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Analog TV is the system adopted for TV transmissions in 1941 (Black & White), and 1953 (Color) in the United States. The standard is called NTSC which is named for the committee (National Television Standards Committee).
An Analog TV signal consists of a Video carrier (Picture), an Audio carrier (Sound), and a Chroma carrier (Color). These carriers are transmitted in a 6 MHz wide RF channel.
Digital TV on the other hand, is the transmission of either Audio and Video signals that have been digitized, or true Digital signals that are generated from a Digital Video Camera. Digital TV separates the picture into separate picture elements called Pixels.
A TV screen normally referred to by the number of scan lines horizontally (Vertical frame resolution) X the number of Pixels per line, I/E 480 x 640. Depending on how the vertical lines are “painted on a screen” i (Interlaced) or P (progressive), the resolution is stated differently such as 720P (1280 x 720 pixels) or 1080i (1920 x 1080 pixels).
Depending on the definition (Standard Definition SD, or High Definition, HD) each line of a picture will have from 480 (SD) pixels to 1080 (HD) pixels. Obviously, the greater number of pixels will result in a higher resolution or higher definition.
These raw digital television signals are then Compressed; using a technique called Compression algorithms (MPEG2 or MPEG4). These algorithms allow transmission of only part of the image per frame, as other aspects of the image are predictable and can be re-constituted by the receiver. A typical standard definition TV signal has a bit rate of 27 Mbits, and a 1080i HD signal has 158 Mbits. These signals are much too large to transmit on a standard 6 MHz TV channel. Typically an SD TV signal is compressed to between 3 and 8 Mbits, and an HD TV signal is compressed to between 11 and 19 Mbits.
We then need to modulate these Compressed Digital TV signals for transmission. Transmission via Satellite uses QPSK modulation, Broadcast uses 8VSB modulation, and Cable TV uses QAM modulation. If we look at QAM modulation for Cable TV, we currently have two standards, QAM64 and QAM 256 with QAM 256 being the most common. QAM 256 is capable of transmitting 38.8 Mbits on a single 6 MHz wide TV channel. This then means that more than one digital TV signal (Program), can be combined together (multiplexed) for transmission on a signal TV channel. For instance 8 SD programs at 4.75Mbits can be transported on one QAM256 Channel (4.75 x 8 = 38.0), or 2 HD at 15 Mbits each and 2 SD at 4 Mbits each.
This is how a Cable TV system that used to carry 80 analog programs, can now carry 400 programs. It is also why the industry has changed from calling TV programs a Channel to a Program.
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