[mod_dptools:playback] fix audio speed change to preserve pitch

When  a  file  is  being played  with  the  playback
application  and  the  speed  is  changed  with  eg.
uuid_fileman  speed:+1  the audio  pitch  increases.
This commit tries to preserve the pitch even when the
speed is changed.

How?
+ Use longer sample chunks (where original freq is preserved)
+ Search for the best cut position using cross-correlation
+ Crossfade adjacent chunks to each other

This updated co-authored commit from the original attempts
to optimize memory usage by allocating memory on the heap
for samples of the previous frame, but using stack memory
for the modified data that is flushed to the buffer.

Co-authored-by: hari <lharangozo@feastfox.com>
Co-authored-by: Joseph Nadiv <ynadiv@corpit.xyz>

src/switch_ivr_play_say.c

@@ -1360,6 +1360,14 @@ SWITCH_DECLARE(switch_status_t) switch_ivr_play_file(switch_core_session_t *sess
 
 
 	for (cur = 0; switch_channel_ready(channel) && !done && cur < argc; cur++) {
+		const int sp_fadeLen = 32;
+		const int sp_cut_src_rng = 64;
+		int16_t sp_ovrlap[32];
+		int sp_has_overlap = 0;
+		int sp_prev_idx = 0;
+		int sp_prev_cap = 0;
+		int16_t *sp_prev = NULL;
+
 		file = argv[cur];
 		eof = 0;
 
@@ -1859,42 +1867,91 @@ SWITCH_DECLARE(switch_status_t) switch_ivr_play_file(switch_core_session_t *sess
 
 			if (!switch_test_flag(fh, SWITCH_FILE_NATIVE) && fh->speed && do_speed) {
 				float factor = 0.25f * abs(fh->speed);
-				switch_size_t newlen, supplement, step;
+				int16_t* bp = write_frame.data;
-				short *bp = write_frame.data;
+				switch_size_t supplement = (int) (factor * olen);
-				switch_size_t wrote = 0;
+				switch_size_t newlen = (fh->speed > 0) ? olen - supplement : olen + supplement;
+				int src_rng = (fh->speed > 0 ? supplement : sp_cut_src_rng)* sp_has_overlap;
+				int datalen = newlen + src_rng + sp_fadeLen;
+				int extra = datalen - olen;
+				int16_t data[datalen * 2];
+				int16_t* currp = NULL;
+				int best_cut_idx = 0;
 
-				supplement = (int) (factor * olen);
+				memset(data, 0, sizeof(data));
-				if (!supplement) {
+
-					supplement = 1;
+				if (!sp_prev) {
+					sp_prev_cap = olen * 3 + sp_prev_idx;
+					switch_zmalloc(sp_prev, sp_prev_cap);
 				}
-				newlen = (fh->speed > 0) ? olen - supplement : olen + supplement;
-
-				step = (fh->speed > 0) ? (newlen / supplement) : (olen / supplement);
 
 				if (!fh->sp_audio_buffer) {
 					switch_buffer_create_dynamic(&fh->sp_audio_buffer, 1024, 1024, 0);
 				}
 
-				while ((wrote + step) < newlen) {
+				if (extra > 0) {
-					switch_buffer_write(fh->sp_audio_buffer, bp, step * 2);
+					int cont = 0;
-					wrote += step;
+					currp = data;
-					bp += step;
+
-					if (fh->speed > 0) {
+					if (extra > sp_prev_idx) {
-						bp++;
+						// not enough data
+						switch_buffer_write(fh->sp_audio_buffer, bp, newlen * 2);
+						cont = 1;
 					} else {
-						float f;
+						memcpy(currp, sp_prev + sp_prev_idx - extra, extra * 2);
-						short s;
+						memcpy(currp + extra, bp, olen * 2);
-						f = (float) (*bp + *(bp + 1) + *(bp - 1));
+					}
-						f /= 3;
+
-						s = (short) f;
+					if (olen * 3 + sp_prev_idx > sp_prev_cap) {
-						switch_buffer_write(fh->sp_audio_buffer, &s, 2);
+						sp_prev_cap = olen * 3 + sp_prev_idx;
-						wrote++;
+						sp_prev = realloc(sp_prev, sp_prev_cap);
+						sp_prev_idx = 0;
+					}
+
+					if (sp_prev_idx > olen) {
+						memmove(sp_prev, sp_prev + sp_prev_idx - olen, olen * 2);
+						sp_prev_idx = olen;
+					}
+
+					memcpy(sp_prev + sp_prev_idx, bp, olen * 2);
+					sp_prev_idx += olen;
+
+					if (cont) {
+						continue;
+					}
+				} else {
+					sp_prev_idx = 0;
+					currp = bp;
+				}
+
+				if (sp_has_overlap) {
+					double best = INT_MIN;
+					for (int idx_src = 0; idx_src < src_rng; idx_src++) {
+						double cc = 0;
+						for (int i = 0; i < sp_fadeLen; i++) {
+							cc += sp_ovrlap[i] * (*(currp + idx_src + i));
+						}
+						if (cc > best) {
+							best = cc;
+							best_cut_idx = idx_src;
 						}
 					}
-				if (wrote < newlen) {
-					switch_size_t r = newlen - wrote;
-					switch_buffer_write(fh->sp_audio_buffer, bp, r * 2);
 				}
+
+				currp += best_cut_idx;
+				switch_buffer_write(fh->sp_audio_buffer, currp, newlen * 2);
+				currp += newlen;
+
+				if (sp_has_overlap) {
+					int16_t* ret = (int16_t*)((unsigned char*)switch_buffer_get_head_pointer(fh->sp_audio_buffer) + switch_buffer_inuse(fh->sp_audio_buffer) - 2 * newlen);
+					// crossfade
+					for (int i = 0; i < (newlen < sp_fadeLen ? newlen : sp_fadeLen); i++) {
+						double factor = ((double)i) / sp_fadeLen;
+						*(ret + i) = (int16_t)(sp_ovrlap[i] * (1 - factor) + *(ret + i) * factor);
+					}
+				}
+
+				memcpy(sp_ovrlap, currp, sp_fadeLen * 2);
+				sp_has_overlap = 1;
 				last_speed = fh->speed;
 				continue;
 			}
@@ -2056,6 +2113,9 @@ SWITCH_DECLARE(switch_status_t) switch_ivr_play_file(switch_core_session_t *sess
 		if (fh->sp_audio_buffer) {
 			switch_buffer_destroy(&fh->sp_audio_buffer);
 		}
+
+		switch_safe_free(sp_prev);
+
 	}
 
 	if (switch_core_codec_ready((&codec))) {