[sdiy] More of that S/PDIF stuff.

[Jay Schwichtenberg]

Batz,

It maybe ok to drive multiple input devices (3-4 at most) from a single
source though not recommended. Personally if a run is longer than about 2
meters I'd go optical. Sometimes there is a 75 ohm resistor across the
inputs to provide the 75 ohm termination/input impedance. Some devices have
it built in either with the chips they use or if they use a transformer. Now
if the manufactures of the input devices has given you the option of
strapping the termination in or out here is what you do. Strap all but one
WITHOUT the termination resistor and leave only one strapped with
termination on. Also this will work with AES-EBU only the termination is 110
ohms.

RCA SPDIF:
Next make your 75 ohm octopus cable and KEEP IT SHORT! This will form waht
we call star distribution (center output with input arms coming out from the
center). Hook it up and see what happens.

BNC SPDIF:
If your system uses this it is better. You'll form a 'chain' of devices with
the last one terminated. What you'd do is buy 75 ohm T adapters for all the
input devices except for the last one. Remove all the input termination
except for the last device in the chain. Again it wouldn't hurt to keep the
cables very short. Connect up all the devices with the output device hooking
to the first input, first input -> second input, *** until you get to the
second to last input device. From the second to last device hook a cable
from it to the last device which doesn't have a T and which is terminated.
Basiclly the output device is the first in the chain and the terminated
device is the last in the chain. MAKE A SINGLE PATH THROUGH THE CHAIN, DO
NOT BRANCH OFF OF IT!

There are a couple of digital patch bays out there. One from MIDI MAN and I
just saw one from Frontier Design. They aren't cheap. If you're just doing
SPDIF/AES-EBU it wouldn't be hard to make your own one in/4-6 out device. It
would take a transformer on input, input buffer->TTL (TTL output outputs
onto a bus that all the drivers are on), output drivers and a small 5V power
supply. Hardest thing to get is the transformers but they are out there.
Crystal (CS8402/CS8412 if I remember right) and AKM have SPDIF receiver and
transmitter chips. Look at the PDF data sheets for those devices to get some
idea of what you need. There are also driver chips out there that are
needed. I 've used DS7545X before but I think there are better chips out
there that cost a little more. Can't remember what they are, it was about 3
years ago now.

Hope this provides some insight.
Jay

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Batz Goodfortune
> Sent: Thursday, May 02, 2002 9:54 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] More of that S/PDIF stuff.
>
>
>
> Anyway, the route question goes a little like this. "How messed up do you
> think the S/PDIF signal would become if I tried to drive multiple inputs
> from a single output?" Now I know in video, you get a bit of a
> signal loss
> but nothing the receiving end can't handle. [USUALLY] But there
> is no kind
> of AGC in an S/PDIF or AES/EBU receiver is there?
>
>