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Digital Volume Control

Rishab Sharma
Prodigy 145 points
Other Parts Discussed in Thread: TAS5756M, PCM5242, PCM5142, TPA3118D2, TAS5508C

I was looking at the TAS5756m IC and volume control is implemented digitally. Now if you you have 16bit audio and digitally increase the volume you will lose the LSBs unless you increase the number of bits (say to 24 bits) so that there is some headroom for lossless volume increase.. That brings me to my question. Does the TAS5756m internally increase the bits used to represent the audio to allow for lossless digital volume control?

I was also wondering how mobile phones do volume control over bluetooth. Do they simply throw away the LSBs and transmit the corrupted digital data to the source?

  • 0
    Genius 3055 points
    Hi Rishab,

    digital volume control is never losses - digitally attenuated signals lose its dynamic, SNR.
    My mobile phone, every app I have, changing volume digitally change the digital dynamic / bit resolution.

    Tomasz
  • 0 in reply to Tomasz Chumiecki
    Prodigy 145 points

    Hi Tomasz

    Are you 100% sure this is always the case?  page 51 of the tas5756m datasheet doesn't go into detail into how volume control is implemented. It simply says you should write to an 8bit register to change the volume.

    Cheers 

  • 0 in reply to Rishab Sharma
    Genius 3055 points
    Hi Rishab,

    you right, the datasheet doesn't go into detail but the TAS5756M consists similar (or this same) audio processing part like PCM5142/PCM5242 and the volume control there is regulated digitally on the input, before DSP section. It does no mater how fine the volume is calculated and what the word length is, digital volume control decreases signal dynamic.

    But the question is, can you hear the 24 bit, or even 16bit dynamic :) listening to the the music quietly?

    Tomasz
  • 0 in reply to Tomasz Chumiecki
    Genius 3055 points
    Hi Rishab,

    In fact every volume control decreases the output signal dynamic - it is a nature of volume control. But analog volume control, simple potentiometer between DAC and amplifier input, attenuates also the DAC output noise and the dynamic is there limited just by amplifier noise floor level. Many high end audio systems have analog volume control implemented after the DAC section.

    Tomasz
  • 0 in reply to Tomasz Chumiecki
    Prodigy 145 points
    Thanks for your response Tomasz

    It is very likely that I will not be able to hear the difference between analog and digital volume control in most circumstances. However I am building a high end system and I am trying to minimise quantisation noise. every source of noise adds up and can end up being quite noticeable.

    I guess I will give digital volume control a shot and see how bad/good it is and figure out whether or not it is worth implementing analog volume control.
  • 0 in reply to Rishab Sharma
    Genius 3055 points
    The TAS5756M is a great choice for a high grade audio system, no doubt about it.

    Have you got some prototype already running?

    I have the combo PCM5242 + TPA3118D2 running at the moment and I'm super satisfied with the audio performance.
    Looking into the TAS5756M datasheet it should give even better performance than my design.
    The one important issue for me is the output idle noise - have you measured what is the real output noise of the TAS5756M playing digital zero data? In the TAS5756M datasheet, on the first page, is written:

    - Idle Channel Noise = 56µVrms (A-Wth)
    - SNR = 104 A-Wth (Ref. to THD+N = 1%)...

    but I do not think the values are valid for the complete audio path including unmuted DAC even playing digital zero data...

    Tomasz
  • 0
    TI__Expert 4320 points
    Rishab, Tomaz

    I think a good implemented digital Volume control works quite well, but make sure the analog gain is not higher than needed.

    There is a implementation of volume control than does not reduce the dynamic range: PSVC as used in the TAS5508C with an open loop powerstage like tas5261.
    This scheme is used in highend amps like Lyngdorf and Steinwaylyngdorf.

    Rgds
    Kim

    Den 30/03/2015 kl. 11.08 skrev Rishab Sharma <bounce-4310751@mail.e2e.ti.com>:

     

    A Message from the TI E2E™ Community
    Texas Instruments

     

    Rishab Sharma replied to Digital Volume Control.

    Thanks for your response Tomasz

    It is very likely that I will not be able to hear the difference between analog and digital volume control in most circumstances. However I am building a high end system and I am trying to minimise quantisation noise. every source of noise adds up and can end up being quite noticeable.

    I guess I will give digital volume control a shot and see how bad/good it is and figure out whether or not it is worth implementing analog volume control.

     
     

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  • 0 in reply to Kim Nordtorp Madsen
    Prodigy 145 points
    Hi Kim

    Would sampling the 16bit audio as 32bit on the tas5756m allow for greater volume without quantisation errors?

    example of what im talking about at page 15 of this pdf: www.esstech.com/.../digital-vs-analog-volume-control.pdf
  • 0 in reply to Rishab Sharma
    Genius 3055 points
    Rishab,

    do not forget, the dynamic range of TAS5756M is defined in the datasheet as 104 dB - it is only 17-18 bits... you should never hear the bits 19 - 32 even is they are processed in DAC because the are below floor noise level of the DAC... Kim, am I right?

    If you feed the amplifier with digital data at 0dBfs you can get full dynamic range 104dB on the output. Setting the volume digitally at -20dB your output dynamic range decreases to 84dB because noise floor stays unchanged...

    Tomasz
  • 0 in reply to Tomasz Chumiecki
    Prodigy 145 points

    Nope not yet. Waiting for parts to arrive to prototype the tas5756m.

    I'll let you know how it goes

  • 0 in reply to Tomasz Chumiecki
    Genius 3055 points
    Kim,

    "There is a implementation of volume control than does not reduce the dynamic range: PSVC as used in the TAS5508C with an open loop powerstage like tas5261"

    as far as I understand the things, PSVC used in systems based on TAS5508C modulator is a mix of digital volume control and digitally controlled analog volume control (changing of the output power stage supply voltage). The method causes no dynamic loss only in the range where power stage supply voltage is regulated. It is nothing to compare with pure digital volume control based on dividing digital sample values.

    Tomasz