AM stereo

AM Stereo is a term given to a series of mutually incompatible techniques for 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 sideband (ISB) systems, promoted principally by American broadcast engineer Leonard Kahn; and quadrature amplitude modulation (QAM) multiplexing systems (conceptually closer to FM stereo).

History

Early experiments with stereo AM radio involved two separate stations (both AM or sometimes one AM and one FM) broadcasting the left and right audio channels. This system was not very practical, as it required the listener to use two separate radios. Synchronization was problematic, often resulting in "ping-pong" effects between the two channels. Reception was also likely to be different between the two stations, and many listeners used mismatching models of receivers.

After the early experiments with two stations, a number of systems were invented to broadcast a stereo signal in a way which was compatible with standard AM receivers.

FM Stereo was first implemented in 1961. In the United States, FM overtook AM as the dominant broadcast radio band in the late 1970s and early 1980s

Timeline

1924 - WPAJ broadcast in stereo from New Haven, CT. using two transmitters. One on 1120 KHz and the other on 1320 KHz. However stereo seperation was poor (to preserve compatability for mono listeners). ^[1]
1960 - AM stereo first demonstrated on XETRA-AM, Tijuana, Mexico, using the Kahn independent sideband system.
1963 - WHAZ runs a stereo program on eight AM stations, four on each channel.
1980 - After five years of testing the five systems, the United States Federal Communications Commission (FCC) selected the Magnavox system as the official AM stereo standard. The FCC's research is immediately accused of being flawed and incomplete.
1982 - After a series of lawsuits and accusations, the FCC decides to "let the marketplace decide" and revokes the Magnavox certification as the AM stereo standard for political reasons. Belar drops out of the AM stereo race, leaving Motorola C-QUAM, Harris Corporation, Magnavox, and the Kahn/Hazeltine independent sideband system.
1984 - General Motors, Ford, Chrysler, and a number of import automakers begin installing C-QUAM AM Stereo receivers in automobiles, beginning with the 1985 model year. Harris Corporation abandons its AM stereo system and puts its support behind C-QUAM (Harris continues to manufacture C-QUAM equipment today).
1985 - AM Stereo broadcasting officially begins in Australia, with the C-QUAM standard.
1988 - Canada and Mexico adopt C-QUAM as their standard for AM Stereo.
1992 - Japan adopts C-QUAM as its standard for AM Stereo.
1993 - The FCC makes C-QUAM the AM stereo standard for stations in the U.S., and also requires Expanded Band AM (1610-1700 kHz) stations to broadcast in AM stereo (although that rule was never actively enforced).
1993 - The AMAX certification program began. This was to set an official manufacturing standard for high-quality AM radio receivers, with a wider audio bandwidth for higher fidelity reception of strong signals, and optionally C-QUAM AM stereo. Despite the availability of AMAX receivers from companies like Sony, General Electric, Denon, and AMAX-certified car radios from the domestic and Japanese automakers, most electronics manufacturers did not wish to implement the more costly AMAX tuner design in their radios, so most AM radios today remain in mono with limited fidelity.
2006 to present - AM stereo gains new life through the support for C-QUAM decoding in most receivers designed for HD Radio. These new digital radios receive AM stereo signals, although with limited fidelity due to the design of the HD Radio chipset.

Broadcasting systems

The Magnavox PMX, Harris Corporation V-CPM, and Motorola C-QUAM (Compatible - Quadrature Amplitude Modulation) were all based around modulating the phase and amplitude of the carrier, placing the stereo information in the phase modulated portion, while the standard mono (L+R) information is in the amplitude modulation. The systems all did this in similar (but not completely compatible) ways. The original Harris Corporation system was later changed to match the Motorola C-QUAM pilot tone for indicating the station was in stereo, thus making it compatible with all C-QUAM receivers.

Harris System

This system was developed by Harris Corporation, a major manufacturer of radio/TV transmitters. Harris is the successor to the pioneer Gates radio line. Harris was the early leader in the AM Stereo wars. It was implemented by a large number of stations in the 1980s, but the FCC temporarily rescinded their approval of the Harris system, causing most to switch to Motorola's C-QUAM system. This Harris system eventually changed their pilot tone to be compatible with C-QUAM. CKLW in Windsor, Ontario, Canada (also serving nearby Detroit, Michigan) was among the first stations to broadcast in Harris AM Stereo. The Harris system is currently no longer used in its original form.

Magnavox System

This system was developed by electronics manufacturer, Magnavox. It is a phase modulation system. It was initially declared the AM Stereo standard by the FCC in 1980, but the FCC later declared that stations were free to choose any system. As with the Harris system, it was popular in the 1980s, but most stations stopped broadcasting in stereo, or upgraded to the C-QUAM system as time went on. 1190 WOWO in Fort Wayne, Indiana was the (then) 50,000-watt clear channel Magnavox flagship station.

Motorola C-QUAM

C-QUAM was developed and promoted primarily by Motorola, a long time manufacturer of two-way radio equipment. It became the dominant system by the late 1980s, and was declared the official standard by the FCC in 1993. While many stations in the USA have since discontinued broadcasting in Stereo, many still have the necessary equipment to do so. C-QUAM is still popular in other parts of the world, such as Canada, Japan, and Australia which it was declared the official standard.

C-QUAM is a non-linear derivation of QUAM (QUadrature Amplitude Modulation); a method which uses 2 reference carriers, in quadrature (90 degrees phase offset) from each other, also referred to as the Inphase (I) and Quadrature (Q) reference carriers. These frequency-locked, phase shifted carriers, along with two channels of information, feed a pair of balanced modulators. Since the modulation input of the first modulator is a DC term plus the information to be sent, the output of the "I" modulator is a reference carrier plus a pair of sidebands which convey the first information terms. The second balanced modulator is fed with the Quadrature carrier reference plus a second channel of information containing no DC term. In practice, since the relationship between the Inphase and Quadrature carriers is known, only one of the two - the Inphase carrier - is actually needed. The quadrature carrier is, therefore, suppressed. The outputs of the two modulators are summed, resulting in a composite signal that contain a reference carrier, an envelope term, and a phase modulation term that can be expressed by (expression to be added). QUAM was first described in a thesis of "Day's Theorem" in the 1930's and later formed the basis for the NTSC color transmission system utilized for half a century in the US and elsewhere. It is also the basis for nearly all modern communications methods in use today including those used in wired modems and over-the-air transmission of higher order constellations in single and multi-carrier systems.

In all AM Stereo systems, there are essentially 2 channels of information that need to be conveyed: Left and Right audio. To send this in a form that is compatible with existing monaural receivers, the audio is fed to a matrix, just as it is done in the FM stereo system, resulting in Left plus Right (L+R, sometimes referred to as the "Sum" channel and Left minus Right (L-R) information, also referred to as the "Difference" channel.

Linear QUAM has many advantages, including its conservation of spectrum while having the ability to send more than one channel of information, but it also had drawbacks; especially in terms of compatibility with existing monophonic receivers and with respect to implementation requirements. The Harris CPM and V-CPM systems utilized a linear derivatives of QUAM but it was found cumbersome to adapt some older transmitters with sufficient performance to broadcast the signal. Motorola developed a non-linear version of QUAM, known as Compatible Quadrature Modulation or, simply, C-QUAM. The mathematical expression deriving C-QUAM will follow, but, at the transmission side, unless the broadcast transmitter consisted of a linear amplifier, all AM stereo systems required the signal to be broken into its two fundamental components: Envelope and Phase. This was done using a system known as Envelope Elimination and restoration (EER). Note: Linear amplification was used in rare cases such as the Gates Vangard 1kW, single tube transmitter of the late 1960's and an English-made transmitter, also 1kW, possibly manufacture red by Marconi that was utilized for AM stereo tests in Ipswich, UK. In addition, in the early days of AM broadcasting, so-called "Composite" transmitters were utilized to generate large power levels. Usually, a low power (1 - 10kW) high level, plate modulated transmitter was used to generate an intermediate signal. That signal was then fed to linear amplifiers to generate the final power level; perhaps 50kW or more. Some of these linear stages were simply Class B linear amplifiers running very low efficiency while others utilized methods such as Doherty high efficiency designs. A classic example of such a composite approach was the Continental Electronics 317B series of transmitters wherein a 10kW signal was generated by means of screen modulation while the final conversion from 10kW to 50kW was performed by a Weldon modified (grounded grid) Doherty amplifier.

The phase term is constant envelope in nature; therefore, it can be applied to a low level stage of the transmitter. Linear amplification is not necessary and, in many cases, the crystal oscillator stage was simply replaced with a phase modulated external oscillator that conveyed the proper information to phase modulation mapping required for each system. The envelope term in all cases except for the Harris system consisted of 1+Left+Right, where "1" represents the DC carrier term and Left plus Right represents the monaural audio to be conveyed to all receivers; mono or stereo. It was simply applied to the high level modulator of the transmitter. In the unique case of the Harris system, the envelope audio term had to contain both monaural and stereo difference audio information to produce the linear sideband structure of that system.

It was in the reception and decoding process of the C-QUAM signal that the C-QUAM signal was unique. In all but the Harris system, an envelope detector, which is insensitive to phase-modulated information, is utilized to derive the L+R audio term. In the case of the Harris system, the envelope detector was replaced with an Inphase detector (much like a product detector utilized for SSB reception in shortwave radios except that the local oscillator is phase locked to the carrier of the incoming RF signal). In the case of Magnavox and Belar, phase demodulators are used to derive the L-R (stereo difference) information; Magnavox utilized direct, "linear" phase modulation while Belar utilized FM at low frequencies, reverting to phase modulation at higher frequencies. Motorola and Harris utilized Quadrature detectors - a product detector that is phase locked to the incoming carrier term, but shifted 90 degrees. In addition, and unique to the C-QUAM system, the output of an unmodified Quadrature detector would produce distortion that was directly related to the cosine of the phase modulated term. To remove this distortion, the cosine term was derived using a high gain feedback loop and through direct comparison of the Inphase and Envelope detectors. Since the Envelope detector is phase-insensitive but the In-phase detector is phase-sensitive, the error term produced by the feedback loop, in conjunction with an analog divider feeding the input of only the Inphase detector, was the cosine phase term. By taking the now-forced "divide by cosine of the phase modulation angle" term and also feeding it to a divider proceeding the Quadrature detector, undistorted L-R, difference audio could be demodulated. Since the dividers are identical, only one is actually required; the inputs of the Inphase and Quadrature detectors are both fed with the same RF signal post divider stage.

C-QUAM was termed by at least one Japanese company as "elegant"; it took most of the good qualities of QUAM, allowed full envelope modulation to occur without distortion except under extreme stereo conditions, and filled the gap left by earlier attempts at AM Stereo such as the CBS linear QUAM (the basis of the Harris V-CPM system) and Philco Arcsin proposals of the 1950's and avoided the noise effects that occurred under heavy negative envelope modulation of the early RCA and later Belar and Magnavox systems.

While C-QUAM was a good solution of the day, considering where the industry has gone and the incorporation of HD Radio on AM - a system many feel is fraught with problems - one would have to think that AM radio would be better off simply using a derivative of linear QUAM that also incorporates audio phase shift networks to generate linear Independent Sideband (ISB). Although such a system does have compatibility issues with existing monaural radios, such compatibility issues do not exist unless substantial amounts of stereo information are present. Considering the primarily-talk content of AM radio today, the noise and propagation issues related to the wider bandwidth HD Radio system, and the fact that electronics do get replaced at a faster rate today than they did some 20+ years ago, perhaps it is time to reconsider utilization of linear ISB for AM broadcasting once again. Hilmer Swanson lead the way at Harris in the design of modern transmitters that can handle the additional envelope modulation requirements of linear systems when he designed the original DX-series, DAC-based transmitters and all manufacturers have followed by designing transmitters that can properly handle envelope elimination and restoration(originally conceived by Leonard Kahn in the 1950's, including the required DC and near-DC terms needed to support a linear system. David Herschberger (W9GR), formerly of Harris and co-inventor of the Harris V-CPM system, proposed this decades ago. Perhaps it's time has now come.

C-QUAM has been long criticized by the Kahn-Hazeltine system's creator, Leonard Kahn as being inferior to his system. First generation C-QUAM receivers suffered from "platform motion" effects when listening to stations received via skywave. Later improvements by Motorola minimized the platform motion effect and increased audio quality and stereo separation, especially on AMAX certified receivers in the 1990s.

Kahn-Hazeltine

The Kahn-Hazeltine system was developed by American engineer Leonard Kahn and the Hazeltine Corporation. This system used an entirely different principle; using independently modulated upper and lower sidebands. While a station using the system would sound best with proper decoding, it was also possible to use two standard AM radios (one tuned above and the other below the primary carrier) to achieve the stereophonic effect, although with poor stereo separation and fidelity compared to a proper Kahn system AM Stereo receiver. One of the best known stations to use the Kahn system was 890 WLS, Chicago. WLS still transmits in AM stereo today but uses the Motorola C-QUAM system instead.

However, the Kahn system suffered from lower stereo separation above 6 kHz (reaching none at 10 kHz whereas FM stereo has 40 db or more separation at 15 kHz) and the antenna array on directional AM (common on a lot of nighttime and some daytime stations) had to have a flat response across the entire 20 kHz AM channel. If the array had a higher reactance value (also known as Standing wave ratio) on one side of the frequency vs the other, it would affect the audio response of that channel and thus the stereo signal would be affected. Also, Kahn refused to license any radio receivers manufacturers with his design, although multi-system receivers were manufactured by various companies such as Sony, Sansui, and Sanyo, which could receive any of the four AM Stereo systems.

Nonetheless, this system remained competitive with C-QUAM into the late 1980s and Kahn was very vocal about its advantages over Motorola's system. Kahn filed a lawsuit claiming that the Motorola system did not meet FCC emission bandwidth specifications, but by that time, C-QUAM had already been declared as the single standard for AM Stereo in the USA.

Kahn's AM Stereo design was later revamped for monaural use and used in the Power-Side system, in which a decreased signal in one sideband is used to improve coverage and loudness, especially with directional antenna arrays. Power-Side became the basis for CAM-D, Compatible AM Digital, a new digital system being promoted by Leonard Kahn and used on several AM stations.

Kahn receiver chips have also been used as an inexpensive method for providing high frequency (world band) receivers with synchronous detection technology.

Belar System

The Belar system was used in limited number of stations, such as WJR. The Belar system was a simple Phase Modulated/AM modulation system, with the L-R phase modulating the carrier (with a 400uS pre-emphasis) and the L+R doing the normal "high level" AM modulation (usually referred to as Plate Modulation in transmitters using a tube in the final stage, where the audio is applied to the plate voltage of the tube; in solid state transmitters, various different techniques are available that are more efficient). The Belar system (by the company of the same name) was dropped due to issues with its design though it was much easier to implement than the other systems. It and the Kahn system did not suffer from platform motion (which was a killer for AM stereo at night; platform motion is where the stereo balance would shift from one side to the other and then back to center) but the use of low level phase modulation with no QUAM information did not permit a high separation of L and R channels.

Adoption in the United States

In 1975, the Federal Communications Commission (FCC) started a series of five year tests to determine which of the five competing standards would be selected. By the end of the testing period, the Belar system was dropped. In 1980, the FCC announced that the Magnavox system would become the standard. This announcement was met with harsh criticism and a series of lawsuits. On March 4, 1982, the FCC revoked their endorsement to the Magnavox standard and let the marketplace decide, meaning that all four standards were allowed. After the 1982 decision, many stations implemented one of the four standards. Initially, all systems remained competitive, but by the later 1980s, Motorola C-QUAM had a clear majority of stations and receivers. Around this same time, Harris Corporation dropped their system and instead endorsed C-QUAM. During this time, radio manufactures either made receivers which decoded just one system, or decoded all four. The multiple systems used greatly confused consumers and severely impacted consumer adoption. As a result of this confusion, and the continued growth of the FM band, interest in AM Stereo dwindled.

In 1993, the FCC declared Motorola's C-QUAM system the standard. To ensure that all AM Stereo receivers maintained the same sound quality, the National Association of Broadcasters and the Electronic Industries Association started the AMAX certification program.

Global adoption

In the early 1980s, other countries, most notably Canada, Australia and Japan approved and implemented AM Stereo systems. Most governments approved a single standard, usually Motorola's C-QUAM, which greatly reduced confusion and increased user adoption.

In many countries, especially those where the AM band is still dominant, AM Stereo radios are still manufactured and stations still broadcast stereo signals.

Current status

Globally, interest in and use of AM Stereo has been declining steadily since the 1990s, as many music stations have continued to move to the FM band. As a result, the vast majority of AM stations broadcast News/Talk or Sports/Sports Talk formats. Many of the stations that initially implemented AM Stereo are clear-channel 50,000 watt stations, and are more concerned with listening range than stereo sound (although there is no proof that use of AM Stereo affects listening range). As a result, these stations still have the necessary equipment to broadcast in stereo, but it is left unused (or converted to HD Radio). Also, many former AM Stereo stations were bought up by broadcasting conglomerates, which generally discourage AM Stereo broadcasting. In the United States, most stations currently using AM Stereo are small, independently owned and broadcast a variety or music format.

Japan: AM Stereo is popular in Japan because of the limited number of FM stations there.
Australia: AM Stereo was also popular in Australia, because AM covers a wide geographic area compared to FM, in addition to the Government adopting a single standard (Motorola C-QUAM) several years sooner than the USA. As of June 2008 no Melbourne AM stations broadcast C-QUAM AM stereo; however some country stations such as 3GG still do. At its peak popularity in the late 1980s the majority of stations did.
Europe: After some experiments in the 1980s, AM Stereo was deemed to be unsuitable for the crowded band conditions and narrow bandwidths associated with AM broadcasting in Europe. However, Motorola C-QUAM AM Stereo remains in use today on a handful of stations in France, Italy, and Greece.
Canada: AM Stereo was more widely adopted in Canada than in the USA. This may have been due to the Canadian Government's decision to use a single standard, and the Canadian Radio-television and Telecommunications Commission (CRTC) licensing stations by format and their hit/non-hit rules for FM (hence, more music stations on AM). However, unlike in the USA, some former AM Stereo stations have left the AM band altogether instead of simply reverting to mono.

Decline in use

Radio stations around the world are converting to various systems of digital radio, such as Digital Radio Mondiale, DAB or HD Radio (in America). Some of these digital radio systems, most notably HD Radio have "hybrid modes" which let a station broadcast a standard AM signal along with the digital information. While these transmission modes allow standard AM, they are not compatible with any AM Stereo system (meaning you can't broadcast both at the same time).

Digital AM broadcasting systems, such as HD Radio have been criticized by supporters of AM Stereo as sounding 'harsh' and 'artificial', but supporters of Digital systems argue that the extended frequency response, increased dynamic range, lack of noise and lower distortion make up for the compression artifacts. However, HD Radio also increases adjacent channel noise due to the digital sidebands, which poses serious problems for nighttime broadcasts. Some have proposed to use HD Radio in the daytime and AM Stereo at night. Many HD radios are based on a common chipset that decodes C-QUAM.

Some users have reported that these radios decode the C-QUAM signal with the channels flipped around (left in the right, right in the left). Later revisions of the Accurian radio removed this feature, but a representative from Sangean stated that they have no plans to remove it.

External links

Sites related to AM Stereo

References

^ http://www.wa2fnq.com/fnq/amstrdoc.htm

[1] ttp://www.wa2fnq.com/fnq/amstrdoc.htm

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