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PCM2903C: Hissing/Static when there is no signal from S/PDIF in

Yutong Gu
Prodigy 60 points
Part Number: PCM2903C

Hi there,

I'm getting hissing/static when there is no audio playing to the S/PDIF connection to the PCM2903C. As soon as I unplug the S/PDIF connection from the socket, I begin to hear the hearing hissing sound. It's almost immediate. The same happens when I turn off the audio source, causing no input light signal from the S/PDIF in. Is there anything that can be done about this?

Best,

Yutong

  • 0
    Prodigy 60 points

    For some added context, I am using alsa with a rate converter and softvol plugin to allow for volume adjustments and normalizing to 44.1KHz when capturing the audio. However, even with no conversions, I see that when there is some sort of S/PDIF input, the PCM output is all 0 (as expected). But once the S/PDIF is removed or turned off, I start getting random values corresponding to the hissing sound.

  • 0 in reply to Yutong Gu
    TI__Mastermind 46225 points

    Hi Yutong,

    How are your analog inputs connected? When the SPDIF data is disconnected, the device automatically switches to the analog inputs, which may be picking up noise.

    Best regards,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 60 points

    Ah, that's probably the issue. The analog inputs are left NC so that would explain why I am getting static. Is there any way to override this behavior?

  • 0 in reply to Yutong Gu
    Prodigy 60 points

    Scratch that. I just checked and the analog inputs are grounded. What are some other potential causes?

  • 0 in reply to Yutong Gu
    TI__Mastermind 46225 points

    Hi Yutong,

    For reference, no this behavior cannot be overridden. Can you try AC coupling the inputs to ground? Noise can still permeate through GND and we typically recommend input signals be AC coupled to GND instead of a direct connection for this reason. 0.47uF or any other similarly sized cap will be fine. 

     Also how are you recording and monitoring this signal? Depending on your gain stages you might just be hearing noise from high gain.

    Best regards,
    Jeff McPherson