Generation loss
Encyclopedia
Generation loss refers to the loss of quality between subsequent copies
Copying
Copying is the duplication of information or an artifact based only on an instance of that information or artifact, and not using the process that originally generated it. With analog forms of information, copying is only possible to a limited degree of accuracy, which depends on the quality of the...

 or transcodes 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. File size increases are a common result of generation loss, as the introduction of artifacts
Compression artifact
A compression artifact is a noticeable distortion of media caused by the application of lossy data compression....

 may actually increase the entropy of the data through each generation.

Analog generation loss

In analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 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 cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

s, amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

s, mixers, recording equipment and anything else between the source and the destination. Poorly adjusted distribution amplifier
Distribution amplifier
In electronics, a distribution amplifier or simply distribution amp, is a device that accepts a single input signal and provides this same signal to multiple isolated outputs....

s 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 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 an 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.

Digital generation loss

Digital technology used correctly can eliminate 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 unauthorized copying. Before digital technology was widespread, a record label
Record label
In the music industry, a record label is a brand and a trademark associated with the marketing of music recordings and music videos. Most commonly, a record label is the company that manages such brands and trademarks, coordinates the production, manufacture, distribution, marketing and promotion,...

, for example, could rest easy in knowing that unauthorized copies of their music tracks were never as good as the originals.

Techniques that cause generation loss in digital systems

In digital systems
Digital Systems
Digital Systems was the first Internet service provider in Bulgaria. The company was established in 1989 with their main business activities being computer systems and networks. It has been offering Internet services since 1991....

, several techniques, used because of other advantages, may reintroduce generation loss and must be used with caution. However, copying a digital file itself incurs no generation loss — the copied file is identical to the original, provided a perfect copying channel is used.

Some digital transforms are reversible, while many are not:
lossless compression is, by definition, fully reversible, while lossy compression throws away some data which cannot be restored. Similarly, many DSP
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...

 processes are not reversible.

Thus careful planning of an audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 or video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 signal chain from beginning to end and rearranging to minimize multiple conversions is important to avoid generation loss. 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.

Similarly, when using lossy compression, it will ideally only be done once, at the end of the workflow involving the file, after all required changes have been made.

Transcoding

Converting between lossy formats – be it decoding and re-encoding to the same format, between different formats, or between different bitrates or parameters of the same format – causes generation loss.

Repeated applications of lossy compression and decompression can cause generation loss, particularly if the parameters used are not consistent across generations.
Ideally an algorithm will be both idempotent, meaning that if the signal is decoded and then re-encoded with identical settings, there is no loss, and scalable, meaning that if it is re-encoded with lower quality settings, the result will be the same as if it had been encoded from the original signal – see Scalable Video Coding
Scalable Video Coding
Scalable Video Coding is the name for the Annex G extension of the H.264/MPEG-4 AVC video compression standard. SVC standardizes the encoding of a high-quality video bitstream that also contains one or more subset bitstreams. A subset video bitstream is derived by dropping packets from the...

. More generally, transcoding between different parameters of a particular encoding will ideally yield the greatest common shared quality – for instance, converting from an image with 4 bits of red and 8 bits of green to one with 8 bits of red and 4 bits of green would ideally yield simply an image with 4 bits of red color depth and 4 bits of green color depth without further degradation.

Some lossy compression algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...

s are much worse than others in this regard, being neither idempotent nor scalable, and introducing further degradation if parameters are changed.

For example, with JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

, changing the quality setting will cause different quantization constants to be used, causing additional loss. Further, as JPEG is divided into 16×16 blocks (or 16×8, or 8×8, depending on chroma subsampling
Chroma subsampling
Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance....

), cropping that does not fall on an 8×8 boundary shifts the encoding blocks, causing substantial degradation – similar problems happen on rotation. This can be avoided by the use of jpegtran or similar tools for cropping. Similar degradation occurs if video keyframes do not line up from generation to generation.

Editing

Digital resampling
Resampling
Resampling may refer to:* Resampling , several related audio processes* Resampling , resampling methods in statistics* Resampling , scaling of bitmap images* Sample rate conversion-See also:* Downsampling* Upsampling...

 such as image scaling
Image scaling
In computer graphics, image scaling is the process of resizing a digital image. Scaling is a non-trivial process that involves a trade-off between efficiency, smoothness and sharpness. As the size of an image is increased, so the pixels which comprise the image become increasingly visible, making...

, and other DSP
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...

 techniques can also introduce artifacts or degrade signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

 (S/N ratio) each time they are used, even if the underlying storage is lossless.

Resampling causes aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...

, both blurring low-frequency components and adding high-frequency noise, causing jaggies
Jaggies
"Jaggies" is the informal name for artifacts in raster images, most frequently from aliasing, which in turn is often caused by non-linear mixing effects producing high-frequency components and/or missing or poor anti-aliasing filtering prior to sampling....

, while rounding off computations to fit in finite precision introduces quantization
Quantization (signal processing)
Quantization, in mathematics and digital signal processing, is the process of mapping a large set of input values to a smaller set – such as rounding values to some unit of precision. A device or algorithmic function that performs quantization is called a quantizer. The error introduced by...

, causing banding
Colour banding
Colour banding is a problem of inaccurate colour presentation in computer graphics. While in 24 bit colour modes, 8 bits per channel should be enough to render images in the full visible spectrum, in some cases there is a risk of producing abrupt changes between shades of the same colour...

; if fixed by dither
Dither
Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images...

, this instead becomes noise. In both cases, these at best degrading the signal's S/N ratio, and may cause artifacts. Quantization can be reduced by using high precision while editing (notably floating point numbers), only reducing back to fixed precision at the end.

Often, particular implementations fall short of theoretical ideals.

See also

  • Signal-to-noise ratio
    Signal-to-noise ratio
    Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

  • Editing digital images
  • Lossless data compression
    Lossless data compression
    Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange...

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