Preparing your stimuli

Audio and video files

Most experiments will involve using audio and/or video files! You are responsible for hosting these somewhere (contact MIT if you need help finding a place to put them).

For basic editing of audio files, if you don’t already have a system in place, we highly recommend Audacity. You can create many “tracks” or select portions of a longer recording using labels, and export them all at once; you can easily adjust volume so it’s similar across your stimuli; and the simple “noise reduction” filter works well.

File formats

To have your media play properly across various web browsers, you will generally need to provide multiple file formats. For a comprehensive overview of this topic, see MDN.

MIT’s standard practice is to provide mp3 and ogg formats for audio, and webm and mp4 (H.264 video codec + AAC audio codec) for video, to cover modern browsers. The easiest way to create the appropriate files, especially if you have a lot to convert, is to use the command-line tool ffmpeg. It’s a bit of a pain to get used to, but then you can do almost anything you might want to with audio and video files.

Here’s an example command to convert a video file INPUTPATH to mp4 with reasonable quality/filesize and using H.264 & AAC codecs:

ffmpeg -i INPUTPATH -c:v libx264 -preset slow -b:v 1000k -maxrate 1000k -bufsize 2000k -c:a libfdk_aac -b:a 128k

And to make a webm file:

ffmpeg -i INPUTPATH -c:v libvpx -b:v 1000k -maxrate 1000k -bufsize 2000k -c:a libvorbis -b:a 128k -speed 2

Converting all your audio and video files can be easily automated in python. Here’s an example script that uses ffmpeg to convert all the m4a and wav files in a directory to mp3 and ogg files:

import os
import subprocess as sp
import sys

audioPath = '/Users/kms/Dropbox (MIT)/round 2/ingroupobligations/lookit stimuli/audio clips/'

audioFiles = os.listdir(audioPath)

for audio in audioFiles:
    (shortname, ext) = os.path.splitext(audio)
    print shortname
    if not(os.path.isdir(os.path.join(audioPath, audio))) and ext in ['.m4a', '.wav']:
        sp.call(['ffmpeg', '-i', os.path.join(audioPath, audio), \
               os.path.join(audioPath, 'mp3', shortname + '.mp3')])
        sp.call(['ffmpeg', '-i', os.path.join(audioPath, audio), \
               os.path.join(audioPath, 'ogg', shortname + '.ogg')])

Directory structure

For convenience, several of the newer frames allow you to define a base directory (baseDir) as part of the frame definition, so that instead of providing full paths to your stimuli (including multiple file formats) you can give relative paths and specify the audio and/or video formats to expect (audioTypes and videoTypes).

Images: Anything without :// in the string is assumed to be a relative image source.

Audio/video sources: you will be providing a list of objects describing the source, like this:

[
    {
        "src": "http://stimuli.org/myAudioFile.mp3",
        "type": "audio/mp3"
    },
    {
        "src": "http://stimuli.org/myAudioFile.ogg",
        "type": "audio/ogg"
    }
]

Instead of listing multiple sources, which are generally the same file in different formats, you can alternately list a single source like this:

[
    {
        "stub": "myAudioFile"
    }
]

If you use this option, your stimuli will be expected to be organized into directories based on type.

• baseDir/img/: all images (any file format; include the file format when specifying the image path)
• baseDir/ext/: all audio/video media files with extension ext

Example: Suppose you set baseDir: 'http://stimuli.org/mystudy/ and then specified an image source as train.jpg. That image location would be expanded to http://stimuli.org/mystudy/img/train.jpg. If you specified that the audio types you were using were mp3 and ogg (the default) by setting audioTypes: ['mp3', 'ogg'], and specified an audio source as [{"stub": "honk"}], then audio files would be expected to be located at http://stimuli.org/mystudy/mp3/honk.mp3 and http://stimuli.org/mystudy/ogg/honk.ogg.