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'[OT] AES/BEU to SPDIF'
1999\07\18@213054 by Thomas Brandon

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I read a FAQ on digital audio that stated that AES/BEU and SPDIF were not
convertable. Yet, the only differences I could see where:
1) Voltage levels
   SPDIF is much lower voltage level (very easily wrong).
2) Protocol
   The framing is different. One uses one more bit for it's protocol (not a
data bit).

I assume by not convertable they simply mean you can't make a cable that
does the conversion (unless there's a MCU/PLD in it).
Has anyone made a device to convert AES/BEU<->SPDIF. If so, were there any
problems? Is it a simple matter of converting levels and reinterpretting the
framing?

Thanks,
Tom.

1999\07\18@233918 by Eric Smith

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Thomas Brandon <spam_OUTtomTakeThisOuTspamPSY.UNSW.EDU.AU> writes:
> I read a FAQ on digital audio that stated that AES/BEU and SPDIF were not
> convertable. Yet, the only differences I could see where:
> 1) Voltage levels
>     SPDIF is much lower voltage level (very easily wrong).

Somewhat.  I've seen people wire them together with no problem.  But it
would be nearly trivial to interface them if they don't work directly.

> 2) Protocol
>     The framing is different. One uses one more bit for it's protocol (not a
> data bit).

The framing is the same.  The interpretation of the C (channel status) bit
can be either "consumer" or "professional", depending on one of the C bits.
The IEC 958 spec defines both.  Some devices don't pay any attention to the
C bit anyhow.  In my (limited) experience, the only problem with having the
wrong C format is that a consumer DAT recorder with SCMS will assume that the
digital input is copy-protected.

1999\07\18@235953 by Ross Bencina

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Eric Smith wrote:

>Thomas Brandon <.....tomKILLspamspam@spam@PSY.UNSW.EDU.AU> writes:
>> I read a FAQ on digital audio that stated that AES/BEU and SPDIF were not
>> convertable. Yet, the only differences I could see where:
>> 1) Voltage levels
>>     SPDIF is much lower voltage level (very easily wrong).
>
>Somewhat.  I've seen people wire them together with no problem.  But it
>would be nearly trivial to interface them if they don't work directly.


I've wired AES/EBU to spdif directly, and it has worked for me with short
cables, so I would agree that electrically that you can fudge it, but I
wouln't sell a product based on those assumptions (although I suspect it has
been done)...

>> 2) Protocol
>>     The framing is different. One uses one more bit for it's protocol
(not a
>> data bit).
>
>The framing is the same.  The interpretation of the C (channel status) bit
>can be either "consumer" or "professional", depending on one of the C bits.
>The IEC 958 spec defines both.  Some devices don't pay any attention to the
>C bit anyhow.  In my (limited) experience, the only problem with having the
>wrong C format is that a consumer DAT recorder with SCMS will assume that
the
>digital input is copy-protected.

As far as product labeling goes at least, AES/EBU dosn't necessarily mean
"professional" subcode and vice versa. I have used semi-pro gear that uses
rca connectors for professional AES/EBU code, and vice versa (especially
older gear - Sony DTS10(?) comes to mind).

You can buy little boxes to remove SCMS from a digital stream, nothing you
couldn't do with a 56k DSP, I'm not sure about bit-banging it with a PIC
though! The pros talk about "Clock Jitter" - it seems that the digital
signal is often unbuffered and that the DAT work clock is synced to the
incoming data stream (?) so clocking innacuracies are heard at the output -
go figure.

Ross.

1999\07\19@152023 by Dwayne Reid

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>I read a FAQ on digital audio that stated that AES/BEU and SPDIF were not
>convertable. Yet, the only differences I could see where:
>1) Voltage levels
>    SPDIF is much lower voltage level (very easily wrong).
>2) Protocol
>    The framing is different. One uses one more bit for it's protocol (not a
>data bit).

Check out the Crystal Semiconductor line of digital audio transmitter and
receiver chips.  These contain all the logic necessary to turn SPDIF and
AES/EBU data into seperate audio and subcode data streams.  Essentially,
configure the receiver as one format and the transmitter as the other.  Feed
the bitstreams from one into the other.  You need an appropriate clock
source and there is some other minor bit twidling required, but the chips do
most of the work for you.  This configuration also allows you to modify
subcode info (more work but can be done.

dwayne


Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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