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LM4931: 3.478Khz tone during silence

Wimpie
Prodigy 30 points
Part Number: LM4931

we use a voip module with a LM4931 codec. After standby or powerdown ( mode 0 or 1) we switch to 'headphone call mkde' (0x03) the codec output is silence (except from some noise) as long as we play only zero's on the PCM input. After playing sounds, there remains a (weak) 3.478Khz tone (at 8Khz sample rate) when we play only zero's.

The tone frequency changes with the sample frequency. The amplitude changes with the hp_gain setting.  It can be heard as while when playing small sounds.

I expect it is generated by a digital low-pass / interpolation filter while the signal seems to be very stable.

We can hear the tone and measured it with a micophone and soundcard (see plot. The LM4931 was setup with 8Khz samplerate)

  • 0
    TI__Expert 5645 points
    Hi Wimpie,
    What is your MCK freq? Also, do you have PLL active? Would you be able to share the entire configuration for LM4931? This will help me resolve the issue w. the tone. It's quite possible that the PLL is not locked and hence we have the tone that you are seeing.

    But we will need additional info to confirm the same and help resolve the issue. Thanks again.

    Best regards,
    Ravi
  • 0 in reply to Ravi Patil
    Prodigy 30 points

    The PLL is disabled. An external  12.288 oscillator is used.

    PMC_CONFIG = 0x02

    CLK_MUX = 0x1c 

    INTERFACES = 0x14

    BASIC_CONFIG = 0x03

    VOICE_CONFIG = 0x00

    HP_GAIN=  0x1F   We have used differend settings. The tone amplitude changes according to the gain setting. So it seems to be generated before the HP amplifier.

    Beset Gerads,

    Jan Willem de Wit