Martin Dragomirecky (mxd@cisco.com)
Wed, 13 Dec 2000 14:29:13 -0800
hi charles,
there appears to be a few minor errors in draft-ema-vpim-wav-00.
1st error is size of fields in header. this may have been caused by
mis-interpreting the types WORD and DWORD from the .wav C header as 32bit and
64bit values, rather than 16bit and 32bit values.
// from MMREG.H rev 04/05/94
typedef struct tWAVEFORMATEX
{
WORD wFormatTag; /* format type */
WORD nChannels; /* number of channels (i.e. mono, stereo...) */
DWORD nSamplesPerSec; /* sample rate */
DWORD nAvgBytesPerSec; /* for buffer estimation */
WORD nBlockAlign; /* block size of data */
WORD wBitsPerSample; /* Number of bits per sample of mono data */
WORD cbSize; /* The count in bytes of the size of
extra information (after cbSize) */
} WAVEFORMATEX;
2nd issue is that for non-PCM formats the above structure is used and includes
an additional cbSize field and implies a variable length format chunk. This
also means that the offset of the data chunk in the file is not fixed.
It is not clear from the header file whether the g711 ulaw and g.726 32kADPCM
formats are considered PCM w/ a fixed length format chunk, but apparently
soundrecorder under win2k always uses the extended format chunk, even for PCM
formats and with value of cbSize = 0.
attached are suggested changes to the draft, modified lines marked w/ 'x'.
on a related topic, for VoIP environments support of the G.723.1 codec is
required by the VoIP Forum Interop agreement. Would it be possible to get a
description of the .wav file format for the MSG723 codec (0x42), similar to the
msgsm draft? While not part of VPIM, ability to record and play G723.1 encoded
audio files on win32 would be highly useful for VoIP-based messaging systems.
Btw, Soundrecorder shows this codec as one of the options for save as, but it is
unable to complete the save. I'm not sure if this is a licensing restriction on
the codec or some other issue.
thanx much,
_______________________________
2.2.2 The Format Chunk
The Format chunk specifies the characteristics of the audio data
necessary to decompress it and play it. Each audio/wav file MUST
include one and only one Format chunk. This chunk MUST include
the following fields:
OFFSET LENGTH VALUE DESCRIPTION
12 4 bytes 'fmt ' The chunk ID.
x 16 4 bytes Length of the chunk excluding the 8
x bytes for the chunk ID and length.
x 20 2 bytes The codec ID.
x 22 2 bytes The number of channels.
x 24 4 bytes Samples per second.
x 28 4 bytes Average bytes per second.
x 32 2 bytes Block alignment.
x 34 2 bytes Bits per sample.
x 36 2 bytes count in bytes of the size of
x extra information in this chunk
x (codec specific)
2.2.3 The Data Chunk
The Data chunk contains the compressed audio data. This chunk
MUST be preceded (though not immediately) by the Format chunk.
The Data chunk MUST adhere to the following format:
OFFSET LENGTH VALUE DESCRIPTION
x 4 bytes 'data' The chunk ID.
x 4 bytes Length of the data
(chunk size minus (-) 8 bytes.
x The compressed audio.
2.2.4 The Fact Chunk
All audio/wav files MUST include a Fact chunk as they contain
compressed data. The Fact chunk MUSt contain one field indicating
the size (in sample points) of the audio data after decompression.
The Fact chunk MUST adhere to the following format:
OFFSET LENGTH VALUE DESCRIPTION
4 bytes 'fact' The chunk ID.
x 4 bytes 4 Chunk size minus (-) 8 bytes.
x 4 bytes Sample length.
Charles Eliot wrote:
>
> Greg is no longer responsible for this draft. Pls send me your comments.
>
> -----Original Message-----
> From: James P. Salsman [mailto:bovik@best.com]
> Sent: Tuesday, November 14, 2000 3:56 PM
> To: Greg Baribault
> Cc: vpim@lists.neystadt.org
> Subject: [VPIM] Laile, co-author of draft-ema-vpim-wav-00, no longer at
> Microsoft
>
> Greg,
>
> My mail to <lailed@microsoft.com> bounced, so you're the
> remaining author of:
>
> http://www.ema.org/vpimdir/specs/draft-ema-vpim-wav-00.txt
>
> If you don't fix that error that I pointed out in my previous
> message and resubmit it, I will in a week or two, or sooner
> if you just let me know.
>
> audio/wav is an important registration that needs to be made
> before VPIM can proceed on the standards track.
>
> Cheers,
> James
This archive was generated by hypermail 2.0b3 on Thu Dec 14 2000 - 01:31:41 IST