Sign in with
Sign up | Sign in
Your question

Audio synch issues with CBS HD

Last response: in Home Theatre
Share
Anonymous
September 19, 2005 3:08:08 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

Hello,

I've been receiving over-the-air HD signals for almost 3 years now.
I've always used a set-top from either DirecTV or Dish (depending on
who I was currently subscribed to). I have a question about the HD
broadcast from CBS. I notice that on all HD broadcasts the audio is
about 1/2 - a full second behind the video. This applies to all CBS
shows in HD. If it's an SD show the audio/video is fine. CBS is the
only station I notice this on. I receive about 10 other OTA digital
signals and the audio/video on their HD broadcasts are fine on all of
them. I seem to remember this problem starting when I switched from
DirecTV to Dish and switched set-top receivers. I wouldn't think the
receiver had anything to do with it (since it only happens on one
station) but I had it replaced just in case and I still have the
problem. I contacted our local CBS affiliate (KDKA - I'm in the
Pittsburgh area) and talked to their HD technician. He said he checked
their equipment and everything seemed fine. Occasionaly he said they
can get out of synch but tend to correct themselves (or someone there
does it).
I can't imagine this is a problem with CBS or else there would be a lot
of complaints. I can't even think this is a problem with the local
affiliate or else there would some complaints, I would think (unless
I'm the only person watching CBS HD over-the-air in Pittsburgh). I
can't seem to pinpoint what could be causing the problem. It's very
distracting to watch prime-time shows or Letterman when the audio
doesn't match the video. Does anyone have any ideas or maybe you are
experiencing the same thing?

Thanks.

More about : audio synch issues cbs

Anonymous
September 19, 2005 8:40:13 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

(brewman_63@yahoo.com) wrote in alt.tv.tech.hdtv:
> I wouldn't think the
> receiver had anything to do with it (since it only happens on one
> station) but I had it replaced just in case and I still have the
> problem. I contacted our local CBS affiliate (KDKA - I'm in the
> Pittsburgh area) and talked to their HD technician. He said he checked
> their equipment and everything seemed fine. Occasionaly he said they
> can get out of synch but tend to correct themselves (or someone there
> does it).

It's completely the fault of the station *if* you are not feeding the audio
through any processing. If you are, that might be exaggerating a slight
problem with that station. Try and see if anybody else has this problem
by reading the Pittsburgh area thread in the "Local HDTV" forum at AVS Forum.

Despite what the tech said, often nobody from the station is actually
monitoring the digital signal, so many things go wrong and nobody notices.
"Checking the equipment" just says it is set up they way they think it should
be, not that the signal is correct. The only thing you can do is complain
and complain some more.

Even if a station can't afford to have somebody sitting and watching the
digital signal (as received, not "in house"), they could still afford to
record it every night and take a look at the recording the next day. It
should keep problems from repeating like they seem to.

--
Jeff Rife |
| http://www.nabs.net/Cartoons/OverTheHedge/Workaholic.gi...
Anonymous
September 22, 2005 5:32:17 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

I've tried the audio both ways - directly from the set-top receiver to
the TV and also through my AV receiver and both ways it is delayed. I
haven't heard of anyone having problems when I've asked before but now
that I know others have experienced it I will try to contact the
station again and ask at the AVS forum if anyone else has noticed this
or has any ideas. Thanks.
Related resources
Anonymous
September 22, 2005 5:32:18 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

<brewman_63@yahoo.com> wrote in message
news:1127421137.929253.47260@z14g2000cwz.googlegroups.com...
> I've tried the audio both ways - directly from the set-top receiver to
> the TV and also through my AV receiver and both ways it is delayed. I
> haven't heard of anyone having problems when I've asked before but now
> that I know others have experienced it I will try to contact the
> station again and ask at the AVS forum if anyone else has noticed this
> or has any ideas. Thanks.
>

It must be local. No synch problems in the LA area on 2 different cable
systems.
September 22, 2005 10:23:49 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

Unfortunately, in the design of current digital TV systems, the audio and
video signals are independently compressed using two entirely different
methods, then mixed / muxed together with some time stamps to allow the
receiver to get things back into synch.

This bifurcation of the data can and often is corrupted by DVD authoring
programs, retransmission of the signal by local broadcasters, and video
editing, compositing, or studio switching equipment. We are still witnessing
the infancy of the HDTV broadcast industry with a lot of glitches and
problems.

The local affiliate / station's chief engineer is the guy to talk to.

Smarty



<brewman_63@yahoo.com> wrote in message
news:1127421137.929253.47260@z14g2000cwz.googlegroups.com...
> I've tried the audio both ways - directly from the set-top receiver to
> the TV and also through my AV receiver and both ways it is delayed. I
> haven't heard of anyone having problems when I've asked before but now
> that I know others have experienced it I will try to contact the
> station again and ask at the AVS forum if anyone else has noticed this
> or has any ideas. Thanks.
>
Anonymous
September 22, 2005 11:53:20 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

On Thu, 22 Sep 2005 18:23:49 -0400, Smarty wrote:

> Unfortunately, in the design of current digital TV systems, the audio
> and video signals are independently compressed using two entirely
> different methods, then mixed / muxed together with some time stamps to
> allow the receiver to get things back into synch.
>
> This bifurcation of the data can and often is corrupted by DVD authoring
> programs, retransmission of the signal by local broadcasters, and video
> editing, compositing, or studio switching equipment. We are still
> witnessing the infancy of the HDTV broadcast industry with a lot of
> glitches and problems.
>
I've run into this while editing HDTV captures, and converting them to
formats that will fit on a DVD-R (the largest storage format on my HTPC).

Some software grabs the first time stamps, calculates the difference
between the audio and video, and assumes that the delay will stay the
same.

That's a bad assumption. It may stay the same for a long period. But it
may change at any time. Once the system starts ignoring the the incoming
timestamps, then the timing is lost for everything downstream.

-- hac
September 23, 2005 6:06:00 AM

Archived from groups: alt.tv.tech.hdtv (More info?)

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.

Smarty


"hac" <redacted@none.invalid> wrote in message
news:p an.2005.09.23.02.53.20.79183@none.invalid...
> On Thu, 22 Sep 2005 18:23:49 -0400, Smarty wrote:
>
>> Unfortunately, in the design of current digital TV systems, the audio
>> and video signals are independently compressed using two entirely
>> different methods, then mixed / muxed together with some time stamps to
>> allow the receiver to get things back into synch.
>>
>> This bifurcation of the data can and often is corrupted by DVD authoring
>> programs, retransmission of the signal by local broadcasters, and video
>> editing, compositing, or studio switching equipment. We are still
>> witnessing the infancy of the HDTV broadcast industry with a lot of
>> glitches and problems.
>>
> I've run into this while editing HDTV captures, and converting them to
> formats that will fit on a DVD-R (the largest storage format on my HTPC).
>
> Some software grabs the first time stamps, calculates the difference
> between the audio and video, and assumes that the delay will stay the
> same.
>
> That's a bad assumption. It may stay the same for a long period. But it
> may change at any time. Once the system starts ignoring the the incoming
> timestamps, then the timing is lost for everything downstream.
>
> -- hac
>
Anonymous
September 24, 2005 1:56:06 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

On Thu, 22 Sep 2005 19:53:20 -0700 hac <redacted@none.invalid> wrote:
| On Thu, 22 Sep 2005 18:23:49 -0400, Smarty wrote:
|
|> Unfortunately, in the design of current digital TV systems, the audio
|> and video signals are independently compressed using two entirely
|> different methods, then mixed / muxed together with some time stamps to
|> allow the receiver to get things back into synch.
|>
|> This bifurcation of the data can and often is corrupted by DVD authoring
|> programs, retransmission of the signal by local broadcasters, and video
|> editing, compositing, or studio switching equipment. We are still
|> witnessing the infancy of the HDTV broadcast industry with a lot of
|> glitches and problems.
|>
| I've run into this while editing HDTV captures, and converting them to
| formats that will fit on a DVD-R (the largest storage format on my HTPC).
|
| Some software grabs the first time stamps, calculates the difference
| between the audio and video, and assumes that the delay will stay the
| same.
|
| That's a bad assumption. It may stay the same for a long period. But it
| may change at any time. Once the system starts ignoring the the incoming
| timestamps, then the timing is lost for everything downstream.

Actually there are two assumptions being made here. If the delay will
not be the same, then something is already wrong in the incoming A/V.
That could be a missing section of one or the other. That could be bad
software doing the original digitizing. But the big problem is, how do
you know whether a jump in timestamp difference is due to missing data
or due to miscalculated timestamps. Given no other means to syncronize,
one assumption or the other has to be made. You have to either assume
the timestamps themselves are the authority for syncronization, or you
have to assume the quantity (time length) is consistent. Either of these
assumptions could be inconsistent with what you get when dealing with any
possible software bugs in the environment creating that A/V content.

But I did say two assumptions are being made. In addition to assuming
how content is supposed to stay in sync, there is also the initial
assumption of how to get it in sync in the first place. If software
can be faulty and produce inconsistent timestamps for consistent length
material, then using the starting difference makes sense (until there is
some missing content). However, if software can result in some missing
content or otherwise unexplained timestamp shifts, then assuming that
the starting point itself is in sync isn't even valid.

Given a world where software developers so frequently do things wrong,
such as inocrrect timing calculations, or poor error handling that lets
physical errors corrupt data streams, no assumption can cover all the
possible problems.

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 Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------
Anonymous
September 24, 2005 2:17:46 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

<phil-news-nospam@ipal.net> wrote in message
news:D h37rm02rov@news2.newsguy.com...
>> 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
Anonymous
September 24, 2005 2:31:31 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

On Fri, 23 Sep 2005 02:06:00 -0400 Smarty <nobody@nobody.com> 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/ |
-----------------------------------------------------------------------------
September 24, 2005 2:31:32 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

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)


<phil-news-nospam@ipal.net> wrote in message
news:D h39u302u0m@news2.newsguy.com...
> On Fri, 23 Sep 2005 02:06:00 -0400 Smarty <nobody@nobody.com> 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/ |
> -----------------------------------------------------------------------------
Anonymous
September 29, 2005 12:49:43 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

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).
Anonymous
September 29, 2005 11:29:52 PM

Archived from groups: alt.tv.tech.hdtv (More info?)

On 29 Sep 2005 08:49:43 -0700 brewman_63@yahoo.com <brewman_63@yahoo.com> 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/ |
-----------------------------------------------------------------------------
!