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DAC shuts off with all 0 input?

MRC01

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#1
What is a DAC supposed to do when the input signal is all zeroes? I've seen some that turn off or disable output, others that stay on and emit a "flat" analog signal. This makes DACs behave differently between tracks or similar situations.
Is there a specification for the proper way for a DAC to treat this?
 

DonH56

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#2
The DAC itself shouldn't care, but I have had issues with some processors that treat a digital 0 as some sort of idle signal and do strange things like mute or glitch. I am not sure there is a standard that says what to do, if anything, so am curious as to what our more protocol-knowledgeable members contribute.
 

KSTR

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#3
With pro level and good consumer grade stuff I wouldn't expect that it uses auto-mute. And even if they do, it must be transparent and glitch-free anyway, otherwise I'd consider the gear broken :-(
No spec I'm aware of, it's all up to the system designer.
 

Vincent Kars

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#4
Wonder if this is a property of the DAC
What I know is when driving a DAC using a PC, the driver could do 2 things
  • Dropping the connection (no signal is send) hence after a while the DAC mutes
  • Sending zero’s, DAC remains active and plays zero’s hence silence but it won’t mute
 

MRC01

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#5
Related question: imagine a DSP that dithers by randomizing the LSB of each sample. If this DSP is fed a signal of all zeroes, it should still randomize the LSB, right? If so, the digital signal it emits would never be all zeroes. So if even if your DAC was muting on zero signal, it should not do that with this DSP upstream from it. Right?
 

Vincent Kars

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#6
Most of the time DSP is applied on an audio stream. Hence if the audio stops playing, all the DSP stops as well.
 

MRC01

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#7
It might be helpful for me to elaborate the scenario - 3 separate devices:

digital source (disc player) --> DSP --> DAC

Consider a digital source playing all zeroes (say a test signal). The DSP dithers it, so its output is never all zeroes (the LSB is randomized). Thus, as long as the source is playing something, the DAC will never see all zeroes. Thus whether the DAC auto-mutes on zero input becomes irrelevant. It will never auto-mute.

Unless:
1. A DSP that dithers, stops dithering when it detects an all-zero signal.
2. The DAC auto-mutes on all zeroes ignoring the LSB.

Either of the above 2 conditions would seem incorrect/broken to me - they shouldn't happen - right?

Put more simply: I've discovered that one of my DACs auto-mutes on all-zero signal. I put a DSP in front that dithers the signal, and the DAC still auto-mutes! Thus, something must be broken - right? Either the DSP isn't properly dithering, or the DAC is ignoring the LSB.
 

Vincent Kars

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#8
or the DAC is ignoring the LSB
Most DACs are 24 bit today
None of them can resolve -144 dBfs
You might wonder as a DACs can resolve 20/21 bits properly if dither (randomizing bit 24) really make sense at all.
Probably not
 

MRC01

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#9
Sure, even the thermal noise of a metal film resistor can be louder than that!
However, it's more of a question of logic. The question isn't whether the LSB actually makes any difference. It's the fact that if it's randomized, then a test signal of all zeroes, gets turned into something that is not all zeroes. So a DAC that auto-mutes on zero input, should never auto-mute with a dithered signal.
That's what I'm asking.
 

MRC01

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#10
Related question: what does a disc player's digital output (PCM over coax or toslink) do when the disc isn't playing? Does it emit continuous zeros or shut off entirely?
My sound card (ESI [email protected]) has a digital input, so I could test this. But if it records nothing (flat line) from the digital input, how would I know whether the input was continuous zeros, or simply turned off?
I suppose my question is really about the SPDIF protocol.
Is there an app for PCs that shows the raw data coming over the SPDIF interface?
 
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sergeauckland

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#11
My NTP bitstream analyser shows a normal S-PDIF stream when the CD player is not playing anything, with all the non-audio bits set the same as when playing. I get the same result with a DVD player and my SBT, so assume that's the norm.

That makes sense, as you wouldn't want an external DAC to lose lock every time the CD was paused or a new CD inserted or indeed in going from one track to the next.

S
 

MRC01

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#12
That makes sense, what I would call "sending all zeros".
Reading the spec, seems there is a bit 28 "validity bit" that tells the receiver there's no data.
I wonder if there's a software analyzer I could use with my sound card, to capture or monitor the raw bitstream.
 

MRC01

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#13
Oppo support tells me, for my BDP-83: when you hit PAUSE it continues sending the SPDIF stream with the audio bits in each data frame set to zero. When you hit STOP, it stops the SPDIF stream.
 

MRC01

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#14
Follow-up: I contacted the manufacturer of the DAC. It had an auto-mute that triggered when the digital input dropped below a threshold level (not just all zeroes). That's why it muted a dithered (non-zero) signal; the dither was below the threshold. I sent it back, they updated the firmware so it can play any level of digital signal without muting.

In the disc player, when you hit pause or stop, the SPDIF data keeps flowing with zeroes in the audio frame. If you leave it paused or stopped, after a few minutes it stops the SPDIF data stream entirely.
 

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