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Short for Quadrature Amplitude Modulation, QAM is a method of transmitting digital cable signals. In order for a television to receive these signals, a QAM tuner is needed. QAM tuners may be internal or external, but since the late 2000s, most are placed within the television sets, especially in high definition televisions (HDTVs). As a result, a QAM tuner can allow viewers to receive unencrypted digital cable television without requiring a set-top converter or box.
A QAM tuner functions automatically to receive and decode signals when a user plugs in the television set. Sets with this type of tuner do not need an external digital or analog decoder to receive and decode signals. If the digital cable is encrypted, however, as if often the case for premium cable channels, an external box or converter may still be required.
When the digital cable signal reaches the television, a QAM tuner is the instrument that is used to decrypt or decode the signals to create the picture on the TV set. The tuner has a card that converts the digital signals sent via the cable into a format that can be processed by the TV. In many cases, this is automatic, but in some cases the tuner has to be tuned to properly receive and decode the signals. The operating instructions to tune the QAM tuner are usually provided with new televisions, and optimizing the image is generally a relatively simple process.
Digital cable channels are made up to two numbers: one major or physical channel and one minor or subchannel. This means that digital television viewers may watch channel 5.1 or 5.2 instead of having only channel 5. Some cable systems avoid this by mapping digital subchannels to "virtual" channels; by this method, channel 5.2 might be mapped to channel 728 to make it easier for the viewer to find and remember.
It is important to note that while all QAM tuners receive digital cable signals, those signals are not necessarily in high definition (HD). Although all broadcasters in the US, and many broadcasters in other countries, transmit their programming in the digital format, in most cases there is no requirement that those signals be in high definition. Due to demand, however, more and more HD programming is becoming available.
Digital television signals that are broadcast over the air (OTA) cannot be read by a QAM tuner; this type of tuner is only used for receiving and modulating digital cable signals. In order for OTA digital TV signals to be picked up, the television must have an ATSC tuner. In the Us, all new televisions are legally required to have an ATSC tuner.