From 72787c3fee472809fb3f819558c7176066f0ee44 Mon Sep 17 00:00:00 2001 From: Martijn van Beurden Date: Fri, 24 Nov 2023 13:45:21 +0100 Subject: [PATCH] Add documentation on foreign metadata storage format --- doc/foreign_metadata_storage.md | 39 +++++++++++++++++++++++++++++++++ 1 file changed, 39 insertions(+) create mode 100644 doc/foreign_metadata_storage.md diff --git a/doc/foreign_metadata_storage.md b/doc/foreign_metadata_storage.md new file mode 100644 index 00000000..874bd1af --- /dev/null +++ b/doc/foreign_metadata_storage.md @@ -0,0 +1,39 @@ +This document specifies three metadata application IDs to store foreign +metadata. + +- For WAVE and RF64 files, ID "riff" (0x72696666) is used. +- For AIFF and AIFF-C files, ID "aiff" (0x61696666) is used. +- For Wave64 files, ID "w64 " (0x773634C0) is used. + +When converting one of the aforementioned filetypes to FLAC and storage +of the foreign metadata is desired, all chunks are copied into metadata +blocks, one for each chunk. All chunks are copied completely, except for +the first chunk (which contains all others) and the chunk containing the +audio, of which only the headers (identifier and size) are copied. It is +important for proper restoration that the order of the chunks is +retained. + +This format is used by the `flac` command line tool when supplied with +the `--keep-foreign-metadata` option. + +It might seem superfluous to store the header of the main chunk and of +the chunk carrying audio. Similarly, much data in the WAVE fmt chunk +is duplicated in the FLAC streaminfo metadata block. However, these +chunks are kept to simplify restoring them and as an extra integrity +check. Additionally, by duplicating the header of the chunk containing +the audio data, it is possible to find out which chunks should precede +the audio data and which chunks should trail it. + +As an example, consider a very simple WAVE file consisting of the main +RIFF chunk, the WAVE subchunk, the fmt subchunk and the data subchunk. +When converted with `flac --keep-foreign-metadata`, the resulting FLAC +file will have 4 application metadata blocks with ID "riff". The first +block only contains the header of the main chunk and therefore will have +8 bytes of content (and thus be 12 bytes long excluding header because +of the 4-byte ID and 16 bytes long including header). The second +metadata block will have 4 bytes of content ("WAVE"). The third metadata +block holds the complete fmt chunk. The fourth metadata block contains +only the header of the data chunk and therefore has only 8 bytes of +content. + +For RF64, AIFF, AIFF-C and Wave64, the procedure is the same.