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Notes about WDM-KS host API
---------------------------
Status history
--------------
5th September 2004:
This is the first public version of the code. It should be considered
an alpha release with zero guarantee not to crash on any particular
system. So far it has only been tested in the author's development
environment, which means a Win2k/SP2 PIII laptop with integrated
SoundMAX driver and USB Tascam US-428 compiled with both MinGW
(GCC 3.3) and MSVC++6 using the MS DirectX 9 SDK.
It has been most widely tested with the MinGW build, with most of the
test programs (particularly paqa_devs and paqa_errs) passing.
There are some notable failures: patest_out_underflow and both of the
blocking I/O tests (as blocking I/O is not implemented).
At this point the code needs to be tested with a much wider variety
of configurations and feedback provided from testers regarding
both working and failing cases.
What is the WDM-KS host API?
----------------------------
PortAudio for Windows currently has 3 functional host implementations.
MME uses the oldest Windows audio API which does not offer good
play/record latency.
DirectX improves this, but still imposes a penalty
of 10s of milliseconds due to the system mixing of streams from
multiple applications.
ASIO offers very good latency, but requires special drivers which are
not always available for cheaper audio hardware. Also, when ASIO
drivers are available, they are not always so robust because they
bypass all of the standardised Windows device driver architecture
and hit the hardware their own way.
Alternatively there are a couple of free (but closed source) ASIO
implementations which connect to the lower level Windows
"Kernel Streaming" API, but again these require special installation
by the user, and can be limited in functionality or difficult to use.
This is where the PortAudio "WDM-KS" host implementation comes in.
It directly connects PortAudio to the same Kernel Streaming API which
those ASIO bridges use. This avoids the mixing penatly of DirectX,
giving at least as good latency as any ASIO driver, but it has the
advantage of working with ANY Windows audio hardware which is available
through the normal MME/DirectX routes without the user requiring
any additional device drivers to be installed, and allowing all
device selection to be done through the normal PortAudio API.
Note that in general you should only be using this host API if your
application has a real requirement for very low latency audio (<20ms),
either because you are generating sounds in real-time based upon
user input, or you a processing recorded audio in real time.
The only thing to be aware of is that using the KS interface will
block that device from being used by the rest of system through
the higher level APIs, or conversely, if the system is using
a device, the KS API will not be able to use it. MS recommend that
you should keep the device open only when your application has focus.
In PortAudio terms, this means having a stream Open on a WDMKS device.
Usage
-----
To add the WDMKS backend to your program which is already using
PortAudio, you must undefine PA_NO_WDMKS from your build file,
and include the pa_win_wdmks\pa_win_wdmks.c into your build.
The file should compile in both C and C++.
You will need a DirectX SDK installed on your system for the
ks.h and ksmedia.h header files.
You will need to link to the system "setupapi" library.
Note that if you use MinGW, you will get more warnings from
the DX header files when using GCC(C), and still a few warnings
with G++(CPP).
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