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Digital audio protocol for 100M copper
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mc73:


 I've been tasked with updating an audio communication system used in the fast food industry for drive ins (e.g. 3 order takers to 30-60 order stalls). We're converting an old simplex analog audio system to duplex. There are long runs of conduit with multiple wires. We're moving to digital to avoid crosstalk and interference that was previously avoided by the simplex communication. On the PCB we'll be working with I2S, AKM DSPs and TI PurePath ICs.  Ideally it needs to run over copper, typically cat 5e or 6 in runs of up to 100m. Wireless is not an option to the stalls. In general signal robustness is more important than quality. We frequently deal with poor wire termination, etc.

I don't have any experience with long distance digital audio protocols and am hoping someone might a have a recommendation.

Thanks in advance for any advice.

coppice:
If you need to send something 100m over CAT5e just use ethernet. There are cheap devices for it, and making the link robust and reliable is a well established practice.
T3sl4co1l:
Yeh, what bandwidth? :)

Are these links point-to-point or multi-drop?  Any clue about signal quality?  (If it's just telephone cord in conduit, and probably some low-voltage power lines or switches or lights or such signals, it's probably not too horrible.  If it's a spaghetti mess with multiple terminal blocks along the way, ehh...)

RS-485 is probably good enough over that distance, assuming typical bandwidth for audio (22kHz 8-bit PCM would be overkill?).

As mentioned, Ethernet is rather good, although using the full Ethernet stack is really heavy weight here.  That said, if you can leverage standard products, it may be enticingly easy to install (just put modular plugs on all the ends, an off-the-shelf router/switch at the central hub, and some kind of audio/intercom device at the end -- I've got to imagine these exist already..?)

Although one downside is, 10/100 Ethernet is half-duplex, i.e. two pairs are needed.  If you only have one pair down the conduit, you'll need something a bit different.  (Are there full duplex converter dongles out there?  This also seems like a common enough problem that a solution exists, hmm.)

Tim
Jeroen3:
Is this for one establishment, or an entire chain?
If for one site, what are the motivations to developing something custom? Aren't there some vandal-proof screens and speaker/microphone units you can use?
A small industrial fanless PC running some software seems a lot easier to replace then a broken custom board.

Also, why don't drive trough work with your smartphone yet?

Otherwise, nothing beats ethernet for available parts or software.
mc73:
It's voice communication so we can limit to about 300Hz to 3400Hz.

The previous system utilizes a punch block for all the stalls. The order taker selects a stall number, and relays isolate that connection.

The system is being designed for 3000+ stores. We're designing from the ground up. The system will also handle signalling in for service, blinking button lights, etc. Off the shelf is not an option feature or cost wise. We'd prefer not to use the full ethernet stack. The systems needs to be robust and isolated. If the system goes down it essentially shuts the store down. Reliability was the primary selling point of our old analog simplex system which has been in stores for about 20 years now. Our customers have tried some ethernet products and had reliability and latency issues.

We can have as many pairs of wire as needed. Pulling wire will be part of the install.

I'll look into PCM over RS-485.

Thanks Tim.




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