Quiet Frame Decompress

EditDate	2025-03-16

Related Threads: FLAC Agent Build

Page created to understand how fLaC decompresses the 0 Quiet Frame so it can be incorporated into analAI

Key differences:

1. Mid-side compression

Could well be the key lossless transformation mid = (left + right)/2 and side = left − right.Web References https://www.reddit.com/r/audioengineering/comments/u2t2o4/questions about the way flac works relatively/

Issues:

1. Warmup does not appear in wave or raw file which I don't understand as I get the same warmup values in analAI as FLaC seems to produce.

Seems I may have to include additional step for mid side channel de-correlation. lossless transformation mid = (left + right)/2 and side = left − right. . One sure issue with this is the side channel requires an extra bit (25 vs 24) which might explain the missed warmup values but right now I don’t think so. Either way it needs to be fixed.

Instruction trace flac for analQuiet.flac

Skip metadata not pertinent

Sync Frame

FLAC__STREAM_DECODER_SEARCH_FOR_FRAME_SYNC

Line Number	Code Package	Module	Value	Opertion Description
1047	SDC	Find Metadata	0xFFF8	FLAC__STREAM_DECODER_SEARCH_FOR_FRAME_SYNC FFF8
1071	SDC	stream decoder process	0xFFF8	FLAC__stream_decoder_process_until_end_of_stream
2041	SDC	read_frame	Unk	read_frame_(FLAC__StreamDecoder *decoder, FLAC__bool *got_a_frame, FLAC__bool do_full_decode)
350	BRC	FLAC__bitreader_reset_read_crc16	Unk	(FLAC__BitReader *br, FLAC__uint16 seed)
2277	SDC	read_frame_header_	Unk	read_frame_header_(FLAC__StreamDecoder *decoder)byte.
381	BDC	FLAC__bitreader_is_consumed_byte_aligned	0	(const FLAC__BitReader *br)
2319	SDC	read in the raw header as bytes so we can CRC it, and parse it on the way	if(!FLAC__bitreader_read_raw_uint32(decoder->private_->input, &x, 8))
418	BDC	FLAC__bitreader_read_raw_uint32	variables below	(FLAC__BitReader *br, FLAC__uint32 *val, uint32_t bits)

Read Frame Sync Fixed Header Channel Side 

Starting at 0x_1192 FFF8 C38C

& value 007F

• 0b_1111 1111 1111 10 Frame Header sync'. 14 bits
• 0b_0 mandatory 0 value Validation Bit 15
• 0b_0 0 = fixed blocking strategy Fixed Blocks or Not Bit 16
• 0b_1100 0x_C 12-8 = 4 so 2 to the 4th power = 16 time 256 = 4096 samples. ( 2358 SDC read frame header) frame blocksize
• 0b_0011 0x3 3 sample rate 192,000 ( 2379 SDC read frame header) samplerate'
• 0b1010 & 0b0111 = Case 2 FLAC__CHANNEL_ASSIGNMENT_MID_SIDE Channel Handling
• 0b110 = C 24 bits per sample bps
• 0b_0 Mandatory 0. Validation

32 bits consumed.

Variables

Variable	Decimal	Hex	Location Flac File
Bits	8	8
br->word		0xfff8c3ac005e	1192
br->consumed_bits	32	0x20

Read Frame Header

Read Frame Header

Line Number	Code Package	Module	Value	Opertion Description
2319	SDC	read_frame_header_	Variables Above	(FLAC__StreamDecoder *decoder) read in the raw header as bytes so we can CRC it, and parse it on the way
420	BDC	FLAC__bitreader_read_raw_uint32	0xC3 0xAC	(FLAC__BitReader *br, FLAC__uint32 *val, uint32_t bits)
2358	SDC	read_frame_header_	0xC3	frame.header.blocksize = 256 << (x-8) = 4098 =0x1000 (x = 0xC3 >> 4 = C = 12)
2379	SDC	read_frame_header_	0xC3	case 3 x &0x0F (x=0xC3) frame.header.sample_rate = 192,000
2429	SDC	read_frame_header_	0xAC	case 2 frame.header.channel_assignment = FLAC__CHANNEL_ASSIGNMENT_MID_SIDE;
2467	SDC	read_frame_header_	0xAC	case 6 (x = (uint32_t)(raw_header[3] & 0x0e) >> 1) frame.header.bits_per_sample = 32;

Read Subframe

Read Sub Frame Header

2082	SDC	read_frame	NA	Allocate channels +1 bit for channel 1 in MID_SIDE
2093	SDC	read_frame	channel 0 then 1 bps 24 then 25	if(!read_subframe_(decoder, channel, bps, do_full_decode))
2600	SDC	read_subframe_	channel 0 then 1 bps 24 then 25	(FLAC__StreamDecoder *decoder, uint32_t channel, uint32_t bps, FLAC__bool do_full_decode)
2606	SDC	read_subframe_	x = 78 = 0x4E then x = 76 = 0x4c (returned values)	FLAC__bitreader_read_raw_uint32(decoder->private_->input, &x, 8)
2625	SDC	read_subframe_	channel 0 then 1 bps 24 then 25	decoder->private_->frame.subframes[channel].wasted_bits = 0;
2666	SDC	read_subframe_	predictor_order = 8(chan 0) then 7(chan 1)	uint32_t predictor_order = ((x>>1)&31)+1
2672	SDC	read_subframe_	channel 0 then 1 bps 24 then 25 predictor_order = 8	if(!read_subframe_lpc_(decoder, channel, bps, predictor_order, do_full_decode))
2806	SDC	read_subframe_lpc	channel 0 then 1 bps 24 then 25 order = 8	(FLAC__StreamDecoder *decoder, uint32_t channel, uint32_t bps, const uint32_t order, FLAC__bool do_full_decode)
2821	SDC	read_subframe_lpc	see warmup values below	FLAC__bitreader_read_raw_int64(decoder->private_->input, &i64, bps)

Variables

Variable	Decimal	Hex	Location Flac File
Bits	8	8
br->word		0x4e000000fffff200	1192
br->consumed_bits	0	0x20

Warmup Values

Below warmup channel 0 bps 24

Warmup Channel 0 bps 24

0 i64	FLAC__int64	0	0x0
1 i64	FLAC__int64	-14	0xfffffffffffffff2
2 i64	FLAC__int64	28	0x1c
3 i64	FLAC__int64	-2	0xfffffffffffffffe
4 i64	FLAC__int64	12	0xc
5 i64	FLAC__int64	37	0x25
6 i64	FLAC__int64	-12	0xfffffffffffffff4
7 i64	FLAC__int64	32	0x20

Little Endian Memory Dump
0000000000000000 
F2FFFFFFFFFFFFFF
1C00000000000000
FEFFFFFFFFFFFFFF
0C00000000000000
2500000000000000
F4FFFFFFFFFFFFFF
2000000000000000

Memory Dump as Dump

From fLaC

00000000 00000000 F2FFFFFF FFFFFFFF 1C000000 00000000 FEFFFFFF FFFFFFFF
0C000000 00000000 25000000 00000000 F4FFFFFF FFFFFFFF 20000000 00000000

From analAI

00000000 00000000 F2FFFFFF FFFFFFFF 1C000000 00000000 FEFFFFFF FFFFFFFF
0C000000 00000000 25000000 00000000 F4FFFFFF FFFFFFFF 20000000 00000000

Warmup Channel 1 bps 25 predictor_order 7

0 i64	FLAC__int64	0	0x0
1 i64	FLAC__int64	13	0xd
2 i64	FLAC__int64	4	0x4
3 i64	FLAC__int64	-13	0xfffffffffffffff3
4 i64	FLAC__int64	48	0x30
5 i64	FLAC__int64	-40	0xffffffffffffffd8
6 i64	FLAC__int64	48	0x30