pcm1794a DAC Mute function

hug96

Member
2008-12-22 12:13 am
Hi,

I just built a CS8416 + PCM1794a DAC, inspired by several circuits I found around the web. I started the project when I realized I was either playing LPs or listening through my computer, but had not used my CD player in a year.

It is working wonderfully excepted that there is a noise when no signal is applied (something like "shhhh" of "fffffffff").

I shorted the zero and the mute pin together, as seen on several other circuits, and when the noise appears Zero and mute are both high, behaving as expected...

I'm using a passive I/V with 200ohms resistors followed by a opa2134 as differential amplifier.

I don't know if I misunderstood the mute function or if my chip is behaving weirdly, but would like to get some advice, as this is a bit wasting the amazing listening experience when the hiss starts everytime whatever i'm playing is over...

Thank you for your help,
Hugues.
 

hug96

Member
2008-12-22 12:13 am
Hi,

the noise occurs when there is no signal applied at all: cable unplugged or in the case of my mac computer about 30 seconds after a sound has been emitted (i think it sends "silence" for a certain period after a sound is emitted, before turning off the output).

But when the noise occurs the chip recognizes that there is no input signal, as the "zero"pin is high, and in my configuration "mute" as well.

It's the noise of the cs8416 trying to lock onto the S/PDIF stream.

do you think it would pass through the dac chip even if the output is supposedly muted?
 

hug96

Member
2008-12-22 12:13 am
It's the noise of the cs8416 trying to lock onto the S/PDIF stream.


I think that you might be right!

I just measured the zero pin when "real" silence is sent and it is high as well but there is no noise.

What might be happening is: as the cs8416 is trying to lock on something, the dac could be coming in and out of mute really fast? making that noise?

what would be the solutions then?

On some circuits that i've seen the "mute" pin is grounded or left floating, do you think it would solve my problem? (I don't want to cut a track on my pcb to experiment if there is no chance it might work..).
 
...What might be happening is: as the cs8416 is trying to lock on something, the dac could be coming in and out of mute really fast? making that noise?...what would be the solutions then?

If the problem is that the input receiver is intermittently going in and out of lock, it is not easy to say why. It could be due to anything from a bad digital cable, to a poor digital interface ground, to poor power supply bypassing, to an inaccurate PLL loop filter implementation, to any number of lesser causes.
 
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hug96

Member
2008-12-22 12:13 am
Correct me if I'm wrong, but you should be able to use pin 14 from the CS8416 (nv/rerr) to activate mute of the PCM. The PCM mute is active if set to high (5V)

That's what I was wondering, can I just short these two?

I included the schematic, it's a bit messy, my first eagle try...
 

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de_smits

Member
2010-02-04 8:37 pm
I'm working in the same way. Copy from different schedules and try to understand data sheets. As far as I understood you should be able to connect (via resistor?) these pins. I purchased also a Hong Kong kit a year ago. I tested the mute to connect 5V and that worked. If this is solving your issue, don't know to be honest.
Anyhow I can confirm that without mute you will have a sissssssss when there is no signal at the spif.
 

hug96

Member
2008-12-22 12:13 am
If the problem is that the input receiver is intermittently going in and out of lock, it is not easy to say why. It could be due to anything from a bad digital cable, to a poor digital interface ground, to poor power supply bypassing, to an inaccurate PLL loop filter implementation, to any number of lesser causes.

Hi, the noise appears when there is no signal applied, otherwise it works perfectly...
What I get is the "sisssss" noise that de_smits mentions in the previous post.

Correct me if I'm wrong, but you should be able to use pin 14 from the CS8416 (nv/rerr) to activate mute of the PCM. The PCM mute is active if set to high (5V)

I just tried that and... no luck. nv/rerr is high, mute pin reads 3.3V but no mute...

Could the mute be defective on my chip?
 
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de_smits

Member
2010-02-04 8:37 pm
Assuming you did dis-connect the mute pin to earth and checked availability of 5V, there should be something else.
I was using a passive I/V + tube output, when I checked manually the mute function applying 5V to the mute pin. The noisy sisssss disappeared at that time.
Could be your chip or.... maybe somebody else has another idea.
 

de_smits

Member
2010-02-04 8:37 pm
The PCM has already an internal reset. You can try without using this pin connection as well. A simple test to check mute function is to connect the mute pin via a switch to 5V. Play the music and activate the mute without disconnection signal. Should work always.
 

hug96

Member
2008-12-22 12:13 am
Hi,

I just did what you suggested and the mute functions properly when playing music (I didn't think about that...)!

But still not when there's no input... This is really frustrating.

I'm just starting to think I will haveto put a relay to ground the output, activated by the nv/rerr pin of the receiver.

Maybe the PCM is not muting because there is nothing to mute as the dac recognizes there is zero input?
 

de_smits

Member
2010-02-04 8:37 pm
Look at schematic kkdac http://www.diyaudio.com/forums/digital-source/86621-dac-project-pcm1794-1798-a.html
Pin14 connected to mute, but pin14 also not in nverr status through ground. In this schematic pin14 is connected to VDD. The thread des not tell if it works. Nevertheless I interpreted the data sheet you need to connect pin14 to ground via resistor and use same pin via resistor and/or transistor to mute pin at PCM. I was able to get indication using led+resistor at pin14, but dd nt test mute yet. Led is connected to ground and is ON if there is no signal and shuts OFF having spdif signal. Should be able to turn this into proper signal for mute function without extra components.