Flash audio primer

When you're developing audio for SWF applications, you need to be aware of the capabilities and requirements of audio in Adobe Flash Player. This section covers everything you need to know to get started.

Overview

The general workflow for audio production in Flash CS4 is the same as with other non-vector media. In most cases the audio asset will be acquired and edited externally from the application. When the audio file is prepared, it is then imported into the FLA file or loaded at runtime using ActionScript. This is the first big decision to be made in the process; do you embed the audio or use audio that is external to the Flash movie?

Using embedded audio allows you to import a range of audio formats and has the benefit of visual authoring in the Adobe Flash interface (you don't need to use any coding to implement it). Embedded audio also has the advantage of visual synchronization with graphic content. The disadvantages are the incurred larger file size to the SWF file and the lack of flexibility for changes and runtime manipulation.

Using external audio is generally the way to go for more complex projects. External audio has the advantage of remaining flexible for edits and dynamic play list driven content. It also has the advantage of excluding the audio's file size from the SWF file. The primary disadvantage is that it requires some ActionScript knowledge to implement it. Some tasks like voice synchronization with a character animation on the Timeline work best if the audio is embedded directly on the Timeline.

Workflows

The following list is divided into the four facets of production that should be considered: file preparation, working with embedded audio, working with external audio, and editing your audio.

General steps in the preparation workflow include the following:

1. Record or acquire an audio source in a high resolution format (AIFF or WAV format for example).
2. Save a copy of the raw source file as a backup.
3. Save the source file in the Adobe Sound Document (ASND) format to retain lossless access to your original file and edit history.
4. Edit the source file in Soundbooth CS4.

General steps in the embedded audio workflow include the following:

1. Create a new file in Flash CS4 or open an existing FLA file.
2. Import the audio file to the Library (File > Import > Import to Library).

3. Decide whether to attach the sound to a timeline at authortime or dynamically work with the sound using the Sound object at runtime.

   • If attaching sound to a timeline, select a keyframe on the target timeline and attach the audio using the Sound properties in the Property inspector
   • If using ActionScript, activate the Export for ActionScript option on the sound in the Library, and then add ActionScript to the movie to manipulate the sound dynamically at runtime

General steps in the external audio workflow include the following:

1. Create a new file in Flash or open an existing FLA file.
2. Add ActionScript to the movie that loads the external audio file—either manually assign the audio file path in the ActionScript code or use an XML play list to dynamically assign file paths using an external file.
3. Manipulate and manage the audio file at runtime using ActionScript.

General steps in the editing workflow include the following:

1. Open the audio file in Soundbooth CS4.

   • If the file is embedded in a FLA file, select the file in the Library and choose the Edit in Soundbooth option from the content menu.
   • If the file is external to the FLA file, open it directly in Soundbooth.
2. Edit the file as desired using the range of editing tools in Soundbooth.

3. Save the source file.

   • If the audio file was embedded in a FLA file, it automatically updates when in the movie when saved in Soundbooth.
   • If the audio file was external to the FLA file, then export the updated source file in the format that the ActionScript code expects (MP3, FLV).

Before I go further into production techniques, let's take a look at the types of audio formats that Flash Player supports.

Flash Player capabilities

It is important to understand that Flash Player is designed to play audio in a few specific formats. By itself, Flash Player cannot record audio streams. However the player does have access to the microphone object on the end viewer's computer and can be combined with the Flash Media Server to record and store sound files on a server or stream the sound to other SWF application instances. The Flash Media Server greatly expands the possibilities of what can be done with Flash audio, but Flash Player by itself works well for streaming sounds for playback.

Bit rates and sample rates supported in Flash

The Flash Player supports 8-bit or 16-bit audio at sample rates of 5.512, 11.025, 22.05, or 44.1 kHz. The audio can be converted into lower sample rate during publishing, but it is recommended that you use an audio editing application to resample and edit the audio files—outside of Flash CS4. If you want to add effects to sounds in Flash, it's best to use 16-bit sounds. If you have limited RAM on your system, keep your sounds short and use 8-bit audio.

Tip: Your sound needs to be recorded in a format sampled at 44.1 kHz, or an even factor of 44.1, to assure proper playback in the SWF file. Working in an audio production tool built to produce "Flash-friendly" audio can save a lot of headaches dealing with subtle details. Soundbooth CS4 is a solution designed specifically for Adobe Flash audio production and integration with other Adobe tools.

Audio formats supported when embedding audio

Flash supports a range of audio formats for importing and embedding sounds in a SWF file. You'll need to have QuickTime 4 or higher installed on your computer to take full advantage of the supported formats during authoring (see Table 1).

Table 1. Supported audio formats when importing audio into Flash

Format	Type	Platform	QuickTime 4
AIFF	Audio Interchange File Format	Win/Mac	N/A
ASND	Adobe Soundbooth	Win/Mac	N/A
AU	Sun File Format	Win/Mac	Required
MOV	Sound Only QuickTime Movies	Win/Mac	Required
MP3	MPEG Layer 3	Win/Mac	N/A
SD2	Sound Designer 2	Mac	Required
WAV	Waveform Audio Format	Win/Mac	N/A

Note: The ASND format is a nondestructive audio file format native to Adobe Soundbooth. ASND files can contain audio data with effects, sound scores, and multitrack compositions that can be modified later.

When you embed the audio, Flash CS4 bundles the sound with the SWF file. In most cases the embedded sound will be compressed along with the rest of the assets in the file during publishing. So in the case of embedded audio, you also have to think about the exported audio format as well (see Table 2).

Table 2. Supported audio formats when exporting embedded audio in a SWF

Format	Type	Platform	QuickTime 4
ADPCM	Adaptive differential pulse code modulation compression	Win/Mac	N/A
MP3	MPEG Layer 3 compression	Win/Mac	N/A
Raw	Uncompressed	Win/Mac	N/A
Speech	ASAO compression	Win/Mac	N/A

Notice that while QuickTime is required for importing the full range of supported audio formats, it is not needed to export them or play the published movie. Flash Player handles the playback of the four export formats. The default and most commonly used audio format is MP3. Also notice that all formats are supported on Mac and Windows regardless of whether the file required a Mac during the authoring process.

The export audio format can be set globally in the Publish Settings dialog box or set per sound file. To adjust audio settings globally, edit the event and streaming fields in the Publish Settings (File > Publish Settings). To adjust audio settings per sound, right-click the sound in the Library to launch the Sound Settings dialog box (see Figure 1).

Figure 1. Sound Settings dialog box for adjusting compression, bit rate, and quality settings

Working with event audio and streaming audio

When you work with embedded audio that is attached to a timeline, you have to decide whether to handle each sound as "event" audio or "streaming" audio in Flash Player. This is a concept that specifies how the audio relates to its timeline (or not). Streaming audio signals Flash Player to synchronize the audio to the timeline to which it is attached, and to start playing the sound as it downloads. When you stream audio, you attach the sound to a timeline and the audio playback is directly synched to the length of that timeline. This approach is commonly used for synchronization with animated content and for streaming playback of larger content files.

Event audio signals Flash Player to handle the sound's playback without regard to its timeline. If its timeline contains a limited number of frames, such as a button, the sound can play from start to finish. Event audio has to download completely before it can play, and therefore is most commonly used for short sounds, such as button clicks.

Audio formats supported when playing external audio

Flash Player supports playback of external audio in MP3 and FLV format. The MP3 format has been a mainstay with Flash developers since the Flash 4 era whereas the FLV format became an option in Flash MX (6) when the Flash Media Server implemented the format for Flash video and audio streaming.

Tip: Flash Player 9,0,115 and later supports the HE-AAC audio codecs along with H.264 video.

The MP3 format is commonly used because it is familiar to developers from other areas of web production. MP3 formatted files are relatively easy to produce and easy to share with other web-based applications. While this may be the case, there are some advantages to working with the FLV format. FLV formatted files can hold metadata, such as cue points for synchronization. You can also manipulate FLV audio files using the FLVPlayback component, which allows you to load the sound and create a playback interface with little to no ActionScript knowledge. As audio editing tools such as Soundbooth now export source audio to FLV format, FLV has become a viable option for developers working outside of the Flash Media Server environment.

Table 3. Supported audio formats when playing external audio in Flash

Format	Type	Platform	QuickTime 4
FLV	Flash Video	Win/Mac	NA
MP3	MPEG Layer 3	Win/Mac	NA

Working with audio metadata

Flash Player 6 and later supports metadata for both the MP3 and FLV formats. MP3 files support the ID3 v1.0 and v1.1 standard. FLV files support FLV video metadata parameters, including cue points for content synchronization and custom parameter entries. In both cases the metadata can be retrieved at runtime using event handler functions in ActionScript.

Tip: Flash CS4 FLA files and FLV files created with Adobe Media Encoder CS4 can also contain XMP metadata, which describes the audio related to the file along with other file information. XMP metadata conforms to W3C standards and can be used by web-based search engines to return meaningful search results about the SWF file and its internal content.

See Working with sound metadata in the Programming ActionScript 3 for Flash online documentation for more information on MP3 metadata. Also check out Using cue points and metadata to learn more about working with FLV metadata.

A few words about audio in Adobe Flash Lite

Flash Lite is a runtime engine used to display SWF files on consumer electronics and mobile devices. This article focuses on implementing audio for playback in Flash Player; however it's interesting to note that Flash Lite supports the playback of device sounds such as MIDI and SMAF, among others. See Import and play a device sound in the Developing Flash Lite 2.x and 3.0 Applications online documentation for more information on working with Flash Lite and deploying audio in mobile devices.

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