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doc: clarify the dither.noise

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Fixes #4057
This commit is contained in:
Wim Taymans 2024-06-13 11:38:26 +02:00
parent 0115042adb
commit b421331275
2 changed files with 26 additions and 4 deletions
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22
doc/dox/config/pipewire-client.conf.5.md
View file

@ -460,10 +460,21 @@ This is only active when the `psd` up-mix method is used.
\endparblock
@PAR@ client.conf dither.noise = 0
This option will add N bits of random data to the signal. This can be used
to keep some amplifiers alive during silent periods. This is usually used together with
\parblock
This option will add N bits of random data to the signal. When no dither.method is
specified, the random data will flip between [-(1<<(N-1)), 0] every 1024 samples. With
a dither.method, the dither noise is amplified with 1<<(N-1) bits.
This can be used to keep some amplifiers alive during silent periods. One or two bits of noise is
usually enough, otherwise the noise will become audible. This is usually used together with
`session.suspend-timeout-seconds` to disable suspend in the session manager.
Note that PipeWire uses floating point operations with 24 bits precission for all of the audio
processing. Conversion to 24 bits integer sample formats is lossless and conversion to 32 bits
integer sample formats are simply padded with 0 bits at the end. This means that the dither noise
is always only in the 24 most significant bits.
\endparblock
@PAR@ client.conf dither.method = none
\parblock
Optional [dithering](https://en.wikipedia.org/wiki/Dither) can be done on the quantized
@ -472,8 +483,11 @@ output signal.
There are 6 modes available:
1. none No dithering is done.
2. rectangular Dithering with a rectangular noise distribution.
3. triangular Dithering with a triangular noise distribution.
2. rectangular Dithering with a rectangular noise distribution. This adds random
bits in the [-0.5, 0.5] range to the signal with even distribution.
3. triangular Dithering with a triangular noise distribution. This add random
bits in the [-1.0, 1.0] range to the signal with triangular distribution
around 0.0.
4. triangular-hf Dithering with a sloped triangular noise distribution.
5. wannamaker3 Additional noise shaping is performed on the sloped triangular
dithering to move the noise to the more inaudible range. This is using

8
spa/plugins/audioconvert/fmt-ops.c
View file

@ -468,6 +468,8 @@ int convert_init(struct convert *conv)
const struct noise_info *ninfo;
uint32_t i, conv_flags, data_size[3];
/* we generate int32 bits of random values. With this scale
* factor, we bring this in the [-1.0, 1.0] range */
conv->scale = 1.0f / (float)(INT32_MAX);
/* disable dither if not needed */
@ -483,6 +485,9 @@ int convert_init(struct convert *conv)
switch (conv->noise_method) {
case NOISE_METHOD_NONE:
conv->noise_method = NOISE_METHOD_PATTERN;
/* the pattern method does not use a random number
* but flips the noise between [-(1<<(N-1)), 0] every
* 1024 samples. */
conv->scale = -1.0f * (1 << (conv->noise_bits-1));
break;
case NOISE_METHOD_RECTANGULAR:
@ -490,10 +495,13 @@ int convert_init(struct convert *conv)
SPA_FALLTHROUGH;
case NOISE_METHOD_TRIANGULAR:
case NOISE_METHOD_TRIANGULAR_HF:
/* Amplify the random noise, with additional
* N-1 bits of noise. */
conv->scale *= (1 << (conv->noise_bits-1));
break;
}
}
/* RECTANGULAR dither goes from [-0.5, 0.5] */
if (conv->noise_method < NOISE_METHOD_TRIANGULAR)
conv->scale *= 0.5f;