S/PDIF Lock Noise

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Let me start by saying I manufacture and sell a commercial DAC. I browsed the forum rules and didn't see anything forbidding participation in this capacity, but if I missed something please let me know. I found references to noise in general from a forum search, but nothing that looks like my specific problem.

I am having an issue where when the audio signal is first sent from the transport, my DAC outputs a brief crackle or noise. I am not 100% sure if the music is present or not during this time. This only happens with a couple of transports out of many, and only when the signal goes from off (not even a silent signal) to on. It sounds similar to if you forcibly create a poor optical connection by not fully connecting the optical cable and wiggling it around. It occurs with both optical and coaxial connections.

The noise does not occur during playback and is clean for the duration of the signal lock, and it does not occur with the vast majority of transports.

Any help?
 
DC Offset Adjustment

The noise is due to DC offset adjustment. I believe the typical solution is to use a coupling capacitor at the analog signal output. But I was hoping there might be another solution that does not require me to introduce any components in series with the analog signal.
 
I don't think this problems is caused by DC offset...

I would guess that you use CS8416 as SPDIF receiver. With the CS8416 you can see these kind of noises with some CD-drives.
If you have a lock indicator, you will see that this will flash when you hear the noise.
I is causes by these CD-drives, that does not send real SPDIF signal when not in paly-mode, and will stop as soon as it starts to play.
Typically you have connected the lock signal from CS8416 to your DAC-chip mute/RST input. And then the DAC-chip will go in and out of mute/RST along with the flashing lock indicator. And this generates the noise.

You can remove this by NOT letting the DAC-chip go into mute/RST or maybe by using a delay between the lock signal and the mute/RST on the DAC.
 
Thanks for the reply. I'm using a WM8804 to PCM1794A. I do indeed see the signal lock indicator fluctuate. But I have not wired the signal lock to the PCM1794A mute. The mute is always off.

I experimented with wiring it so the signal lock would trigger mute, but I haven't yet been able to confirm this fixes the problem. It sounds like from what you're saying it may actually cause a problem. I thought about adding a delay, but that would mean you miss some music.

I can see the analog output DC level change on my oscilloscope when this happens. It looks like it drops and then comes back up.
 
OK....
Anyway, the noise is generated by the SPDIF datastream from the CD-drive. As I said, you can remove this noise by adding some kind of delay in the mute-signal.
Make so that as soon as the lock is lost, the PCM1794A goes into mute, and that the lock must be present for a given period before the PCM1794A is un-muted again. I would think that 250ms om delay will do the job.

How about that PCM1794A?? You like it?? What kind of analog stage are you using?
We did test the PCM1794A, but choose something else ;-)
We found the PCM1794A to be a bit to skinny, even using an well known discrete analog stage. Be we were VERY impressed by the noise performance of the PCM1794A. I have never seen better noise performance from any DAC. Also the mid and the high frequency was impressive. Only the low frequency area had some trouble. But I believe that this could be solved by more tweaking with the analog stage.
 
Yeah, I figured mute might be an option, especially with a delay. Thought there might be another solution though.

I really like the PCM1794A. I'm using a completely passive transformer-coupled output stage. It really works well. That is part of the reason I don't want to use a coupling capacitor--I don't want it to contribute any impedance that would affect the output.

Thanks. :)
 
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