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PCM1794 blob sounds when switching clock

Tihomir Tomsic
Prodigy 40 points
Other Parts Discussed in Thread: PCM1794, PCM1794A

Hi everybody,

I am using a PCM1794 as DAC which is fed by a PC via I2S. When playback is going to start the I2S clocks named LRCK and BCK stop in order to calculate the new clock rates suited. But when this happens the PCM gives out a blob sound even when it's muted. Same when LRCK and BCK are starting anew, a second blob sound can be heard. The blobs come from a DC offset from 0.4V during LRCK and BCK switched off.

 

Is this normal? What to do to supress these blobs?

 

Thank you!

  • 0
    Prodigy 40 points

    *push*

    HELP

    I  made an almost fluent transit of I2S clocks when switching to another Frequency - BCK is strechted about 4 times for just one period. But just this is enough to create a crackle on the output even when mute is activated. I can't believe that this can be true.... or am I doing anything wrong??

  • 0 in reply to Tihomir Tomsic
    TI__Guru**** 189401 points

    Hi, Tihomir,

    I am not very familiar with this device, but I noticed this in the d/s:

    Soft Mute

    The PCM1794A supports mute operation. When MUTE (pin 10) is set to HIGH, both analog outputs are transitioned

    to the bipolar zero level in –0.5-dB steps with a transition speed of 1/fS per step. This system provides pop-free muting

    of the DAC output.

    So, you need to assert mute, wait for a number of fS for it to actually get to mute, and then change your clocks.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 40 points

    Hi,

    Thank you for your answer! Should there be a difference between PCM1794 and PCM1794A reagrding MUTE function? I set PCM1794 to MUTE for many fS but always heard a very short crackle when changing clocks. I'll try the PCM1794 anyway, maybe there will be a change.

     

  • 0 in reply to Tihomir Tomsic
    TI__Guru**** 189401 points

    Tihomir,

    It's not clear to me which version of the IC you are using. You should not be using the non-A version. It is NRND (not recommended for new designs).

    I doubt there was a change made with respect to this between the non-A and -A versions.

    Unfortunately, I don't have details on what changed between non-A and -A versions.

    -d2

  • 0 in reply to Tihomir Tomsic
    Prodigy 240 points

    Hi,

    you could try gating some of the clocks during ta clock rate change.

    A question to TI experts:

    how to guarantee that the MUTE works during clock change?

    You could gate some of the clocks before rate  change and ungate them after the clocks are Ok again?

    Would this help?

    Is there a good way for doing this?

    br,

    Esa

  • 0 in reply to Esa Harma
    Prodigy 40 points

    Hi Esa,

    I could fix this issue by using the PCM1794A! Even without using MUTE!

    [...]

    regards