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Original question:

TAS2781: cancel echo using TAS2781

TAS2781: TAS2781: loopback signal clarification question

Simon Yuh
Prodigy 110 points
Part Number: TAS2781


Hi Sir

We would like to review loopback signal, but we found the problem here.

We found that the "Audio In" signal level was -24dB different from the input signal, and "Audio out" signal level always remained at -9dB until we trimed input to -16dB then the  "Audio out" signal level became to -10dB.

Is there any gain limit inside(Audio in and Audio out signal)? How can we release the limit?

Should we expect the digital input = digital output without any smartAmp, EQ and DRC?

  • 0
    TI__Mastermind 21195 points

    Hi Simon, 

    the "audio in" is the audio input to the speaker protection block, which is after the audio enhancement block, so the "audio in" data is affected by the EQ,DRC,etc (but not speaker protection)

    the DSP headroom is 30dB, so before entering the audio enhancement block the audio data is shifted down by 30dB. (to allow for even 0dBFS signals to be boosted.) this 30dB is added back after the speaker protection.

    "Audio In" = (input audio) + (-30dB) + (gain from Audio Enhancement)

    "Audio out" = "audio in" + (30dB) + (gain from speaker protection)

    if all blocks are disabled, such as device being in ROM mode. then yes, input audio = output audio

    please let me know if you have further questions \

    regards, Arthur

  • 0 in reply to Arthur Brown
    Prodigy 110 points

    Hi Sir


    "Audio In" = (input audio) + (-30dB) + (gain from Audio Enhancement)

    "Audio out" = "audio in" + (30dB) + (gain from speaker protection)

    Base on the formula, the situation of my previous comment means that the (gain from Audio Enhancement) = 6dB, and the (gain from speaker protection) is limit at -9dB?

    Can you show me how to turn off all blocks except the loopback function?
    I'd like to see "input audio" = "Audio In" = "Audio out" (Is this the same as your mention of Input Audio = Output Audio?)

  • 0 in reply to Simon Yuh
    TI__Mastermind 21195 points

    Hi Simon, 

    yes it sound reasonable that the speaker protection is applying -9dB of gain. have you applied positive gain from volume control, EQ or DRC expansion?

    there is not a way to transmit both "audio in" and "audio out" when input audio = audio in = audio out. however when the devices DSP is disabled it is possible to transmit the unprocessed audio data. 

    to do this you need to use the "Device Control" Panel. select the device to be in ROM mode, enter the "advanced mode", and then configure the transmitter to have the audio data in Slot 0, and then press "apply"

    Regards,
    Arthur

  • 0 in reply to Arthur Brown
    Prodigy 110 points

    Hi Sir

    Follow your step, I can't record any signal from digital output.


    Originally, I would like to review the signal after passing DSP.

    But when I finished TDM "Audio OUT" signal setting, the loopback signal was weird, so I'd like to confirm the input signal which enter in the TAS2781.

    But when I confirmed "Audio in" signal which is from TAS2781, it is different from the input singal.

    Let me repeat my question.

    My request is I want to see the loopback signal which is through DSP is the same as the input signal.

    Then I can base on this to tune the EQ and DRC.

    How can I do that?

  • 0 in reply to Simon Yuh
    TI__Mastermind 21195 points

    Hi Simon, 

    It is not possible to completely disable the speaker protection when the DSP is enabled, 

    For this reason you should check the "audio in", and amplify it by 30dB. also make sure that all tuning blocks are disabled and the volume control is set to 0dB.

    Regards,

    Arthur

  • 0 in reply to Arthur Brown
    Prodigy 110 points

    Hi Sir


    "Audio In" = (input audio) + (-30dB) + (gain from Audio Enhancement)

    Regarding above formula, if we can't bypass -30dB, can we disable "gain from Audio Enhancement" ? How to do it?

    Because we saw that "Audio In" only has a -24dB difference from the "input audio", that means "gain from Audio Enhancement" has +6dB?

  • 0 in reply to Simon Yuh
    TI__Mastermind 21195 points

    Hi Simon,

    "gain from audio enhancement" is any gain applied by the feature in the "audio processing" panel, such as EQ, DRC or volume control., by default in PPC3 these gains will be 0dB. can you try with a new ppc3 file that has not been changed?

    I just tested this on my EVM and I found the expected result. the "audio in" level when the input audio was 0dBFS the -30dB, 

    Regards,
    Arthur

  • 0 in reply to Arthur Brown
    Prodigy 110 points

    Hi Sir

    This is the default measurement, as shown below(left side). I did measure -30dB when I used the default settings, but there was some loss in the data.

    So I change Transmitter offset from 1 to 0, the data is fine(right side), and the level is -24dB. Could you confirm that?

  • 0 in reply to Simon Yuh
    TI__Mastermind 21195 points

    Hi Simon, 

    this does make sense to me. a 1 bit shift is equivalent to 6dB. essentially by setting the TX offset to 0bits the level will be increased 6dB. however it does look like some data is being dropped,

    can you please confirm that the Fs and channels are 48kHz and 8 channels selected in windows sounds for the EVM? also double check in the software you are using to record.

    Regards,
    Arthur