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Fix choice of delta sign.

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Ensure that the delta goes in the correct direction (i.e. use positive delta for prev <= sample, and negative delta for prev > sample)
This commit is contained in:
luckytyphlosion 2021-01-05 13:56:26 -05:00
parent fb396395e9
commit 8855623ad0
1 changed files with 22 additions and 1 deletions
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23
tools/aif2pcm/main.c
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@ -351,6 +351,12 @@ const int gDeltaEncodingTable[] = {
-64, -49, -36, -25, -16, -9, -4, -1, -64, -49, -36, -25, -16, -9, -4, -1,
}; };
#define POSITIVE_DELTAS_START 0
#define POSITIVE_DELTAS_END 8
#define NEGATIVE_DELTAS_START 8
#define NEGATIVE_DELTAS_END 16
struct Bytes *delta_decompress(struct Bytes *delta, unsigned int expected_length) struct Bytes *delta_decompress(struct Bytes *delta, unsigned int expected_length)
{ {
struct Bytes *pcm = malloc(sizeof(struct Bytes)); struct Bytes *pcm = malloc(sizeof(struct Bytes));
@ -418,12 +424,27 @@ struct Bytes *delta_decompress(struct Bytes *delta, unsigned int expected_length
return pcm; return pcm;
} }
#define U8_TO_S8(value) ((value) < 128 ? (value) : (value) - 256)
int get_delta_index(uint8_t sample, uint8_t prev_sample) int get_delta_index(uint8_t sample, uint8_t prev_sample)
{ {
int best_error = INT_MAX; int best_error = INT_MAX;
int best_index = -1; int best_index = -1;
int delta_table_start_index;
int delta_table_end_index;
int sample_signed = U8_TO_S8(sample);
int prev_sample_signed = U8_TO_S8(prev_sample);
for (int i = 0; i < 16; i++) // if we're going up (or equal), only choose positive deltas
if (prev_sample_signed <= sample_signed) {
delta_table_start_index = POSITIVE_DELTAS_START;
delta_table_end_index = POSITIVE_DELTAS_END;
} else {
delta_table_start_index = NEGATIVE_DELTAS_START;
delta_table_end_index = NEGATIVE_DELTAS_END;
}
for (int i = delta_table_start_index; i < delta_table_end_index; i++)
{ {
uint8_t new_sample = prev_sample + gDeltaEncodingTable[i]; uint8_t new_sample = prev_sample + gDeltaEncodingTable[i];
uint8_t sample_diff_1 = (sample - new_sample) & 0xff; uint8_t sample_diff_1 = (sample - new_sample) & 0xff;