Generation loss
From Wikipedia, the free encyclopedia
Generation loss refers to the loss of quality between subsequent copies of data. Anything that reduces the quality of the representation when copying, and would cause further reduction in quality on making a copy of the copy, can be considered a form of generation loss.
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[edit] Analog generation loss
In analog systems (including systems that use digital recording but make the copy over an analog connection), generation loss is mostly due to noise and bandwidth issues in cables, amplifiers, mixers, recording equipment and anything else between the source and the destination. Poorly adjusted distribution amplifiers and mismatched impedances can make these problems even worse. Repeated conversion between analog and digital can also cause loss.
Generation loss was a major consideration in complex analog audio and video editing, where multi-layered edits and were often created by making intermediate mixes which were then "bounced down" back onto tape. Careful planning was required to minimize generation loss, and the resulting noise and poor frequency response.
One way of minimizing the number of generations needed was to use a audio mixing or video editing suite capable of mixing a large number of channels at once; in the extreme case, for example with a 48-track recording studio, an entire complex mixdown could be done in a single generation, although this was prohibitively expensive for all but the best-funded projects.
The introduction of professional analog noise reduction systems such as Dolby A helped reduce the amount of audible generation loss, but were eventually superseded by digital systems which vastly reduced generation loss.
[edit] Digital generation loss
Digital technology was invented to minimize generation loss. Copying a digital file gives an exact copy if the equipment is operating properly. This trait of digital technology has given rise to awareness of the risk of unauthorised copying. Before digital technology was widespread, a record label, for example, could rest easy in knowing that unauthorised copies of their music tracks were never as good as the originals.
[edit] Techniques that bring back generation loss in digital systems
In digital systems, several techniques, used because of other advantages, may reintroduce generation loss and must be used with caution.
- "Compression" (more accurately "recoding with a technique that uses fewer bits" ) is often used to minimize storage or communications time used. Lossless compression is, by definition, fully reversible. Lossy compression throws away some of the information which cannot be restored. Ideally, when its use is appropriate, it would only be done once, at a carefully planned spot (namely, at the end of the workflow involving the file, ideally after all required changes have been made). Repeated applications of lossy compression and decompression can cause generation loss. Some lossy compression algorithms are much worse than others in this regard. Generation loss caused by lossy compression can be made worse if the parameters used are not consistent across generations. For example, with JPEG, changing the quality setting will cause different quantisation constants to be used, causing a lot of extra loss. Shifting the image by a number of pixels that is not a multiple of the block size will also cause a lot of extra loss. However, copying a digital file as-is, without decompression and recompression, even if it is in a lossy format, incurs no generation loss — the copied file is identical to the original.
- Digital resampling, picture scaling, and other DSP techniques can also introduce artifacts each time they are used. Often, particular implementations fall short of theoretical ideals.
Careful planning of an audio or imaging signal chain from beginning to end and rearranging to minimize multiple conversions is important. Often, arbitrary choices of numbers of pixels and sampling rates for source, destination, and intermediates can seriously degrade digital signals in spite of the potential of digital technology for eliminating generation loss completely.
