PCM1754DBQR questions about POP noise and mute function

Hi Charlie,

What do you mean by HDMI switching? And I am guessing is SPDIF coming over HDMI, so what part is between the HDMI and the PCM1754 to convert it from SPDIF to PCM data? Do the clocks stop completely? What happens with data?

We have a post in the audio FAQ that talks about eliminating pop with the PCM1754 that could help: e2e.ti.com/.../3120.pcm1754-minimizing-pop-and-click

Justin

Dear Justin

Thanks for your rapid reply and sorry to confuse you.

HDMI switching means when I change 720p to 1080p(etc...)  POP noise occurs.

My HDMI signal come over from SOC , not SPDIF, and there is few part between SOC and PCM1754. Please see my schematics as below.

The clocks stops most of times , sometimes not.  

The data stop completely.

I read the URL you sent to me about eliminating pop with the PCM1754 . I learnd from it that we can use MCU I/O to eliminate pop. But I don't think it helps because pop noise occurs before MCU I/O work.

Is there any other way to eliminate pop ?

thanks very much

Charlie

Hi Charlie,

Would it be possible to get some scope shots of the noise? You said it doesn't matter if mute is enabled or not, that there is still output from VOUT, does this mean that the mute is not stopping the pop, or that the mute isn't stopping any audio at all?

If you stop the clocks abruptly to the part, or the data suddenly goes from full scale output to 0 or somewhere in between, the part has no way of stopping this from happening. In the first case, the part is losing all of its clocks and therefore cannot operate/the output will go from some value to 0 immediately. In the second case, the data is telling the part to go from some value to 0 as fast as possible, which produces a pop, this should be avoidable with using MUTE, but would have to be enabled before the data goes to 0 abruptly.

Justin

Hi Justin

This is Charlie from China.

Sorry to reply you so late. I just came back from Chinese new year vacation.

The mute can stop the audio except pop.

I took some waveform of the noise, please see attach file.

Sorry to bother you so many times.

Thank you very much for your help.

Charlie

POP issue.xls

Hi Charlie,

In the attached file you say that pop happens before MUTE enables, are you talking about before the SOC can pull mute high? Is it possible to connect the function that changes HDMI sources to a mute for the PCM1754, so that as soon as a switch is requested the SOC will mute the PCM1754? This part relies on the mute control or an external mute before clocks change or any random audio comes over the I2S line.

Justin

Hi Justin

Yes, pop happens before the SOC can pull mute high.

I'm thinking about adding a signal swtich after PCM1754 to mute the POP.

Thank you very much.

Charlie

Hi Charlie,

A possible solution is to use a pert like the SN74LVC1G123, which could act as a detector when there is no digital input, and trigger the mute pin through a transistor.

Justin

Hi Justin

Good day!
Thanks for your advice. We are willing to use SN74LVC1G123.

Could you please give me a detailed description about how to use SN74LVC1G123 ?
How to use SN74LVC1G123 to trigger the MUTE pin when switch the HDMI signal ?
A schedemics is appreciated if possible.

Thanks and best regards.
Charlie.

Hi Charlie,

A rough mock up is below, this would trigger mute as soon as the clocks stop.

Just replace the SRC4382 with the PCM1754.

Justin