From 3a2b1ac3aa62be7078fdfac46be0e9d764664baf Mon Sep 17 00:00:00 2001 From: Josh Coalson Date: Tue, 27 Mar 2001 23:50:51 +0000 Subject: [PATCH] update with new format changes: frame crc-16, wasted bits field, longer sync code, longer blocksize code --- doc/format.html | 150 ++++++++++++++++++++++++++++++++++-------------- 1 file changed, 107 insertions(+), 43 deletions(-) diff --git a/doc/format.html b/doc/format.html index f8a864e4..f0df84bd 100644 --- a/doc/format.html +++ b/doc/format.html @@ -175,16 +175,16 @@

@@ -227,7 +227,7 @@ The audio data is composed of one or more audio frames. Each frame consists of a frame header, which contains a sync code, info about the frame like the block size, sample rate, number of channels, et cetera, and an 8-bit CRC. The frame header also contains either the sample number of the first sample in the frame (for variable-blocksize streams), or the frame number (for fixed-blocksize streams). This allows for fast, sample-accurate seeking to be performed. Following the frame header are encoded subframes, one for each channel, and finally, the frame is zero-padded to a byte boundary. Each subframe has its own header that specifies how the subframe is encoded.

  • - Since a decoder may start decoding in the middle of a stream, there must be a method to determine the start of a frame. A 9-bit sync code begins every frame. The sync code will not appear anywhere else in the frame header. However, since it may appear in the subframes, the decoder has two other ways of ensuring a correct sync. The first is to check that the rest of the frame header contains no invalid data. Even this is not foolproof since valid header patterns can still occur within the subframes. The decoder's final check is to generate an 8-bit CRC of the frame header and compare this to the CRC stored at the end of the frame header. + Since a decoder may start decoding in the middle of a stream, there must be a method to determine the start of a frame. A 14-bit sync code begins each frame. The sync code will not appear anywhere else in the frame header. However, since it may appear in the subframes, the decoder has two other ways of ensuring a correct sync. The first is to check that the rest of the frame header contains no invalid data. Even this is not foolproof since valid header patterns can still occur within the subframes. The decoder's final check is to generate an 8-bit CRC of the frame header and compare this to the CRC stored at the end of the frame header.

  • Again, since a decoder may start decoding at an arbitrary frame in the stream, each frame header must contain some basic information about the stream because the decoder may not have access to the STREAMINFO metadata block at the start of the stream. This information includes sample rate, bits per sample, number of channels, etc. Since the frame header is pure overhead, it has a direct effect on the compression ratio. To keep the frame header as small as possible, FLAC uses lookup tables for the most commonly used values for frame parameters. For instance, the sample rate part of the frame header is specified using 4 bits. Eight of the bit patterns correspond to the commonly used sample rates of 8/16/22.05/24/32/44.1/48/96 kHz. However, odd sample rates can be specified by using one of the 'hint' bit patterns, directing the decoder to find the exact sample rate at the end of the frame header. The same method is used for specifying the block size and bits per sample. In this way, the frame header size stays small for all of the most common forms of audio data. @@ -239,7 +239,7 @@ FLAC specifies a subset of itself as the Subset format. The purpose of this is to ensure that any streams encoded according to the Subset are truly "streamable", meaning that a decoder that cannot seek within the stream can still pick up in the middle of the stream and start decoding. It also makes hardware decoder implementations more practical by limiting the blocking such that decoder buffer sizes can be easily determined. "flac" generates Subset streams by default unless the "--lax" command-line option is used. The Subset makes the following limitations on what may be used in the stream: