PsychPortAudio(‘Open’)

Psychtoolbox>PsychPortAudio.{mex*} subfunction

pahandle = PsychPortAudio(‘Open’ [, deviceid][, mode][, reqlatencyclass][, freq][, channels][, buffersize][, suggestedLatency][, selectchannels][, specialFlags=0]);

Open a PortAudio audio device and initialize it. Returns a ‘pahandle’ device
handle for the device.

On most operating systems you can open each physical sound device only once per
running session. If you feel the need to call ‘Open’ multiple times on the same
audio device, read the section about slave devices and the help ‘PsychPortAudio
OpenSlave?’ instead for a suitable solution.
All parameters are optional and have reasonable defaults. ‘deviceid’ Index to
select amongst multiple logical audio devices supported by PortAudio. Defaults
to whatever the systems default sound device is. Different device id’s may
select the same physical device, but controlled by a different low-level sound
system. E.g., Windows has about five different sound subsystems. ‘mode’ Mode of
operation. Defaults to 1 == sound playback only. Can be set to 2 == audio
capture, or 3 for simultaneous capture and playback of sound. Note however that
mode 3 (full duplex) does not work reliably on all sound hardware. On some
hardware this mode may crash hard! There is also a special monitoring mode == 7,
which only works for full duplex devices when using the same number of input-
and outputchannels. This mode allows direct feedback of captured sounds back to
the speakers with minimal latency and without involvement of your script at all,
however no sound can be captured during this time and your code mostly doesn’t
have any control over timing etc.
You can also define a audio device as a master device by adding the value 8 to
mode. Master devices themselves are not directly used to playback or capture
sound. Instead one can create (multiple) slave devices that are attached to a
master device. Each slave can be controlled independently to playback or record
sound through a subset of the channels of the master device. This basically
allows to virtualize a soundcard. See help for subfunction ‘OpenSlave’ for more
info.
‘reqlatencyclass’ Allows to select how aggressive PsychPortAudio should be about
minimizing sound latency and getting good deterministic timing, i.e. how to
trade off latency vs. system load and playing nicely with other sound
applications on the system. Level 0 means: Don’t care about latency or timing
precision. This mode works always and with all settings, plays nicely with other
sound applications. Level 1 (the default) means: Try to get the lowest latency
that is possible under the constraint of reliable playback, freedom of choice
for all parameters and interoperability with other applications. Level 2 means:
Take full control over the audio device, even if this causes other sound
applications to fail or shutdown. Level 3 means: As level 2, but request the
most aggressive settings for the given device. Level 4: Same as 3, but fail if
device can’t meet the strictest requirements. ‘freq’ Requested playback/capture
rate in samples per second (Hz). Defaults to a value that depends on the
requested latency mode. ‘channels’ Number of audio channels to use, defaults to
2 for stereo. If you perform simultaneous playback and recording, you can
provide a 2 element vector for ‘channels’, specifying different numbers of
output channels and input channels. The first element in such a vector defines
the number of playback channels, the 2nd element defines capture channels. E.g.,
[2, 1] would define 2 playback channels (stereo) and 1 recording channel. See
the optional ‘selectchannels’ argument for selection of physical device channels
on multi- channel cards.
‘buffersize’ requested size and number of internal audio buffers, smaller
numbers mean lower latency but higher system load and some risk of overloading,
which would cause audio dropouts. ‘suggestedLatency’ optional requested latency
in seconds. PortAudio selects internal operating parameters depending on
sampleRate, suggestedLatency and buffersize as well as device internal
properties to optimize for low latency output. Best left alone, only here as
manual override in case all the auto-tuning cleverness fails.
‘selectchannels’ optional matrix with mappings of logical channels to device
channels: If you only want to use a subset of the channels present on your sound
card, e.g., only 2 playback channels on a 16 channel soundcard, then you’d set
the ‘channels’ argument to 2. The ‘selectchannels’ argument allows you to
select, e.g., which two of the 16 channels to use for playback.
‘selectchannels’ is a one row by ‘channels’ matrix with mappings for pure
playback or pure capture. For full-duplex mode (playback and capture),
‘selectchannels’ must be a 2 rows by max(channels) column matrix. row 1 will
define playback channel mappings, whereas row 2 will then define capture channel
mappings. In any case, the number in the i’th column will define which physical
device channel will be used for playback or capture of the i’th PsychPortAudio
channel (the i’th row of your sound matrix). Numbering of physical device
channels starts with zero! Example: Both, playback and simultaneous recording
are requested and ‘channels’ equals 2, ie, two playback channels and two capture
channels. If you’d specify ‘selectchannels’ as [0, 6 ; 12, 14], then playback
would happen to device channels zero and six, sound would be captured from
device channels 12 and 14. Please note that channel selection is currently only
supported on some sound cards. The parameter is silently ignored on non-capable
hardware or driver software.

‘specialFlags’ Optional flags: Default to zero, can be or’ed or added together
with the following flags/settings:
1 = Never prime output stream. By default the output stream is primed. Don’t
bother if you don’t know what this means.
2 = Always clamp audio data to the valid -1.0 to 1.0 range. Clamping is enabled
by default.
4 = Never clamp audio data.
8 = Always dither output audio data. By default, dithering is enabled in normal
mode, and disabled in low-latency mode. Dithering adds a stochastic noise
pattern to the least significant bit of each output sample to reduce the impact
of audio quantization artifacts. Dithering can improve signal to noise ratio and
quality of output sound, but it is more compute intense and it could change very
low-level properties of the audio signal, because what you hear is not exactly
what you specified.
16 = Never dither audio data, not even in normal mode.

###See also: Close GetDeviceSettings