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Compression

Compression: Plain and simply, compression makes your digital media files smaller in terms of file size but you lose some quality. By analogy, imagine taking every 15th word out of this definition. You would still get most of the meaning, but might miss a little. Real digital compression involves a similar process, but selects words that won’t slur the meaning too much, though there is still a degradation in quality. Examples of compression include the Mp3 or H.264. Compressing an audio or video data file involves encoding the data information utilizing an algorithm that compacts the data into a smaller file size than the original or uncompressed version.  “Lossy” compression removes information that, ideally, will not be perceived and is very efficient in obtaining smaller file sizes. Although getting more efficient, lossy compression codecs still yield degradation. “Lossless” compression, less common for audio and video formats, minimizes loss and degradation but yields much larger files sizes.

Audio compression: “Lossy compression” for audio (such as Mp3 or Vorbis compression) involves an algorithm that applies psycho-acoustic principles to determine what can be removed from an audio signal in order to make the file smaller while minimizing the perception of loss. This inherently involves degrading the audio signal however and should not be associated with long-term preservation. Although these algorithms are making degradation more and more imperceptible, however, if any derivative files are made from your master recording (that was originally recorded using compression) digital artifacts will emerge and the derivative files will be extremely low quality.

Compounding compression–or recompressing an already compressed audio recording, will seriously degrade audio quality.  Flash and HDD storage have dramatically dropped in price and therefore, compressed recording is, again, not recommended for interviewing. Most common for audio is the Mp3 format, however, Marantz has used Mp2 on some of their recorders.

MPEG recording will dramatically decrease your data footprint and thus increase your recording time.  The consequence of this compression will degrade your recording quality, however it is ideal for web deliverable files.  Mp3 files have become a standard uncompressed codec almost universally accepted by most computer players. Mp3 files are usually measured by “bit rate” rather than by sample rate and bit depth.  Also included among more proprietary audio compression codecs are the Windows Media Audio Files, and Real Audio file. There are several “lossless” compression codecs available including FLAC (Free Lossless Audio Codec) and the Apple Lossless Audio Codec (ALE/ALAC), however, at this time, most recorders to not include them as a recording option.

Video compression: Uncompressed digital video is prohibitively large in size and very uncommon. Most digital video cameras automatically use compression when capturing digital video. Video compression utilizes codecs that minimize visual redundancy and achieve more realistic data file sizes. Like audio, “lossy” video compression removes unnecessary information from the data package. Even some of the most professional High Definition digital video formats utilize a great deal of lossy compression that passes unperceived to the human eye. MPEG-2 or H.264 are lossy compression codecs, which greatly minimizes data sizes while still presenting professional quality video. Just as with audio, compounding compression–or recompressing an already compressed video recording, will begin to degrade video quality. Transcoding is the process of converting an already compressed file to another form of compression. This is often associated with the creation of access versions of an oral history interview. JPEG2000 is an example of a lossless compression codec for video. Although loss is minimized, file sizes remain quite large.

Video codecs are often proprietary and should be closely monitored for obsolescence. [ Audio Glossary ] [ Video Glossary ] [ Archive Glossary ]

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