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Old 20th April 2014, 01:42 AM   #1
TJ52 is offline TJ52  United Kingdom
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Default How does dual wire AES work?

Hi All,

My name is Ted. Can someone please explain how Dual AES on transports works?

Does it send left and right channel down separate cables with half the sample rate ? Or does it just half the single and send both left and right channel down both?

Thanks in advance!
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Old 20th April 2014, 02:31 AM   #2
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by TJ52 View Post
Hi All,

My name is Ted. Can someone please explain how Dual AES on transports works?

Does it send left and right channel down separate cables with half the sample rate ? Or does it just half the single and send both left and right channel down both?

Thanks in advance!
Dual AES? Do you mean AES/EBU which usually is sent over 2 wires and a shield in an XLR cable?
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Old 20th April 2014, 08:19 AM   #3
TJ52 is offline TJ52  United Kingdom
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Yes. Like on esoteric and DCS dacs. I am wondering how the digital signal is sent. If it is split from left to right or if the signal is halved.
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Old 20th April 2014, 12:08 PM   #4
dmills is offline dmills  United Kingdom
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The protocol is almost exactly the same as spdif, just transmited differentially at a different voltage and impedance level (To make it more robust when used across very long lines).

You can easily send an AES3 signal 100m down a line traversing an electrically noisy environment like a TV studio, spdif does not do nearly as well under these conditions, but the major difference is just in the electrical interface, the bitstream is very close to being identical.
Electrically AES3 is pretty much RS422 with the usual manchester coding that allows for easy transformer coupling.

It is in fact often possible to feed AES3 to a spdif recever with nothing but a resistive pad to adapt the levels and it will often work fine at least over short distances.

Canford used to have an amusing party trick at the broadcast trade shows where they sent AES3 down two bits of string stretched over a salt water bath, that is how robust it is.

No really good reason to favour AES3 over spdif in a domest setup where the cables are short and the equipment is all within a few metres.

Regards, Dan.
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Old 20th April 2014, 12:13 PM   #5
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by TJ52 View Post
Yes. Like on esoteric and DCS dacs. I am wondering how the digital signal is sent. If it is split from left to right or if the signal is halved.
Like dmills said AES/EBU is just SPDIF but at higher level and balanced signals. Great if you need to transmit long distances.

The actual format is digital in nature so you don't need to think in ways that you send half the signal in a single cable: the answer is you send both signals at the correct sampling rate. It's just bits to transmit and as long as the bandwidth exceed channels * sampling rate * resolution all is well.
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Old 20th April 2014, 12:13 PM   #6
TJ52 is offline TJ52  United Kingdom
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Thank you for your reply. Is dual wire AES simply split up left and right channel?
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Old 20th April 2014, 12:30 PM   #7
OllBoll is offline OllBoll  Sweden
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Originally Posted by TJ52 View Post
Thank you for your reply. Is dual wire AES simply split up left and right channel?
If I remember correctly at each sample you basically get two packages of data, one package for the left channel and one for the right. I think it was somewhere in the header a bit is set to 0/1 depending on which channel it is. At least it is that way in i2s and I expect that SPDIF is similar.

So yes, AES/EBU sends both channels data on the same wires and the receiving chip analyzes the data and extracts the two channels from it. Kinda like how a picture file can have several pixels, AES has more than 1 channel.

Last edited by OllBoll; 20th April 2014 at 12:32 PM.
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Old 20th April 2014, 12:42 PM   #8
dmills is offline dmills  United Kingdom
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It is basically the same TDM protocol that spdif uses, almost the only difference is the voltage and impedance level, so no it is NOT one wire per channel, the voltage difference between the wires carries the information for both channels.

Regards, Dan.
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Old 21st April 2014, 09:18 AM   #9
Dr_EM is offline Dr_EM  United Kingdom
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Yes, it is identical to S/PDIF data, with both channels multiplexed together, only 1 bit differs in the header which tells the receiver (if it is looking at this bit at all) whether the stream is consumer or professional.

Definitely not 1 wire per channel, nor a separate clock as some people believe. It is simply a balanced transmission, just like professional analogue audio is, and hence requires a hot and a cold line. As mentioned, the characteristic impedance for AES3 is 110 Ohm whereas for S/PDIF over coax it is 75 Ohm.
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