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PCM1794A: Noise issue with device in mute.

Lenny
Intellectual 700 points
Part Number: PCM1794A

I am using the PCM1794A in a application where it receives the I2S signal from an S/PDIF receiver chip (Cirrus CS8416).

The CS8416 has an output which indicates loss of signal on the S/PDIF side, I am using this signal to mute mute the PCM1794A on pin 10.

The issue that I am having is that while the PCM1794A is muted in this way there is noise on the output which sounds like hissing.

  • 0
    TI__Guru 57800 points

    HI, Lenny,

    Thanks for your question. The PCM1794A mute process is designed to be performed in steps to reduce any possible pop noise, when the process is complete the device should not have any noise. Is it possible t replicate the noise issue by tying the MUTE pin directly to high level? Is this issue present in different boards?

    Thanks and Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Intellectual 700 points

    In my design there are two ways of muting the PCM1794A, one way is using the device's zero (pin 13) output, the other way is with the S/PDIF error signal.

    The zero signal and the error signal are put though an OR gate that way either will mute the PCM1794A. When the device is muted though the zero signal there is no noise.

    When the device is muted by the error signal there is noise. I don't understand the difference.

  • 0 in reply to Lenny
    TI__Guru 57800 points

    Hi, Lenny,

    Sorry for the delay. There should be no difference in the way the device detects the mute signal as the data is coming from a logic gate. My only thought is that somehow the signal sent by the S/PDIF receiver is not valid, so when the S/PDIF is lost, the noise comes to the PCM1794A. There is no reason for the device to stream noise when  muted. 
    Can you please double check for any abnormal behavior on the S/PDIF error signal?. What is the status of the PCM1794A input I²S signals when S/PDIF is lost?

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Intellectual 700 points

    When the SPDIF is lost the I2S lines behave irregularly. The WCLK is 2.9kHz. Should this matter when the device is muted?

    The mute signal behaves identically regardless of where it originated.

    Should I be putting the device into reset instead of mute in this situation?

  • 0 in reply to Lenny
    TI__Guru 57800 points

    Hi, Lenny,

    Thanks for the additional information. The PCM1794A requires a valid audio sample rate for a correct operation, so if the WCLK frequency changes when the S/PDIF signal is lost, the DAC might not operate as expected. If the sampling rate in the I²S lines changes each time S/PDIF signal is lost, then you might try putting the DAC into reset state.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer