Use the new PortAudio option to enable WASAPI Shared conversions.

This feature was introduced in PortAudio commit
aa0748a5b59491ba2cfa9943825653cde6e1f748. It is enabled by default
since that makes WASAPI Shared much easier to use; it can be
disabled through the new FlexASIO wasapiAutoConvert option.

Fixes #32.

Author: dechamps

BACKENDS.md

@@ -6,10 +6,6 @@ wanting to optimize their audio pipeline. In particular, this document provides
 the necessary information to understand what the [`backend`][backend] and
 [`wasapiExclusiveMode`][wasapiExclusiveMode] FlexASIO configuration options do.
 
-**Note:** this document is a work in progress, and does not go into much detail
-regarding the specific differences between the backends. For now, you will have
-to experiment for yourself. Comments and suggestions are welcome.
-
 A `backend` is just another term for what PortAudio calls the *host API*.
 FlexASIO uses the term "backend" to avoid potential confusion with the term
 "ASIO host".
@@ -105,8 +101,9 @@ ms, if not more.
 For example, it might expose 8 channels for a 5.1 output, downmixing the rear
 channels pairs.
 
-Modern versions of Windows implement the MME API by using WASAPI internally,
-making this backend a "second-class citizen" compared to WASAPI and WDM-KS.
+Modern versions of Windows implement the MME API by using WASAPI Shared
+internally, making this backend a "second-class citizen" compared to WASAPI and
+WDM-KS.
 
 ## DirectSound backend
 
@@ -122,7 +119,7 @@ if that's really the case in practice. The DirectSound backend has been observed
 to [behave very poorly][issue29] with small buffer sizes on the input side,
 making it a poor choice for low-latency capture use cases.
 
-Modern versions of Windows implement the DirectSound API by using WASAPI
+Modern versions of Windows implement the DirectSound API by using WASAPI Shared
 internally, making this backend a "second-class citizen" compared to WASAPI and
 WDM-KS.
 
@@ -147,16 +144,10 @@ used. The two modes behave very differently; in fact, they should probably be
 seen as two separate backends entirely.
 
 In *shared* mode, WASAPI behaves similarly to MME and DirectSound, in that the
-audio goes through most of the normal Windows audio pipeline. One important
-limitation of this mode is that there is no sample rate conversion. Therefore,
-initialization will fail if the application sample rate is different from the
-sample rate of the input or output devices, as configured in the Windows sound
-settings. (Corollary: if the input and output devices are configured with
-different sample rates in Windows, WASAPI Shared won't work, period.) There is
-also no support for upmixing nor downmixing; the channel counts must match
-exactly. These limitations [are inherent to WASAPI itself][wasapisr]. It is
-reasonable to assume that this mode will provide the best possible latency for
-a shared backend.
+audio goes through most of the normal Windows audio pipeline. Indeed, in
+modern versions of Windows, MME and DirectSound are just thin wrappers
+implemented on top of WASAPI. For this reason it is reasonable to assume that
+this mode will provide the best possible latency for a shared backend.
 
 In *exclusive* mode, WASAPI behaves completely differently and bypasses the
 entirety of the Windows audio pipeline, including mixing and APOs. As a result,
@@ -220,9 +211,8 @@ Streaming.
 [wasapiExclusiveMode]: CONFIGURATION.md#option-wasapiExclusiveMode
 [Windows Audio Session API]: https://docs.microsoft.com/en-us/windows/desktop/coreaudio/wasapi
 [Windows Driver Model]: https://en.wikipedia.org/wiki/Windows_Driver_Model
-[wasapisr]: https://docs.microsoft.com/windows/desktop/CoreAudio/device-formats
 [WDM-KS issue]: https://github.com/dechamps/FlexASIO/issues/21
 
-<!-- Use the converter at http://http://gravizo.com/ to recover the source code
-of this graph. -->
-[diagram]: https://g.gravizo.com/svg?digraph%20G%20%7B%0A%09rankdir%3D%22LR%22%0A%09style%3D%22dashed%22%0A%09fontname%3D%22sans-serif%22%0A%09node%5Bfontname%3D%22sans-serif%22%5D%0A%0A%09subgraph%20clusterApplicationProcess%20%7B%0A%09%09label%3D%22Application%20process%22%0A%0A%09%09Host%5Blabel%3D%22ASIO%20host%20application%22%5D%0A%0A%09%09subgraph%20clusterFlexASIO%20%7B%0A%09%09%09label%3D%22FlexASIO%22%0A%09%09%09FlexASIO%5Blabel%3D%22ASIO%20driver%22%5D%0A%0A%09%09%09subgraph%20clusterPortAudio%20%7B%0A%09%09%09%09label%3D%22PortAudio%22%0A%0A%09%09%09%09PortAudio%5Blabel%20%3D%20%22Frontend%22%5D%0A%09%09%09%09subgraph%20%7B%0A%09%09%09%09%09rank%3D%22same%22%0A%09%09%09%09%09node%20%5Bcolor%3D%22red%22%3B%20penwidth%3D3%5D%0A%0A%09%09%09%09%09PortAudioMME%5Blabel%3D%22MME%22%5D%0A%09%09%09%09%09PortAudioDirectSound%5Blabel%3D%22DirectSound%22%5D%0A%09%09%09%09%09PortAudioWASAPI%5Blabel%3D%22WASAPI%22%5D%0A%09%09%09%09%09PortAudioWDMKS%5Blabel%3D%22WDM-KS%22%5D%0A%09%09%09%09%7D%0A%09%09%09%7D%0A%09%09%7D%0A%09%7D%0A%0A%09subgraph%20clusterWindows%20%7B%0A%09%09label%3D%22Windows%20audio%20subsystem%22%0A%09%09subgraph%20%7B%0A%09%09%09rank%3D%22same%22%0A%09%09%09MME%0A%09%09%09DirectSound%0A%09%09%09WASAPIShared%5Blabel%3D%22WASAPI%20(shared)%22%5D%0A%09%09%09WASAPIExclusive%5Blabel%3D%22WASAPI%20(exclusive)%22%5D%0A%09%09%09WDMKS%5Blabel%3D%22Kernel%20Streaming%22%5D%0A%09%09%7D%0A%0A%09%09SampleRateConversion%5Blabel%3D%22Sample%20rate%20conversion%22%5D%0A%09%09PreMix%5Blabel%3D%22Pre-mix%20APOs%22%5D%0A%09%09Mix%5Blabel%3D%22Mixing%22%5D%0A%09%09PostMix%5Blabel%3D%22Post-mix%20APOs%22%5D%0A%09%7D%0A%0A%09subgraph%20clusterHardware%20%7B%0A%09%09label%3D%22Audio%20hardware%22%0A%09%09HardwareDriver%5Blabel%3D%22Driver%22%5D%0A%09%09HardwareDevice%5Blabel%3D%22Device%22%5D%0A%09%7D%0A%0A%09Host-%3EFlexASIO%0A%09FlexASIO-%3EPortAudio%0A%0A%09PortAudio-%3E%7B%0A%09%09PortAudioMME%0A%09%09PortAudioDirectSound%0A%09%09PortAudioWASAPI%0A%09%09PortAudioWDMKS%0A%09%7D%0A%0A%09PortAudioMME-%3EMME%0A%09PortAudioDirectSound-%3EDirectSound%0A%09PortAudioWASAPI-%3EWASAPIShared%0A%09PortAudioWASAPI-%3EWASAPIExclusive%0A%09PortAudioWDMKS-%3EWDMKS%0A%0A%09MME-%3ESampleRateConversion%0A%09DirectSound-%3ESampleRateConversion%0A%09SampleRateConversion-%3EWASAPIShared%0A%09%0A%09WASAPIShared-%3EPreMix%0A%09WASAPIExclusive-%3EHardwareDriver%0A%09PreMix-%3EMix%0A%09Mix-%3EPostMix%0A%09PostMix-%3EHardwareDriver%0A%09%0A%09WDMKS-%3EHardwareDriver%0A%09%0A%09HardwareDriver-%3EHardwareDevice%0A%7D%0A
+<!-- Use the converter at http://gravizo.com/ to recover the source code of this
+graph. -->
+[diagram]: https://g.gravizo.com/svg?digraph%20G%20%7B%0A%09rankdir%3D%22LR%22%0A%09style%3D%22dashed%22%0A%09fontname%3D%22sans-serif%22%0A%09node%5Bfontname%3D%22sans-serif%22%5D%0A%0A%09subgraph%20clusterApplicationProcess%20%7B%0A%09%09label%3D%22Application%20process%22%0A%0A%09%09Host%5Blabel%3D%22ASIO%20host%20application%22%5D%0A%0A%09%09subgraph%20clusterFlexASIO%20%7B%0A%09%09%09label%3D%22FlexASIO%22%0A%09%09%09FlexASIO%5Blabel%3D%22ASIO%20driver%22%5D%0A%0A%09%09%09subgraph%20clusterPortAudio%20%7B%0A%09%09%09%09label%3D%22PortAudio%22%0A%0A%09%09%09%09PortAudio%5Blabel%20%3D%20%22Frontend%22%5D%0A%09%09%09%09subgraph%20%7B%0A%09%09%09%09%09rank%3D%22same%22%0A%09%09%09%09%09node%20%5Bcolor%3D%22red%22%3B%20penwidth%3D3%5D%0A%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20PortAudioMME%5Blabel%3D%22MME%22%5D%0A%09%09%09%09%09PortAudioDirectSound%5Blabel%3D%22DirectSound%22%5D%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20PortAudioWASAPI%5Blabel%3D%22WASAPI%22%5D%0A%09%09%09%09%09PortAudioWDMKS%5Blabel%3D%22WDM-KS%22%5D%0A%09%09%09%09%7D%0A%09%09%09%7D%0A%09%09%7D%0A%09%7D%0A%0A%09subgraph%20clusterWindows%20%7B%0A%09%09label%3D%22Windows%20audio%20subsystem%22%0A%09%09subgraph%20%7B%0A%09%09%09rank%3D%22same%22%0A%09%09%09MME%0A%09%09%09DirectSound%0A%09%09%09%0A%09%09%7D%0A%09%09subgraph%20%7B%0A%09%09%09rank%3D%22same%22%0A%09%09%09WASAPIShared%5Blabel%3D%22WASAPI%20(shared)%22%5D%0A%09%09%09WASAPIExclusive%5Blabel%3D%22WASAPI%20(exclusive)%22%5D%0A%09%09%09WDMKS%5Blabel%3D%22Kernel%20Streaming%22%5D%0A%09%09%7D%0A%0A%09%09PreMix%5Blabel%3D%22Pre-mix%20APOs%22%5D%0A%09%09Mix%5Blabel%3D%22Mixing%22%5D%0A%09%09PostMix%5Blabel%3D%22Post-mix%20APOs%22%5D%0A%09%7D%0A%0A%09subgraph%20clusterHardware%20%7B%0A%09%09label%3D%22Audio%20hardware%22%0A%09%09HardwareDriver%5Blabel%3D%22Driver%22%5D%0A%09%09HardwareDevice%5Blabel%3D%22Device%22%5D%0A%09%7D%0A%0A%09Host-%3EFlexASIO%0A%09FlexASIO-%3EPortAudio%0A%0A%09PortAudio-%3E%7B%0A%09%09PortAudioMME%0A%09%09PortAudioDirectSound%0A%09%09PortAudioWASAPI%0A%09%09PortAudioWDMKS%0A%09%7D%0A%0A%09PortAudioMME-%3EMME%0A%09PortAudioDirectSound-%3EDirectSound%0A%09PortAudioWASAPI-%3EWASAPIShared%0A%09PortAudioWASAPI-%3EWASAPIExclusive%0A%09PortAudioWDMKS-%3EWDMKS%0A%0A%09MME-%3EWASAPIShared%0A%09DirectSound-%3EWASAPIShared%0A%09%0A%09WASAPIShared-%3EPreMix%0A%09WASAPIExclusive-%3EHardwareDriver%0A%09PreMix-%3EMix%0A%09Mix-%3EPostMix%0A%09PostMix-%3EHardwareDriver%0A%09%0A%09WDMKS-%3EHardwareDriver%0A%09%0A%09HardwareDriver-%3EHardwareDevice%0A%7D%0A

CONFIGURATION.md

@@ -320,6 +320,42 @@ wasapiExclusiveMode = true
 
 The default behaviour is to open the stream in *shared* mode.
 
+#### Option `wasapiAutoConvert`
+
+*Boolean*-typed option that determines if WASAPI Shared is allowed to convert
+the sample rate and channel count of the audio stream.
+
+This option is ignored if the backend is not WASAPI. See the
+[`backend` option][backend]. Furthermore, it is only effective when WASAPI is
+used in *Shared mode*; WASAPI never converts *Exclusive mode* streams. See the
+[`wasapiExclusiveMode` option][wasapiExclusiveMode].
+
+If set to `true`, WASAPI will automatically convert the stream's sample rate
+and channel count (upmixing/downmixing) if it doesn't match the *shared format*,
+i.e. the format configured in the Windows audio control panel for that device.
+
+If set to `false`, WASAPI will not do any sample rate and channel count
+conversions and will only accept streams whose sample rate and channel count
+match the one configured in the Windows audio control panel for that device. If
+the sample rate or channel count don't match, FlexASIO will fail to initialize.
+Note that WASAPI Shared might still do additional processing besides sample rate
+and channel count conversion (e.g. sample format conversions, mixing, APOs).
+
+Example:
+
+```toml
+backend = "Windows WASAPI"
+
+[output]
+wasapiAutoConvert = false
+```
+
+The default behaviour is to allow conversions.
+
+(Note: as explained in [BACKENDS][], in modern versions of Windows, DirectSound
+and MME use WASAPI Shared behind the scenes, and they implicitly enable the same
+automatic conversion mechanism as the one this option controls.)
+
 [backend]: #option-backend
 [BACKENDS]: BACKENDS.md
 [bufferSizeSamples]: #option-bufferSizeSamples
@@ -334,3 +370,4 @@ The default behaviour is to open the stream in *shared* mode.
 [suggestedLatencySeconds]: #option-suggestedLatencySeconds
 [TOML]: https://en.wikipedia.org/wiki/TOML
 [WASAPI]: BACKENDS.md#wasapi-backend
+[wasapiExclusiveMode]: #option-wasapiExclusiveMode

FAQ.md

@@ -40,7 +40,18 @@ settings. Here are some common issues:
 
  - **Invalid values for configuration options** (e.g. wrong type, typos in
    backend names or device names) will result in initialization failures.
- - **When using WASAPI Shared…**
+ - **When using an exclusive backend (i.e. WASAPI Exclusive, WDM-KS)…**
+   - The **sample rate** selected in the ASIO Host Application must be natively
+     supported by the hardware audio device.
+   - The **channel count** that FlexASIO is configured to use must be natively
+     supported by the hardware audio device.
+ - **WDM-KS will fail to initialize if the selected device is already in use**
+   by any other application, even if no audio is actually playing. This means
+   that WDM-KS is unlikely to initialize successfully on the Windows default
+   devices; this can be worked around using the
+   [`device` configuration option][device].
+ - **When using WASAPI Shared with the [`wasapiAutoConvert` configuration
+   option][wasapiAutoConvert] disabled…**
    - **Only one sample rate is supported**: the one configured in the Windows
      audio device settings for the input *and* output devices.
      - If the ASIO Host Application uses any other sample rate, FlexASIO will
@@ -52,17 +63,6 @@ settings. Here are some common issues:
      audio device settings for the selected device.
      - FlexASIO will fail to initialize if it is configured to use any other
        channel count.
-   - These limitations are [inherent to WASAPI itself][wasapisr].
- - **When using an exclusive backend (i.e. WASAPI Exclusive, WDM-KS)…**
-   - The **sample rate** selected in the ASIO Host Application must be natively
-     supported by the hardware audio device.
-   - The **channel count** that FlexASIO is configured to use must be natively
-     supported by the hardware audio device.
- - **WDM-KS will fail to initialize if the selected device is already in use**
-   by any other application, even if no audio is actually playing. This means
-   that WDM-KS is unlikely to initialize successfully on the Windows default
-   devices; this can be worked around using the
-   [`device` configuration option][device].
  - A **FlexASIO (or PortAudio) bug**. If you believe that is the case, please
    [file a report][report].
    - In particular, please do file a report if FlexASIO fails to initialize with
@@ -231,4 +231,4 @@ wasapiExclusiveMode = true
 [report]: README.md#reporting-issues-feedback-feature-requests
 [sampleType]: CONFIGURATION.md#option-sampleType
 [suggestedLatencySeconds]: CONFIGURATION.md#option-suggestedLatencySeconds
-[wasapisr]: https://docs.microsoft.com/windows/desktop/CoreAudio/device-formats
+[wasapiAutoConvert]: CONFIGURATION.md#option-wasapiAutoConvert

src/flexasio/FlexASIO/config.cpp

@@ -100,6 +100,7 @@ namespace flexasio {
 			SetOption(table, "sampleType", stream.sampleType);
 			SetOption(table, "suggestedLatencySeconds", stream.suggestedLatencySeconds, ValidateSuggestedLatency);
 			SetOption(table, "wasapiExclusiveMode", stream.wasapiExclusiveMode);
+			SetOption(table, "wasapiAutoConvert", stream.wasapiAutoConvert);
 		}
 
 		void SetConfig(const toml::Table& table, Config& config) {

src/flexasio/FlexASIO/config.h

@@ -15,6 +15,7 @@ namespace flexasio {
 			std::optional<std::string> sampleType;
 			std::optional<double> suggestedLatencySeconds;
 			bool wasapiExclusiveMode = false;
+			bool wasapiAutoConvert = true;
 		};
 		Stream input;
 		Stream output;

src/flexasio/FlexASIO/flexasio.cpp

@@ -521,6 +521,10 @@ namespace flexasio {
 				if (config.input.wasapiExclusiveMode) {
 					input_wasapi_stream_info.flags |= paWinWasapiExclusive;
 				}
+				Log() << (config.input.wasapiAutoConvert ? "Enabling" : "Disabling") << " auto-conversion for input WASAPI stream";
+				if (config.input.wasapiAutoConvert) {
+					input_wasapi_stream_info.flags |= paWinWasapiAutoConvert;
+				}
 				input_parameters.hostApiSpecificStreamInfo = &input_wasapi_stream_info;
 			}
 		}
@@ -545,6 +549,10 @@ namespace flexasio {
 				if (config.output.wasapiExclusiveMode) {
 					output_wasapi_stream_info.flags |= paWinWasapiExclusive;
 				}
+				Log() << (config.output.wasapiAutoConvert ? "Enabling" : "Disabling") << " auto-conversion for output WASAPI stream";
+				if (config.output.wasapiAutoConvert) {
+					output_wasapi_stream_info.flags |= paWinWasapiAutoConvert;
+				}
 				output_parameters.hostApiSpecificStreamInfo = &output_wasapi_stream_info;
 			}
 		}