Re: what video (and audio) format for distribution?

On Sep 7, 2:01 am, Roderick Stewart
<r...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
In article <b9ae74e9-ef98-4fe7-b23f-

16bf99cbc...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Dtvaust wrote:
The distinction that has to be made between the formats used to send
pictures to a TV station as contribution material and material that is
broadcast
as distribution is that a much higher data rate is used for
contribution.

That's what I meant when I said an important principle had been
abandoned in order to implement digital broadcasting. There is a
difference, but there wasn't one before.

In the days of analogue television, the signal the broadcaster would
obtain from the camera, the signal they would feed through any editing
and distribution system, the signal any external contributor would feed
into this system, and the signal that would eventually be fed to the
transmission system to be broadcast to the viewers at home would all be
specified as *exactly the same signal* without any intended changes. Any
impairments or degradations were due to imperfections in the equipment
and transmission, but theoretically if these were perfect the viewers
would see exactly the same signals the broadcasters saw.

Digital transmission requires such enormous bandwidth to do the job
properly that we effectively don't do the job properly. Viewers at home
see a degraded version of what the cameras produced, the degradations
having been introduced deliberately by the broadcasters before the
signal is broadcast, simply to reduce the bandwidth to manageable
values. Even if the transmission path were theoretically perfect nobody
would ever see an exact rendition of what came out of the camera.

The abandonment of the principle that the viewers should be given the
same signal that came out of the camera requires some new technical
specifications. We don't just need to specify how much natural
degradation of the signal from unavoidable causes we are prepared to
accept, but we now also need to specify how much we think it is
acceptable to introduce deliberately, because that is what we are doing.

Rod.
--
Virtual Access V6.3 free usenet/email software fromhttp://sourceforge.net/projects/virtual-access/

ALthough there is some truth in what you say about deliberately
deciding how much degradation is allowable in the distribution
network and transmission, this is not all the story.

When a signal is compressed using DCT as is the case with
MPEG2 the resolution is independant of the transmission
data rate. Even if a very low data rate is used you will still have
full resolution but you will get more "Artifacts" such as blocking
if there is a lot of movement in the picture. If there is little
movement as with a"Talking Head" very low data rates can be
used.

When a signal is compressed and then decompressed and
then compressed again different artifacts start to appear in
the video. Quantisation noise and situations where the DCT
coefficients and movement vectors are different to the ones
used in the first compression are used cause degradation
that rapidly increases with each time the signal is compressed
again.

This is why a high data rate and use of 4:2:2 is used ideally
until the last time the video is compressed for transmission.
It is particularly noticeable where video has been compressed
at a remote site with a sports event, sent by satellite using a
limited bandwidth channel, decompressed at the studio,
supers added, recompressed for transit by microwave to
a transmitter site and then recompressed to a different rate
for transmission, received at a cable headend and again
has the data rate changed in a "Cherry picker" for inclusion
on a cable channel.
These recompressions give artifacts from each stage so the
end result can be quite poor in such a case.
The end result compared to the original camera pictures can
be a huge degradation.

Where satellite transmission is involved and possibly use
of standards conversion for use in a country using CCIR 50 Hz
standards is necessary quite horrible end results are common.

Use of some of the newer techniques for compression make
things better such as JPEG2000 compression. Schemes like
this used for Digital Cinema are mathematically transparent
with no quality loss in the decompressed signal if a suitable
data rate transmission path is available.
Transmission of a 1.5 Gb/s HD signal at 70Mb/s with
minimal artifacts is possible with JPEG2000.

.

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