QAM transcoders are used by operators who want
to offer digital programming to their subscribers. A transcoder is used to
receive one satellite broadcast transponder and convert it to a cable TV
channel. This channel may then be sent down a typical coaxial distribution
system for reception at the customers TV set. A set top converter box is
required at each television to convert the QAM signal into a format that the TV
will understand.
Typically a satellite transponder
will carry between 8 and 12 standard
definition television channels or 2
to 3 HDTV channels. By using a QAM transcoder,
you are able to send the entire satellite
transponder down a single 6MHZ cable
TV channel. With a little math, you
can see how it is now possible to send
hundreds of channels down a single coaxial
cable.
As you can see from the illustration. TV channels are uplinked to satellites that in turn send the signals back to the earth for reception by a small (1 meter) antenna. The QPSK satellite transponders are then "cherry picked" by the transcoder and sent out on a cable channel in a QAM format. These QAM channels may be combined with analog channels and/or High Speed Internet to complete the cable tv offering.
An example of a QPSK to QAM conversion could be:
Satellite 148, transponder 11 converted to CATV channel 82. This would put 12 digital channels in the 6MHZ channel bandwidth between 570mhz and 576mhz.
