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QAM is used in a variety of communications systems such as Dial-up modems and WiFi. In cable systems, a QAM tuner is linked to the cable in a manner that is equivalent to an ATSC tuner which is required to receive over-the-air (OTA) digital channels broadcast by local television stations when attached to an antenna. Most new HDTV digital ...
In addition to receiving inputs from digital networks, most tuners are also capable of tuning analog broadcasts. Tuners can be divided into three broad categories: Broadcast In-band (IB) Tuner. Once the signal arrives from the physical transmission media, the IB tuner will isolate a physical from a multiplex of channels and convert to baseband.
ATSC and DVB-T specify the modulation used for over-the-air digital television; by comparison, QAM is the modulation method used for cable. The specifications for a cable-ready television, then, might state that it supports 8VSB (for broadcast TV) and QAM (for cable TV). 8VSB is an 8-level vestigial sideband modulation.
Any converter box converts the digital signal for the current digital sub-channel to an analog signal (at the reduced screen resolution of the analog standard), outputs that signal onto analog channel 3 or 4 (set by the user to avoid any conflict with local over-the-air channels) and sends that signal to the analog tuner on the TV.
A "181-channel tuner" receives 125 on cable (1 to 125), plus 10 (126 to 135) more for digital cable ready TVs, plus the 56 (14 to 69) which are not identical in both (2 to 13). Other cable channels, 0, 00 and 1, which along with channels 136-158 are ill-defined and thus rarely used, and often not included in otherwise cable-ready tuners.
QAM Mapper: the bit sequence is mapped into a base-band digital sequence of complex symbols. There are 5 allowed modulation modes: 16- QAM , 32-QAM, 64-QAM, 128-QAM, 256-QAM. Base-band shaping: the QAM signal is filtered with a raised-cosine shaped filter, in order to remove mutual signal interference at the receiving side.
Tuning adaptors and tuning adaptor interfaces have been added to provide communication back to the headend needed for switched video.) CableCARDs are not necessary for viewing unscrambled digital cable channels if the user has a QAM tuner—a feature in some televisions and DVRs. CableCARD support is most common on higher end televisions that ...
Some countries have also introduced digital television and ended analog over-the-air transmissions. Most new TVs feature a digital tuner which allows reception of digital over-the-air TV without need of an external device such a converter box. If using a TV set without a digital tuner, an external converter box must be purchased and used.