[Flac-dev] ACM codec

Ingo Ralf Blum ingoralfblum at gmx.de
Tue Oct 2 18:05:03 PDT 2001

Hi Daniel,

> Here's an excerpt from the DirectX SDK documentation:
> ...
> The particular format of a sound can be described by a WAVEFORMATEX or
> The wFormatTag member of this structure contains a unique identifier
> assigned by Microsoft Corporation. The only tags valid with DirectSound are
> the following:
> WAVE_FORMAT_PCM. This tag indicates PCM, an uncompressed format.
> WAVE_FORMAT_EXTENSIBLE. Multichannel audio format; valid only with WDM
> drivers.
> ...

Which says nothing. The above text applies to renderers and capture filters, not
to intermediate filters like compressors. Especially all standard AVI and WAV
filters support the WAVEFORMATEXTENSIBLE structure, whether the docs state
something different or not.

> The ACM driver responds windows messages with WAVEFORMATEX structures. The
> ACM driver isn't responsible for keeping the data type information, the
> client software is. AVI files are RIFF files, so the information returned by
> the ACM driver in a WAVEFORMATEX is wrote in the RIFF structure. That's why
> I said that there will be some data repeated. If VirtualDub splits FLAC
> data, it's responsible for keeping the data type in the two result RIFF
> structures. The data that the ACM driver will receive from the second part
> when decoding will have not the "fLaC" marker and the metadata blocks. But
> since the FLAC format is streamable (it has frame headers), it's not so bad.

So when the driver has to decompress the second file it creates a marker and the
metadata and feeds it to the FLAC encoder. Or do you use a lower level API than
the stream en/decoder from libFLAC?

> The standard behaviour for an ACM codec is return a set of standard formats
> for the data type. It's about samples per second, bits per sample and
> channel number and not about compression options. The compression options
> are set in a dialog provided by the ACM driver. The client software can ask
> for a nonstandard format.

May I ask how?

> In VirtualDub you see a list of data types and a list of standard formats
> for each data type. The strange thing is that there are sub-types. Looking
> in the Windows Registry, I saw that each sub-type is registered as a
> diferent type that use the same driver. The driver I wrote is based in the

This may be a dumb question, but where in the registry did you find this?

> IMA ADPCM driver code, that registers only one data type. The compression
> options' dialog isn't available in VirtualDub for Audio. That's odd, because
> the compression options' dialog is available for video when the driver
> provide it, and the VCM is similar to the ACM. Anyway, the dialog is
> available in the Control Panel.

So, then which message shows the compression options dialog for ACM drivers.

All drivers support the DRV_CONFIGURE message, which shows the control panel
dialog. but the DDK says:

The DRV_CONFIGURE message is one of the standard driver messages. A client sends
the message by calling the driver's DriverProc entry point, passing the
specified parameter values. Typically, this message is sent by the Control
Panel's Multimedia applet.

Drivers display a dialog box to obtain configuration parameters from the system
administrator. Your driver must confirm that the client has Administrator

which is an excerpt from the DRV_CONFIGURE reference in the DDK. Additionally
the DRV_CONFIGURE message is usually sent to a driver, that was opened with no
parameters (0 as argument to DRV_OPEN). So the options would have to be common
for one driver instance. Unfortunately one driver instance can be used to open
multiple streams, which should have their own diffferent options each.

Another dialog is the ACMDM_DRIVER_ABOUT dialog, which has its counterpart in
the ICM_ABOUT message.

But only the ICM drivers have the ICM_CONFIGURE message for changing compression
options. I can't find a similar message for the ACM drivers. Can you enlighten
me here.

As you already mentioned, Virtual Dub does not display a dialog for selecting
compression options. But I assume some others do and you know at least one of
them. Can you please give me a download link for an application that shows such
a dialog. It would be helpful for testing.



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