TechNote (Archived)

Understanding Sound: Streaming MP3 sound always compressed at Publish Setting values

Issue

The majority of Flash users compress final Macromedia Flash Player Movies (SWF files) containing audio with MP3 audio compression. This provides the best balance of file size to audio quality. However, there are some issues that developers may not be aware of.

For example, for streaming sounds with MP3 compression, the "Override sound settings" checkbox in the Flash tab of Publish Settings is actually ignored. Streaming MP3 audio is always recompressed at the quality specified in Publish Settings.

Note: This information applies only to streaming sounds with MP3 compression. Uncompressed audio will not show this behavior.

Reason

MP3 Event Sounds are exported to SWF without recompression when the following conditions are true:

MP3 Compression is used.
MP3 sample rate is greater than 5 and less than 18 bits per sample.
The 'Override sound settings' checkbox is not checked in the 'Publish Settings' dialog box

As mentioned above, Streaming MP3 audio is always re-compressed on SWF Export, regardless of the 'Override sound settings' checkbox in Publish Settings. Historically, this is so that each frame in the SWF contains just enough MP3 sound data to keep the sound in sync with the SWF frame rate.

Ref. (137528)