Psychtoolbox>PsychPortAudio.{mex*} subfunction

Change the current settings for the “direct input monitoring” feature on device
The device must be open for this setting to take effect. Changed settings may or
may not persist across closing and opening the device, this is hardware
dependent and not to be relied on.
So-called “Zero latency direct input monitoring” is a hardware feature of some
modern (and usually higher end) soundcards. It allows to directly feed audio
signals that are received at the audio input connectors of the soundcard back to
the output connectors, without any extended intermediate processing of the audio
signals by either the sound hardware or the host computer and its software. Due
to this direct signal path, which only applies selectable amplification and some
stereo panning and rerouting, the feedback latency from input to output (e.g,
microphone to speakers) is as minimal as technically possible. On many high-end
cards it is instantaneous!

The ‘enable’ flag is mandatory: If set to zero, monitoring will be disabled for
the given ‘inputChannel’. A setting of one will enable input live-monitoring of
the given ‘inputChannel’ to the given ‘outputChannel’ with the selected other
All following settings are optional and have reasonable defaults. Depending on
your hardware, some or all of them may be silently ignored by your sound
The optional ‘inputChannel’ argument specifies which input audio channels
monitoring settings should be modified. If omitted or set to -1, all input
channels settings will be modified, or at least tried to be modified.
The optional ‘outputChannel’ specifies the index of the base-channel of a
channel stereo-pair to which the ‘inputChannel’ should be routed. It must be an
even number like 0, 2, 4, …. If omitted, channel 0, i.e., the first output
channel stereo pair will be used. This at least on ASIO soundcards under
The optional ‘gainLevel’ defines the desired amplifier gain for the routed
signal. The value should be negative for signal attenuation (i.e., negative
gain) and positive for amplification (i.e., positive gain). As specification of
gains is not standardized across operating systems, the numbers you’ll have to
pass in for a desired effect will vary across operating systems and audio
hardware. However, the default setting of zero tries to set a neutral gain of
zero decibel - the signal is passed through without change in intensity. On
MS-Windows with ASIO soundcards, values between 0.0 and 1.0 will select gains
between 0 and 12 dB. Values between 0.0 and -1.0 will select negative gains
between 0 and -infinity dB, ie., full attenuation. The setting may get
completely ignored or only approximately implemented by given hardware.
Double-check your results!
The optional ‘stereoPan’ parameter allows to select panning between the two
output channels of a selected stereo output channel pair if the hardware allows
that. Range 0.0 - 1.0 selects between left-channel and right channel, with the
default of 0.5 selecting a centered output with equal distribution to both

In the optional return argument ‘result’ the function returns a status code to
report if the requested change could be carried out successfully. A value of
zero means success. A value of 1 means some error, e.g., invalid parameters
specified. A value of 2 means that your combinatin of operating system, sound
system, soundcard device driver and soundcard hardware does not support direct
input monitoring, at least not for the given configuration. A setting of 3 means
that your PortAudio driver plugin does not support the feature - You may need to
update your plugin from the Psychtoolbox Wiki.

The current PsychPortAudio driver supports direct input monitoring on MacOS/X
with certain sound hardware (but not the builtin sound) and on Microsoft Windows
systems with ASIO-2.0 capable sound hardware, and only if the latest
portaudio_x86.dll ASIO plugin is installed from our Wiki. Even then, only a
subset of ASIO-2 hardware may support this feature and only a subset of these
may support all parameters. According to vendor documentation, some soundcards
from Creative Labs and many of RME’s cards do support this feature.

###See also: Open GetDeviceSettings