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Add wwise_ima comment about alternate nibble expand

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bnnm 2018-02-17 19:44:18 +01:00
parent 47bdada1e8
commit 7c41cb22bf
1 changed files with 16 additions and 1 deletions
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17
src/coding/ima_decoder.c
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@ -719,7 +719,6 @@ void decode_wwise_ima(VGMSTREAM * vgmstream,VGMSTREAMCHANNEL * stream, sample *
std_ima_expand_nibble(stream, byte_offset,nibble_shift, &hist1, &step_index);
outbuf[sample_count] = (short)(hist1);
sample_count+=channelspacing;
//todo atenuation: apparently from hcs's analysis Wwise IMA decodes nibbles slightly different, reducing dbs
}
}
@ -729,7 +728,23 @@ void decode_wwise_ima(VGMSTREAM * vgmstream,VGMSTREAMCHANNEL * stream, sample *
stream->adpcm_history1_32 = hist1;
stream->adpcm_step_index = step_index;
}
//todo atenuation: apparently from hcs's analysis Wwise IMA expands nibbles slightly different, reducing clipping/dbs
/*
From Wwise_v2015.1.6_Build5553_SDK.Linux
<_ZN13CAkADPCMCodec12DecodeSampleEiii>:
10: 83 e0 07 and $0x7,%eax ; sample
13: 01 c0 add %eax,%eax ; sample*2
15: 83 c0 01 add $0x1,%eax ; sample*2+1
18: 0f af 45 e4 imul -0x1c(%rbp),%eax ; (sample*2+1)*scale
1c: 8d 50 07 lea 0x7(%rax),%edx ; result+7
1f: 85 c0 test %eax,%eax ; result negative?
21: 0f 48 c2 cmovs %edx,%eax ; adjust if negative to fix rounding for below division
24: c1 f8 03 sar $0x3,%eax ; (sample*2+1)*scale/8
Different rounding model vs IMA's shift-and-add (also "adjust" step may be unnecessary).
*/
/* MS-IMA with possibly the XBOX-IMA model of even number of samples per block (more tests are needed) */
void decode_awc_ima(VGMSTREAMCHANNEL * stream, sample * outbuf, int channelspacing, int32_t first_sample, int32_t samples_to_do) {
int i, sample_count;