"Richard Clark" wrote in message
...
On Thu, 08 Nov 2007 00:40:50 -0000, MRW wrote:

How is 19.2Mb/sec calculated?

It's called digital compression.

73's
Richard Clark, KB7QHC


http://www.maxim-ic.com/appnotes.cfm/appnote_number/750

has an interesting chart that shows the analog bandwidth requirements for
various resolutions, including the 1080i and 720p HD formats, which are both
shown as 26 MHz. This is misleading, however, since digitizing the signal
results in data rates in the range of 1 Gb/s to 1.6 Gb/sec before
compression, according to one source.

The compression algorithm can be "tweaked" to put the original program into
almost any bandwidth. (The US Navy's "TV-Direct-to-Sailors" puts three TV
programs into a 3.6 Mbps data stream.
http://www.sia.org/2007DoDSatcomWork...y/DoD/Navy.ppt I've seen it and
it's OK.) Most compression algorithms are termed "lossy" because they lose
some of the original data. The resulting deficiencies, called "artifacts"
may be visible, depending on the amount of compression, the observer and the
program material.

There are several modulation methods which will all put about the same data
rate into a 6 MHz-wide channel.