[igt-dev] [PATCH i-g-t 4/5] lib/igt_audio: detect noise and pops

Simon Ser
2019-05-29T13:32:46Z

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.

Signed-off-by: Simon Ser <simon.ser@intel.com>
---
lib/igt_audio.c | 44 +++++++++++++++++++++++++++++++++++++++-----
1 file changed, 39 insertions(+), 5 deletions(-)

diff --git a/lib/igt_audio.c b/lib/igt_audio.c
index 08c0fb6af0db..423615427a4d 100644
--- a/lib/igt_audio.c
+++ b/lib/igt_audio.c
@@ -39,6 +39,8 @@
#define CHANNELS_MAX 8
#define SYNTHESIZE_AMPLITUDE 0.9
#define SYNTHESIZE_ACCURACY 0.2
+#define MIN_FREQ 200 /* Hz */
+#define NOISE_THRESHOLD 0.0005

/**
* SECTION:igt_audio
@@ -108,6 +110,7 @@ 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 +307,12 @@ 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 +337,11 @@ 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 (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 +352,7 @@ 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 +362,36 @@ 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;

--
2.21.0