Power, Serial & AES3

[QuIcK]

Hello,
I'm investigating the possibility of maybe looking at the applications of perhaps doing something along the lines of this:
some background:
AES3 is a digital audio protocol. the long & the short of it is that it runs at 24MHz (max, dependant on audio samplerate) on a balanced signal cable. The signal is galvanicly isolated at each end.
There are some very nice Analogue to Digital converters that translate directly to AES3, with minimal circuitry. And there are some very nice digitally controlled audio preamplifiers (for microphones) out there.
The analogue & digital circuit design is not a problem, I know someone that is pretty awesome at this sort of thing.

What I'm wanting to do is inject a serial-modulated DC onto the AES signal, and recover this at the other end. Its one-way communications, theres no need for any feedback. its going to be designed that the AD will clip before the preamp. if the AD clips, it will show up in our computer or on the audio interfaces.
The 24v would be dealt with to provide Audio power for the mic preamp & AD analogue side, and digital power for a control picaxe, AD digital side & preamp. power consumption would be about 150mA.

I've attached a very bad drawing of what I'm wanting to achieve. The resistors are to use CMR to remove the balanced AES signal from the injection/extraction.

I know that there has been previous projects of model train control over modulated DC, and other power&spi over DC projects, but I've had a look around, and I can't find them.
Is this as simple as having a 24v DC supply, then using a transistor to modulate this from the USART of a picaxe and inject to the aes. then at the other end, using an RC filter to remove DC (and maybe a schmitt trigger) to feed the picaxe USART, and a clamping diode (or similar) to remove the serial from the DC power?

 

[QuIcK]

Looking about, i think I might need to use ASK in order to transmit the serial over DC... that sound right?
or is there a better way?

 

[hippy]

Rather than the simplistic idea of somehow being able to add DC and serial signal to the AES stream by simply connecting to the existing cable ( as in the diagram ) it might be better to extract/recover AES highs and lows and combine that, the serial highs/lows and the DC and then transmit to the other end. The other end untangling it all to create DC, serial and AES.

In theory that's easy enough to do, having got the high/low AES and serials signals, output 24V DC + 1V x AES + 2V x Serial with comparators at the other end stripping out the individual signals. As for implementing it ...

 

[QuIcK]

The problem is that the communications are in opposite directions.
Even if it wasn't, I dont think I would want to intercept & superimpose data on the AES stream. its a very solid standard for transmitting data, but I dont want to potentially corrupt it.
The AES Signal is balanced, so anything I impose on it will be cancelled by the transformer.
Are your worries that the AES signal would effect the serial/DC?
Ill draw up a better diagram! (zipped due to image height of 500px) It doesnt include any DC transmission.

Looking around the internet, I've found this:
http://www.nxp.com/documents/application_note/AN10903.pdf UART Powerline Modem
http://www.nxp.com/documents/data_sheet/TDA5051A.pdf Also UART powerline Modem.
Do you recon they might be what I need? Theres no application notes regarding DC powerlines

 

[hippy]

The problem is that the communications are in opposite directions.

Ah ... that wasn't clear to me.

Even if it wasn't, I dont think I would want to intercept & superimpose data on the AES stream. its a very solid standard for transmitting data, but I dont want to potentially corrupt it.

Yes that's the challenge in achieving what you want to do.

The AES Signal is balanced, so anything I impose on it will be cancelled by the transformer.

In theory, in a perfect world ...

Are your worries that the AES signal would effect the serial/DC?

My main concern was how what you did would actually affect things ( it's a bit like superimposing DC and serial on an active ethernet link between two PC's ), and I don't know what the consequences of doing that would be, but I can imagine that it's not quite so simple as it may appear to be.

Circuits, especially high-speed signalling circuits, are usually quite complexly designed ( for example, look at the pulse transformer circuit couplings used in ethernet NIC's ) and altering anything can upset the delicate balance. You'd probably need a detailed understanding of what's going on in the circuit in order to alter things safely or without corruption.

 

[QuIcK]

I figured some extensive testing would be required. What should I be looking for? I think this is getting more into my mates area of expertise. Im pretty good with PICs & digital stuff, but when its actually in the analogue domain I fall over.
I have plenty of AES interfaces etc kicking about, so I might put a scope on it, and see whats happening. See if applying DC to the AES can be done cleanly & safely, and if it saturates the transformers.

I've figured out that I need to modulate the DC using amplitude shift keying, but I have no idea how to go about this. my trawling of the internet has come up with the previously mentioned ICs (AN10903, TDA5051). perhaps thats where I need to start for that. Any suggestions?

 

[QuIcK]

http://www.scientificonversion.com/AES1998.pdf Page 36 shows Phantom Power injection for AES. Which means this is possible. I just need to find a suitable AES transformer with a center tap.
Then test modulation of this.