Digital Audio Fault Finding

[Bigglez]

Our entertainment television set up is the last
link in a long chain, of which we only have access
to the final few steps.

On some broadcast (and non-broadcast) channels
the digital audio randomly drops out briefly for about
one second. It takes another second to recover. The
picture is not disturbed during these events.

The satellite receiver outputs a fibre optics cable
to the AV receiver/amp. Currently there is no
analog path to compare with this digital path, which
supports Dolby digital surround sound with 48Kbs
PCM audio encoding. The AV receiver has an
alphanumeric display that indicate whether
Dolby digital, or normal surround sound, or stereo
audio, are being received and decoded form the
satellite feed.

The audio glitch is quite common when source
materials are switched by the TV station (in
particular if the local station inserts a commercial
that is not digital audio). The AV receiver drops
back to surround sound or just stereo decoding.

During shows where the dialog is important the
random dropouts are very annoying. (I don't
care if the first two seconds of a commercial
are clipped...).

The channels with the highest drop out rate are
not linked by being on the same transponder or
even the same bird.

I have checked the satellite received signal
strength and there is no correlation between
audio channel problems and signal strength.

At another time the misalignment of the dish
caused dropouts in the picture, and generally
across several channels. (high winds caused
the misalignment - only once in about two years).

Any suggestions on how to isolate the problem
or identify which system component is at fault?

I dread calling to India for help at this point.

[Externet]

Good day!

That is my daily headache at work, trying to see if a digital audio signal on the scope is valid and trace where in the DSP chain the flaw originates.

Do the dropouts appear without the Dolby decoder selected ? Is that a new misbehavior of your TV or been always there?

From what I have experienced in other circuits, changes in sampling rates are not tracked fast enough, due to a missing/bad pullup/pulldown resistor in such sampling rate detector chip.

Not of much help, hope you nail it.
Miguel

[Janitor Tzap]

There are any number of causes...
Take your pick.

Sun Spot Activity.
Local Weather.
Weak Signal.
Refracted Signal.
High Voltage Electrical Interference.
Sudden Spike, or Electrical Surge. {Messes with the Tuner.}
Signal Interference.{High Frequency Ham Operators, High powered CB,
High Frequency Transmissions from Military Aircraft or Commercial that are close by.}

I see this more with the Over Air Broadcasts then with the Satellite Broadcasts.
But I have an older RCA Satellite Receiver that just suddenly drops out.
Without warning, then resets itself.
I'm pretty sure the box is the culprit, because the other receivers work fine.

Good luck with your hunt.


Signed: Janitor Tzap

[Bigglez]

That is my daily headache at work, trying to see if a digital audio signal on the scope is valid and trace where in the DSP chain the flaw originates.

Exactly! I have no easy way of knowing the source
of the problem.

Do the dropouts appear without the Dolby decoder selected ? Is that a new misbehavior of your TV or been always there?

The current configuration has been in place
since Jan 07. The problem has only been there
since the summer. It's a very low failure rate!
Initially I only noticed it on the satellite feed
originating from the local FOX affilliate (KTUV).
Now its on KGO and HIST too.

The decoder is set to "auto Decode" so that it
drops back to surround sound decoding or just
stereo audio if the Dolby Digital is not present.

From what I have experienced in other circuits, changes in sampling rates are not tracked fast enough, due to a missing/bad pullup/pulldown resistor in such sampling rate detector chip.

Possibly. I can't consider a component level
fault until I know what box has the fault. No
amount of troubleshooting will work if the
signal I get from the satellite receiver is bad.

I wish I had access (loaner? friend with same model?)
to a substitute for either the satellite box or
the AV receiver. Would be good to know where
this fault lies. If it is widespread I would expect
customer complaints would bring attention.
In the early days of digital TV is was not uncommon
for a local station to accidently "turn off" the
audio or for a DVR to not record audio or video.

After this long I should be past those problems
with my system set up.

[Bigglez]

There are any number of causes...
Take your pick.

Agreed there are many ways the signal form the bird
fails. This is a case of the audio failing while the PIX
is still good.

I see this more with the Over Air Broadcasts then with the Satellite Broadcasts.
But I have an older RCA Satellite Receiver that just suddenly drops out.
Without warning, then resets itself.
I'm pretty sure the box is the culprit, because the other receivers work fine.

In your experience does the PIX or the sound
fail first?

The digital sound is embedded in the data stream.
Destroying that stream would affect both equally.

Eother the sound data stream is wrong or my
sound decoder is unable to 'lock' to it.

Could be as simple as a drifted PLL or data slicer,
or as complex as a fundemental mismatch in
how my decoder manipulates the data (due to
revised encoding standards). Perhaps the
decoder I have is sensistive to pathalogical
data that only rarely appears in normal data.

The received data stream is separated within
the satellite receiver. Even a program recorded
to DVR and played back has the audio gaps in
the same place each time its played. This
tells me the 'gaps' are stored on the HDD
along with the correct PIX data.

Any system level ideas for trouble shooting?

[rolerbe]

My experience from work with IP based Audio/Video suggests excessive jitter in the packet stream. Depending on the video encoding method (and I don't know what digital TV uses), the pix may be tolerant of a late packet (e.g. MPEG-4 interpolative encoding, etc.), but the audio can't fill the gap. The resynch delay (about 2 sec?) seems long in that case, but if the audio buffer gets clobbered then it may take this long to refill.

[Janitor Tzap]

There are any number of causes...
Take your pick.

Agreed there are many ways the signal from the bird
fails. This is a case of the audio failing while the PIX
is still good.

I see this more with the Over Air Broadcasts then with the Satellite Broadcasts.
But I have an older RCA Satellite Receiver that just suddenly drops out.
Without warning, then resets itself.
I'm pretty sure the box is the culprit, because the other receivers work fine.

In your experience does the PIX or the sound
fail first?

Unfortunately, there doesn't seem to be a pattern with the Satellite Box.

The digital sound is embedded in the data stream.
Destroying that stream would affect both equally.

Either the sound data stream is wrong or my
sound decoder is unable to 'lock' to it.

Could be as simple as a drifted PLL or data slicer,
or as complex as a fundemental mismatch in
how my decoder manipulates the data (due to
revised encoding standards). Perhaps the
decoder I have is sensistive to pathalogical
data that only rarely appears in normal data.

The received data stream is separated within
the satellite receiver. Even a program recorded
to DVR and played back has the audio gaps in
the same place each time its played. This
tells me the 'gaps' are stored on the HDD
along with the correct PIX data.

Any system level ideas for trouble shooting?

I've noticed with the RCA 800 Series DTV Converter boxes.
If you switch channels too fast.
You loose the sound, and it will take up to 10 - 15 Seconds before you'll
get it back.
Or sometimes I have to turn it off.
So it resets the buffer memory.

Hmm........
I would think the problem is in the Satellite Receiver,
and not the A/V Amplifier.
Disconnect the Satellite Receiver, and connect it directly to the Television.
{Use the RCA Composite Output Jacks, or use RF output CH 3-4 if it has it.}
If the Audio still is dropping out, then it is the Satellite Receiver.

I would look into a software upgrade for the Tuner/Receiver.
This may only be possible by sending the unit into the factory for the upgrade.


Signed: Janitor Tzap

[Bigglez]

Unfortunately, there doesn't seem to be a pattern with the Satellite Box.

So you loose audio and keep PIX, or vice versa
(at random with no other changes)?

I've noticed with the RCA 800 Series DTV Converter boxes.
If you switch channels too fast.
You loose the sound, and it will take up to 10 - 15 Seconds before you'll
get it back.

This is normal operation. The decoder has to lock
the data stream and start decoding. It takes
several frames of data to build a whole PIX.
No matter the brand or signal source (bird, OTA,
DVR) any compressed data stream will do this.
Its going to be a bummer for the channel-surfers
as each click will take several seconds to display
a new channel.

I would think the problem is in the Satellite Receiver,
and not the A/V Amplifier.

Why?

Disconnect the Satellite Receiver, and connect it directly to the Television.
{Use the RCA Composite Output Jacks, or use RF output CH 3-4 if it has it.}
If the Audio still is dropping out, then it is the Satellite Receiver.

We don't have any "television" The flat panel
is connected by HDMI, and AC power only.

I would look into a software upgrade for the Tuner/Receiver.
This may only be possible by sending the unit into the factory for the upgrade.

DirecTV will send us new firmware automagically
(at 3am) and has done so a few times a year.
No need to go looking for it or send the box back
for exchange or upgrading.

The AV receiver/amp, on the other hand, has not
been upgraded since new (2003 or 2004). Not sure
that Sony still makes this model?

[Janitor Tzap]

Unfortunately, there doesn't seem to be a pattern with the Satellite Box.

So you loose audio and keep PIX, or vice versa
(at random with no other changes)?

Well, when it happens the box will then automatically reset.

I've noticed with the RCA 800 Series DTV Converter boxes.
If you switch channels too fast.
You loose the sound, and it will take up to 10 - 15 Seconds before you'll
get it back.

This is normal operation. The decoder has to lock
the data stream and start decoding. It takes
several frames of data to build a whole PIX.
No matter the brand or signal source (bird, OTA,
DVR) any compressed data stream will do this.
Its going to be a bummer for the channel-surfers
as each click will take several seconds to display
a new channel.

I would think the problem is in the Satellite Receiver,
and not the A/V Amplifier.

Why?

Unless the A/V Amplifiers have changed a whole lot in the past ten years.
It should only be acting as a Signal Amplifier and Director.

Disconnect the Satellite Receiver, and connect it directly to the Television.
{Use the RCA Composite Output Jacks, or use RF output CH 3-4 if it has it.}
If the Audio still is dropping out, then it is the Satellite Receiver.

We don't have any "television" The flat panel
is connected by HDMI, and AC power only.

Ok, Does this Monitor have the RCA Composite Input Jacks?
Yes?
Then connect the Satellite Receiver that way.
If the picture is fine and the audio.
Then reconnect the A/V Amplifier, and see if the problem reappears again.

I would look into a software upgrade for the Tuner/Receiver.
This may only be possible by sending the unit into the factory for the upgrade.

DirecTV will send us new firmware automagically
(at 3am) and has done so a few times a year.
No need to go looking for it or send the box back
for exchange or upgrading.

The AV receiver/amp, on the other hand, has not
been upgraded since new (2003 or 2004). Not sure
that Sony still makes this model?

One thing that you can try is the old unplug all the units.
Wait about 1 minute, and let them reset.
Then plug them all back in.
I've seen so many CPU based electronic equipment that are on constantly.
Thus, the Buffer stack in the CPU doesn't get cleared.
And causes all sorts of strange behavior, until you reset them.


Signed: Janitor Tzap

[Bigglez]

So you loose audio and keep PIX, or vice versa
(at random with no other changes)?

Well, when it happens the box will then automatically reset.

I didn't describe it clearly. In my case the
PIX is fine (no break up, glitches, freezing) but
the sound goes to silence for about one second
(no glitches, pops, clicks).

When the bird's signal is lost (clouds, rain.
dish misalignment) the picture freezes and or
goes to missing blocks. The sound goes to
silence.

Unless the A/V Amplifiers have changed a whole lot in the past ten years.
It should only be acting as a Signal Amplifier and Director.

This model has multiple inputs and surround sound
amplifiers (six channels plus sub-woofer line level out).
The digital inputs are optical. Obviously, inside the
box the optical is made electrical, decoded to Dolby
6.1 or surround sound or just stereo, converted to
analog and feeds the six internal amplifiers, plus the
external powered sub-woofer.

Ok, Does this Monitor have the RCA Composite Input Jacks?

Yes?

Then connect the Satellite Receiver that way.
If the picture is fine and the audio.
Then reconnect the A/V Amplifier, and see if the problem reappears again.

This is a fool's errand. It doesn't help
isolate the problem because it bypasses all
the digital signal path. Doing so would feed
analog audio in stereo directly from the satellite
box to the monitor's audio speakers.
It would not be possible to monitor the Dolby
digital decoding at all, and it does nothing to
test whether the problem is in the satellite
decoder, or it's optical output, or the optical
"cable" itself, or the AV receiver's digital decoder.
The drop outs are one secod duration and if
present vary from five to fifteen minute spacing.
Switching to another channel or to DVR playback
stops the problem (hint: its not the cables).

One thing that you can try is the old unplug all the units.
Wait about 1 minute, and let them reset.

I understand your goal, but it would not explain
why this problem is on only one or two channels.

It looks more like the signal source is bad than
what I'm doing with it once it comes out of the
satellite box.

Anyway thanks for your help and insight. If this
problem was easy to solve I think we'd have
done so by now.

[haklesup]

AFAIK the audio and video signals are compressed together during transmission. If it were a reception problem, it should be both, not just one.

Ideally for diagnostics, you would view these stations on a separate TV with analog audio to see a difference. You say there is no Analog audio. Are you using HDMI only or a combination of composite and fiberoptic for audio. Is there any analog audio output at all from the satellite reciever.

I know that my HDMI audio occassionally drops off and I need to either change channel or power off/on to get the sound back but this never happens with composite or analog outputs. I currently have never tried fiberoptic as I normally use HDMI.

I am of the opinion that the audio/digital decoder your set uses has issues with missing or unsynched data in the audio stream. There may be a latent delay when switching modes and occasionally this is significant or results in long timeouts.

From your 2nd to last post it seems as though the encoder in the satellite box is the troublemaker. Are there any setup options for audio. Can you force stereo only mode for example?

If a sattelite station were transmitting anomalous data in its audio stream that on particular boxes causes decoding errors that result in silence, then you should inquire with the satellite company. Occasionally, they do figure things out (at least the cable company can). Heck, you just might find out about a firmware u/g.

[Janitor Tzap]

Ok,

I see I don't have enough information about your setup,
to give you some sort of idea as to where to start looking.

The quickest, and easiest way to figure out which unit is giving you the problem.
Is too sub in a known good A/V Amp, or Satellite Receiver Box.

Sorry, I couldn't help you out more.
Thou, I think haklesup may be on to something.


Signed: Janitor Tzap

[afarawayland]

having worked as an audio op at a local TV station, I can tell you that some discrepancies can be caused by operator/production error.

I would think that high production network stuff is pretty flawless, but when it comes to low level production such as local affiliates, they are operating in and out of time windows. Basically, some times, for one reason or another, things like bad/no/wrong audio or picture occur.

just a thought.

Also, some local affiliates are still mono. Meaning they broadcast and do local commercial work in house..in mono. They don't really spend much post production time on it.

If your receivers doing something like pro logic or whatnot, since it is really taking the stereo received signal and applying DSP to make it surround, instead of receiving a discrete 5.1 signal. If this was the case, and a mono signal was fed to a matricing DSP, it would most certainly sound different than the normal stereo/surround would.

[Bigglez]

I would think that high production network stuff is pretty flawless, but when it comes to low level production such as local affiliates, they are operating in and out of time windows.

Agreed. I know of local market errors where
network feeds were aired with no digital audio.
As more viewers (listeners?) get digital only
kit this will not go unnoticed for long.

In my case I doubt the audio drop outs are
widespread as this is a major market and other
viewers would complain. I have this problem on
two local stations and one cable only station
(all by satellite service).

It may be that I'm the only one affected
due to a defective component or inappropriate
configuration in my system.

FWIW, the most recent programs with the
problem were:
24: Redemption (2008) via KTVU-TV
Episodes of "Life on Mars" (2008) via KGO-TV

As the "guy on the end of a long chain" I'm clueless
what might have happened at any step upstream
from me. (I'm pretty clueless about the
boxes I have in my set up - I plumbed them together
based on the diagrams in the manuals).

Also, some local affiliates are still mono.

Unlikely as this is the fourth largest TV market in the USA.

If your receivers doing something like pro logic or whatnot, since it is really taking the stereo received signal and applying DSP to make it surround, instead of receiving a discrete 5.1 signal. If this was the case, and a mono signal was fed to a matricing DSP, it would most certainly sound different than the normal stereo/surround would.

I'm at the limit of my knowledge, perhaps these will help
you (or someone) discover my problem?

Satellite Receiver Manual
AV Receiver Manual
Flat Panel Televison Manual

Wiring was straight forward, I can make a diagram
if anyone is interested.

[afarawayland]

Thats a nice Sony, Ive got their older DG500.

I do not know how many channels broadcast in surround, nor whether or not there is a set broadcast standard for it. For film it can be anywhere all over the place. There are standards for film, but each film chooses from a list of options concerning format. 5.1, 6.1, 7.1, etc...

At least on mine, I make a selection as to what kind of surround format I use. I choose between direct multichannel input (discrete 5.1) or some of the other various formats, Dolby Digital has several different variations, Pro Logic, Pro Logic ii, iix, ect... even variations of these types to fit certain situations, like, Game, Movie, etc...

So, check your AV receiver setup to the type of surround you have selected. Unfortunately, they have their differences.

that said, whatever is being provided to your AV receiver is being supplied by the satellite box.

I have a DVD player hooked up in a similar fashion. Perhaps if you have another piece of digital gear, you could test the AV's handling of PCM audio code with other components, and see if a similar problem occured.

If so, the problem is most likely with the AV receiver, if not..

option 1. the AV receiver has problems (isn't happy) with the received digital intelligence from the Sat box,

option 2. the signal from the Sat box has the errors, which cause dropouts in the AV receivers hardware/software.

option 3. the signal from the Sat box has the errors, due to broadcast, reception, and or programming, which are accurately reproduced by the AV box.