BRIGHAM YOUNG UNIVERSITY

  Humanities Technology and Research Support Center

Introduction to Audio Playback and Recording

As you would expect for a multimedia development environment, Revolution has a robust set of audio playback and recording tools. Let's look at the basic tools for playing and recording in common audio formats. This introduction will only scratch the surface of what is possible. Later when we look at player objects in depth we will discover many more capabilities.

Common Audio Formats

Revolution was designed to play back several of the most common audio formats. Since it was developed as a cross-platform development tool, the original author designed it to play the following formats natively; that is, without the need for a special player. These formats are WAVE, most common on the Windows platform, Audio Interchange File Format (AIFF or AIF) used most commonly on the Macintosh operating system, and Sun Microsystems' µ-law ("mu-law", usually denoted by a .au file extension), common in Unix-based operating systems. Note that with the exception of µ-law, Revolution will only play back uncompressed files of this type natively. Any other file types such as MP3 or compressed AIFF must be played using a player object.

Revolution can play these audio formats as imported audio clips or as external files. To import an audio file use the File menu > Import as Control > Audio Clip. Once imported, the clip becomes part of the stack file and will always be available for playback, regardless of where the stack resides. However, just as in the case of imported images, these audio clips can exponentially increase the size of the stack. Therefore you may find it more appropriate to leave the audio files as external files and play them back by referencing the file path.

The play command

Not surprisingly, the command to play audio files is play. It has the following basic syntax:

play {filePath | clipName} [looping] -- For playing a clip.

play stop -- For stopping playback.

If an audio clip has been imported you would play it back like this:

play mySound.aif

Playing an external audio file differs only in the requirement to specify the whole file path:

play "/Users/myuser/Desktop/mySound.aif" --Mac or Unix file path
play "c:/Documents and Settings/localuser/My Documents/mySound.wav" --Windows file path

Naturally, you can also set the defaultFolder property to give you the folder that contains the sound file, just as you can with external image or text files.

You can also play sound files from the web, assuming they are in one of the three allowed formats. Simply specify the URL following the play command:

play "http://chum210.byu.edu/resources/audio/kewl.wav"

If you want to play a clip repeatedly use the looping option, like this:

play mySound.aif looping

Stopping playback is equally straightforward, using the somewhat paradoxical form:

play stop

Remember that the play command only works with WAVE, µ-law (.au) and uncompressed AIFF files.

The sound function

There is a function called the sound that tells us if there is a currently playing audio clip. If there is an audio clip playing, the function returns the name of the clip. If there is no clip playing the function returns the constant done. Here are some examples:

If you are playing the audio clip "mySound.aif" and you check the sound function:

put the sound into currSndName

the value of the currSndName variable would be "mySound.aif". Once the clip was through playing the value of currSndName would be "done".

This means that you can use the sound function to prevent anything else from happening until the sound is through playing:

play "mySound.aif" 
wait until the sound is done
answer "The clip has finished playing." -- (or whatever you want to do here.) 

Recording audio files

Revolution also has a very easy-to-use audio recording capability. Of course, it depends upon the host computer having a sound card that supports audio input (virtually all Mac and Windows systems today do, as well as many Linux systems.) You must have a microphone connected to the computer, with the computer's sound input setting set to recognize the microphone.

The record command has the following basic syntax:

record sound file filePath

This command will start the recording process. The audio will be recorded to the file named in the filePath parameter. As expected, this will record to the defaultFolder if you only give a file name, or to the file path if you designate an absolute file path.

The record sound file command will keep recording forever, or until the hard drive runs out of memory, whichever comes first. So it's important to remember to stop the recording with the command:

stop recording

If you intend to play back this recording using the play command, you must also make sure the following two recording-related properties are set:

set the recordFormat to "aiff" -- or "wave" or "ulaw"
set the recordCompression to "raw " -- note the trailing space!

An easy way to set the recordCompression property, as well as several other recording properties is to use the answer record command:

answer record

This will present a dialog box with several options for setting recording parameters:

We will not deal with all of these options at this point, but in general these options affect the size and quality of the audio file created by the record to file command. Remember that, with the exception of µ-law format (which allows µlaw 2:1 compression), you can't use compression if you're going to use the play command to play these files. Remember also that the recordFormat is set to "aiff" by default, and that it must be set explicitly using a set command, as shown above.

More Useful Commands

We used the following commands and functions to help us in building the Audio Jukebox stack.

answer folder - Brings up a standard system "folder picker" dialog box. The folder chosen is put into the it variable.

the files - returns a list of all the files in the defaultFolder

  answer folder "Where are the sound files?"
  if it is not empty then
    set the defaultFolder to it
  end if 
  put the files into field "playlist"

the selectedText - While not really new (we covered it in the lesson on Working with Text,) we haven't seen it in the context of a list style field. A list field is a field with its listbehavior property set to true. Lines in a list field automatically highlight when they are clicked, and you can get the text of the selected line with the selectedText function:

put the selectedText of field "playlist" into sndName
play sndName

Summary

We've seen that using just a few simple commands we can play back audio, either imported or external to the stack. It is equally simple to start and stop a recording in Revolution. It's another example of how Revolution takes care of the behind-the-scenes details to allow us to rapidly implement powerful multimedia capabilities in our projects.