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

FLAC__STREAM_DECODER_SEARCH_FOR_FRAME_SYNC

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	-	2429	SDC	read_frame_header_	0xAC	case 2 frame.header.channel_assignment = FLAC__CHANNEL_ASSIGNMENT_MID_SIDE;

Edit To Here

Read Frame Last Half Header Metadata 

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