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TAS5825MEVM: TAS5825M no audio at 32KHz SR

Part Number: TAS5825MEVM
Other Parts Discussed in Thread: TAS5825M,

Hello TI Team,

we use the TAS5825M with the Base/Pro 2.0/48KHz mode. Biquad filters are used. The input rates are 32KHz, 44.1KHz or 48KHz. I  tested with the TAS5825MEVM, PPC motherboard and latest PPC software. For variable input sample rates, I use an external audio interface with optical SPDIF because the 32kHz firmware I had programmed on the motherboard does not play audio on my Windows 10 PC, although communication with the PPC software works. By using the optical input, I was able to successfully verify (oscilloscope) the sample rates on the TAS5825MEVM.

Our SRC setting is Base Sample Rate = 48KHz, Input Sample Rate = Auto-Detected. As far as I know, this setting would be ideal, as the DSP would then always operate at a fixed rate. Unfortunately we have no sound when the input signal has a sample rate of 32kHz (bitclock is 1.024MHz). When playing back at 32kHz, the FS monitor displays 0x06, which means 32kHz. The device is not muted during playback (auto mute is enabled). Clock Detection Status 0x39 reports 0x08, which indicates a PLL overrate! Global Fault 0x71 displays 0x04, indicating a "Clock Error".

With the SRC setting Base Sample Rate = As Input, Input Sample Rate = 48KHz, audio works at all three desired rates, and it doesn't seem to make any difference which input rate is selected (I haven't tested 16KHz). If I keep this setting, I have the following questions:
1. at what rate does the DSP operate with an externally supplied audio signal with a sampling rate of 32KHz, 44.1KHz, and 48KHz? If the DSP rate always follows the input rate, what does the SRC do?
2. what about the coefficients at 32KHz, 44.1KHz and 48KHz respectively?
Does the DSP need to be told externally via the input sample rate how to convert the coefficients, or what does the input sample rate setting do here?
3. i read here in the forum that the PWM frequency also changes when the DSP frequency change. I could not measure this behavior with Base Sample Rate = As Input, Input Sample Rate = 48KHz mode with different applied input rates. Do I understand something wrong here?

Thanks n d Regards

Mario

  • Hi Mario

        1. It's based on your settings. As you set with Base Sample Rate = 48KHz, Input Sample Rate = Auto-Detected, DSP will always works in 48KHz sample rate.

        2. DSP can't convert the coefficients, actually DSP can't calculate the coefficients, DSP can only use the coefficients to deal with the music signal. Coefficients is calculate by PPC3 software, that's why you need to set input sample rate.

        3. PWM frequency will not change according to different input sample rate. May I know where you got that information?

  • Hello Shadow He,

    1. I'm interested in the behavior with a setting of Base Sample Rate = As Input.
    - The DSP works with a frequency depending on what is applied on the input and the coefficients are calculated once before in the PPC software?
    - So the coefficients do not always fit optimally?
    - Which task has the SRC when "As Input" is selected? 
    2. understood.
    3. I read this: e2e.ti.com/.../3602593 "If you don't mind the PWM switching frequency fluctuating between 384kHz and 354.2kHz, you can use the default configuration for auto-detection."

    In conclusion, do I understand this correctly?
    - In Base/Pro 2.0/48KHz mode, 32KHz does not work with SRC setting Base Sample Rate = 48KHz, Input Sample Rate = Auto-Detected.
    - In Base/Pro 2.0/48KHz mode with Base Sample Rate = As Input. The setting 32KHz now works but the DSP then works with different rates and in two cases the coefficients are not optimal.

    Thanks and Regards

    Mario

  • Hi Mario

    1. Set "as input", DSP works under the input sample rate. Coefficients don't always fit, as it's value is calculate by PPC3, can't be changed. SRC doesn't need to work.

    3. This PWM shift is because our DSP won't distinguish between 48KHz and 44.1KHz, the 10% margin is needed to avoid clock fault always coming up. So when the sample rate change between this two value, the PWM may also shift a little. But won't change under other sample rate, as we can use the sample rate value to calculate the correct PWM.

    In Base/Pro 2.0/48KHz mode, 32KHz does not work with SRC setting Base Sample Rate = 48KHz, Input Sample Rate = Auto-Detected.

       Yes, can't work. The SRC can't transfer 32KHz to 48KHz.

    In Base/Pro 2.0/48KHz mode with Base Sample Rate = As Input. The setting 32KHz now works but the DSP then works with different rates and in two cases the coefficients are not optimal.

       Yes, can work but the coefficients are still under 48KHz

       You can consider choose 96KHz process flow and Auto-Detect, SRC can transfer 32KHz and 48KHz into 96KHz successfully.