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TAS5825M: Running the amp at 44.1 kHz

Markus Krautter
Intellectual 450 points
Part Number: TAS5825M

Hi,

I'm designing an amp with tas5825m chip. Prototype is up and running. I will have to use the amp at 44.1 kHz sample rate. I use the 2.0 48 kHz Base/Pro process flow and configured the amp in ppc3 software.

Due to the fact, that biquads are calculated for 48 kHz, it's needed to do a sample rate conversion from 44.1 to 48 kHz. I found some info in the datasheet on page 48:

I have 3 questions:

1. Is it necessary to set SIG_CH_CTRL register manually to 44.1 kHz when using that as input sample rate?

2. Does the src module convert 44.1 kHz to 48 kHz to have correct filter setting? Or does the src just do some minimal resampling to have the correct multiplication factor between lrclk and sclk?

3. Is that a typo in the datasheet, because FS_MODE is just bit 3, but the configs require 4 bit, so it should be bits 0-3 for FS_MODE?

Kind regards,

Markus

  • 0
    TI__Genius 15080 points

    Hello Markus,

    The default configuration is autodetection so it doesn't necessarilly need to be configured manually.

    For SRC module, "As input" can support 32, 48/44.1, 88.2/96 and 192KHz depending on the process flow.

    You are correct it should be 4 bits for FS_MODE.

    best regards,

    Luis

  • 0 in reply to Luis Fernandes de Pina
    Intellectual 450 points

    Hi Luis,

    if the autodetection is able to recognize 44.1 kHz, why is it written in datasheet to manually set that register if 44.1 kHz is used? Doesn't make sense to me yet.

    I have a general question concerning the sample rate which is set in ppc3:
    When I choose for example a 48 kHz based process flow. I suggest that the biquads will be calculated for 48 kHz (sample rate is a parameter for biquad calculation). If I set the base sampling frequency to "As input", will the filters be recalculated inside tas5825m if the sample rate differs from the originally sample rate the process flow was exported for?
    Or how can that actually work? Please give some detailed information on how the sample rate and it's conversion is handled in tas5825m.

    Is it possible in SRC to convert from 44.1 to 48 kHz? E.g. I use a 48 kHz process flow and supply a 44.1 kHz input signal.

    Kind regards, Markus

  • 0 in reply to Markus Krautter
    TI__Genius 15080 points

    Hello Markus,

    Sorry for the confusion.

    The reason for "setting it manually If the input Fs is 44.1kHz/88.2kHz/176.4kHz" is that TAS5825 does not make a difference with 48kHz & 44.1kHz, 96kHz & 88.2kHz... which means the PLL frequency factors are the same.

    If you do not mind the PWM switching frequency varies between 384kHz & 354.2kHz you can use default Auto detection configuration.

    For the process flow you are using you would need to change the base sampling rate(which is the DSP sample rate) to as input so the process flow will be operating properly otherwise you might need to use a process flow with a bank switch to keep the Biquad frequency response in sync.

    Best regards,

    Luis

  • 0 in reply to Luis Fernandes de Pina
    Intellectual 450 points

    Hi Luis,

    thanks for the first piece of information. We're slowly getting to the point now.

    What I want to understand: when I set the sample rate in ppc3 to "as input", how does the chip know which sample rate that will be? How will the biquads be calculated? I guess the filters must be calculated e.g. with 48 kHz sample rate and then when the amp is running there must be a sample rate conversion if the input is not equal to 48 kHz. I don't see any other option, in case you don't use a process flow with a second biquad bank for 44.1 kHz.

    Please answer these questions precisely.

    Regards, Markus

  • 0 in reply to Markus Krautter
    TI__Genius 15080 points

    Hello Markus,

    When you set the Base sampling rate(which is the DSP sample rate) it will calculate the biquad coefficients based on what you set as the input sampling rate.

    The biquad banking is used specifically to make sure the frequency response is matched closely between 44.1k/48k input signal as without bank switching the frequency response will be still be very similar but not exact since the coefficients would have been calculated for one specific sampling rate as the DSP when processing data won't really distinguish between 44.1k/48k and 88.2k/96k and 176.4k/196k.

    The SRC overall has up and down sampling for these multiples, but essentially treats 44.1k/48k the same it doesn't convert 44.1k to 48k.

    1/12x, 1/6x, 1/4x, 1/3x, 1/2x, 2/3x, 1x, 3/2x, 2x, 3x, 4x, 6x, 8x, 12x, 24x.

    If you test on an EVM 44.1k will work on a 48k flow but EQ frequency response might be slightly different compared to a 48k, and the switching frequency will be different.

    Best regards,

    Luis