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decoder/mad: Fix decode of LAME peak value 

6d91b5c7b2 ("fix double promotions") changed
how LAME peak values are decoded, producing large incorrect values that
cause some MP3 files to play silently.

Restore the original decode from MAD fixed-point format to double and
document what it's doing.

Fixes #1823
This commit is contained in:
Simon Arlott 2023-05-27 20:13:53 +01:00 committed by Max Kellermann
parent fc9626e2f4
commit 6ee3d0102b
2 changed files with 17 additions and 1 deletions
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2
NEWS
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@ -1,4 +1,6 @@
ver 0.23.14 (not yet released)
* decoder
- mad: fix calculation of LAME peak values
ver 0.23.13 (2023/05/22)
* input

16
src/decoder/plugins/MadDecoderPlugin.cxx
View File

@ -562,7 +562,21 @@ parse_lame(struct lame *lame, struct mad_bitptr *ptr, int *bitlen) noexcept
mad_bit_skip(ptr, 16);
lame->peak = MAD_F(mad_bit_read(ptr, 32) << 5); /* peak */
/* The lame peak value is a float multiplied by 2^23 and stored as an
* unsigned integer (it is always positive). MAD's fixed-point format uses
* 28 bits for the fractional part, so shift the 23 bit fraction up before
* converting to a float.
*/
unsigned long peak_int = mad_bit_read(ptr, 32);
#define LAME_PEAK_FRACBITS 23
#if MAD_F_FRACBITS > LAME_PEAK_FRACBITS
peak_int <<= (MAD_F_FRACBITS - LAME_PEAK_FRACBITS);
#elif LAME_PEAK_FRACBITS > MAD_F_FRACBITS
peak_int >>= (LAME_PEAK_FRACBITS - MAD_F_FRACBITS);
#endif
lame->peak = mad_f_todouble(peak_int); /* peak */
FmtDebug(mad_domain, "LAME peak found: {}", lame->peak);
lame->track_gain = 0;