is contained in the sideband
Sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...
s, adjacent channel interference can cause problems. Finally, when only part of a sideband is attenuated (as often happens to skywave
Skywave
Skywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...
signals reflecting off the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
), an effect known as selective fading
Selective fading
Selective fading or frequency selective fading is a radio propagation anomaly caused by partial cancellation of a radio signal by itself — the signal arrives at the receiver by two different paths, and at least one of the paths is changing...
, very unpleasant effects result; hence, the C-QUAM system is not often if ever used for shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
broadcasting, nor by stations which receive a great deal of skywave interference.
User base
, there are still several AM radio stations in North America broadcasting in C-QUAM stereo. Among those stations are WWBFWWBF
WWBF is a full-time commercial broadcast radio station located in Bartow, Florida, USA. As the capital city of Polk County, Bartow is centrally located in the Lakeland-Winter Haven Metropolitan Area....
1330 kHz, Bartow, Florida; CFCO 630 kHz Chatham, Ontario (covering SW Ontario, Eastern Michigan and Northern Ohio); KCJJ
KCJJ
KCJJ is a radio station licensed to serve Iowa City, Iowa, USA. The station is owned by Steve Bridges' River City Radio. The station was assigned the KCJJ call sign by the Federal Communications Commission in 1982.-Programming:...
1630 kHz, Iowa City, Iowa (covering Iowa and several states at night); WLS
WLS (AM)
WLS is a Chicago clear-channel AM station on 890 kHz. It uses C-QUAM AM stereo and transmits with 50,000 watts from transmitter and towers on the south edge of Tinley Park, Illinois....
890 kHz during night hours, Chicago, Illinois
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...
; WNMB
WNMB
WNMB is an oldies music formatted radio station licensed to North Myrtle Beach, South Carolina, USA, and serves the greater Myrtle Beach area. The Norman Communications, Inc., outlet is licensed by the Federal Communications Commission to broadcast at 900 kHz with a TPO of 500W...
900 kHz, North Myrtle Beach, South Carolina; WXNI
WXNI
WBLQ is a radio station licensed to serve Westerly, Rhode Island. The station is owned by Diponti Communications.-1999:After WERI was sold, the callsign was changed to WXNI on January 4...
1230 kHz, Westerly, Rhode Island
Rhode Island
The state of Rhode Island and Providence Plantations, more commonly referred to as Rhode Island , is a state in the New England region of the United States. It is the smallest U.S. state by area...
; WREF, Ridgefield, Connecticut; and WLAD
WLAD
WLAD is a radio station licensed to serve Danbury, Connecticut, United States. The station is owned by The Berkshire Broadcasting Corp. It airs a News/Talk format....
, Danbury, Connecticut. In addition to FCC-Licensed C-QUAM AM broadcast stations, low-powered (<100mW) 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.
Technological obsolescence
C-QUAM is incompatible with IBOC digital radioDigital radio
Digital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
. The IBOC (In Band On Channel) 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, which is further complicated due to proprietary nature of the company, iBiquity
IBiquity
iBiquity Digital Corporation is a company formed by the merger of USA Digital Radio and Lucent Digital Radio, with the goal of creating an in-band on-channel digital radio system for the United States and around the world...
, which not only sells and licenses the associated equipment to broadcasters, but also maintains control over its deployment, use, and distribution. This includes 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.
A very few AM radio stations that broadcast with IBOC HD Radio
HD Radio
HD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals...
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. Many HD radio tuners have a limited ability to decode C-Quam stereo transmissions, i.e. at lower bandwidth, and as a result, reduced audio quality than what could be expected from a specifically designed C-Quam only tuner.