USING OTHER CODECS

To use the library with other ACM-supported codecs besides
TrueSpeech, the following things are required:

  1. DLL file supporting the codec
  2. correct format tag for the codec
  3. correct WAVEFORMATEX structure for the codec

For codecs which are supported by mplayer, the easiest
way to get items 1 and 2 is by looking at the audio codec
section of mplayer's codec status table:
  http://mplayerhq.hu/DOCS/codecs-status.html
If mplayer says it is using a .acm file to support the 
audio codec, then most likely it can be used with this
library.  If mplayer is using another method to support
that codec, you should probably consider using that
same method in your Linux program instead of this library,
because it is most likely a native way of supporting the
codec.  Using a native codec should allow greater portability
of your code to other architectures besides x86
and does not require your users to have Windows DLLs available.

Determining item 3, the correct WAVEFORMATEX structure
for the codec, is the hardest part.  If your code is 
simply playing files encoded by the codec this is easy.
Your code must register the mapping between format tags
and DLL filenames (items 1 and 2 above) by making repeated
calls to MSACM_RegisterDriver().  Note that you will 
often have to call MSACM_RegisterDriver() twice for each
codec, once to register it for the codec's format tag for
decoding, and once to register it with the PCM format tag
for encoding.  Then to play a file you read the .wav header 
from the beginning of the file and use that to create a
WAVEFORMATEX structure which you pass to the acmStreamOpen()
function.  From there on out everything can be the same
as the example program.

On the other hand, if your user is choosing the codec to 
use for some encoding tasks, you do not have a .wav header
to read.  The easiest thing to do in this case is probably
to create a .wav file in the desired format under Windows using
the Windows sound recorder.  Then examine this file with
a hex editor and determine the appropriate values of the
fields to use in the WAVEFORMATEX structure.  Note that
most codecs use extra fields beyond the basic WAVEFORMATEX
structure.  test_truespeech.c shows how to create an extended
length WAVEFORMATEX and how to initialize the extra parts
for the TrueSpeech codec.  A thorough writeup of the .wav
format headers is at
  http://www.borg.com/~jglatt/tech/wave.htm
  

Most codecs will support 16-bit signed linear PCM, but probably
do not support a direct conversion to another proprietary
codec.  To convert from one proprietary codec format to 
another, typically a conversion to an intermediate 16-bit PCM
form is required.