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TLV320AIC3268: noise reduction in audio

NIKHIL O
Prodigy 50 points

Part Number: TLV320AIC3268

Hi,

I am currently working with the TLV320AIC3268 using analog routing (not MiniDSP yet). I would appreciate your guidance regarding startup pop noise and GPS related interference issues.

Current Configuration

Inputs:

IN2 (SkyVoice, also carries GPS data via RS232 in the same system)
IN3 (AoA source)
ASI2 (Bluetooth I2S input)

Routing:

All three signals are mixed using analog routing.
IN2 and IN3 are routed through LPGA.
ASI2 is routed through DAC.
Final mix output goes to LOL/LOR, then to an external Class-D amplifier and 8 ohm speaker.

Parallel outputs are taken from:
RECP/RECM (differential output)
HPL (used as single ended and differential feed to headphone amplifier)

Issues Observed

  1. Startup Pop and Hissing Noise

On power up, there is a noticeable pop followed by hissing noise from the speaker.
The noise persists for a short duration before stabilizing.

  1. GPS / RS232 Data Interference

One of the analog input sources shares the system with a GPS module transmitting RS232 data.
When GPS data is actively streaming, audible digital noise is introduced into the mixed audio output.

Attempted Mitigation

I mixed IN2L and IN3L together.
I tied IN2R and IN3R to CM1 to avoid floating negative inputs.

This reduced the noise significantly.
However, IN3 output became completely silent after this change.

Questions

  1. Is there any firmware level configuration (register settings) to:

  • Reduce startup pop and hiss?

  • Improve rejection of digital noise coupling into analog inputs?

  • Properly configure unused negative inputs without muting the channel?

  1. Are there recommended LPGA, input common mode, biasing, or soft start register settings to reduce pop noise during power up?

  2. Is there a recommended way to configure pseudo differential versus single ended inputs in this case to improve noise immunity without external hardware changes?

  3. Would enabling internal input impedance changes, CM routing, or digital soft stepping help reduce the interference?

At this stage, I am trying to avoid hardware modifications and would like to explore all possible firmware level optimizations first.

Looking forward to your guidance.

Best regards,
Nikhil

  • 0
    TI__Guru 51190 points

    Hi Nikhil,

    Speaker Pop and Hiss:

    The best practice here is to power up the codec and let the common mode of the line out drivers stabalize before you power up the external Class-D amp. Pops are a result of sudden changes in voltage, such as the step function created when the line out driver is powered on. To get the quietest start up, you should only power things on in order of the signal flow:

    - Host / Inputs -> Codec -> External Amp -> Speaker

    The hiss I'm less familiar with, but if it's dispersing then it might be related to the ramp of the line out. Or it might have something to do with the speaker itself and how the amp it handling it. Regardless I think proper power sequencing will take care of this. However there are no register settings to handle the pop on the line outs. Those options are only available on the headphone and receiver outs.

    PsuedoDiff vs SE:

    For a single ended input, the negative terminal of the PGA should be selected to CM like you mentioned. For any differential signal, you connect one of the corresponding R signals to the negative side instead. If you have a PsuedoDiff signal then you connect it the same way. But since there are AC coupling caps, the device will set its own common mode on the device pins. The end result is still the same just 6dB less than if it were true differential.

    From the description I'm not sure why IN3 went silent but I suspect you accidentally canceled out that signal. 

    You can attempt to try different input impedances (on both the signal and the CM connections while being mindful of the divider created) to see if they have an impact on the noise. It will change the filter response and the driving strength that the noise has and there might be a benefit. Just be mindful that a high input impedance like 40k will have a slight hit on dynamic range and SNR due to the thermal noise of the resistor.

    Unfortunately the only way I see to get the digital noise off the signal is to filter it (depending on the source frequency) or inject the same noise on the negative inputs of that PGA so that the CMRR takes care of it (aka PsuedoDiff). From the ADC point of view, it will take whatever signal you give it, so there's not a lot it can do to help since it doesn't have any clue as to what is noise vs intended signal. 

    Best regards,
    Jeff McPherson