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Old 1st January 2011, 01:12 AM   #1
novec is offline novec  Norway
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Default Balanced AES/EBU to/from optical S/PDIF

Yes, I'm sure this has been answered before, but I can't find any good answers by searching. Sorry...

I've got a Lynx AES16 digital sound card with balanced AES/EBU inputs and outputs, and need to convert them to and from optical and coax S/PDIF. Circuits for balanced AES/EBU to coax S/PDIF are abundant, but I haven't found anything for optical. Anyone have anything up their sleeves?
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Old 1st January 2011, 02:43 PM   #2
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Look up the TOTX requirements and you should be able to cobble something together. Optical IS SPDIF.

Best, Bill
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Old 1st January 2011, 10:59 PM   #3
novec is offline novec  Norway
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Finally something that's actually really simple!

Happy new year!
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Old 2nd January 2011, 12:19 AM   #4
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Maybe not so simple. Putting aside the issue of professional and consumer flags and that some consumer gear flatly refuse to work with professional flags and the sample rate indicators on my old school Dacmagic won't light up with them, the output levels are different with AES/EBU at 2-7 Vpp and SPDIF at 0.5 - 0.6 Vpp. Not only is the output of an optical receiver ill-suited to driving such levels, it is also single ended. In your shoes, I would seriously consider an actively driven transformer based solution.
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Old 3rd January 2011, 01:52 PM   #5
novec is offline novec  Norway
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Input is fortunately very simple, and since the Lynx has output transformers I'm just adding a voltage divider on the coax S/PDIF outputs. The optical outputs can take the AES/EBU voltage directly.

Flags on the outputs may undoubtedly cause problems in certain situations, but I'll tackle that problem if it arises. Their main functions will be to interface with home made DACs if I ever need more channels.
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Old 3rd January 2011, 05:06 PM   #6
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Good call. Most of, if not all, the modern receiver chips can also handle a 5V signal, and with proper termination, a 7V AES/EBU signal is knocked down to 3.5V.
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Old 3rd January 2011, 05:30 PM   #7
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Quote:
Originally Posted by novec View Post
Input is fortunately very simple,....
Not to the Lynx
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Old 4th January 2011, 11:25 AM   #8
novec is offline novec  Norway
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Bill: In that case I might get away with terminating the return wire and run the live directly into coax for a 3.5V output. Would make for a cleaner signal path when running XLR, too. And I'd need to get the actual output voltage from Lynx to get correct resistor values for the voltage divider to keep S/PDIF coax voltage within specs.

Bottom line: Do as little as possible, and fix it if things blow up

rfbrw: How so? The Lynx can take both AES3 and S/PDIF code, and AES has an input voltage range of 0.2-7V to cater for both S/PDIF coax, optical and AES/EBU.

Last edited by novec; 4th January 2011 at 11:28 AM.
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Old 4th January 2011, 11:46 AM   #9
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To allay any fears, look up the datasheet for the CS8416 receiver chip, and check out Rane's website. They have quite a bit of info about terminations and other useful stuff. Any time you match impedances you attenuate the signal by 50%.

If you ever do need to buffer or raise the level of an SPDIF output you can do it easily with a flip-flop chip like the 74hc74. Easy as pie.

Best, Bill
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Old 4th January 2011, 01:41 PM   #10
gmarsh is offline gmarsh  Canada
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I've built plenty of these cards. I wouldn't call these "audiophile", but neither is AES/EBU (who the hell decided XLR was a connector worth using for >1mbps data, anyway?)

For SPDIF-AES, use a TORX177 receiver and feed a 74HCT86 XOR gate with it - usually I'll use all four gates to buffer the '177 output, two configured as inverting and two as non-inverting. Put a 100 ohm resistor on every gate output, tie the inverting/non-inverting lines together after the resistors, put a 1uF ceramic in series with one of them and feed an AES/EBU transformer with it. Output side of the transformer is your AES/EBU output.

For AES-SPDIF, feed the AES/EBU input into a transformer, terminate the secondary of the transformer with 110 ohms (I use a pair of 1% 221's) then feed the input into a fast (>=10mbps) RS422/RS485 receiver chip. Connect the receiver output to a TOTX177 input.

In each case power the thing off a 5V wall wart, or if you're lazy, put a USB connector on the card and power it from a USB port on your PC. And make sure you decouple everything well, especially the HC86 in the first design.
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