ref: e6d1e3cad1eb9ef8f59afb597f5dddddb5bf86aa
dir: /stretch.c/
////////////////////////////////////////////////////////////////////////////
// **** AUDIO-STRETCH **** //
// Time Domain Harmonic Scaler //
// Copyright (c) 2019 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.
#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 1024 /* maximum allowable pitch period */
struct stretch_cnxt {
int num_chans, inbuff_samples, shortest, longest, tail, head, fast_mode;
short *inbuff, *calcbuff;
float outsamples_error;
unsigned int *results;
};
static void merge_blocks (short *output, short *input1, short *input2, int samples);
static int find_period_fast (struct stretch_cnxt *cnxt, short *samples);
static int find_period (struct stretch_cnxt *cnxt, short *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.
*/
StretchHandle stretch_init (int shortest_period, int longest_period, int num_channels, int fast_mode)
{
struct stretch_cnxt *cnxt;
if (fast_mode) {
longest_period = (longest_period + 1) & ~1;
shortest_period &= ~1;
}
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 * 8;
cnxt->inbuff = calloc (cnxt->inbuff_samples, sizeof (*cnxt->inbuff));
cnxt->calcbuff = calloc (longest_period * num_channels, sizeof (*cnxt->calcbuff));
cnxt->results = calloc (longest_period, sizeof (*cnxt->results));
}
if (!cnxt || !cnxt->inbuff || !cnxt->calcbuff || !cnxt->results) {
fprintf (stderr, "stretch_init(): out of memory!\n");
return NULL;
}
cnxt->head = cnxt->tail = cnxt->longest = longest_period * num_channels;
cnxt->shortest = shortest_period * num_channels;
cnxt->num_chans = num_channels;
cnxt->fast_mode = fast_mode;
return (StretchHandle) cnxt;
}
/*
* Process the specified samples with the given ratio (which is clipped to the
* range 0.5 to 2.0). Note that the number of samples refers to total samples for
* both channels in stereo and can be as large as desired (samples are buffered
* here). The exact number of samples output is not possible to determine in
* advance, but it will be close to the number of input samples times the ratio
* plus or minus 3X the longest period.
*/
int stretch_samples (StretchHandle handle, short *samples, int num_samples, short *output, float ratio)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
int out_samples = 0;
num_samples *= cnxt->num_chans;
if (ratio < 0.5)
ratio = 0.5;
else if (ratio > 2.0)
ratio = 2.0;
/* while we have samples to do... */
do {
/* if there are more samples and room for them, copy in */
if (num_samples && cnxt->head < cnxt->inbuff_samples) {
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, do so */
while (cnxt->tail >= cnxt->longest && cnxt->head - cnxt->tail >= cnxt->longest * 2) {
int period = cnxt->fast_mode ? find_period_fast (cnxt, cnxt->inbuff + cnxt->tail) :
find_period (cnxt, cnxt->inbuff + cnxt->tail);
float process_ratio;
/*
* 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 (output + 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 (output + out_samples, cnxt->inbuff + cnxt->tail, period * 2 * sizeof (cnxt->inbuff [0]));
cnxt->outsamples_error += (period * 2.0) - (period * 2.0 * ratio);
out_samples += period * 2;
cnxt->tail += period * 2;
}
else if (process_ratio == 1.5) {
memcpy (output + out_samples, cnxt->inbuff + cnxt->tail, period * sizeof (cnxt->inbuff [0]));
merge_blocks (output + out_samples + period, cnxt->inbuff + cnxt->tail + period,
cnxt->inbuff + cnxt->tail, period);
memcpy (output + 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 (output + 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 we're almost done with buffer, copy the rest back to beginning */
if (cnxt->head == cnxt->inbuff_samples) {
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;
}
} while (num_samples);
return out_samples / cnxt->num_chans;
}
/* flush any leftover samples out at normal speed */
int stretch_flush (StretchHandle handle, short *output)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
int samples_to_copy = (cnxt->head - cnxt->tail) / cnxt->num_chans;
memcpy (output, cnxt->inbuff + cnxt->tail, samples_to_copy * cnxt->num_chans * sizeof (*output));
cnxt->tail = cnxt->head;
return samples_to_copy;
}
/* free handle */
void stretch_deinit (StretchHandle handle)
{
struct stretch_cnxt *cnxt = (struct stretch_cnxt *) handle;
free (cnxt->calcbuff);
free (cnxt->results);
free (cnxt->inbuff);
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, short *samples)
{
unsigned int sum, diff, factor, best_factor = 0;
short *calcbuff = samples;
int period, best_period;
int i, j;
period = best_period = cnxt->shortest / cnxt->num_chans;
if (cnxt->num_chans == 2) {
calcbuff = cnxt->calcbuff;
for (i = j = 0; i < cnxt->longest * 2; i += 2)
calcbuff [j++] = (samples [i] + samples [i+1]) >> 1;
}
/* accumulate sum for shortest period size */
for (sum = i = 0; i < period; ++i)
sum += abs (calcbuff [i]) + abs (calcbuff [i+period]);
/* this loop actually cycles through all period lengths */
while (1) {
short *comp = calcbuff + period * 2;
short *ref = calcbuff + period;
/* compute sum of absolute differences */
diff = 0;
while (ref != calcbuff)
diff += abs (*--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. Care
* must be taken here to avoid possibility of overflow.
*/
factor = diff ? (sum * 128) / diff : UINT32_MAX;
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 += abs (calcbuff [period * 2]);
sum += abs (calcbuff [period++ * 2 + 1]);
}
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, short *samples)
{
unsigned int sum, diff, 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 */
if (cnxt->num_chans == 2)
for (i = j = 0; i < cnxt->longest * 2; i += 4)
cnxt->calcbuff [j++] = (samples [i] + samples [i+1] + samples [i+2] + samples [i+3]) >> 2;
else
for (i = j = 0; i < cnxt->longest * 2; i += 2)
cnxt->calcbuff [j++] = (samples [i] + samples [i+1]) >> 1;
/* accumulate sum for shortest period */
for (sum = i = 0; i < period; ++i)
sum += abs (cnxt->calcbuff [i]) + abs (cnxt->calcbuff [i+period]);
/* this loop actually cycles through all period lengths */
while (1) {
short *comp = cnxt->calcbuff + period * 2;
short *ref = cnxt->calcbuff + period;
/* compute sum of absolute differences */
diff = 0;
while (ref != cnxt->calcbuff)
diff += abs (*--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. Care
* must be taken here to avoid possibility of overflow.
*/
cnxt->results [period] = diff ? (sum * 128) / diff : UINT32_MAX;
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 += abs (cnxt->calcbuff [period * 2]);
sum += abs (cnxt->calcbuff [period++ * 2 + 1]);
}
if (best_period * cnxt->num_chans * 2 != cnxt->shortest && best_period * cnxt->num_chans * 2 != cnxt->longest) {
int high_side_diff = cnxt->results [best_period] - cnxt->results [best_period+1];
int low_side_diff = cnxt->results [best_period] - cnxt->results [best_period-1];
if (low_side_diff > high_side_diff * 2)
best_period = best_period * 2 + 1;
else if (high_side_diff > low_side_diff * 2)
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.
*/
static void merge_blocks (short *output, short *input1, short *input2, int samples)
{
int i;
for (i = 0; i < samples; ++i)
output [i] = (((input1 [i] + 32768) * (samples - i) +
(input2 [i] + 32768) * i) / samples) - 32768;
}