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.^[1]

QAM-based HD programming of local stations is sometimes available to analog cable subscribers, without paying the additional fees for a digital cable box. The availability of QAM HD programming is rarely described or publicized in cable company product literature.

QAM stands for "quadrature amplitude modulation," the format by which digital cable channels are encoded and transmitted via cable. 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. QAM carries nearly twice the data of over-the-air ATSC 8VSB but since it requires a significantly cleaner signal path it is appropriate for digital cable.

An integrated QAM tuner allows the free reception of unscrambled digital programming sent "in the clear" by cable providers, usually local broadcast stations or cable radio 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.^[2]

In the United States a television that is labelled digital cable ready can have a CableCARD installed by the cable provider to unscramble the protected channels, allowing subscribers to tune all authorized digital channels without the use of a set-top box.^[3]

Technical considerations

Unlike the case with ATSC tuners there is no FCC requirement that QAM tuners be included in new television sets, as the ATSC standard calls for 8VSB modulation. Since cable providers can deliver twice as much data using QAM than using 8VSB, they petitioned the FCC to allow the use of QAM instead of 8VSB. As the same hardware is often used for both, ATSC and QAM are commonly included in most receivers.

QAM is only a modulation. It does not specify the format of the digital data being carried via this modulation. However, when used in context of digital cable television in ATSC areas, 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.

Although technically most digital and high-definition programming on cable uses QAM, the term is generally reserved among viewers for discussions of unlabeled channels. These are not included in guide information on devices like TiVo DVRs, and can be unexpectedly moved from channel to channel. Local channels of the major networks are typically broadcast via clear [free] QAM, usually in high definition.

This non-standard numbering appears to be the result of inter-operation with various divergent existing numbering schemes:

• Analog cable channels are numbered based on a fixed frequency table, with channels 2-13 matching their over-the-air equivalents
• Digital "package" receivers, as supplied by cable and direct-broadcast satellite providers, use an entirely virtual channel numbering scheme, where channel numbers are uniquely assigned within one provider's channel lineup but without any fixed correlation between channel number and assigned carrier frequency.
• ATSC digital television is based on using the channel's 6 MHz bandwidth to accommodate multiple digital programs. Its virtual numbering scheme therefore is a two-part number; the virtual number of a standard analog channel, followed by a '.' or '-' separator and a sequential sub-channel number to specify an individual program within that specific ATSC transmission.

As the QAM tuner in this case is an adaptation of existing ATSC-compatible hardware, the television set's channel numbering will follow ATSC-like conventions. If what appears as "channel 300" on the cable company's package receivers is physically on frequencies corresponding to an analog cable converter's "channel 77", an ATSC-compatible digital-cable ready TV will most likely display this as "channel 77-300."

This makes watching QAM channels frustrating for the casual viewer, potentially encouraging them to purchase a "digital cable package" which includes a set top box and guide data.

Clear QAM has become important to home theater computer enthusiasts. QAM tuners are available for computers and many software DVR options exists to work in conjunction with QAM, namely Microsoft's Windows Media Center (Vista only), SageTV (native support of high definition QAM), and MythTV (also native support for HD QAM).

References

1. ↑ Terms & Definitions, Sceptre.com
2. ↑ Video Noise Does Your Next Video Display Need to Have a QAM Tuner?, February 2004
3. ↑ Residential Systems How Federal Regulations Affect the Products You Install, Michael Heiss Jul 8, 2004

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