Chuck,
In digital times, the main reason for creating delays is due to the
temporal compression of MPEG-2 and MPEG-4. Production quality is either
not compressed or JPEG-2000 compressed. If you do not compress at all,
delay structure can be similar to that of analog video days. JPEG-2000
typically requires the full frame to be grabbed before serious crunching
can be done, due to the 2D wavelet processing. There exists low-delay
compression schemes which is in the handful of lines (about 16) of
delay. MPEG-2 requires a re-arraning of transmission order in order for
the IBBBPBBBP... sequence requires the I (or preceeding P) and following
P be sent before the B frames interpolating between them.
Add that many buffer management systems is horrible, especially when
going over IP.
Doing long distance (22500 km) 4K uncompressed video has been done with
only 750 ms delay. That delay is probably trimable if you really need to.
The one feature you have with digital video, is that you can create
delays by mistake so easy, and that is gravely misused feature to this
day. Let's say that most systems does not impress me.
Cheers,
Magnus
On 01/01/2015 04:31 PM, Chuck Harris wrote:
It is not that they don't care about time sync, it is that they
have to follow the rules of causality.
Because the whole digitization, broadcast, and display process
of digital TV processes seconds to minutes of material at a time,
You cannot make an event show at an exact time unless the event
was pre-staged... How do you pre-stage a live event that must
happen at a specific time, such as the ringing in of the new year?
In the old days of analog TV, the problem was similar, but the
smallest unit of data was a screen, which took about 1/30th of
a second... added to the un avoidable transport delays... fiber,
microwave, or satellite hop.
-Chuck Harris
Rex wrote:
TV doesn't seem to care about time sync much these days. It also
depends a lot on the
path getting to you,
I get most of my TV via satellite (Dish network). The receiver I have
also can get
OTA. I have happened to notice, once, that I had a local channel on
two TVs. One was
receiving the local via satellite and one was tuned to OTA local
broadcast. The
satellite was many seconds (at least 5, probably more) behind the OTA.
I walked from
one room to the other and had a brief period of deja vu. Hmm, just
occurred to me, an
earphone on the early one while watching the later one with friends
would make you a
living room Jeopardy game show super star.
But that satellite delay all makes sense.
One thing annoys me though. Many channels don't care much about start
and stop times.
If I program something to record using the schedule, often I miss the
end of it. They
frequently go over the half-hour or hour mark by a minute or two.
Occasionally they
complicate it more by starting a show a little early too. That irks me.
But for New Years, I didn't try to measure anything exactly, but I
know they were off
by about 3 hrs. I live in California. I was watching New York's events
on my TV and
the ball dropped at about midnight local time. I am enough of a time
nut to know that
should have happened at 9 PM local time.
See, you just can't trust the media for accuracy these days.
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