CAM-D
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Compatible Amplitude Modulation - Digital or CAM-D is a proposed hybrid digital radio format for AM broadcasting, put forth by well-known broadcast engineer Leonard R. Kahn.
While little is known about it, it is an in-band on-channel technology that uses the sidebands of any AM radio station. Analog information is still used up to a bandpass of about 7.5kHz, with standard amplitude modulation. The missing treble information that AM normally lacks is then transmitted digitally beyond this. Audio mixing in the receiver then blends them back together.
Unlike other IBOC technologies like iBiquity's HD Radio, Kahn's apparently does not provide a direct path to all-digital transmissions, nor any multichannel capability[citation needed]. The advantage however is that it takes up far less of the sidebands, thereby causing far less interference to adjacent channels, hence the "Compatible" in the name. The interference problem has plagued HD Radio on AM, along with the fact that it, like CAM-D, is proprietary.
Digital Radio Mondiale, commonly used in shortwave broadcasting, uses less bandwidth than AM, to provide high quality audio, however, there is no "compatible" mode. Digital Radio Mondiale requires digital detection circuitry not present in conventional AM radios to decode programming. Some CAM-D programming can be heard on current analog AM receivers. Special CAM-D receivers provide the benefit of better frequency response and a slow auxiliary data channel for display of station ID, programming titles, etc.
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[edit] Issues
[edit] Availability of Receivers
At present, there are a limited amount of special receivers capable of receiving CAM-D. According to Leonard Kahn however, the system has seen potential growth within the past year (2007)[1].
[edit] Upgradeability of existing transmitters
KCI (Kahn Communications, Inc., the company that invented CAM-D) uses as one of its main selling points that some older transmitters are not directly upgradeable to transmit HD Radio. It is true that older transmitters may lack the transmission purity needed to support HD Radio's digital transmission standard, but more recent transmitters are more likely to be capable of upgrading to transmit HD Radio.
CAM-D is said to work with a wider variety of existing transmitters, which is a selling point for smaller broadcasters with limited budgets.
The price of the CAM-D hardware is $65,600 as of July 1, 2007.[2]
[edit] Technical Issues
CAM-D is said to add high-frequency program information digitally transmitted and then overlaid onto the existing low- to mid-frequency analog program. This provides enhanced fidelity that under adequate analog signal conditions may allow AM stations to transmit music and other program content with a more life-like sound.
However, a significant disparity between Ibiquity's AM HD Radio and CAM-D is that of time diversity. Namely, HD Radio's AM scheme[3] broadcasts two copies of the program offset by a few seconds, allowing signal to be briefly lost and maintain uninterrupted program audio (for example, driving under an overpass, multipath in urban areas, lightning, switching transients, etc.).[citation needed]
Analog AM radio, including CAM-D, since CAM-D requires the analog sidebands to transmit most of the program audio, will suffer loss/disruption of main program audio in these and other circumstances.[citation needed]
These outstanding technical issues present a significant limit to the utility of CAM-D, especially in urban areas, where the transmitter site is often located outside the urban area and thus lacks a more direct path to the receivers (and even with a more direct path, these noise sources can still be quite disruptive to program audio without the time diversity).[citation needed]
An additional issue is the fact that the system provides no noise immunity for the analog signal[citation needed]. A substantial problem of analog AM transmission is that it is subject to poor signal to noise ratio due to radio frequency noise from man made and natural sources, such as fluorescent lamps, motors, switches and lightning. Simply adding high frequency information does not produce high fidelity. HD Radio signals on AM, however, use more bandwidth signal wise and often create "hash" on surrounding frequencies, up to 30 kHz from the center frequency. This interference, which is audible mainly on analog radios, was one of the reasons Citadel Communications pulled night broadcasts of their AM HD station's signals.[4] CAM-D does not produce interference on any channels 10 kHz or more from the center frequency.
CAM-D is a system that is applicable only to medium wave and possibly shortwave transmission. Since receivers are designed for multiple bands, a receiver that requires a substantially different architecture for CAM-D will require special circuitry that will cause an increase in its cost. Since all new broadcast radios are moving to full digital formats that decode with nearly identical hardware (XM, Sirius, HD Radio(in digital mode), Digital Audio Broadcasting, Digital Radio Mondiale) in the long term, the overhead of analog circuitry will cause a cost penalty to implement CAM-D in future radios.
[edit] Stations Broadcasting in CAM-D
- KDYL, 1060 AM, South Salt Lake, Utah [5]
- WSRF, 1580 AM, Fort Lauderdale, Florida
- WKBF, 1270 AM, Rock Island, Illinois
- KZGX, 1600 AM, Minneapolis, Minnesota
[edit] References
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