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PCM1780: out of band noise when implimenting the upsampling from 44.1kHz to 88.2kHz.

Shunsuke Yamamoto
Expert 5936 points
Part Number: PCM1780

Hi team,

My customer plans to change the sampling frequency from 44.1kHz to 88.2kHz. Does this change improve the out of band noise? My customer would like to reduce the out of band noise.

Best regards,

Shunsuke Yamamoto

  • 0
    TI__Expert 5936 points

    Hi,

    Can the upsampling from 44.1kHz to 88.2kHz reduce the out of band noise which doesn't include foldback noise? Does the upsampling impact on the out of band noise?

    Best regards,

    Shunsuke Yamamoto 

  • 0 in reply to Shunsuke Yamamoto
    TI__Expert 6485 points

    Shun-San,

    Increasing the sampling frequency within the acceptable range generally improves SNR by pushing the noise floor level lower in magnitude.

    When it comes to out of band noise attenuation, the output stage of the PCM1780 has an integrated filter with a noise shaping characteristic that improves out-of-band noise rejection, but according to the DS connecting an external active low-pass filter would further improve the out-of-band noise attenuation.  

    I hope I was able to answer your question, if not feel free to ask for more help.

    Regards, 

    Ore.

  • 0 in reply to Ore Ogundana
    TI__Expert 5936 points

    Hi Ore-san,

    Thank you so much for the answer. What I would like to know is that the out-of-band-noise. Not SNR. The upsampling can improve the out-pf-band-noise(OBN)?

    I know the way to reduce the out-of-band-noise, but my customer doesn't want to add external low pass filters.

    Best regards,

    Shunsuke Yamamoto

  • +1 in reply to Shunsuke Yamamoto
    TI__Expert 6485 points

    Understood, 

    As far as improving the out-of-band noise, upsampling(or oversampling) would not improve the out-of-band noise. 

    Any impact the PCM1780 has on the out-of-band noise comes from the noise-shaping effect from its delta-sigma modulator and the integrated LPF at its output stage.

    The delta-sigma modulator noise-shapes. Which means pushing the quantized noise to a frequency range near the oversampling frequency which is really high, and it is a multi-order modulator so inherently the quantized out-of-band noise lowers with each order. Here is an example of what noise-shaping looks like:

    https://www.ti.com/lit/an/slyt423a/slyt423a.pdf

    (from DS)

    and to further lower the quantized noise, the output stage has an integrated LPF. So the out-of-band noise is being taken care of within the DAC...but with all these features, an external LPF is advised for optimal out-of-band noise attenuation. This should further reduce the potential noise that could be amplified on the output stage. 

    Yes, this application would require more board space but it's either the customer is okay with the noise-reduction from the DAC or they implement the external filter for optimal noise performance from both the on-chip solution and discreet solution.

    I hope I was able to answer your question, if not feel free to clarify. 

    Regards,

    Ore.  

    Disclaimer: All design are up to the customer to make, TI is not responsible for customer design.