Audio+gaps

= Audio gaps = toc

How understanding audio gaps can help the LIS profession
Facilities which may be making digital representations of other recorded works need to understand how gaps in digital audio works, to provide the best showcase for their efforts and to provide users with optimal digital presentation. For facilities managing sociolinguistic data, moving older files to newer forms may present a problem where pauses are concerned. There are historical sound media which contain gaps as a necessary element of their presentation. There are media which contain gaps as necessary elements for proper implementation, such as lesson files where a student may need to repeat something in a pause.

A little about gaps
Gaps are the pauses between tracks on a CD or in an MP3 Playlist. The gaps often have sound in them - fading out of the previous song, the introduction of the next song. In CD format, the gaps are attached to the end of the track, unless specified otherwise when the CD is being burnt, but are only played if the list is run in order. Playing pieces out of sequence removes the gaps. For most people, the gaps, which are most often two seconds long, are not a problem. For some applications, or for some uses, longer gaps are essential.

Bit rates
Even more modern devices, with larger storage capacity, have their limits. File size is an important consideration. So is sound quality. Choosing a file size that will give quality while not taking up too much space is a concern. A good rule of thumb to remember is, the higher the bit rate, the better the sound quality and, the larger the file size. When portions of a file, which will not affect the sound quality, are shrunken, the file can be smaller while still maintaining higher quality sound. By varying the rate at which the different blocks of a file are delivered, based on the content of those blocks, higher quality can be maintained with a lower sized file.

Variable bit rate (VBR)
There are two settings to choose between when recording an audio file. Variable bit rate (VBR), reads segments to find out how large they need to be. Gaps, pauses, repeated elements can be shrunken down without noticeably altering the sound. This allows for a lower bit rate when there is not much happening in the file. Greater compression can be achieved without sacrificing sound quality.

Constant (or, Fixed) bit rate (CBR)
Constant, or, fixed, bit rate (CBR) keeps the entire file at a constant bit rate setting, ensuring predictable file sizes. The bit rate for each segment remains constant, whether something is happening in the file, or not. CBR is an older format that is supported by all MP3 players; VBR is not supported by all players, though newer ones will support it.

Gaps
When VBR is used, gaps are shrunken to provide the most efficient compression, in order to deliver quality sound. This quality is a drawback where gaps are important. CBR provides a stable environment which includes full-length gaps. When the file contains voice but no music, the file can be set to a lower bit rate in order to off-set the larger content size. There are several applications where a gap, or a series of gaps, may be important, so it is necessary, when looking into recording audio files, to look at the type of file that is being recorded. Music will need the flexibility of VBR in order to maintain sound quality and to provide a faithful representation of the original. Speeches, discussions, instruction, language learning, and other voice files, can be recorded at a lower bit rate without sacrificing faithful representation, using CBR to preserve gaps, pauses, and other elements which might be compressed with VBR.

Examples

 * Longer gaps can be used in language learning, to allow students to say the phrase between the instructor's prompt and the subsequent modeling.
 * Longer gaps give time between tracks, providing time for other events to occur outside of the file. People who both operate a player without a remote and who need to get to a different location before the next track begins, need an adequate gap between tracks. One example is of a dance instructor who needs to begin a song on a player off the dance floor but who, then, needs the time to get onto the dance floor.
 * For some combination CD players / amplifiers, it is necessary to have a CD in and playing for the amplification to work. A CD of silence would provide the playing CD while allowing the amplification function to be used for announcements.
 * A famous historical example of a gap is the eighteen and a half missing minutes from the Nixon Watergate tapes.

**References**
[|Slicing and Dicing MP3 bit rates] [|Famous gap: Nixon White House Tapes] [|RealJukebox and Jukebox Plus FAQ] [|Forum thread: Help with homework - tech]

Pictures: [|Representation of gap and track on a CD]