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PCM5121: DPS coefficients for custom bitrates

AndrIach
Intellectual 390 points
Part Number: PCM5121
Other Parts Discussed in Thread: TLV320ADC3140, , PCM5141

Our application has a custom bit rate (near 32khz), without master clock.

Internal PLL lock the frequency and the system works.

Now we need to use the fixed flow dsp for equalization, but pure path studio is  locked on 44.1khz or and 48khz.

How we do this? 

  • 0
    TI__Guru 52683 points

    Hi Andrlach,

    You can operate the DSP on the 48kHz settings in PPS.  There is no difference in the number of instructions that you are able to use.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Intellectual 390 points

    Hi Paul,

    we can't operate at 48Khz, our hardware is limited to 32kHz (31250 Hz exactly) and  this sample rate can't be selected on PPS.

    So the situation is this:

    - PPS doesn't permit to choose custom frequency

    - Coefficients depends on frequency

    - Rules to generate the coefficients in unknown.

    For example for the TLV320ADC3140 the datasheet specify how to obtain the correct format for the coefficient.

    On PCM5121:

    - It is not clear the format 4.20 for the biquad filter.

    - It is unknown what to write in the DRC, Volume control, etc. (i expect thy depends from bit rate).

    We don't need hifi quality so i think the difference from 31250 and 32000 Hz in not relevant, but i need to select this on PPS or the knowledge about the value of the registers.

    Regards,

    Andrea Iachini.

  • 0 in reply to AndrIach
    TI__Guru 52683 points

    Hi Andrea,

    I suspect that using all of the same coefficient settings for the 48kHz for a 32kHz input will work okay for your application.  The device performance is roughly divided into 3 groups: quad rates, double rates, and single rates.  In this case, 32kHz would be considered a quad rate.  As the ROM process flow only supports 44.1kHz/48kHz in PPS, you will need to configure the DSP as if it is in 48kHz, but then configure the PLL for 32kHz.  The DSP clock should be approximately the same.  You can then do a listening test or frequency response evaluation to confirm if your settings are acceptable for your application.  

    The way PPS generates coefficients is quite enigmatic, but we never expected customers to generate new coefficients on the fly.  We are using the 4.20 format as well, see section 12.1.3 Coefficient Data Formats.

    If you feel that you still want to use coefficient values for specifically 32kHz, then you will need to use the PCM5141 instead.  That device does not use the ROM process flow, so you can change the sample rate and modify the flow as you see fit.

    Thanks,
    Paul