>Psychtoolbox>PsychDemos

DelayedSoundFeedbackDemo([reqlatency=150 ms][, duplex=0][, freq]=48000[, minLatency=10 ms][, device][, channels=2])

CAUTION: TEST TIMING OF THIS SCRIPT WITH MEASUREMENT EQUIPMENT IF YOU

DEPEND ON ACCURATE FEEDBACK TIMING!!!

Demonstrates usage of the new Psychtoolbox sound driver PsychPortAudio()
for audio feedback with a controlled delay.

Sound is captured from the default recording device and then - with a
selectable delay - played back via the default output device.

Parameters and their meaning:

‘reqlatency’ Wanted feedback latency between sound input and output in
milliseconds. A value of zero will ask for the lowest possible latency on
the given setup. Defaults to 150 msecs. Please notice that the minimum
achievable latency will be constrained by the capabilities of your
operating system, sound card driver, computer hardware and sound
hardware. Only very high quality systems will be able to go below 5 msecs
latency, good systems will be able to go below 20 msecs, but less capable
setups may only allow for a latency much larger than 20 msecs. In order
to achieve low latency reliably without timing glitches or audible
artifacts, you may need to tune both the parameters for this demo and
your system setup carefully. The optimal parameter set varies from setup
to setup.

‘duplex’ = Select between full-duplex and half-duplex mode:

Depending on your sound hardware you’ll have to either leave ‘duplex’ at
its default of zero (2 times half-duplex mode, aka simplex mode) or set
it to 1 (full-duplex mode). On a given system, only one of these will work
reliably (or at all):
On OSX it depends on the sound hardware. IntelMacs are happy with half-
duplex mode, some PowerMacs may need full-duplex mode. However, except for
‘reqlatency’ == 0 minimal latency mode, simplex mode provides much higher
accuracy and reliability on OSX at least with the built-in soundchips on
Intel based Macintosh computers. On Linux, performance varies depending on
the card at use.

‘freq’ = Sampling frequency (Hz). Defaults to auto-detect. The maximum achievable
value depends on your specific soundcard. Intel Mac’s built in soundchips
allow for a maximum of 96000 Hz, high-end soundcards may allow for 192000
Hz under some circumstances. Increasing the frequency reduces minimum
latency but increases system load and the probability of glitches.

‘minLatency’ is a tuning parameter for the driver and a hard-constraint
on the mininum achievable latency for feedback. It is ignored on OSX,
but can be tused for tuning latency vs. reliability on Linux and on
MS-Windows. High-end cards may allow for much lower than the default 10
msecs, low-end cards may malfunction at lower settings.

‘device’ Optional device index of soundcard to use.

‘channels’ Number of input and output channels to use. Defaults to 2.

Specific tips for different setups:

On OSX with builtin sound chip on Intel Macs, choose duplex = 0 for
feedback with controlled low-latency, and a freq’ency of 96000 Hz. For
lowest latency mode, you may try reqlatency = 0 and duplex = 1.

On MS-Windows use reqlatency = 0 for feedback with minimal latency, positive
values for feedback with controlled latency. Play around with the ‘minLatency’
parameter, set it as low as possible - to the lowest value that doesn’t cause
any error messages by our driver or audible artifacts like crackling noises or
static. Try to set ‘freq’uency as high as possible. Check the manual of your
soundcard for the highest value that can be used for capture + playback. E.g.,
the Soundblaster Audigy ZS 2 seems to be limited to max. 48000 Hz in this
mode.

On Linux, no general statements can be made, only that some soundcards
allow for extremely low latencies of < 2 msecs if properly tuned. Search
the Internet for tips.

If you need low-latency, make sure to read “help InitializePsychSound”
carefully or contact the forum.