High pitched noise when in silence

#1

Hi,

I’ve been implementing a “full hearing aid software” based on the WDRC_8BandFIR_Stereo example from the Tympan examples, and I’ve added noise reduction, which works great. But lately I’ve been experiencing a very loud high pitched noise mostly whenever there’s silence/no one speaking, if I have noise reduction off, independent from compression (doesn’t matter if it’s on or off). This happen if I have a global digital gain of like 5.0 dB and the following parameters of compression:

BTNRH_WDRC::CHA_DSL dsl_left = {5, // attack (ms)
50, // release (ms)
115, // maxdB. calibration. dB SPL for input signal at 0 dBFS. Needs to be tailored to mic, spkrs, and mic gain. (original = 115)
0, // 0=left, 1=right…ignored
N_CHAN, // num channels
{317.1666, 502.9734, 797.6319, 1264.9, 2005.9, 3181.1, 5044.7}, // cross frequencies (Hz)
{0.7, 0.7, 0.7, 0.7, 0.7, 0.7, 0.7, 0.7}, // compression ratio for low-SPL region (ie, the expander…values should be < 1.0)
{30.0, 30.0, 30.0, 30.0, 30.0, 30.0, 30.0, 30.0}, // expansion-end kneepoint
//{55, 55, 65, 65, 85, 85, 85, 85}, // Audiogram values
{21, 21, 29, 29, 45, 45, 45, 45}, // compression-start gain
{0.7f, 0.9f, 1.0f, 1.1f, 1.2f, 1.4f, 1.5f, 1.5f},
{30.0, 30.0, 30.0, 30.0, 30.0, 30.0, 30.0, 30.0}, // compression-start kneepoint (input dB SPL)
{79.f, 88.f, 91.f, 93.f, 98.f, 103.f, 102.f, 100.f} // output limiting threshold (comp ratio 10)
};

When I say I turn off compression, I just load up another preset with the CR’s set to 1.0, but with the same gain values as the preset above.

Any tips?

Thanks!

#2

More troubles. Bummer!

Questions about your setup:

  • What sample rate and block size are you using? (should be near the top of your main *.ino file)
  • For the “high pitched noise”, what kind of noise is it?
    • Is it like a steady tone? Can you estimate the pitch of the high-pitched tone? 1kHz? Higher like 4kHz? Really high like 8 kHz? Really, really high like 16 kHz?
    • Or, is it not a tone…is it more like static noise? Is it steady or does it pulse or otherwise change with time?
  • For your audio input, are you using the microphones built into the Tympan? Or, an external microphone via the pink jack? Or, a line audio source (like your phone/PC) via the pink jack?
  • For your audio output where you hear the noise, are you using earphones? Or, do you have the Tympan plugged into loudspeakers?

And questions about your description of the conditions:

  • You provided the compression parameters (thank you!) but did you also say that it doesn’t matter whether the compression is on or off? Can you confirm whether the compression matters?
  • The compressor settings that you provided includes the variable “N_CHAN”. Presumably, you’ve defined this somewhere in your code? What value is it set to?
  • You said that if you set a global digital gain of 5.0 dB, you hear the high-pitched noise:
    • When you increase this gain value, does the noise get louder?
    • When you turn it down a little bit (1 dB at a time), does the noise suddenly go away when crossing a magic gain value? Or does it steadily get quieter for each step down in gain until it fades out to be too quiet to hear?

Sorry for so many questions…I want to help you solve the problem!

Chip