RaspberryPiSetup – How to setup your Pi optimally for Psychtoolbox.

There are two major families of the RaspberryPi with features suitable for
meaningful use for visual and auditory stimulation with Psychtoolbox-3, the
older variants RaspberryPi 2 and 3, and the new RaspberryPi 4/400 family.

Psychtoolbox is currently only tested with up to date versions of the official
RaspberryPi Linux operating system distribution known as “RaspberryPi OS”, also
known under its former name “Raspbian”. Only 32-Bit Raspbian is supported by
Psychtoolbox as provided directly by us via DownloadPsychtoolbox et al., even
on 64-Bit processors. Whatever is the most recent stable Raspbian + GNU/Octave
is what PTB will support, with some jiggle room, as we do not keep close track
of development an updates to Raspbian. Raspbian itself also provides versions
of Psychtoolbox, installable via “sudo apt install octave-psychtoolbox-3”, and
64-Bit versions of Raspbian would ship such packages as 64-Bit packages. Other
Linux distributions for RaspberryPi, like Ubuntu for RaspberryPi, will also
ship Psychtoolbox packages as 32-Bit or 64-Bit variants via the sudo apt install
octave-psychtoolbox-3 method. While these packages will likely work just fine,
these are currently not tested by the Psychtoolbox developers, or supported in
case of questions or problems. These packages are maintained by the NeuroDebian
maintainers, and they are usually quite a bit out of date wrt. the current PTB
beta releases, as the Linux distributions usually ship whatever PTB was the most
recent version at a time sometime before the distributions was released. Those
packages do not get updates after initial distribution release, not even bug
fixes. E.g., Raspbian at early 2021 ships some Psychtoolbox 3.0.15 variant,
whereas PTB betas are now at version 3.0.17 or later.

The RaspberryPi 2B is tested for compatibility with Psychtoolbox. The test system
is a RaspberryPi 2B with 1 GB of RAM. Other models of RaspberryPi 2 and 3 are
expected to work equally well (or better for the more powerful RaspberryPi 3),
but not tested. It provides:

  • 40 pin digital i/o, e.g., for TTL trigger emission or reception, hardware
    interfacing etc. See RaspberryPiGPIODemo.m for how to use it.

  • Working stereo audio output with ok latency and good timing via the headphone
    jack. Tested and verified by use of a VPixx Datapixx. Working audio output or
    input via UAC compliant USB audio cards.

  • Working video capture with external USB connected web cams.

  • Single display support via HDMI for up to 1920x1080 pixels full HD, with 8 bit
    of color precision, for standard 24 bit color depth / 16.8 Millions of colors,
    or 256 levels of red, green, blue, grey. No support for floating point framebuffers
    or textures, and thereby no support for stimulus drawing or post-processing
    which would require such buffers.

The RaspberryPi 4, and its keyboard form-factor variant RaspberryPi 400, apart
from providing more RAM and faster processor and graphics, do support:

  • 40 pin digital i/o, e.g., for TTL trigger emission or reception, hardware
    interfacing etc. See RaspberryPiGPIODemo.m for how to use it. Untested so far,
    but should work the same as on the RaspberryPi 2 and 3.

  • Presumably working stereo output like the RPi 2 and 3 on models with headphone
    jack. This is untested by us so far, but reasonable to assume it is working.

  • Audio output via HDMI audio: This is tested to be working for video playback
    with the Chrome webbrowser, and with Psychtoolbox built-in [GStreamer](GStreamer) based
    movie/video/audio playback engine. You can play back movies with sound, or
    sound files in common formats like .wav or .mp3 or .ogg audio etc., whatever
    is supported by [GStreamer](GStreamer’s collection of plugins.

    CAUTION: Audio output over HDMI via our PsychPortAudio driver currently does
    not work at all! Audio output over headphone jacks - if any - is supposed to
    work, but not tested. Working audio output or input via UAC compliant USB audio

  • Working video capture with external USB connected web cams.

  • Dual-display support via two micro-HDMI connectors up to 4k UHD resolution.
    So far tested is single-display operation, due to lack of a 2nd adapter cable.

    The new VideoCore-6 gpu has slightly higher performance than the older VideoCore
    4 gpu in RPi 2 and 3. More importantly, the gpu supports full 32 bit floating
    point framebuffers and textures, with the restriction that framebuffer blending/
    alpha-blending is only supported on 16 bit floating point framebuffers. This
    opens up many new use cases for more complex visual stimuli, e.g., it makes
    AdditiveBlendingForLinearSuperpositionTutorial.m demo work, as well as
    MorphDemo and MorphTextureDemo, FDFDemo, GarboriumDemo and ProceduralGarboriumDemo
    and various other complex shader-based or alpha-blending based stimuli. It also
    allows to take full advantage of high-end visual stimulators and display devices
    from VPixx, if you are inclined enough to drive a multi-thousand dollar stimulator
    with a 50$ microcomputer. Ofc. while accuracy is excellent, performance is more
    limited than with a desktop or laptop graphics card from Intel, AMD or NVidia.
    Testing showed that stimuli generated without use of alpha-blending can be output
    at the full 16 bit or 14 bit precision of VPixx or CRS visual stimulators, while
    stimuli created via alpha blending with 16 bit floating point framebuffers can
    achieve up to 11 bit precision, depending on specific stimulus.

    Movie playback of full HD video at 24 fps is possible with suitably encoded
    movies and proper playback settings. For playback of H264 encoded movies, the
    builtin H264 hardware decoder of the RaspberryPi 4 is used. You must specify
    an explicit ‘pixelFormat’ of 6 in Screen(‘OpenMovie’, …., pixelFormat); for
    this to work at good performance for high resolution movies. At default settings
    movie playback would be very slow, e.g., 1-2 fps for full HD content. Additionally,
    the H264 hardware decoder is picky wrt. proper content encoding. While footage
    e.g., from YouTube or other professionally encoded sources works, badly encoded
    footage may not. Regrettably our own included demo movie DualDiscs.mov, as used
    by SimpleMovieDemo and PlayMoviesDemo by default, does not work at all, the
    decoder chokes on it. Many other movie formats play back with software decoding,
    at potentially lower performance. To summarize: Good performance playback of HD
    content at good framerates works if you do everything right wrt. playback parameters
    and encoding. Otherwise your mileage may vary both wrt. performance and reliability.

  1. Update your Raspbian to the latest version “sudo apt update” and “sudo apt upgrade”.

  2. If you are on a RaspberryPi 4 or 400, make sure the Linux kernel is at least of
    version 5.10 or later. If “uname -a” reports a lower version, upgrade the kernel
    via “sudo rpi-update” and some reboot. This step is not needed for the RPi 2 and 3.

  3. Switch from the firmware kms video driver to the proper fully open-source kms
    video driver. For this, edit the file /boot/config.txt - the section at the end,
    then reboot. The section should look like this on a RaspberryPi 4 / 400:

    # Enable DRM VC4 V3D driver on top of the dispmanx display stack


    You could increase gpu_mem to numbers larger than 256 MB if you run into
    graphics performance trouble or other glitches. Important for proper visual
    stimulation with proper timing and reliability is that the dtoverlay parameter is
    set to vc4-kms-v3d-pi4 instead of vc4-fkms-v3d.

    On a RaspberryPi 2 or 3, the dtoverlay parameter should be vc4-kms-v3d instead
    of vc4-fkms-v3d.

  4. On the RaspberryPi 4 / 400 with editions of Raspbian / RaspberryPi OS older than
    February 2021, you should either update your OS to the latest, and reboot, or run
    PsychLinuxConfiguration, XOrgConfCreator, and XOrgConfSelector to install a custom
    xorg.conf file, which will work around a bug in Raspbian. Reboot! If the bug is
    present and this workaround from step 4 is not applied, Psychtoolbox will complain
    angrily about “pageflipping not being used for flips”, visual stimulation timing
    will be grossly wrong, and you will observe massive tearing artifacts, flicker and
    other stimulus anomalies.

With all setup steps performed properly, and a reboot for good measure, visual
stimulation should work very well, with robust, trustworthy and sub-millisecond
accurate visual stimulus onset timestamps, as verfied by us with external measurement

In general, the RaspberryPi microcomputers are quite capable devices for neuroscience
testing, even for many vision science paradigms. Their builtin gpio support makes them
especially nifty if one needs to receive or send TTL triggers or other i/o.

Further support for improvements to Psychtoolbox wrt. RaspberryPi devices, and
user support will be contingent on labs financially supporting our work. Until
then, consider this work being done on a low effort basis.

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