PCM3070: DAC DRC functionality

Jason Morris

Part Number: PCM3070

Hi, we're using the PCM3070 in our audio amp product, the impressive amount of routing/filtering options has made this a really good one-chip solution.

I'm trying to add some DRC functionality on the DAC output to help compress live speech from a mic, looped through directly from the ADC to DAC. I've loaded up the registers as described in app note SLAU332, but can't see any difference in the transfer function: feeding 1kHz sine in from sig gen - no level change occurs even at -3dBFS output levels. I tried broadening the LPF up to 1.5kHz to get the energy recognised - I do see the DRC flags getting set, but still no effect on output.

How can I do a sanity check on DRC operation? Should I be able to see a non-linear amplitude transfer for a steady-state sinewave input?

Thanks

11 months ago

0 pdjuandi 11 months ago

TI__Mastermind 29745 points

Hi,

Here is an apps. note on DRC from similar devices. You need to set the DRC parameters like threshold, gain etc.

https://www.ti.com/lit/an/slaa446/slaa446.pdf?ts=1691174711299&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTLV320AIC3254

You can test by providing signal that is lower than the threshold and check if the gain is applied for example.

Regards.

0 Jason Morris 11 months ago in reply to pdjuandi

Prodigy 80 points

Thanks, but I'd already tried that - in fact those devices look virtually identical even down to coefficient numbers!

What I observe is that gain is always applied, it never gets reduced even though the DRC seems to be triggered. It's unclear as to the mechanism - does it work for a steady state 1kHz sine wave? Also, I can't see how the HPF/LPF matches the classic description of how DRC works by measuring the signal envelope. When I put the default HPF (~80Hz) and LPF (~16Hz) settings into the Ti Biquad calculator, there's not any visible 'overlap' - they intersect when their gains have already plummeted, so how can they detect any meaningful energy?

0 pdjuandi 11 months ago in reply to Jason Morris

TI__Mastermind 29745 points

Are you testing it with the EVM and the GUI DRC?

0 Jason Morris 11 months ago in reply to pdjuandi

Prodigy 80 points

No - but I can set all the registers with a command line serial interface. In the early proto stages, I used the EVM and hard-wired the I2S & control lines across into our codec for testing, it was really effective. I threw that proto out two weeks ago before this task :( I'm confident the registers are getting set ok, as I can read them back.

Do you expect I will see the above transfer curve on a steady state sinewave? I'm guessing yes. My gut feeling is that the envelope energy measurement isn't working. I see the threshold flags being set, so I know it's being triggered.

0 pdjuandi 11 months ago in reply to Jason Morris

TI__Mastermind 29745 points

Yes, it should work with sinewave and let it run continuously. You can use an instrument like AP and sweep input like the one shown below to check.

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PCM3070: DAC DRC functionality