Re: [igt-dev] [PATCH i-g-t v2 1/3] lib/igt_audio: detect noise and pops

[Martin Peres <martin.peres@linux.intel.com>]

On 04/06/2019 14:48, Simon Ser wrote:
> First, normalize the bin power by dividing it by the number of input samples.
> We need to multiply by 2 since we get half as many bins as input samples.
> 
> Second, check that low frequencies are under a given threshold. If there is a
> pop or some noise, the low frequencies will be affected.

Reviewed-by: Martin Peres <martin.peres@linux.intel.com>
> 
> Signed-off-by: Simon Ser <simon.ser@intel.com>
> ---
>  lib/igt_audio.c | 58 ++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 53 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/igt_audio.c b/lib/igt_audio.c
> index 08c0fb6af0db..e0b1bafe1be2 100644
> --- a/lib/igt_audio.c
> +++ b/lib/igt_audio.c
> @@ -39,6 +39,17 @@
>  #define CHANNELS_MAX 8
>  #define SYNTHESIZE_AMPLITUDE 0.9
>  #define SYNTHESIZE_ACCURACY 0.2
> +/** MIN_FREQ: minimum frequency that audio_signal can generate.
> + *
> + * To make sure the audio signal doesn't contain noise, #audio_signal_detect
> + * checks that low frequencies have a power lower than #NOISE_THRESHOLD.
> + * However if too-low frequencies are generated, noise detection can fail.
> + *
> + * This value should be at least 100Hz plus one bin. Best is not to change this
> + * value.
> + */
> +#define MIN_FREQ 200 /* Hz */
> +#define NOISE_THRESHOLD 0.0005
>  
>  /**
>   * SECTION:igt_audio
> @@ -108,6 +119,7 @@ int audio_signal_add_frequency(struct audio_signal *signal, int frequency,
>  
>  	igt_assert(index < FREQS_MAX);
>  	igt_assert(channel < signal->channels);
> +	igt_assert(frequency >= MIN_FREQ);
>  
>  	/* Stay within the Nyquist–Shannon sampling theorem. */
>  	if (frequency > signal->sampling_rate / 2) {
> @@ -304,6 +316,12 @@ void audio_signal_fill(struct audio_signal *signal, double *buffer,
>  	audio_sanity_check(buffer, signal->channels * samples);
>  }
>  
> +/* See https://en.wikipedia.org/wiki/Window_function#Hann_and_Hamming_windows */
> +static double hann_window(double v, size_t i, size_t N)
> +{
> +	return v * 0.5 * (1 - cos(2.0 * M_PI * (double) i / (double) N));
> +}
> +
>  /**
>   * Checks that frequencies specified in signal, and only those, are included
>   * in the input data.
> @@ -328,6 +346,16 @@ bool audio_signal_detect(struct audio_signal *signal, int sampling_rate,
>  	data = malloc(samples_len * sizeof(double));
>  	memcpy(data, samples, samples_len * sizeof(double));
>  
> +	/* Apply a Hann window to the input signal, to reduce frequency leaks
> +	 * due to the endpoints of the signal being discontinuous.
> +	 *
> +	 * For more info:
> +	 * - https://download.ni.com/evaluation/pxi/Understanding%20FFTs%20and%20Windowing.pdf
> +	 * - https://en.wikipedia.org/wiki/Window_function
> +	 */
> +	for (i = 0; i < data_len; i++)
> +		data[i] = hann_window(data[i], i, data_len);
> +
>  	/* Allowed error in Hz due to FFT step */
>  	freq_accuracy = sampling_rate / data_len;
>  	igt_debug("Allowed freq. error: %d Hz\n", freq_accuracy);
> @@ -338,8 +366,7 @@ bool audio_signal_detect(struct audio_signal *signal, int sampling_rate,
>  		igt_assert(0);
>  	}
>  
> -	/* Compute the power received by every bin of the FFT, and record the
> -	 * maximum power received as a way to normalize all the others.
> +	/* Compute the power received by every bin of the FFT.
>  	 *
>  	 * For i < data_len / 2, the real part of the i-th term is stored at
>  	 * data[i] and its imaginary part is stored at data[data_len - i].
> @@ -349,15 +376,36 @@ bool audio_signal_detect(struct audio_signal *signal, int sampling_rate,
>  	 * The power is encoded as the magnitude of the complex number and the
>  	 * phase is encoded as its angle.
>  	 */
> -	max = 0;
>  	bin_power[0] = data[0];
>  	for (i = 1; i < bin_power_len - 1; i++) {
>  		bin_power[i] = hypot(data[i], data[data_len - i]);
> -		if (bin_power[i] > max)
> -			max = bin_power[i];
>  	}
>  	bin_power[bin_power_len - 1] = data[data_len / 2];
>  
> +	/* Normalize the power */
> +	for (i = 0; i < bin_power_len; i++)
> +		bin_power[i] = 2 * bin_power[i] / data_len;
> +
> +	/* Detect noise with a threshold on the power of low frequencies */
> +	for (i = 0; i < bin_power_len; i++) {
> +		freq = sampling_rate * i / data_len;
> +		if (freq > MIN_FREQ - 100)
> +			break;
> +		if (bin_power[i] > NOISE_THRESHOLD) {
> +			igt_debug("Noise level too high: freq=%d power=%f\n",
> +				  freq, bin_power[i]);
> +			return false;
> +		}
> +	}
> +
> +	/* Record the maximum power received as a way to normalize all the
> +	 * others. */
> +	max = NAN;
> +	for (i = 0; i < bin_power_len; i++) {
> +		if (isnan(max) || bin_power[i] > max)
> +			max = bin_power[i];
> +	}
> +
>  	for (i = 0; i < signal->freqs_count; i++)
>  		detected[i] = false;
>  
> 
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