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tas5706 subwoofer output revisited

Hanns
Expert 1005 points
Other Parts Discussed in Thread: TAS5706B

Hello!

Call me stupid but I'm not getting how the sub- and sub+ outputs of the TAS5706b are meant to be used. I get a nice bass sound if I take either single one of them and apply an LPF with a ~300Hz cut-off frequency. The only difference I can see between the two is a minor phase shift.

Once and for all, which alternative below is true about the sub- and sub+ signal pair of the TAS5706b?

a) I can use either of them (doesn't matter which) and LPF that
b) I must logically AND them together and then LPF that
c) They two are differential outputs to minimize distortion and I must use a differential amplifier on them and then LPF that.
d) I should do something else. If so please explain what to do and how to do it

Could someone please tell my which option is true so that I can sleep well at night :-)

Thank you very much!

  • 0
    TI__Guru**** 189399 points

    Hi, Hanns,

    STILL with the sub-woofer, eh? :)

    Seriously, this is designed to be a differential signal out, but it is controlled by software (the bane of my existence...). Look at register 0x25. It depends on how you route the sub-woofer signal. 

    The default is ch5 to sub_pwm-, and ch6 to sub_pwm+. So, depending on how you have ch5 and 6 configured, you will get different stuff out of the sub_pwm pins.

    -d2

  • 0 in reply to Don Dapkus
    Expert 1005 points

    Hi Don!

    Yep, still struggling with this and believe me, I have certainly been looking at these registers :-)

    Anyway, this is an excerpt of my current config:

    0x20 = 00-89-77-7A (ch6 (BTL–) to ch5—BTL pair to ch6)
    0x21 = 00-00-41-03 (bass management to ch6)
    0x25 = 01-02-13-45 (ch6 to sub+)

    I take it, this means I get the desired bass-signal from the sub+ line and so I could just use that only and leave sub- floating. Correct?

    Or maybe I should change register 0x21:

    0x21 = 00-00-42-03 ((L'+R')/2 to ch6)

    This would get me a mono-downmixed signal on sub+, correct?

  • 0
    TI__Mastermind 22965 points

    Hi Hanns,

    The Sub+ & Sub- can be used either differentially (BTL) or in Single-Ended (SE).

    If you need to use it Single-Ended (i.e. use either one of the outputs), I would recommend to make the following updates..

    1.) Update Register 0x20 to set Ch-6 in AD mode (default is BD mode). AD-mode is set by toggling the Bit-D3 of register 0x20 to '0'.

    2.) Update the ICD register's 0x15 & 0x16 to x04 & xFC respectively. (Default is xD0, x90).

    Register 0x25 can be left at default setting for both cases (BTL/SE).

    Register 0x21 can be set to either down-mix (L+R)/2 or bass-management.

    Regards,

    Ravi

  • 0 in reply to Ravi Singh
    Expert 1005 points

    Hi Ravi and thanks for answering.

    I want SE mode since I'm connecting it to an external active subwoofer (i.e. a regular standalone consumer product thing).

    I'll implement the mods you recommended and in the meantime, here's a couple of follow-up questions for you:

    1) Does the SE-setup mean that I should use Sub-/Sub+ as signal and ground respectively or should I use Sub+ and regular ground?

    2) What is the difference between a) down-mix (L+R)/2 and b) bass-management?

    Again thanks!

  • 0 in reply to Hanns
    TI__Mastermind 22965 points

    Hi Hanns,

    For the SE setup, the regular ground should be used for reference.

    Down-mix is simply the L+R/2 summation, while Bass-Management is a little bit of legacy feature, that combines Ch-6 with a ratio of the other channels. The following block diagram illustrates the differnet mux options for register 0x21

  • 0 in reply to Ravi Singh
    Expert 1005 points

    Hello again Ravi,

    I've tested the configuration changes you suggested and it works well.

    I use the down-mix (L+R)/2 mode and take out the bass signal between sub+ and gnd which is then low pass filtered (130Hz cut-off) and then fed to an active subwoofer.

    Thanks for all the help!

    Hanns