QAM (television)

This article is about North American digital television. For other uses of QAM, see quadrature amplitude modulation.

QAM stands for quadrature amplitude modulation, the format by which digital cable channels are encoded and transmitted via cable television providers. QAM tuners can be likened to the cable equivalent of an ATSC tuner which is required to receive over-the-air (OTA) digital channels broadcast by local television stations; many new cable-ready digital televisions support both of these standards. Although QAM uses the same 6 MHz bandwidth as ATSC, it carries about twice the data (38.47 Mbp/s @256QAM per 6Mhz channel) due to the lack of error correction; however, this requires a significantly cleaner signal path, such as distribution through hybrid fiber-coax digital cable.^[1]

Technical details

QAM is only a modulation format and does not specify the format of the digital data being carried. However, when used in the context of digital cable television, the format of the data transmitted using this modulation is based on ATSC. This is in contrast to DVB-C which is also based on QAM modulation, but uses a DVB-based data format which is incompatible with North American receivers.

Many cable providers offer few or no details about unencrypted QAM channels. It is also common for cable providers to falsely insist that a set top box from the cable company is required to watch all digital cable channels, including unencrypted channels, even though QAM channels may be distributed via their system. QAM channels may move without notification and some channels may have strange numbering schemes.

QAM tuners

In North American digital video, a QAM tuner is a device present in some digital televisions and similar devices which enables direct reception of digital cable channels without the use of a set-top box.^[2] An integrated QAM tuner allows the free reception of unscrambled digital programming sent "in the clear" by cable providers, usually local broadcast stations, cable radio channels, or in the case of providers which have transitioned to do so, Public-access television cable TV channels. Which channels are scrambled varies greatly from location to location and can change over time; the majority of digital channels are scrambled because the providers consider them to be extra-cost options and not part of the "basic cable" package.^[3]

Most US TVs sold after 2006 include a QAM tuner, though some low end and less expensive models still do not include the functionality to tune QAM channels as of 2009. Even though the hardware used to tune ATSC (over the air digital) and QAM (digital cable) channels is similar, there are still plenty of devices on the market as of 2009 which can tune ATSC signals but not QAM signals. Some do have the QAM feature but do not describe it in the operator's manual or on-screen menu options, choosing instead to place it under the category of a channel scan and forcing viewers to delete many scrambled channels after a scan. The FCC mandates that all new TVs sold in the US must include an ATSC tuner, but there are no requirements for QAM tuning functionality. FCC Sec. 76.630 and CFR Title 47, §76.901(a).

Unencrypted QAM tuners are available for computers and many software DVR options exist to work in conjunction with QAM, namely Elgato's EyeTV, SageTV (native support of high definition QAM), MythTV (also native support for HD QAM), and Snapstream's Beyond TV, Microsoft's Windows Media Center software supports QAM on supported tuners (on Vista with the TV Pack update or with Windows 7).

References

1. ^ HD 101: What is ATSC, PSIP, QAM, and 8-VSB?
2. ^ Terms & Definitions, Sceptre.com
3. ^ Video Noise Does Your Next Video Display Need to Have a QAM Tuner?, February 2004