How to convert optical to coaxial digital output? - diyAudio
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Old 23rd June 2010, 07:33 AM   #1
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Default How to convert optical to coaxial digital output?

Hello,

I have an old Onkyo cd player that have an optical out, And I would like to add a coaxial out in it.

There are 3 connection going to the optical transmitter, I would like to ask what specifically I would have to do ,To make a coaxial output in the cd player. Below is the diagram of the optical out .

Click the image to open in full size.
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Old 23rd June 2010, 11:43 AM   #2
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Join Date: Oct 2005
Location: Central Florida
Most coax dig outs use a resistive divider feeding a pulse trafo. The old TTL standard was 5v, the newer LVTTL standard is 3.3v. Look up the chip that supplies the SPDIF and see what the output is and you can copy any existing player's circuit. The power to the torx can just be abandoned.
Look up SPDIF on Wikipedia, there are links to different schemes and lots of info.
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Old 23rd June 2010, 10:14 PM   #3
fzaad is offline fzaad  Europe
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First think you have to determite is what format data comes from the chip. If it in normal format then you have to put it to coax aka ac format just search from google. If you can't find the documentation that you can understand then search for aes/ebu and fit it to spdif standart. optical and coaxcical is the same put coaxcical is ns. ac and optical is just on/off.
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Old 24th June 2010, 11:17 AM   #4
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Quote:
Originally Posted by fzaad View Post
First think you have to determite is what format data comes from the chip. If it in normal format then you have to put it to coax aka ac format just search from google. If you can't find the documentation that you can understand then search for aes/ebu and fit it to spdif standart. optical and coaxcical is the same put coaxcical is ns. ac and optical is just on/off.
Your explanation is worse than mine.

The three wires are power, ground, and signal. The signal is SPDIF, but is at an elevated voltage much higher than the SPDIF standard of .5v p-p. Most DAC receiver chips in use today can handle 5v signals so you can design it a little hotter. You must trace the signal back to its origin and look up the chip to see what you are dealing with and what a proper load would be. I've done this several times and used a different scheme every time, there are scores of ways to do it. The chips generally list their max output current at all output pins so pick around half of that to work into a voltage divider. 390ohms/90ohms comes to mind as an often used divider but that would be around 10ma@5v, that might be too much current for some chips. I'll send you a manual if I can find something suitable.
Bill
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Old 24th June 2010, 12:33 PM   #5
wwenze is offline wwenze  Singapore
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I kinda get what fzaad is saying regarding the alternative current (or voltage) part.

But it doesn't matter, in the computer the TTL version of SPDIF does not send in ac but on/off, and after the transformer it doesn't really matter since only the ac component gets passed. Treat it as an ac signal with a DC offset @ half TTL voltage, if you would.
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