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tuck1s 12th October 2012 11:17 AM

S/PDIF monitoring / display tool?
Does anyone know of (or have a design for) a simple gadget that can be used to decode and display what's coming out of a S/PDIF (coax) or TOSLINK (optical) stream?

I have a variety of devices e.g. Popcorn Hour, Squeezebox Touch, running into a Cambridge Audio DacMagic. The DacMagic shows up the incoming sample rate via LEDs. But I've always wondered whether, when playing higher bitrate material (e.g. Linn 24 / 96 Studio Master flacs) whether the devices are REALLY playing 24 bits, or whether they are truncating down to 16 bits.

Maybe some other DACs would display this, but it seems to be something that ordinary consumers (even audiophiles) don't really think about. But I hope that we DIY'ers care about this !!

The S/PDIF encoding doesn't seem to be too complicated - Manchester clock encoding etc - so I was thinking that some kind of simple logic could be used to decode and display the bits on the wire. I was thinking of two rows (for left / right) of 24 LEDs, or maybe even 32, and some good old-fashioned logic gates :-)

Would appreciate your thoughts on this. Not even sure what to call it - hence hard to Google for answers! Maybe a "digital audio truth-teller" :-)


DF96 12th October 2012 12:15 PM

They could truncate to 16 bits but still send 24 bits with zero-stuffing so the absence of the bits says nothing. You could check the lower bits to see if they are all zero, but then someone could just put a bit of random noise in there. The only way to tell is to check the bit value statistics for genuine randomness, but this is not easy.

tuck1s 12th October 2012 01:42 PM

Agreed. Both the Popcorn Hour and the Squeezebox Touch have digital volume controls (when set at 100%, bits out should equal bits in). So I think it might be useful to look at the output at very low volume levels, to see if the lower 8 bits contain actual music.

Do have any suggestions on a S/PDIF decoding circuit?

tuck1s 12th October 2012 04:56 PM

... actually I found a much simpler way to prove that my system is doing 24-bits all the way through. I made test .WAV files which have a 440Hz sine wave, very low amplitude (so that only bits 17 to 24 are actually used), i.e. full scale amplitude is 1/65536. The tone is modulated on/off every second (to make it easier to tell whether it's there or not).

I made three versions - 44.1, 48 and 96 KHz sampling rates. You can download the files here:

Playing this tone carefully through my system, with the amp volume wound briefly around to maximum, the tone can be clearly heard with an ear pressed against the speaker grille.

Only do this test yourself if you KNOW that your system will play files without any clicks/pops at the start/end. Keep mobile phones (in fact anything that emits radio waves) out of the room, because you could easily damage your amp, speakers, or hearing with the system up on maximum gain.

tuck1s 12th October 2012 05:48 PM

.. Just to add that the tone is JUST audible, in a quiet room, at loud-ish (but normal for 'attentive' listening) levels. This surprised me as I had thought that the lower-order bits, now shown to be carrying information in a technical sense, may not be carrying content that could affect normal listening.

This test shows that these low-order bits at at least capable of carrying musically meaningful information, in quiet passages, on a mid-budget system (Squeezebox Touch, optical interconnect to DACMagic, Mission Cyrus 2 + PSX, PMC GB1i speakers).

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