Optical output to Coaxial? - diyAudio
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Old 13th June 2008, 02:14 AM   #1
Tenson is offline Tenson  United Kingdom
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Default Optical output to Coaxial?

Hi,

How simple (or not) would it be to get a coaxial digital output from a CD player that only has an optical output? I know there are boxes that can convert, but Id rather take the signal directly from the player if possible.

I read a thread on here about a guy wanting to do a similar thing, but to avoid ground loops with his AV kit he converted the optical output signal to an electrical one. My DAC (with CS8420 input receiver) has transformer isolation anyway, so I'd rather take the electrical signal direct without ever going optical.

Is it simply a matter of tapping off the electrical signal before the optical socket? Or what, any tricks needed?

Thanks!

Edit: Having looked through the data sheet for the input receiver in my DAC I see that the chip also has digital output pins. Does this mean that the CD player is likely to have an 'input receiver' chip such as the CS8420 driving the digital output? I that is the case I could take the feed directly from its output pins and wire it up with a 75R resistor and pulse transformer. But maybe CD players don't work like this at all?!
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Old 13th June 2008, 12:31 PM   #2
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Try this
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Old 13th June 2008, 01:50 PM   #3
Tenson is offline Tenson  United Kingdom
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Hi,

Thanks that could be useful.

The CD player (now I have won the auction) is a Cyrus DAD3.

What I need to know I suppose, is what to look for to find a good place to tap the digital output? What chip would it be coming from?
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Old 13th June 2008, 04:31 PM   #4
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Trace it back from the optical connector.
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Old 13th June 2008, 05:02 PM   #5
Tenson is offline Tenson  United Kingdom
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Trace it back to where, exactly? What should I be looking for?

I'll take some detailed pics when I get the player and pop the lid.
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Old 14th June 2008, 10:03 AM   #6
Jambo is offline Jambo  United Kingdom
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Well if you look at the optical connector, they only have three pins generally. One for supply, one for ground and the other is your SPDIF signal. The SPDIF TTL signal will be either 3.3V or 5V and you should really reduce it to 1V for coaxial. I answered a thread on how to do this here: Optical 2 S/pdif converter?

Hope this helps.
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Old 14th June 2008, 01:42 PM   #7
Tenson is offline Tenson  United Kingdom
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Hi,

Thanks I thought that was the case, and I had seen that thread.
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Old 17th June 2008, 09:21 PM   #8
Tenson is offline Tenson  United Kingdom
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Hi Guys,

Right, well I have got the player now, so I have gone inside and taken some pictures for you. Its a Cyrus DAD3 with Q DAC upgrade board.

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.


Now what I want to do is add a coaxial digital output, and low and behold the PCB already has the spaces for parts to do that! Super dooper! So I take it all apart and probe around to find were things connect. Here is a picture with red line showing where things go.

Click the image to open in full size.

And here is a circuit diagram of the same.

Click the image to open in full size.

Now my question, is what values do I want to use for 'filling in the blanks' as it were, on the coax out? I have some pulse transformers already so thats no problem.

First there is clearly a low pass filter, I guess to reduce noise form other bits in the player. Then there is a voltage divider, so I guess the output directly from the SAA7345GP is too high for a coax out. Though in the datasheet for this chip, it says that the DOBM digital output is in compliance with IEC958, but I guess that doesn't spec voltage? The chip datasheet can be found here - http://matthieu.benoit.free.fr/cross...ts/SAA7345.pdf

After that we have the transformer and then after that for some reason we have another voltage divider. Why do we need two? I can see we want a 75R resistor between the two TX outputs to set the impedance but whats the series one for?

Help on what values to use where and if any of it should just be bypassed is much appreciated!!

Also I have to wonder why they didn't implement it in the first place. Is it likely that the SAA7345GP doesn't have enough current drive to cope with all the voltage dividers? Seems unlikely they would make it not able to provide a coax out.
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Old 17th June 2008, 10:33 PM   #9
rossl is offline rossl  United States
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I'm guessing that you really don't need R1 and C1. They may be there just to pass an EMI emissions regulation.

You probably don't need R4 and R5 either.

Try the values for R2 and R3 as shown in the above link provided by rfbrw.

100 nF coupling caps in the positions provided for R1 and R4 would probably work OK. Leave C1 and R5 position open. Use a pulse transformer and your SPDIF should sing.
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Old 18th June 2008, 12:10 AM   #10
Tenson is offline Tenson  United Kingdom
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I can easily share the same R1 and C1 that the optical conenctor uses. they go to the same source pin on the chip anyway.

Surely the datasheet must state the output voltage of DOBM pin? Am I just not seeing it? Its a bit tiral and error to assume its output is 3V. I want to get the voltage divider right!

Also is there any point having a coupling cap after the TX, if no DC goins in, none is going to come out, is it? I have 0.22uF caps.
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