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Split digital signal

Postby twotoedsloth » Mon Nov 14, 2011 5:27 pm

Is it "safe" to split an AES or SPDIF signal? I have an optical splitter that works great for Toslink SPDIF. Is this even possible with electrical SPDIF or AES/EBU? I remember reading somewhere that AES/EBU was designed to be split up to four times, but Google is not cooperating.

Thanks,

Peter
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Re: Split digital signal

Postby John Willett » Mon Nov 14, 2011 7:18 pm

I always split an AES signal with a proper transformer splitter.

The impedance needs to be 110Ω for balanced AES cable, ot 75Ω for co-axial cable.
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Re: Split digital signal

Postby twotoedsloth » Tue Nov 15, 2011 7:29 pm

So a Y cable won't work?

How about for an electrical (75 ohm) splitter?

I'd like to use a mic pre with a built in ADC to feed three recorders simultaneously.

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Peter
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Re: Split digital signal

Postby twotoedsloth » Thu Nov 17, 2011 5:04 pm

Anyone? Anyone? Bueller?
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Re: Split digital signal

Postby James Perrett » Thu Nov 17, 2011 5:27 pm

You'll need a transformer or distribution amplifier to do it properly. A Y cable might work but this would depend on the device driving the splitter.

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Re: Split digital signal

Postby Hugh Robjohns » Thu Nov 17, 2011 5:57 pm

Er... the original design spec for AES3 (AES/EBU as it was then) specifically allowed for up to a four-way passive split arrangement using a parallel strip or Y-split arrangement -- but with the proviso that each output was correctly terminated with 110 ohms at all times.

That option was withdrawn from the later specs documents becuase of the potential for catastrophic failure with unterminated split outputs... but it does work and I have been known to do it on occasion when necessary. Works best if you can keep the cables very short -- less than a couple of metres. It is also possible to do with S/PDIF (with 75 ohm terms)m, although sincehte starting signal strength is so much lower than AES3 it gets dodgy much sooner!

It is obviously far better (technically) and safer (from a reliability perspective) to use a proper active and independently buffered digital distribution amplifier system, or failing that a proper AES3 transformer split, but in quick and dirty bodge-it situations, a passive parallel split -- provided both outputs are correctly terminated -- can be used quite satisfactorily.

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Re: Split digital signal

Postby twotoedsloth » Thu Nov 17, 2011 6:37 pm

Please forgive me for asking what is probably a stupid question, but what is proper termination? Is this like something from the SCSI days when you'd put a terminator at the end of your device chain?
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Re: Split digital signal

Postby James Perrett » Fri Nov 18, 2011 10:30 am

When you are talking about frequencies above audio the cable characteristics start to become more important and we have to start using transmission line techniques. Every cable has a characteristic impedance which is a function of its capacitance and inductance and each end of the cable must be terminated with a matching network (often just a resistor but it can be more complex) in order to prevent reflections from the ends corrupting the signal.

So the reason for terminators is exactly the same for SCSI as it is for AES/EBU or SPDIF although the value of the resistors may be different.

James

PS - if you've ever looked at the utilities that come with some computer network adaptors, you will often find a cable length tester. This uses the fact that an unterminated cable will reflect a pulse back and measures the time taken for the pulse to return to give the cable length.
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