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-rw-r--r--libavcodec/ac3enc.c90
1 files changed, 89 insertions, 1 deletions
diff --git a/libavcodec/ac3enc.c b/libavcodec/ac3enc.c
index b69ed8b26f..d372e6a202 100644
--- a/libavcodec/ac3enc.c
+++ b/libavcodec/ac3enc.c
@@ -1115,6 +1115,68 @@ static int cbr_bit_allocation(AC3EncodeContext *s)
/**
+ * Downgrade exponent strategies to reduce the bits used by the exponents.
+ * This is a fallback for when bit allocation fails with the normal exponent
+ * strategies. Each time this function is run it only downgrades the
+ * strategy in 1 channel of 1 block.
+ * @return non-zero if downgrade was unsuccessful
+ */
+static int downgrade_exponents(AC3EncodeContext *s)
+{
+ int ch, blk;
+
+ for (ch = 0; ch < s->fbw_channels; ch++) {
+ for (blk = AC3_MAX_BLOCKS-1; blk >= 0; blk--) {
+ if (s->blocks[blk].exp_strategy[ch] == EXP_D15) {
+ s->blocks[blk].exp_strategy[ch] = EXP_D25;
+ return 0;
+ }
+ }
+ }
+ for (ch = 0; ch < s->fbw_channels; ch++) {
+ for (blk = AC3_MAX_BLOCKS-1; blk >= 0; blk--) {
+ if (s->blocks[blk].exp_strategy[ch] == EXP_D25) {
+ s->blocks[blk].exp_strategy[ch] = EXP_D45;
+ return 0;
+ }
+ }
+ }
+ for (ch = 0; ch < s->fbw_channels; ch++) {
+ /* block 0 cannot reuse exponents, so only downgrade D45 to REUSE if
+ the block number > 0 */
+ for (blk = AC3_MAX_BLOCKS-1; blk > 0; blk--) {
+ if (s->blocks[blk].exp_strategy[ch] > EXP_REUSE) {
+ s->blocks[blk].exp_strategy[ch] = EXP_REUSE;
+ return 0;
+ }
+ }
+ }
+ return -1;
+}
+
+
+/**
+ * Reduce the bandwidth to reduce the number of bits used for a given SNR offset.
+ * This is a second fallback for when bit allocation still fails after exponents
+ * have been downgraded.
+ * @return non-zero if bandwidth reduction was unsuccessful
+ */
+static int reduce_bandwidth(AC3EncodeContext *s, int min_bw_code)
+{
+ int ch;
+
+ if (s->bandwidth_code[0] > min_bw_code) {
+ for (ch = 0; ch < s->fbw_channels; ch++) {
+ s->bandwidth_code[ch]--;
+ s->nb_coefs[ch] = s->bandwidth_code[ch] * 3 + 73;
+ }
+ return 0;
+ }
+ return -1;
+}
+
+
+/**
* Perform bit allocation search.
* Finds the SNR offset value that maximizes quality and fits in the specified
* frame size. Output is the SNR offset and a set of bit allocation pointers
@@ -1122,11 +1184,37 @@ static int cbr_bit_allocation(AC3EncodeContext *s)
*/
static int compute_bit_allocation(AC3EncodeContext *s)
{
+ int ret;
+
count_frame_bits(s);
bit_alloc_masking(s);
- return cbr_bit_allocation(s);
+ ret = cbr_bit_allocation(s);
+ while (ret) {
+ /* fallback 1: downgrade exponents */
+ if (!downgrade_exponents(s)) {
+ extract_exponents(s);
+ encode_exponents(s);
+ group_exponents(s);
+ ret = compute_bit_allocation(s);
+ continue;
+ }
+
+ /* fallback 2: reduce bandwidth */
+ /* only do this if the user has not specified a specific cutoff
+ frequency */
+ if (!s->cutoff && !reduce_bandwidth(s, 0)) {
+ process_exponents(s);
+ ret = compute_bit_allocation(s);
+ continue;
+ }
+
+ /* fallbacks were not enough... */
+ break;
+ }
+
+ return ret;
}