OggPCM Draft1

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What is it

OggPCM is a pulse-code modulation (PCM) audio codec for Ogg. Similar to Microsoft's .wav or Apple's .aiff formats, it's a simple way to store and transfer uncompressed audio within an Ogg container.


Why is it

The intention for this format is as an interchange format, especially for use with OggStream. It is also useful for storing time-synced decoded audio/video for development, vs RIFF/WAV (.wav) and YUV4MPEG (.yuv) in seperate files as we did with Theora.

It is also less complex than either .wav (RIFF) or .aiff (AIFF), both of these formats being designed for generic multimedia (audio, video, etc). Full compatability with these formats includes support for non-PCM data.

Using raw PCM data, on the other hand, doesn't give us that all-important header which carries information about the number of channels, sample width, and sample frequency. So what we need is a header followed by raw PCM data - nothing more complicated.

Format

Packets are processed as per the value of their first byte. Packets of unknown ID should be silently ignored, providing a convient way to add future expandability which does not break the data format. Multibyte fields in the header packet are packed in big endian order. Other fields are stored MSB first. Multibyte fields in the data packet are packed in little endian order.

The granule position specified is the total samples encoded after including all samples on the page. Samples must not be split across pages. The rationale here is that the position specified in the frame header of the last page tells how long the data coded by the bitstream is. A truncated stream will still return the proper number of samples that can be decoded fully.

An example of how this can be useful is the proposed ReplayGain extension to .wav format: http://replaygain.hydrogenaudio.org/file_format_wav.html

Note that no such extension is planned, nor is the need for a future format forseen, but history has shown that even the most basic formats eventually become obsolete.

Packet 0, BOS, 12 bytes
 8  0x00   Header Packet ID
24  "PCM"  Codec identifier 
 -
 8  0x01   Version Major (breaks backwards compatability to increment)
 8  0x00   Version Minor (backwards compatable, ie, via extended header)
 8  [int]  Number of Channels (1-256)
 8  [int]  Samples per Second
 -
 4  [int]  Bytes per Sample (*8)
 1  [flg]  False = MSB, True = LSB
 1  [flg]  Float, if >2bytes/sample, Unsigned if 1byte/sample (False=signed int)
 2  [nil]  Padding to byte/int - may be used for "extended" data type
 [ Channel Identifiers ]
Data Packet
 8  0xFF       Data Packet ID
24  "PCM"      Codec identifier, pads data to 32-bits
..  [data]     variable length pcm data

Alternative Format

The primary difference between this format and the one above is that it is intended to support channels from the same source having different sampling parameters.

Packet 0, BOS, tbd bytes
 8  0x00       Header Packet ID
24  "PCM"      Codec identifier 
 8  0x01       Version Major (breaks backwards compatability to increment)
 8  0x00       Version Minor (backwards compatable, ie, via extended header)
 8  [uint]     Source ID (Unique amongst all OggPCM streams in the physical stream)
 8  [uint]     Channel Block
16  [bitfield] Indicates which of the 16 channels in this channel block 
               are present in this logical OGGPCM stream.
 8  [enum]     Sample format (OGGPCM_FMT_U8, OGGPCM_FMT_LE_S16, OGGPCM_FMT_BE_S16, etc) 
24  [uint]     Sample rate
Data Packet
 8  0xFF       Data Packet ID
24  "PCM"      Codec identifier, pads data to 32-bits
..  [data]     variable length pcm data, packing defined by Sample Format field in header

Constraints: This format can support any __documented and registered__ format by since it uses an enumeration. Each logical stream can support up to 16 channels sharing a fixed sample rate. Logical streams from the same source may be multiplexed to provide up to 4096 channels per source, each with their own sample rate. Up to 256 Sources may be multiplexed within a physical Ogg stream, unless an application takes other measures to logically partition the stream.

Discussion: This seems to make it easy to support the simple/normal cases and possible to support the pathological cases, for instance:

Source ID Channel Bitfield Sample Rate Sample Format Comment
0x00 0000 0000 0000 0011 96000 OGGPCM_FMT_LE_S24 Front Stereo Pair
0x00 0000 0000 0011 1100 44100 OGGPCM_FMT_LE_S16 Center And Surrounds
0x00 0000 0000 0010 0000 8000 OGGPCM_FMT_LE_S16 LFE Channel
0x01 0000 0000 0000 0001 8000 OGGPCM_FMT_U8 PC Speaker
0x02 0000 0000 0000 0001 8000 OGGPCM_FMT_U8 Microphone
0x03 0000 0000 0000 0011 8000 OGGPCM_FMT_LE_S16 Voice Chat

Each entry in the table is a logical Ogg stream. I'm not convinced that the source id and channel block are necessary, but figured I'd throw it out there.

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