SPDIF signal question

Thread Starter

coinmaster

Joined Dec 24, 2015
502
I'm trying to repair an expensive audio DAC product and by all means it should currently work but it doesn't.
I'm inputting an spdif music signal from my computer through optical into the dac but there is no spdif signal at the output of the DAC chip.
The only thing I can think of is that it is the signal that is the issue and not the DAC.
The spdif signal is reading 101010101 and it doesn't respond when I turn the music on and off. I'm assuming this is just a blank signal?
 
Last edited:

ebeowulf17

Joined Aug 12, 2014
3,282
No one knows?
Schematics, part numbers, model numbers? Once we've seen those, it would help to get a clearer picture of exactly where you've checked signals.
no spdif signal at the output of the DAC chip.
Depending on what chip you're referring to, I'd expect an analog signal, not a digital one, at the output of a DAC chip.

I can't make any promises, because I'm probably out of my league on these issues anyway, but you're more likely to get help if you provide some relevant details.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
No schematics.
The DAC chip is ES9018.
I don't know what else to say.
I just replaced the entire dac section with a brand new replacement.
The power supply measures right.
The output stage works.
There's no signal at the output of the dac or the output of the I-V stage.
The input signal from the spdif optical is just a repetitive evenly spaced square wave indicating a series of 0s and 1s if I'm not mistaken.
I don't know what that means in spdif language.
 
Last edited:

ebeowulf17

Joined Aug 12, 2014
3,282
No schematics.
The DAC chip is ES9018.
I don't know what else to say.
I just replaced the entire dac section with a brand new replacement.
The power supply measures right.
The output stage works.
There's no signal at the output of the dac or the output of the I-V stage.
The input signal from the spdif optical is just a repetitive evenly spaced square wave indicating a series of 0s and 1s if I'm not mistaken.
I don't know what that means in spdif language.
Have you tried playing your computer optical stream through another DAC to confirm that the stream is good? ... Or tried playing some other optical stream from another device to prove that the stream isn't the problem?

When you say the optical stream is alternating 1010... are you reading the light pulses directly somehow, or reading the voltage output of a spdif chip of some sort? Have you looked at the spdif stream on the computer end to see if it matches what you see on the dac end?
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
Have you tried playing your computer optical stream through another DAC to confirm that the stream is good? ... Or tried playing some other optical stream from another device to prove that the stream isn't the problem?
I don't have another of anything unfortunately.
When you say the optical stream is alternating 1010... are you reading the light pulses directly somehow, or reading the voltage output of a spdif chip of some sort? Have you looked at the spdif stream on the computer end to see if it matches what you see on the dac end?
Well it doesn't look like a chip, it's just a capacitor and some resistors that I see, in any case it's obviously an optical to digital converter. I'm measuring it after the converter and every point between it and the dac chip.
I don't have any equipment to measure the spdif stream from the computer itself but I don't see how a different signal could be magically converted to 1010 at the other end of an optical cable.
 

ebeowulf17

Joined Aug 12, 2014
3,282
I did a little spdif reading:
https://en.m.wikipedia.org/wiki/S/PDIF
https://en.m.wikipedia.org/wiki/Differential_Manchester_encoding

I'm probably going to mangle some terminology here, but that 101010 stream is essentially the carrier/clock signal. If all ones and zeros are evenly spaced, then they actually represent all ones or all zeroes, depending on the relationship between the square wave frequency and the signal's clock frequency.

I can't say for sure that there's no legitimate state in which it would be literally all zeros, but it seems unlikely. The data is grouped into blocks, and each block starts with a control word. The control word is unlikely to be all zeros, so the data stream should include zeros and ones (which because of the differential encoding would appear as differences in the length of high and low pulses.)

I don't see how you can diagnose this any further without either opening up the computer and looking at the data going into its optical hardware, or finding some other device to experiment with as a reference point.

This is going to sound like a dumb question, but is there any chance there's a computer configuration error, like sound is being routed to a different output device, or volume is muted, etc.? Has this computer previously worked with other optical DACs? Is there any evidence for or against trusting the computer as a reliable optical spdif source? Right now it sounds suspect to me, but there's really no way for us to know through the internet!
Well it doesn't look like a chip, it's just a capacitor and some resistors that I see, in any case it's obviously an optical to digital converter.
I was expecting you to have something along the lines of the optical devices on page 2 of this document:
https://media.digikey.com/pdf/Data Sheets/Toshiba PDFs/Fiber-Optic Devices TOSLINK.pdf
I didn't know when I wrote that if there were special circuits handling the optical connections, but now that I've seen the document above, it seems to match what you're describing. The DAC is doing all the work of interpreting the spdif, and the optical parts just have a few passive components with them - so not really a conversion to or from digital, just a conversion from raw light pulses to raw electrical pulses.

I would bet that you don't have a meaningful optical output coming from your computer right now, but why that's happening is anybody's guess. Can't you get your hands on something with an optical out? Surely if you're in the business of repairing high end converters you can locate something with an optical out (cd player, dvd player, digital tv, etc.)
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
I borrowed a friends ps4 and managed to get the dac to work with its spdif output, so apparently it is my computers fault. Now I just need to figure out why.
 

ebeowulf17

Joined Aug 12, 2014
3,282
I borrowed a friends ps4 and managed to get the dac to work with its spdif output, so apparently it is my computers fault. Now I just need to figure out why.
Sweet! Glad to hear the DAC repair was a success.

Sorry it's bad news for your computer. Good luck with that. Is the optical output on your computer a usb or FireWire device, part of a bigger soundcard, built right into the motherboard, a dedicated device just for spdif output? Do you have other audio outputs on your computer that you can test in order to narrow down the scope of the problem?
 
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