I want to create a virtual audio device that gets audio data from the default output (which is an output IOAudioStream) and converts it to an input IOAudioStream.
I went through most of the examples I could find, however they only implement a feature to copy the output IOAudioStream to the input one at most. That means it only converts the audio to an input stream if the audio device is selected as output.
This should be possible, since ScreenFlow allows recording of computer audio by installing a kext that creates a virtual driver.
How can I access the audio data from the default output and send it to my virtual driver?
Take a look at the open-source WavTap, which is a simplified fork of the open-source SoundFlower virtual sound card driver. It is a .kext that I believe does substantially what you want.
For reference, here is how some popular commercial closed-source options work:
Rogue Amoeba's Audio Hijack Pro
-Captures system audio via code based off of the open-source SoundFlower .kext
-Captures an application's audio by substituting a "patch" framework for the normal CoreAudio.framework when launching the application
-Captures an already-running application's audio with the help of the haxie "Application Enhancer" (APE) from Unsanity
These features are branded as their "Instant On" feature (InstantOn.kext).
Ambrosia Software's WireTap Studio
-Captures system audio and application audio via an in-house developed .kext
Telestream's ScreenFlow
-Captures system audio via an in-house developed .kext. (Version 2.x uses varaudio.kext; Version 3.x uses TelestreamAudio.kext)
Macsome's Audio Recorder
-Unknown method
Araelium Group's Screenflick
-Captures system audio using the SoundFlower .kext
UPDATE #1
After reading the author's comments, it appears the underlying goal is to be able to capture the system sound without publishing the virtual audio driver as a device (that would appear in the System Preference's list) and without changing the current default output device (or at least the appearance that the device has changed).
SoundFlower: Adds a sound device to the list upon installation
WavTap: Adds a sound device to the list upon installation; auto-selects the device when the WavTap application is started; auto-deselects the device when the application is shutdown and reselects the previous device
Audio Hijack Pro: Adds a sound device only when audio capture of the default system sound is selected; removes the sound device when audio capture is no longer selected and reselects the previous device
WireTap Studio: Unknown
ScreenFlow: Captures the system sound without changing the current default output device and without publishing the virtual audio driver as a device
UPDATE #2
A quote from Jeff Moore, a CoreAudio Apple engineer, in reference to applications such as WireTap and Audio Hijack Pro:
"There are no APIs on the system that will give you the output of any specific app or the whole mix going to the hardware...[Capturing System Sound] isn't supported by the System and those folks had to be clever. There's nothing stopping you from doing the same thing except how willing you are to get your hands dirty.
The fact is, Mac OS X's audio system was designed first and foremost for performance. This lead us to a design where it is not easy to support the functionality you want without imposing performance penalties. So, we have opted for better performance at the cost of not being able to provide this feature."
If you want to read more on the subject, check out these threads on the CoreAudio API mailing list:
"WireTap, CoreAudio's API, and system capture, and kexts..."
"Another question on capturing audio played back by a software"
"Capturing currently played audio using CoreAudio on Mac"
"'audio hijack'"
"monitoring system audio output like wire tap"
"Capturing audio output to a file"
"Mirroring Audio Output"
"Recording system audio"
Relevant SO Questions:
Hide Audio device using codeless kext
So long story short, you're not likely to find examples from Apple that accomplish this, and you're not likely to find open source code that accomplishes this either, unless someone is feeling very generous. It appears to be too valuable of information.
After additional research, here are some theoretical techniques I came up with that might allow you to accomplish your goal:
Similar to Prosoft Engineering's Hear product, you could create a HAL plugin (user-mode virtual driver) rather than a .kext (kernel-mode virtual driver). Apple has a sample HAL plugin called "SampleHardwarePlugIn" and PulseAudio has one as well. However, with his method I don't think you get access to a pre-mixed system sound stream. You would have to gather up all streams from the various applications (which must use CoreAudio to play sound) and mix them together for pseudo system sound capture.
Create a virtual audio device that is hidden [1][2] from user interaction. When the user wishes to capture the default sound, programmatically create an aggregate device that includes your hidden virtual device and the current default sound device. Temporarily set this aggregate device as the default output. In this manner, you are able to both capture the default sound and hear it.
Side Note: If Mac OS X allows for a hidden device to also be set as the default output device, what would System Preferences show as the selected device? If it instead shows the secondary output device as selected, then you have the added allusion that nothing has changed.
A newer open-source virtual audio device that works with the latest versions of MacOS is BlackHole - it supports multiple audio channels and sampling rates.
It can be used as an audio sink and/or source. It's also handy as part of an aggregate audio device so audio can be heard and re-routed - e.g. using the MacOS Audio MIDI Setup app
Related
At 3:38-4:00 in the session video, it seems Baek San Chang says that AudioDriverKit will not be allowed to be used for virtual audio devices
Video: https://developer.apple.com/videos/play/wwdc2021/10190/
Here is what he says:
Keep in mind that the sample code presented is purely for
demonstrative purposes and creates a virtual audio driver that is not
associated with a hardware device, and so entitlements will not be
granted for that kind of use case.
For virtual audio driver, where device is all that is needed, the
audio server plugin driver model should continue to be used.
The mentioning of sample code is a little confusing; Does he mean the entitlements for hardware access won't be granted for a virtual device? That would seem obvious.
But if he means the entitlements for driver kit extensions (com.apple.developer.driverkit and com.apple.developer.driverkit.allow-any-userclient-access) won't be granted for virtual audio devices, and this is why AudioServerPlugins should still be used, then that's another story.
Are we allowed to use AudioDriverKit Extension for Virtual Devices?
The benefit of having the extension bundled with the app rather than requiring an installer is a significant reason to use an extension if allowed.
I need to create a virtual audio driver that presents a virtual microphone and a virtual speaker to the user. The user can then select these virtual endpoints in 3rd party audio communication apps like Skype, Zoom etc. The virtual audio driver implementation then routes audio between physical devices (selected by the user in the virtual driver userspace control app) and the virtual devices.
It is a requirement that the virtual audio driver and its control app can be published to the Apple app store for users to download and install on their machine without any problems.
How should I go about this?
How should I go about this?
Apply for the entitlements straight away (don't lie on your request form obviously), wait until AudioDriverKit is out of beta, then file a Developer Tech Support TSI and explain what you're trying to do and ask what the policy is. I haven't seen any written policy on this, and the information in the video may or may not be accurate.
Don't forget that you don't just need the entitlements; your virtual audio drivers will also need to pass App Store review, so I'd make sure to get something in writing before you spend all that effort implementing your driver.
One more comment: com.apple.developer.driverkit.allow-any-userclient-access is not generally needed, and whether or not you need to apply for it depends on the architecture you are planning for your driver.
I would like to know whether SoX/LibSoX offers the possibility to access a sound device in exclusive/hog mode. The idea is to prevent other applications from accessing the sound card / DAC that is being used by the focal app.
My main target is OSX CoreAudio output, but I am also eager to know about Linux (OSS/Alsa).
I know this is possible in CoreAudio, because I have seen it implemented in several apps, including this open source one.
On Mac OS X at least, the answer appears to be no. In http://sourceforge.net/p/sox/code/ci/master/tree/src/coreaudio.c SoX uses the default input or output device but there is no provision for hog mode.
I want to use the system's mic in/speaker out jacks to measure and fix voltage drops, but I cannot do this reliably without ensuring that my program has exclusive control of the audio. Is this possible in Windows XP? I have read about various API's that are available (WaveOut, Mixer) but have not found any information on taking exclusive control of an audio device before Windows Vista or 7.
Apparently the WDM kernel streaming API could possibly take exclusive control of the sound card, but this is undefined behavior. I might be able to work around by disabling the sound card by default in system settings, but this is less than an ideal solution (because it would still be possible for other applications to use the sound card).
Grabbing the system audio using 'Stereo Mix' is easy, but most modern drivers don't support that anymore. Steam local streaming somehow managed to do it without, i can clearly hear other apps audio output while streaming an arbitrary game.
How does steam do that? Did they silently install a driver? Is there any other way of grabbing system audio?
From Windows 7 and up that is.
From Vista and later you can use WASAPI. It allows to use loopback for any output device, just use an output as an input with the AUDCLNT_STREAMFLAGS_LOOPBACK flag for this function: http://msdn.microsoft.com/en-us/library/windows/desktop/dd370875(v=vs.85).aspx
A friend of mine and I got into a conversation and realized Windows 7 is missing a key component to its per-application audio settings. You can set volume but you can't stipulate which device each application should use.
Some applications such as ventrilo or skype allow you to select which device to use however, MOST applications simply rely on the current 'Default Audio Device.'
Is there a way to access this? What language would be best used to expose these kinds of functions? Thanks!
Ventrilo and Skype are able to choose which audio device to use because they are coded to directly specify audio output devices instead of just getting the default from the OS. For applications which are coded to use the default Windows device, you can of course change which device is the default device using the sound settings, but this will change the default for the whole system.
Setting different audio devices for separate applications which all use the default audio device isn't something that is necessarily supported by Windows, and many applications use the DirectSound API which complicates the situation further. However, some applications check which device is the default when they initialize and then output exclusively to that device. In this case, you could change the default sound device to one audio device, start a program, then change the default to another audio device, and the first program would continue to use the device which was default when it started up.
However, this is a pretty weak workaround and will only work for specific applications which have been coded in the way described above.