ref: 4b9ccf2823e0244c6ae7fcf4fe6a5d7e8d3868d8
dir: /stretch.c/
////////////////////////////////////////////////////////////////////////////
// **** AUDIO-STRETCH **** //
// Time Domain Harmonic Scaler //
// Copyright (c) 2022 David Bryant //
// All Rights Reserved. //
// Distributed under the BSD Software License (see license.txt) //
////////////////////////////////////////////////////////////////////////////
// stretch.c
// Time Domain Harmonic Compression and Expansion
//
// This library performs time domain harmonic scaling with pitch detection
// to stretch the timing of a 16-bit PCM signal (either mono or stereo) from
// 1/2 to 2 times its original length. This is done without altering any of
// the tonal characteristics.
//
// Use stereo (num_chans = 2), when both channels are from same source
// and should contain approximately similar content.
// For independent channels, prefer using multiple StretchHandle-instances.
// see https://github.com/dbry/audio-stretch/issues/6
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <math.h>
#include "stretch.h"
#define MIN_PERIOD 24 /* minimum allowable pitch period */
#define MAX_PERIOD 2400 /* maximum allowable pitch period */
#if INT_MAX == 32767
#define MERGE_OFFSET 32768L /* promote to long before offset */
#define abs32 labs /* use long abs to avoid UB */
#else
#define MERGE_OFFSET 32768
#define abs32 abs
#endif
#define MAX_CORR UINT32_MAX /* maximum value for correlation ratios */
struct stretch_cnxt {
int num_chans, inbuff_samples, shortest, longest, tail, head, fast_mode;
int16_t *inbuff, *calcbuff;
float outsamples_error;
uint32_t *results;
struct stretch_cnxt *next;
int16_t *intermediate;
};
static void merge_blocks (int16_t *output, int16_t *input1, int16_t *input2, int samples);
static int find_period_fast (struct stretch_cnxt *cnxt, int16_t *samples);
static int find_period (struct stretch_cnxt *cnxt, int16_t *samples);
/*
* Initialize a context of the time stretching code. The shortest and longest periods
* are specified here. The longest period determines the lowest fundamental frequency
* that can be handled correctly. Note that higher frequencies can be handled than the
* shortest period would suggest because multiple periods can be combined, and the
* worst-case performance will suffer if too short a period is selected. The flags are:
*
* STRETCH_FAST_FLAG 0x1 Use the "fast" version of the period calculation
*
* STRETCH_DUAL_FLAG 0x2 Cascade two instances of the stretcher to expand
* available ratios to 0.25X to 4.00X
*/
StretchHandle stretch_init (int shortest_period, int longest_period, int num_channels, int flags)
{
struct stretch_cnxt *cnxt;
int max_periods = 3;
if (flags & STRETCH_FAST_FLAG) {
longest_period = (longest_period + 1) & ~1;
shortest_period &= ~1;
max_periods = 4;
}
if (longest_period <= shortest_period || shortest_period < MIN_PERIOD || longest_period > MAX_PERIOD) {
fprintf (stderr, "stretch_init(): invalid periods!\n");
return NULL;
}
cnxt = (struct stretch_cnxt *) calloc (1, sizeof (struct stretch_cnxt));
if (cnxt) {
cnxt->inbuff_samples = longest_period * num_channels * max_periods;
cnxt->inbuff = calloc (cnxt->inbuff_samples, sizeof (*cnxt->inbuff));
if (num_channels == 2 || (flags & STRETCH_FAST_FLAG))
cnxt->calcbuff = calloc (longest_period * num_channels, sizeof (*cnxt->calcbuff));
if ((flags & STRETCH_FAST_FLAG))
cnxt->results = calloc (longest_period, sizeof (*cnxt->results));
}
if (!cnxt || !cnxt->inbuff || (num_channels == 2 && (flags & STRETCH_FAST_FLAG) && !cnxt->calcbuff) || ((flags & STRETCH_FAST_FLAG) && !cnxt->results)) {
fprintf (stderr, "stretch_init(): out of memory!\n");
return NULL;
}
cnxt->head = cnxt->tail = cnxt->longest = longest_period * num_channels;
cnxt->fast_mode = (flags & STRETCH_FAST_FLAG) ? 1 : 0;
cnxt->shortest = shortest_period * num_channels;
cnxt->num_chans = num_channels;
if (flags & STRETCH_DUAL_FLAG) {
cnxt->next = stretch_init (shortest_period, longest_period, num_channels, flags & ~STRETCH_DUAL_FLAG);
cnxt->intermediate = calloc (longest_period * num_channels * max_periods, sizeof (*cnxt->intermediate));
}
return (StretchHandle) cnxt;
}
/*
* Re-Initialize a context of the time stretching code - as if freshly created
* with stretch_init(). This drops all internal state.
*/
void stretch_reset (StretchHandle handle)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
cnxt->head = cnxt->tail = cnxt->longest;
memset (cnxt->inbuff, 0, cnxt->tail * sizeof (*cnxt->inbuff));
if (cnxt->next)
stretch_reset (cnxt->next);
}
/*
* Determine how many samples (per channel) should be reserved in 'output'-array
* for stretch_samples() and stretch_flush(). max_num_samples and max_ratio are the
* maximum values that will be passed to stretch_samples().
*/
int stretch_output_capacity (StretchHandle handle, int max_num_samples, float max_ratio)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
int max_period = cnxt->longest / cnxt->num_chans;
int max_expected_samples;
float next_ratio;
if (cnxt->next) {
if (max_ratio < 0.5) {
next_ratio = max_ratio / 0.5;
max_ratio = 0.5;
}
else if (max_ratio > 2.0) {
next_ratio = max_ratio / 2.0;
max_ratio = 2.0;
}
else
next_ratio = 1.0;
}
max_expected_samples = (int) ceil (max_num_samples * ceil (max_ratio * 2.0) / 2.0) +
max_period * (cnxt->fast_mode ? 4 : 3);
if (cnxt->next)
max_expected_samples = stretch_output_capacity (cnxt->next, max_expected_samples, next_ratio);
return max_expected_samples;
}
/*
* Process the specified samples with the given ratio (which is normally clipped to
* the range 0.5 to 2.0, or 0.25 to 4.00 for the "dual" mode). Note that in stereo
* the number of samples refers to the samples for one channel (i.e., not the total
* number of values passed) and can be as large as desired (samples are buffered here).
* The ratio may change between calls, but there is some latency to consider because
* audio is buffered here and a new ratio may be applied to previously sent samples.
*
* The exact number of samples output is not easy to determine in advance, so a function
* is provided (stretch_output_capacity()) that calculates the maximum number of samples
* that can be generated from a single call to this function (or stretch_flush()) given
* a number of samples and maximum ratio. It is reccomended that that function be used
* after initialization to allocate in advance the buffer size required. Be sure to
* multiply the return value by the number channels!
*/
int stretch_samples (StretchHandle handle, const int16_t *samples, int num_samples, int16_t *output, float ratio)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
int out_samples = 0, next_samples = 0;
int16_t *outbuf = output;
float next_ratio;
/* if there's a cascaded instance after this one, try to do as much of the ratio here and the rest in "next" */
if (cnxt->next) {
outbuf = cnxt->intermediate;
if (ratio < 0.5) {
next_ratio = ratio / 0.5;
ratio = 0.5;
}
else if (ratio > 2.0) {
next_ratio = ratio / 2.0;
ratio = 2.0;
}
else
next_ratio = 1.0;
}
num_samples *= cnxt->num_chans;
/* this really should not happen, but a good idea to clamp in case */
if (ratio < 0.5)
ratio = 0.5;
else if (ratio > 2.0)
ratio = 2.0;
/* while we have pending samples to read into our buffer */
while (num_samples) {
/* copy in as many samples as we have room for */
int samples_to_copy = num_samples;
if (samples_to_copy > cnxt->inbuff_samples - cnxt->head)
samples_to_copy = cnxt->inbuff_samples - cnxt->head;
memcpy (cnxt->inbuff + cnxt->head, samples, samples_to_copy * sizeof (cnxt->inbuff [0]));
num_samples -= samples_to_copy;
samples += samples_to_copy;
cnxt->head += samples_to_copy;
/* while there are enough samples to process (3 or 4 times the longest period), do so */
while (cnxt->tail >= cnxt->longest && cnxt->head - cnxt->tail >= cnxt->longest * (cnxt->fast_mode ? 3 : 2)) {
float process_ratio;
int period;
if (ratio != 1.0 || cnxt->outsamples_error)
period = cnxt->fast_mode ? find_period_fast (cnxt, cnxt->inbuff + cnxt->tail) :
find_period (cnxt, cnxt->inbuff + cnxt->tail);
else
period = cnxt->longest;
/*
* Once we have calculated the best-match period, there are 4 possible transformations
* available to convert the input samples to output samples. Obviously we can simply
* copy the samples verbatim (1:1). Standard TDHS provides algorithms for 2:1 and
* 1:2 scaling, and I have created an obvious extension for 2:3 scaling. To achieve
* intermediate ratios we maintain a "error" term (in samples) and use that here to
* calculate the actual transformation to apply.
*/
if (cnxt->outsamples_error == 0.0)
process_ratio = floor (ratio * 2.0 + 0.5) / 2.0;
else if (cnxt->outsamples_error > 0.0)
process_ratio = floor (ratio * 2.0) / 2.0;
else
process_ratio = ceil (ratio * 2.0) / 2.0;
if (process_ratio == 0.5) {
merge_blocks (outbuf + out_samples, cnxt->inbuff + cnxt->tail,
cnxt->inbuff + cnxt->tail + period, period);
cnxt->outsamples_error += period - (period * 2.0 * ratio);
out_samples += period;
cnxt->tail += period * 2;
}
else if (process_ratio == 1.0) {
memcpy (outbuf + out_samples, cnxt->inbuff + cnxt->tail, period * 2 * sizeof (cnxt->inbuff [0]));
if (ratio != 1.0)
cnxt->outsamples_error += (period * 2.0) - (period * 2.0 * ratio);
else
cnxt->outsamples_error = 0; /* if the ratio is 1.0, we can never cancel the error, so just do it now */
out_samples += period * 2;
cnxt->tail += period * 2;
}
else if (process_ratio == 1.5) {
memcpy (outbuf + out_samples, cnxt->inbuff + cnxt->tail, period * sizeof (cnxt->inbuff [0]));
merge_blocks (outbuf + out_samples + period, cnxt->inbuff + cnxt->tail + period,
cnxt->inbuff + cnxt->tail, period);
memcpy (outbuf + out_samples + period * 2, cnxt->inbuff + cnxt->tail + period, period * sizeof (cnxt->inbuff [0]));
cnxt->outsamples_error += (period * 3.0) - (period * 2.0 * ratio);
out_samples += period * 3;
cnxt->tail += period * 2;
}
else if (process_ratio == 2.0) {
merge_blocks (outbuf + out_samples, cnxt->inbuff + cnxt->tail,
cnxt->inbuff + cnxt->tail - period, period * 2);
cnxt->outsamples_error += (period * 2.0) - (period * ratio);
out_samples += period * 2;
cnxt->tail += period;
if (cnxt->fast_mode) {
merge_blocks (outbuf + out_samples, cnxt->inbuff + cnxt->tail,
cnxt->inbuff + cnxt->tail - period, period * 2);
cnxt->outsamples_error += (period * 2.0) - (period * ratio);
out_samples += period * 2;
cnxt->tail += period;
}
}
else
fprintf (stderr, "stretch_samples: fatal programming error: process_ratio == %g\n", process_ratio);
/* if there's another cascaded instance after this, pass the just stretched samples into that */
if (cnxt->next) {
next_samples += stretch_samples (cnxt->next, outbuf, out_samples / cnxt->num_chans, output + next_samples * cnxt->num_chans, next_ratio);
out_samples = 0;
}
/* finally, left-justify the samples in the buffer leaving one longest period of history */
int samples_to_move = cnxt->inbuff_samples - cnxt->tail + cnxt->longest;
memmove (cnxt->inbuff, cnxt->inbuff + cnxt->tail - cnxt->longest,
samples_to_move * sizeof (cnxt->inbuff [0]));
cnxt->head -= cnxt->tail - cnxt->longest;
cnxt->tail = cnxt->longest;
}
}
/*
* This code is not strictly required, but will reduce latency, especially in the dual-instance case, by
* always flushing all pending samples if no actual stretching is desired (i.e., ratio is 1.0 and there's
* no error to compensate for). This case is more common now than previously because of the gap detection
* and cascaded instances.
*/
if (ratio == 1.0 && !cnxt->outsamples_error && cnxt->head != cnxt->tail) {
int samples_leftover = cnxt->head - cnxt->tail;
if (cnxt->next)
next_samples += stretch_samples (cnxt->next, cnxt->inbuff + cnxt->tail, samples_leftover / cnxt->num_chans,
output + next_samples * cnxt->num_chans, next_ratio);
else {
memcpy (outbuf + out_samples, cnxt->inbuff + cnxt->tail, samples_leftover * sizeof (*output));
out_samples += samples_leftover;
}
memmove (cnxt->inbuff, cnxt->inbuff + cnxt->head - cnxt->longest, cnxt->longest * sizeof (cnxt->inbuff [0]));
cnxt->head = cnxt->tail = cnxt->longest;
}
return cnxt->next ? next_samples : out_samples / cnxt->num_chans;
}
/*
* Flush any leftover samples out at normal speed. For cascaded dual instances this must be called
* twice to completely flush, or simply call it until it returns zero samples. The maximum number
* of samples that can be returned from each call of this function can be determined in advance with
* stretch_output_capacity().
*/
int stretch_flush (StretchHandle handle, int16_t *output)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
int samples_leftover = cnxt->head - cnxt->tail;
int samples_flushed = 0;
if (cnxt->next) {
if (samples_leftover)
samples_flushed = stretch_samples (cnxt->next, cnxt->inbuff + cnxt->tail, samples_leftover / cnxt->num_chans, output, 1.0);
if (!samples_flushed)
samples_flushed = stretch_flush (cnxt->next, output);
}
else {
memcpy (output, cnxt->inbuff + cnxt->tail, samples_leftover * sizeof (*output));
samples_flushed = samples_leftover / cnxt->num_chans;
}
cnxt->tail = cnxt->head;
memset (cnxt->inbuff, 0, cnxt->tail * sizeof (*cnxt->inbuff));
return samples_flushed;
}
/* free handle */
void stretch_deinit (StretchHandle handle)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
free (cnxt->calcbuff);
free (cnxt->results);
free (cnxt->inbuff);
if (cnxt->next) {
stretch_deinit (cnxt->next);
free (cnxt->intermediate);
}
free (cnxt);
}
/*
* The pitch detection is done by finding the period that produces the
* maximum value for the following correlation formula applied to two
* consecutive blocks of the given period length:
*
* sum of the absolute values of each sample in both blocks
* ---------------------------------------------------------------------
* sum of the absolute differences of each corresponding pair of samples
*
* This formula was chosen for two reasons. First, it produces output values
* that can directly compared regardless of the pitch period. Second, the
* numerator can be accumulated for successive periods, and only the
* denominator need be completely recalculated.
*/
static int find_period (struct stretch_cnxt *cnxt, int16_t *samples)
{
uint32_t sum, diff, factor, scaler, best_factor = 0;
int16_t *calcbuff = samples;
int period, best_period;
int i, j;
period = best_period = cnxt->shortest / cnxt->num_chans;
// convert stereo to mono, and accumulate sum for longest period
if (cnxt->num_chans == 2) {
calcbuff = cnxt->calcbuff;
for (sum = i = j = 0; i < cnxt->longest * 2; i += 2)
sum += abs32 (calcbuff [j++] = ((int32_t) samples [i] + samples [i+1]) >> 1);
}
else
for (sum = i = 0; i < cnxt->longest; ++i)
sum += abs32 (calcbuff [i]) + abs32 (calcbuff [i+cnxt->longest]);
// if silence return longest period, else calculate scaler based on largest sum
if (sum)
scaler = (MAX_CORR - 1) / sum;
else
return cnxt->longest;
/* accumulate sum for shortest period size */
for (sum = i = 0; i < period; ++i)
sum += abs32 (calcbuff [i]) + abs32 (calcbuff [i+period]);
/* this loop actually cycles through all period lengths */
while (1) {
int16_t *comp = calcbuff + period * 2;
int16_t *ref = calcbuff + period;
/* compute sum of absolute differences */
diff = 0;
while (ref != calcbuff)
diff += abs32 ((int32_t) *--ref - *--comp);
/*
* Here we calculate and store the resulting correlation
* factor. Note that we must watch for a difference of
* zero, meaning a perfect match. Also, for increased
* precision using integer math, we scale the sum.
*/
factor = diff ? (sum * scaler) / diff : MAX_CORR;
if (factor >= best_factor) {
best_factor = factor;
best_period = period;
}
/* see if we're done */
if (period * cnxt->num_chans == cnxt->longest)
break;
/* update accumulating sum and current period */
sum += abs32 (calcbuff [period * 2]) + abs32 (calcbuff [period * 2 + 1]);
period++;
}
return best_period * cnxt->num_chans;
}
/*
* This pitch detection function is similar to find_period() above, except that it
* is optimized for speed. The audio data corresponding to two maximum periods is
* averaged 2:1 into the calculation buffer, and then the calulations are done
* for every other period length. Because the time is essentially proportional to
* both the number of samples and the number of period lengths to try, this scheme
* can reduce the time by a factor approaching 4x. The correlation results on either
* side of the peak are compared to calculate a more accurate center of the period.
*/
static int find_period_fast (struct stretch_cnxt *cnxt, int16_t *samples)
{
uint32_t sum, diff, scaler, best_factor = 0;
int period, best_period;
int i, j;
best_period = period = cnxt->shortest / (cnxt->num_chans * 2);
/* first step is compressing data 2:1 into calcbuff, and calculating maximum sum */
if (cnxt->num_chans == 2)
for (sum = i = j = 0; i < cnxt->longest * 2; i += 4)
sum += abs32 (cnxt->calcbuff [j++] = ((int32_t) samples [i] + samples [i+1] + samples [i+2] + samples [i+3]) >> 2);
else
for (sum = i = j = 0; i < cnxt->longest * 2; i += 2)
sum += abs32 (cnxt->calcbuff [j++] = ((int32_t) samples [i] + samples [i+1]) >> 1);
// if silence return longest period, else calculate scaler based on largest sum
if (sum)
scaler = (MAX_CORR - 1) / sum;
else
return cnxt->longest;
/* accumulate sum for shortest period */
for (sum = i = 0; i < period; ++i)
sum += abs32 (cnxt->calcbuff [i]) + abs32 (cnxt->calcbuff [i+period]);
/* this loop actually cycles through all period lengths */
while (1) {
int16_t *comp = cnxt->calcbuff + period * 2;
int16_t *ref = cnxt->calcbuff + period;
/* compute sum of absolute differences */
diff = 0;
while (ref != cnxt->calcbuff)
diff += abs32 ((int32_t) *--ref - *--comp);
/*
* Here we calculate and store the resulting correlation
* factor. Note that we must watch for a difference of
* zero, meaning a perfect match. Also, for increased
* precision using integer math, we scale the sum.
*/
cnxt->results [period] = diff ? (sum * scaler) / diff : MAX_CORR;
if (cnxt->results [period] >= best_factor) { /* check if best yet */
best_factor = cnxt->results [period];
best_period = period;
}
/* see if we're done */
if (period * cnxt->num_chans * 2 == cnxt->longest)
break;
/* update accumulating sum and current period */
sum += abs32 (cnxt->calcbuff [period * 2]) + abs32 (cnxt->calcbuff [period * 2 + 1]);
period++;
}
if (best_period * cnxt->num_chans * 2 != cnxt->shortest && best_period * cnxt->num_chans * 2 != cnxt->longest) {
uint32_t high_side_diff = cnxt->results [best_period] - cnxt->results [best_period+1];
uint32_t low_side_diff = cnxt->results [best_period] - cnxt->results [best_period-1];
if ((low_side_diff + 1) / 2 > high_side_diff)
best_period = best_period * 2 + 1;
else if ((high_side_diff + 1) / 2 > low_side_diff)
best_period = best_period * 2 - 1;
else
best_period *= 2;
}
else
best_period *= 2; /* shortest or longest use as is */
return best_period * cnxt->num_chans;
}
/*
* To combine the two periods into one, each corresponding pair of samples
* are averaged with a linearly sliding scale. At the beginning of the period
* the first sample dominates, and at the end the second sample dominates. In
* this way the resulting block blends with the previous and next blocks.
*
* The signed values are offset to unsigned for the calculation and then offset
* back to signed. This is done to avoid the compression around zero that occurs
* with calculations of this type on C implementations that round division toward
* zero.
*
* The maximum period handled here without overflow possibility is 65535 samples.
* This corresponds to a maximum calculated period of 16383 samples (2x for stereo
* and 2x for the "2.0" version of the stretch algorithm). Since the maximum
* calculated period is currently set for 2400 samples, we have plenty of margin.
*/
static void merge_blocks (int16_t *output, int16_t *input1, int16_t *input2, int samples)
{
int i;
for (i = 0; i < samples; ++i)
output [i] = (int32_t)(((uint32_t)(input1 [i] + MERGE_OFFSET) * (samples - i) +
(uint32_t)(input2 [i] + MERGE_OFFSET) * i) / samples) - MERGE_OFFSET;
}