Optical cable question

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Jim Barrett
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Optical cable question

Post by Jim Barrett » Mon Mar 02, 2009 7:25 am

Good Day folks,
I have a home theater client who is complaining of a switching noise when his system switches from analog audio to optical/digital audio. I replaced the optical cable with no improvement so it would appear that this noise is being generated in either the cable box or the audio amp.
Ordinarily my next step would be to look at the signal from the cable box to see if the noise was evident but I have no clue how to do that with an optical cable.
Any bright (but cheap) ideas? :???:

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CeaSaR
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Re: Optical cable question

Post by CeaSaR » Mon Mar 02, 2009 9:14 am

Ordinarily I would remove one cable (source) and test for noise by switching
between the sources. If noise continues, swap out which cable (source) is
unplugged. If noise continues, remove all source cables and switch between
again. If the noise is still there, it is in the "source switcher". Should the noise
stop when 1 of the sources is unplugged, then you have the problem isolated.

You may find that placement will affect noise. If the regular cable, (phono style
patch cord) is too close to a switching area, it might pick up spurious noise. Try
organizing and routing the cords appropriately.


CeaSaR
Hey, what do I know?

Bigglez
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Re: Optical cable question

Post by Bigglez » Mon Mar 02, 2009 9:25 am

Jim Barrett wrote: I have a home theater client who is complaining of a switching noise when his system switches from analog audio to optical/digital audio.
What sort of noise? A single click? Multiple clicks?
A background noise (hum, buzz, whine)? One time only
or always present with each swtch over?

Is the problem only present in one direction (A to B,
but not B to A)?

Has this just started or a continuing problem?
Intermittent or always present since installation?

Jim Barrett
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Re: Optical cable question

Post by Jim Barrett » Mon Mar 02, 2009 2:57 pm

OK, I've only had a few minutes to look at this so far but I was able to hear the problem.

The cable TV box is connected to the Sony surround sound amp via HDMI but for reasons unknown to me the HDMI does not carry audio signal. Never did & this appears to be normal.
In order to hear surround audio on hi-def channels one must connect the cable box digital audio output (an optical connection) to the digital (optical) input of the AVR.

HOWEVER, commercials are not alway broadcast in digital audio so an analog connection must also be made via the red & white analog inputs, and non-hi-def channels have only analog so the AVR must then be set for autodetect with priority for a digital signal.

NOW, when watching the hi-def channel program the digital audio works as expected with the front of the AVR indicating an optical audio signal. When a commercial comes on and the AVR front panel indicates a switch from optical to analog the speakers give a brief pop, like a speaker would do if you used a switch to cut it off when it was under load (sorry, I have no better way to describe it). At the end of the commercial the AVR switches from analog back to optical and the brief pop occurs.

I have run across this business of the cable box HDMI not outputting audio and having to do this double cable business before but regardless, the AVR should switch with no noise.

When next I go over there I will take a different optical cable with me, just to rule it out (although why a digital signal in a fiberoptic cable should cause such a symptom is beyond me).
If it were a conventional cable I would think about hanging a 'scope in the line to determine whether the noise is comming from the cable box or not.

My question involves an adapter or test jig that would allow one to look at the signal on the optical cable. Noise on that cable should be easily distiguishable from the signal. But, as I think about it, it's more likely the analog connection making noise than the digital.

Still, the fact remains, I can only troubleshoot the optical connection via infrence & substitution. There seems to be a lot of devices available to check loss of signal on optical cables but that was all I found.

Whadayathink?

Bigglez
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Re: Optical cable question

Post by Bigglez » Mon Mar 02, 2009 6:21 pm

Jim Barrett wrote: In order to hear surround audio on hi-def channels one must connect the cable box digital audio output (an optical connection) to the digital (optical) input of the AVR.

HOWEVER, commercials are not alway broadcast in digital audio so an analog connection must also be made via the red & white analog inputs, and non-hi-def channels have only analog so the AVR must then be set for autodetect with priority for a digital signal.
Correct.
Jim Barrett wrote:NOW, when watching the hi-def channel program the digital audio works as expected with the front of the AVR indicating an optical audio signal. When a commercial comes on and the AVR front panel indicates a switch from optical to analog the speakers give a brief pop, like a speaker would do if you used a switch to cut it off when it was under load (sorry, I have no better way to describe it). At the end of the commercial the AVR switches from analog back to optical and the brief pop occurs.
Our AVR doesn't pop. The only indication that it has lost digital
audio is the change on the front panel display, and the collapse
of the surround sound field, usually to stereo.

Further, it takes at least a few seconds to resync each time,
and the audio is muted. We routinely clip the first few seconds
of audio.

I'm surprised the advertisers don't complain about this loss of
delivery to their audience. (For a while I thought I was the only
one who'd noticed this problem).
Jim Barrett wrote:I have run across this business of the cable box HDMI not outputting audio and having to do this double cable business before but regardless, the AVR should switch with no noise.
What brand and model are the TV, AVR, and cable box?
What service provider feeds the cable box?
Jim Barrett wrote:When next I go over there I will take a different optical cable with me, just to rule it out (although why a digital signal in a fiberoptic cable should cause such a symptom is beyond me).
If it were a conventional cable I would think about hanging a 'scope in the line to determine whether the noise is comming from the cable box or not.
I've tried to isolate the problem by reconfiguring the various
cables and units in our current configuration. So far the problem
is systemic (loss of digital audio and delay while muting to switch
to stereo mode).
Jim Barrett wrote:My question involves an adapter or test jig that would allow one to look at the signal on the optical cable. Noise on that cable should be easily distiguishable from the signal. But, as I think about it, it's more likely the analog connection making noise than the digital.
This type of interface is not testable with general purpose gear.
In fact the limited work I've done on serial digital interfaces has
required BER (bit error rate) generators and pathalogical signals.

I posted on this topic last November, but made no progress. We've
decided to live with it.
More info here

Jim Barrett
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Re: Optical cable question

Post by Jim Barrett » Tue Mar 03, 2009 3:35 am

This client is really flakey , so other than my direct observations I give little creedence to his reports & I was only there a few minutes the other day & eventually have to go back. That being said:
Our AVR doesn't pop. The only indication that it has lost digital
audio is the change on the front panel display, and the collapse
of the surround sound field, usually to stereo.
When I originally set the system up I had it set for original program format output but the customer insisted that everything be forced into surround sound so he could hear his surround speakers. I do not know whether the problem existed before that change but I guess it is something to be checked out.
What brand and model are the TV, AVR, and cable box?
What service provider feeds the cable box?
Samsung TV:Series 550, AVR:Sony STR-DG750, I believe the cable box is a Samsung, model unknown & the provider is Time Warner.

Glad to hear someone else has these problems & sometimes the answer is "mysterious are the ways of the cable company". :???:

Jim Barrett
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Re: Optical cable question

Post by Jim Barrett » Tue Mar 03, 2009 3:51 am

Bigglez, I just read the thread you referred me to & it sounds like our problems are remarkably similar with the exception that I'm trying to force the AVR into faking a surround signal. Maybe that accounts for the audible noise.
Thanks for the feedback. Now just gotta see what happens.
I'll update if I get anywhere.

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