How to tell if toslink optical audio cable is bad? - AVS Forum
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Old 01-17-2010, 08:30 PM - Thread Starter
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A couple of days ago the audio started to fail from my cable box (little by little, a few works muted here and there) on my home theater setup. I suspect the optical audio cable (toslink) or the receiver (surround bar) input. I have hdmi from my cable box going to my Sony HDTV, then audio to my yamaha surround bar via toslink optical audio cable. The audio on the yamaha surround bar works from the dvd player (via rca imput). I can see the red LED laser type light on the optical audio before it hits the surround bar...does this mean the signal is getting to the receiver and working? The sound works on the tv speakers when switched to that input by the way

thanks for any help you can give
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Old 01-17-2010, 10:14 PM
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The only way for that cable to go bad would be physical damage of some sort. Can you plug the toslink cable into your CD player, instead?
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Old 01-17-2010, 10:37 PM
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Quote:
Originally Posted by mrshanno View Post

I can see the red LED laser type light on the optical audio before it hits the surround bar...

Then the cable is good.
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Old 01-18-2010, 02:49 AM
 
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Originally Posted by bluesky636 View Post

Then the cable is good.

Maybe. It could be cracked and intermittent. I've seen that many times before, particularly if it's bent in just such a way that it's sort of an intermittently bad gap, but you should be able to see the light go away as you manipulate the cable usually. If you don't see light at the other end of the cable it indicates it's probably cracked or damaged, but just because you DO see light doesn't mean that it is then okay.

In any case, if you have another optical SPDIF source you can test the cable on that, or pick up another optical cable from monoprice for a few bucks to test.
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Old 01-18-2010, 05:08 AM - Thread Starter
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thanks guys, I will get another cable to make sure as I don't have another component that uses optical audio.
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Old 01-19-2010, 06:02 PM - Thread Starter
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thanks guys for your help, I tried a new cable and that was the ticket (despite me being able to see the red light at the end of the old cable)
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Old 01-19-2010, 10:45 PM
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I had one go bad not too long ago. It just went dull. I didn't do anything to damage it either.

I've found that a good fiber works with the connector part or all the way pulled out. If the signal is so weak you need it right plugged in to get sound then it's failing or failed.

If you have more than one then you can compare the light between them using 2 different sources. both should appear the same.

Peter
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Old 10-26-2012, 10:49 AM
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I also have a question about optical cables not working. I use my optical out from stb to receiver for great stereo music via cable channels. But when I want to use it for 5.1 sound on a tv show, the sound is spotty, not steady. The AVR shows Dolby Digital and 5.1 speaker display. I was thinking of buying a digital cable and do away with the optical cable..Can the optical be damaged? thanks in advance..

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Old 11-20-2012, 08:34 AM
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I also have a question about optical cables not working. I use my optical out from stb to receiver for great stereo music via cable channels. But when I want to use it for 5.1 sound on a tv show, the sound is spotty, not steady. The AVR shows Dolby Digital and 5.1 speaker display. I was thinking of buying a digital cable and do away with the optical cable..Can the optical be damaged? thanks in advance..

I have seen a similar issue which I believe to be caused by the optical cable.

I use an optical cable to connect the audio from my HTPC to my audio receiver. It sounds great when I play back stereo music files but there are intermittent drop-outs when I play back movies with AC-3 audio (i.e. Dolby Digital 5.1). It's a fairly long cable (12' - 15') and I can "fix" the symptoms by manipulating the slack a bit--I'm pretty sure it's not with the connectors but with the cable proper. It's pretty old (5 years) so I plan to just replace it.

FYI, the optical cable carries digital data--that's the reason I chose it in the first place (before HDMI became so widespread).
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Old 11-20-2012, 09:33 AM
 
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FYI, the optical cable carries digital data--that's the reason I chose it in the first place (before HDMI became so widespread).

Coax carries digital data as well..so do twisted pairs.
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Old 11-20-2012, 10:16 AM
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Originally Posted by SAM64 View Post

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FYI, the optical cable carries digital data--that's the reason I chose it in the first place (before HDMI became so widespread).

Coax carries digital data as well..so do twisted pairs.

Right.

The first test for Toslink is to pull the receiving end out of the AVR and see if there is a bright red light coming out of the end. Shine it on your hand and look for a brilliant red dot. If you have one and the ends are reasonably tightly held by the sockets at each end then it is probably OK.

The final check is to obtain a known good Toslink cable (test it with someone else's system) and see if it helps.

Basically these guys seem to be saying that their cable providers STBs sound great on stereo but have dropouts and the like if they try to listen to 5.1.

I'd blame the source (DVR or cable provider) because the data rate over toslink for 5.1 is pretty much the same as it is for stereo (44.1 versus 48 which is less than 10%). The stereo is lossless, while the 5.1 is lossy compressed.

Another way to test a toslnk cable - use it to interface the blu ray's audio to the AVR.
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Old 11-20-2012, 10:56 AM
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The data rate for "5.1" on toslink/coax is actually much less than stereo CD. The standard for compression in US for digital transmissions is Dolby Digital which is capped at 440 kbps (can go up to 640 kbps on Blu-ray). This compares to uncompressed CD at 1,400 kbit/sec or about three times higher! Yes, the sampling rate is usually 48 Khz for broadcast vs 44.1 for CD but lossy compression more than compensates for that.

The reason you may notice audio problems more with 5.1 is that compression occurs on a block of data at a time (called a frame). If you lose a few bits in that chunk, the decoder may throw out the entire frame. Further, the receiver may lose "synchronization" not knowing what is the start or the end of the frame. As a result of these factors, you hear a larger gap in audio when compressed 5.1 streams are transmitted and data loss occurs even though it is not taxing the link as much. In the case of playing uncompressed CD the lost data only impacts the specific audio samples and not the adjacent bits so the audible gap may be much smaller.

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Old 11-20-2012, 12:53 PM
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Originally Posted by amirm View Post

The data rate for "5.1" on toslink/coax is actually much less than stereo CD. The standard for compression in US for digital transmissions is Dolby Digital which is capped at 440 kbps (can go up to 640 kbps on Blu-ray). This compares to uncompressed CD at 1,400 kbit/sec or about three times higher! Yes, the sampling rate is usually 48 Khz for broadcast vs 44.1 for CD but lossy compression more than compensates for that.

The reason you may notice audio problems more with 5.1 is that compression occurs on a block of data at a time (called a frame). If you lose a few bits in that chunk, the decoder may throw out the entire frame. Further, the receiver may lose "synchronization" not knowing what is the start or the end of the frame. As a result of these factors, you hear a larger gap in audio when compressed 5.1 streams are transmitted and data loss occurs even though it is not taxing the link as much. In the case of playing uncompressed CD the lost data only impacts the specific audio samples and not the adjacent bits so the audible gap may be much smaller.
I was going to post the same thing. Amir has saved me a lot of typing.

In a nutshell, the link appears marginal, with just a few bits being dropped. With PCM, that means that individual samples are discarded, which are no larger than 23 microseconds and probably inaudible. With AC3 (and DTS), it prevents the entire block from being decoded, so you hear large drop-outs.

Although the problem is most likely the cable, it could also be the driver, receiver or dirt. If the cable was ever over-bent, that can permanently attenuate the light passing through it.
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Old 11-20-2012, 02:28 PM
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Originally Posted by sasmith129 View Post

IIt's a fairly long cable (12' - 15') and I can "fix" the symptoms by manipulating the slack a bit--I'm pretty sure it's not with the connectors but with the cable proper. .

If you can "fix" the symptoms by manipulating the slack, you probably have a cable that's too short and the stress (over time) has compromised the cable/connector. Sharp bends/ stress, especially at 15' , can be a likely possibility.
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