In article ,
mm wrote:
What is the difference between scanning for chhannles and going
directly to one?

Or are digital tuners different from analog, in that scanning first is
essential?

On analog tuners, the frequency is a direct function of the channel
number as seen by the user.

On ATSC digital, that is no longer true. The "channel number" seen by
the user is no longer a 1:1 function of the transmission frequency.

There are actually two channel numbers involved... the transmission-
frequency channel number (which corresponds to the old NTSC analog
channel number) and the station-identification channel number. They
may very well be different.

This architecture was put in place so that stations which were
previously down in the VHF range, could move their transmitters up to
UHF, without losing their commercially-valuable "channel number"
identity.

So, when you tell a digital TV or DVR or set-top box to do a "channel
scan", it's doing several things:

- It scans the frequencies, looking for NTSC and ATSC signals, and
"remembering" which channels are in use.

- When it sees an ATSC digital signal, it pauses briefly, decodes and
parses the signal, and "remembers" which "channel numbers" and
sub-channels are being multiplexed/transmitted on that frequency.

Subsequently, when you tell it to "tune to channel 13", it will look
ints memory. If it previously saw that "channel 13" was NTSC, it just
tunes to the traditional Channel 13 frequency slot. If, however, it
detected a "Channel 13" ID in an ATSC digital stream on *any*
frequency (even if not the traditional "channel 13" frequency) it will
tune to that frequency, start decoding, and begin extracting that
program.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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