Audio synch issues with CBS HD

Phil,

I used DV merely as an illustration of an interleaved format but did not
intend to suggest that it made sense as a transmission format per se. Very
optimized interleaved formats could certainly have been developed for HDTV
and DVD applications.

I agree with your comments, and wanted to add that the "economy" of using
two disparate compression schemes for video and audio and then relying on
time stamps to ensure synch is a bad judgment call IMHO, since both software
bugs (as you state) as well as other 'unexpected' corruptions can and will
cause synch to slip unpredictably.

Take the simple case where a fragmented hard disk or otherwise maxed-out CPU
cannot keep up with frame rates of the stream. It is entirely possible (and
often a real problem) that MPEG editing and DVD authoring (particularly
during the program capture stage) merely run out of resources and record a
stream with dropped frames. Similarly, a noisy RF channel with multipath,
phase distortion, interference, or other fading / attenuation can briefly
experience dropouts. In both cases, there is no forward or backward error
correction code to recover the loss. Rather, there is a "hole" in the data
stream, which for both video and audio cause huge problems since each relies
on interframe (delta) transitions to reconstruct the original waveforms. At
least as troublesome is that the time stamps themselves may be dropped as
well. Even if they aren't, the hole in the audio or video stream prevents
re-synchronization.

I personally feel that the ATSC committee and DVD consortium did a
disservice to the world with their reliance on methods which ignore some of
the harsh realities of satellite links, UHF propagation effects, burst error
statistics in noisy channels, etc. In making their choices they exposed the
entire medium to the multipath, dropouts, and synch issues which now plague
HDTV and DVD delivery systems.

Smarty (KC2OZ)


wrote in message
...
On Fri, 23 Sep 2005 02:06:00 -0400 Smarty wrote:

| I've had the same experience. I personally feel that the decision to
| independently compress the video and audio and then maintain their synch
was
| a short-sighted one. Using an "interleaved" audio/video format with
inherent
| time synchronization like DV tape for example does impart a penalty in
| storage, but makes the whole process of presentation so much more
reliable
| in terms of lip synch. In a world where storage costs drop by a factor
of
| two or more each year, and transmission rates increase in much the same
| manner per dollar, then the design choice to separate the two streams
for
| compression gains seems unfortunate. Getting movie film to use the same
| approach in the last century took a similar path until eventually the
film
| and sound track were unified.

Personally, it seems to me that having to design formats (or protocols)
in a certain way due in order to avoid the possibility of software errors
is wrong. I'd put more blame on the software developers and require them
to "get it right". Unfortunately, software development costs are not
really dropping much, although businesses are trying to push it lower all
the time (and hence, much of the problem).

Still, an integrated interleaved format like DV does have attraction,
including for other purposes like random frame access.

While storage and transmission costs are rapidly declining, there are
some places where limitations exist. DV would not have been practical
for over the air television in the 6 MHz of bandwidth used in North
America and Japan, and a high definition version of an interleaved DV
format would certainly be much more imposing (at potentially 6 times
the needed data rate). Getting that many more bits through 6 MHz, or
more MHz, is just not going to happen. So there is value in the kinds
of compression selected by ATSC (though we have better options now).

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/
http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/
http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

Phil,

Isn't it ironic that the 6 MHz channel assignments, ostensibly used as a
rationale for creating highly compressed data streams, have now given way to
FCC approval of standard definition SD sub-channels. Broadcasters LOVE the
opportunity to sub-divide their HD bandwidth so as to have multiple
"channels" to broadcast infomercials and other crap. So now we have the
worst of both worlds IMHO.........inferior signaling / transmission as a
result of trying to achieve the highest possible bandwidth reductions for
HD, and then.......using the bandwidth to send multiple SD channels of
garbage.....

Smarty


wrote in message
...
On Fri, 23 Sep 2005 02:06:00 -0400 Smarty wrote:

| I've had the same experience. I personally feel that the decision to
| independently compress the video and audio and then maintain their synch
was
| a short-sighted one. Using an "interleaved" audio/video format with
inherent
| time synchronization like DV tape for example does impart a penalty in
| storage, but makes the whole process of presentation so much more
reliable
| in terms of lip synch. In a world where storage costs drop by a factor
of
| two or more each year, and transmission rates increase in much the same
| manner per dollar, then the design choice to separate the two streams
for
| compression gains seems unfortunate. Getting movie film to use the same
| approach in the last century took a similar path until eventually the
film
| and sound track were unified.

Personally, it seems to me that having to design formats (or protocols)
in a certain way due in order to avoid the possibility of software errors
is wrong. I'd put more blame on the software developers and require them
to "get it right". Unfortunately, software development costs are not
really dropping much, although businesses are trying to push it lower all
the time (and hence, much of the problem).

Still, an integrated interleaved format like DV does have attraction,
including for other purposes like random frame access.

While storage and transmission costs are rapidly declining, there are
some places where limitations exist. DV would not have been practical
for over the air television in the 6 MHz of bandwidth used in North
America and Japan, and a high definition version of an interleaved DV
format would certainly be much more imposing (at potentially 6 times
the needed data rate). Getting that many more bits through 6 MHz, or
more MHz, is just not going to happen. So there is value in the kinds
of compression selected by ATSC (though we have better options now).

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/
http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/
http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

wrote in message
...
And it doesn't help that audio sampling rates are not a whole number for
a single frame of video in so many cases (e.g. "NTSC legacy" frame rates
in North America). The methods to deal with that could be the cause of
"strange software".

Phil,

NTSC timecode frame rates and audio sampling rates are two entirely
different things. 48k is 48k at 25fps, 29.97fps and at 30fps drop or
non-drop. Timecode and sample rate are not the same thing. Granted if a
24fps or 30fps film production is pulled down in telecine for NTSC transfer
the sample rate would have to come down as well but that in practice entails
either a sample rate conversion back to 48k or a two way trip through a
DA/AD converter.There are also some field recordists that will record double
system sound for film at 48.048 so the pulldown to NTSC will result in 48.00
if video is the final production or presentation format. Anyway...not to be
a nitpicker but 1 second of time is 1 second of time and 48k is 48k
regardless of the timecode frame rate.

I'd be willing to bet that many of the sync issues that consumers notice are
due to video up-conversion delays in their STB's and televisions which is a
problem no broadcaster can address because of the wide variety of devices
out there. One person's television takes 720P and makes it 1080i, another
takes 1080i and converts to 720P etc etc.

Charles Tomaras
Seattle, WA

I can understand the STB being the culprit if it happened on more than
one OTA signal. If the broadcast is 1080i from the broadcaster and the
STB is processing it at 1080i (no down-conversion) then wouldn't other
1080i signals also have synch issues (assuming it's the STB)? But other
1080i signals do not have the synch issue so I would think I could rule
out the STB and go back to the broadcaster as the source of the problem
(either CBS nationally, which I doubt, or the local affiliate, which is
probably more likely).

On 29 Sep 2005 08:49:43 -0700 wrote:

| I can understand the STB being the culprit if it happened on more than
| one OTA signal. If the broadcast is 1080i from the broadcaster and the
| STB is processing it at 1080i (no down-conversion) then wouldn't other
| 1080i signals also have synch issues (assuming it's the STB)? But other
| 1080i signals do not have the synch issue so I would think I could rule
| out the STB and go back to the broadcaster as the source of the problem
| (either CBS nationally, which I doubt, or the local affiliate, which is
| probably more likely).

It is still possible that it can be a combination of things. It could
be something unusual in that channel's signal that triggers a bug in the
design of that STB.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------