White noise when powering up the TAS5705 EVM2

Hi, Hanns,

Cool! I need to visit Scanian someday - sounds awesome!!!

OK, I'm confused... Which part are you using? TAS5705 or TAS5706?

-d2

Sorry, there was a typo in the subject - I got a TAS5706 EVM2 and that's the one saying "PFZZzzzt" whenever de-asserting the !PDN pin.

Scania is in the very south of Sweden, much smaller than Texas but just as flat, we don't wear hats but we talk equally funny :-)

Hanns,

I'm going to try and re-create this white noise on start-up that you have noticed.  Could you send me any additional details about your set-up?  For instance, what load are you using?

Thanks,

-Clancy

Hanns,

I was able to replicate what you are describing.  PDN doesn't smoothly turn the outputs on and off.  On the otherhand, MUTE does sequence down the volume of the outputs, allowing for a smooth transition.  Generally, you should assert mute before you assert PDN, and then let the device out of mute only after PDN is released.  This will give you noiseless transitions from volume to silence and visa versa.

-Clancy

Hi Clancy!

I'm currently in france and prior to this, I was unable to post on your reply using my iPhone. I thought I already did try according to your suggestion but I'll give it another try when I'm back at the office.

Thanks in the meantime!

Hello again Clancy!

I've now tried muting/unmuting according when toggling !PDN as you described above but I still hear the subtle "pzzzt" sound.

I'm using software mute by writing 0xff to register 0x07 but maybe you mean I should use the h/w !MUTE pin?

Hanns,

Using the hardware MUTE pin should remove the sound (let me know if it doesn't, possibly we are hearing different things)?  I won't have a chance to test the volume register mute until sometime tomorrow.  Another possibility would be to try the soft mute register (0x06).  I'll give you an update tomorrow.

-Clancy

Hanns,

Using the volume register to mute the channels worked fine for me.  I noticed no white noise when I un-muted the outputs.  Could you describe your procedure in more detail?

-Clancy

Hello again Clancy and thank you for taking time to look into this.

I tried muting by writing 0xff to register 0x06 and unmuting by writing 0x00. This pseudo code illustrates the method I'm currently using to toggle power:

if !pdn = 1
{
  register_0x06 = 0xff
  !pdn = 0
}
else
{
  !pdn = 1
  register_0x06 = 0x00
}

Using this method I get a subtle noise when "re-powering" at the point of unmuting in the else leg above. The noise is quite subtle and can easily be mistaken for being part of the music so you have to play some rather soft music to hear it.

EDIT: I have now tried all three ways of doing the muting/unmuting part (i.e. using register 0x06, register 0x07 and the !mute pin) but to no avail in all cases. During all these tests I constantly feed audio from a PCM2707 USB audio chip.

I'm using a microcontroller and I have tried using a delay the way you mention and found that it made no difference. The device has a remote control and I've assigned one button to each method so that I can easily test all three of them repeatedly .

When I tried using much larger delays than before I had a breakthrough since the noise "went away" completely at approx 350ms and this goes for all three methods. I guess the noise doesn't really go away but instead I wait for it to die out before unmuting and I can't help wondering why you don't get the same noise on your rig.

At a delay of 300ms there's only an ever so small "tail" of the noise so it's ok and this kind of delay is acceptable too.

I can settle for this work-around and so I think we've found a good enough solution!

Thanks for all your help!

I'm glad that delay is acceptable.  I was able to recreate the white noie you've described when I unmute the device immediately after de-asserting !PDN.

There is a 120ms delay after PDN goes high before the device begins to start-up.  From there, the device does a soft start and ramps the modulators up to a 50% duty cycle.  Reaching this 50% duty cycle before starting to play audio is important, as it allows for a smooth noiseless transition.  As long as you wait until this start-up sequence is finished, you shouldn't notice any noise when you unmute.

-Clancy

Hi!
Thanks for clarifying this. I can't see that this information is in the datasheet but maybe it ought to be.

Again thanks for your time.

Hanns