An ATSC (Advanced Television Systems Committee) tuner, often called an ATSC receiver or HDTV tuner, is a type of television tuner that allows reception of digital television (DTV) television channels that use ATSC standards, as transmitted by television stations in North America, parts of Central America, and South Korea. Such tuners are usually integrated into a television set, VCR, digital video recorder (DVR), or set-top box which provides audio/video output connectors of various types.
Another type of television tuner is a digital television adapter (DTA) with an analog passthrough.
Technical overview
The terms "tuner" and "receiver" are used loosely, and it is perhaps more appropriately called an ATSC receiver, with the tuner being part of the receiver (see Metonymy). The receiver generates the audio and video (AV) signals needed for television, and performs the following tasks: demodulation; error correction; MPEG transport stream demultiplexing; decompression; AV synchronization; and media reformatting to match what is optimal input for one's TV. Examples of media reformatting include: interlace to progressive scan or vice versa; picture resolutions; aspect ratio conversions (16:9 to or from 4:3); frame rate conversion; and image scaling. Zooming is an example of resolution change. It is commonly used to convert a low-resolution picture to a high-resolution display. This lets the user eliminate letterboxing or pillarboxing by stretching or cropping the picture. Some ATSC receivers, mostly those in HDTV TV sets, will stretch automatically, either by detecting black bars or by reading the Active Format Descriptor (AFD).
Operation
An ATSC tuner works by generating audio and video signals that are picked up from over-the-air broadcast television. ATSC tuners provide the following functions: selective tuning; demodulation; transport stream demultiplexing; decompression; error correction; analog-to-digital conversion; AV synchronization; and media reformatting to fit the specific type of TV screen optimally.
Selective tuning
Selective tuning is the process by which the radio frequency (RF) of the television channel is selected by a receiver from within a band of transmitted RF signals. The tuner usually performs the function of frequency-agile selection, along with rejection of unwanted out-of-band signals.
Demodulation
"Demodulation" means transforming the signal from the tuner into a signal that a TV set can use to produce images and sound without further consideration for the frequency at which it was transmitted. It is separation of a standard baseband signal from the RF carrier that was used to transmit it through the air (or down a coaxial cable or other long-distance medium.) ATSC as implemented in the US uses 8VSB modulation, which requires less power to transmit, as opposed to the also proposed COFDM modulation (used in European DVB-T, which is less prone to multipath distortion and therefore better received in mobile installations).
Transport stream
In the US, multiple digital signals are combined and then transmitted from one antenna source to create over the air broadcasts. By the reverse process (demultiplexing), an ATSC receiver first receives the combined MPEG transport stream and then decodes it to display one of its component signals on a TV set.
Decompression
Since digital signals that are broadcast over the air are compressed (packed smaller), once they are received by the ATSC tuner, these compressed packets of digital data are then decompressed (unpacked to their original size). The ATSC system uses lossy compression, so while the decompressed data size is the same as the original compressed data size, the data produced is not exactly the same as the original data fed into the system at the transmitting site, but it is close enough that most people will not notice a difference.