Optocoupling for electrical S/PDIF connection

Started by Andy-Antsinpants, December 09, 2016, 09:43:04 PM

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Hi guys, I've got a Denon AVR-1707 AV-Receiver, which has 2 electrical S/PDIF inputs (RCA connector) and 2 optical S/PDIF inputs (toslink). The AV-Receiver outputs to a Teufel Concept M 5.1 speaker set and a Loewe 720p LCD TV.

There are three devices taking up the digital inputs: Xbox 360 (toslink), PS3 (toslink) and a Toshiba SD-220E DVD Player (has both electrical and toslink S/PDIF outputs). The DVD Player is necessary for multiregion DVD playback, since I've got no intentions to modify the other two DVD capable devices for the fear of bans. Plus, that DVD Player ist just really handy and serves as a stupidly simple CD Player, too.

I also route the video signals of some devices (PS3, Wii, DVD-Player) through this AV-receiver, using component Y-Pb-Pr video. The AV-receiver then acts as a A/V switch, and it does a pretty fine job. The Xbox 360 is connected directly to the TV using VGA.

And that's where the trouble lies in. Usually, the picture quality is very nice. But as soon as I connect any device (DVD Player, digital TV settop box, etc.) to any of the 2 electrical S/PDIF inputs, the AV-receivers video signal output gets grainy/noisy. It doesn't matter if I go the component video or S-video route.

There's no such interference issue when using the toslink inputs.

So I guess there's trouble with the electrical S/PDIF connections. I've got these options:
A) A toslink switch, which is pretty annoying over time.
B) Try optocoupling for the electrical S/PDIF.
C) Use analog stereo RCA connections for the DVD Player, leaving the electrical S/PDIF inputs unused. But then I'd stick with Dolby Pro Logic II instead of discrete Dolby Digital 5.1

I tried several TVs, and all had shown the same issue.
I'd really like to get into the optocoupling thing, but I don't know of any DIY project or premade circuitry that is suitable for electrical S/PDIF. All I found is for analog stereo connections.

Cheers, Andy and his ants.

EDIT: Well, thinking about it revealed that there's a chance I might not achieve anything with the optocoupling, since the coupling device needs its own power supply and thus could still apply noise revealed on the AV-receivers video signal output. Still, I'd like to give it a try just for the sake of not giving up easily.

Of course it would be better to check the AV-receiver and DVD-Player for any electrical culprits like cold solder joints and bad capacitors. Not too hard on the latter one, but the former is a pita to service.


Oh well, last Saturday night the SD-220E's laser unit finally refused to read Audio-CDs anyway, while reading DVD-Video is still going strong.

So I cleaned the lens and regreased the gears and rails. All the gears and rubber bands are still intact. I see two tiny pots at the laser unit, let's see if I can find a service manual* for that thing to get the Audio-CD part readjusted.

In the meantime, analog stereo connections will do just fine for the Toshiba. I'll let it play the occasional region 1 DVD and (if the repair is a success) Audio CDs, while region 2 DVD-Video and Blu-ray discs will be handled through the PS3 from now on with S/PDIF. And it might do better upscaling and progressive scan anyway.

Thanks for reading!

*EDIT: Found one! -> http://elektrotanya.com/toshiba_sd-120-220-e.pdf/download.html
Looks like an oscilloscope will be on my shopping list. If allowed, I'd like to attach the PDF file to this post, so others can still download it even when the link might be down in the future.

EDIT2: Since it's too large to be handled as a forum attachment, here's a mirror for the file: toshiba_sd-120-220-e.pdf
If anybody is in search for a laser pickup unit, it might not be available individually. The service manual refers to part MD01, which is Toshiba #79070499 (whole DVD mechanism assembly). It should be cheaper to harvest one out of a working DVD player from these Toshiba models:


So I got myself another SD-220E for measly 14 bucks. Earlier serial no. by the way.
And guess what, that one doesn't have any video noise issues with the RCA S/PDIF out.

So that's a win-win. I've got Audio CD functionality back again, can use RCA S/PDIF without inducing stupid video noise, and was also able to get the multiregion DVD playback function working on that device.