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Re: [Phys-l] Digital TV signal delay



On 09/23/2008 08:56 AM, Rauber, Joel wrote:

Our digital TV is pulling signal off of a pair of rabbit ears, and our
channel switching occurs very close to as fast as our older analog TV's

That's an important observation. I interpret it as follows: As
for what happens when switching channels, there doesn't need to
be a big delay. Therefore I am very skeptical of all arguments
as to why there "needs" to be a big multi-second delay.

As for what happens when there are two TVs in two different rooms,
you can be sure it is not delay in the coax cable. Do the physics.
You can easily calculate that signals travel through coax at
nearly the speed of light. Measurements confirm this, if you
have a loooong piece of coax and/or a really fast oscilloscope.

I don't think anybody on this list can afford a two-second length
of coax. If you can, please let me know; I'd like to get to
know you better!

As for what /does/ explain the observed asynchrony, the short
answer is that I don't know. However, here is a hypothesis:
*) We start with the question: Are both TVs watching the
same _channel_, or are they merely watching the same
_program_. It turns that chez moi, the local PBS station
can be received over cable as
-- analog on channel 6
-- digital on channel 103.2
-- hi-def on channel 105.1
... and you cannot easily tell which you are receiving,
because the cable box will display "6" on the front in all
three cases, and the station sends the same callsign on all
three channels. This is infuriating for techies but I
suppose it is "user friendly" for the other 99.99% of the
population, who care more about the program than how it
is encoded or channelized.

Three different TVs watching the same program via three
different channels will experience wildly different encoder
delays.

*) Innumerable other hypotheses are possible, but none
stands out as particularly better than the others.