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82 lines
4.0 KiB
Plaintext
82 lines
4.0 KiB
Plaintext
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Notes about WDM-KS host API
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---------------------------
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Status history
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--------------
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10th November 2005:
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Made following changes:
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* OpenStream: Try all PaSampleFormats internally if the the chosen
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format is not supported natively. This fixed several problems
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with soundcards that soundcards that did not take kindly to
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using 24-bit 3-byte formats.
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* OpenStream: Make the minimum framesPerHostIBuffer (and framesPerHostOBuffer)
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the default frameSize for the playback/recording pin.
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* ProcessingThread: Added a switch to only call PaUtil_EndBufferProcessing
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if the total input frames equals the total output frames
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5th September 2004:
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This is the first public version of the code. It should be considered
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an alpha release with zero guarantee not to crash on any particular
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system. So far it has only been tested in the author's development
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environment, which means a Win2k/SP2 PIII laptop with integrated
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SoundMAX driver and USB Tascam US-428 compiled with both MinGW
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(GCC 3.3) and MSVC++6 using the MS DirectX 9 SDK.
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It has been most widely tested with the MinGW build, with most of the
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test programs (particularly paqa_devs and paqa_errs) passing.
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There are some notable failures: patest_out_underflow and both of the
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blocking I/O tests (as blocking I/O is not implemented).
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At this point the code needs to be tested with a much wider variety
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of configurations and feedback provided from testers regarding
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both working and failing cases.
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What is the WDM-KS host API?
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----------------------------
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PortAudio for Windows currently has 3 functional host implementations.
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MME uses the oldest Windows audio API which does not offer good
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play/record latency.
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DirectX improves this, but still imposes a penalty
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of 10s of milliseconds due to the system mixing of streams from
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multiple applications.
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ASIO offers very good latency, but requires special drivers which are
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not always available for cheaper audio hardware. Also, when ASIO
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drivers are available, they are not always so robust because they
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bypass all of the standardised Windows device driver architecture
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and hit the hardware their own way.
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Alternatively there are a couple of free (but closed source) ASIO
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implementations which connect to the lower level Windows
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"Kernel Streaming" API, but again these require special installation
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by the user, and can be limited in functionality or difficult to use.
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This is where the PortAudio "WDM-KS" host implementation comes in.
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It directly connects PortAudio to the same Kernel Streaming API which
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those ASIO bridges use. This avoids the mixing penatly of DirectX,
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giving at least as good latency as any ASIO driver, but it has the
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advantage of working with ANY Windows audio hardware which is available
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through the normal MME/DirectX routes without the user requiring
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any additional device drivers to be installed, and allowing all
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device selection to be done through the normal PortAudio API.
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Note that in general you should only be using this host API if your
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application has a real requirement for very low latency audio (<20ms),
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either because you are generating sounds in real-time based upon
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user input, or you a processing recorded audio in real time.
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The only thing to be aware of is that using the KS interface will
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block that device from being used by the rest of system through
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the higher level APIs, or conversely, if the system is using
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a device, the KS API will not be able to use it. MS recommend that
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you should keep the device open only when your application has focus.
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In PortAudio terms, this means having a stream Open on a WDMKS device.
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Usage
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-----
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To add the WDMKS backend to your program which is already using
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PortAudio, you must undefine PA_NO_WDMKS from your build file,
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and include the pa_win_wdmks\pa_win_wdmks.c into your build.
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The file should compile in both C and C++.
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You will need a DirectX SDK installed on your system for the
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ks.h and ksmedia.h header files.
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You will need to link to the system "setupapi" library.
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Note that if you use MinGW, you will get more warnings from
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the DX header files when using GCC(C), and still a few warnings
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with G++(CPP).
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