Transform coding is a type of
data compression for "natural" data like
audio
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
*Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
* Digital audio, representation of sou ...
signals or photographic
images. The transformation is typically lossless (perfectly reversible) on its own but is used to enable better (more targeted)
quantization, which then results in a lower quality copy of the original input (
lossy compression
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data si ...
).
In transform coding, knowledge of the application is used to choose information to discard, thereby lowering its
bandwidth. The remaining information can then be compressed via a variety of methods. When the output is decoded, the result may not be identical to the original input, but is expected to be close enough for the purpose of the application.
Colour television
NTSC
One of the most successful transform encoding system is typically not referred to as such—the example being
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
color
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, ...
. After an extensive series of studies in the 1950s,
Alda Bedford showed that the human eye has high resolution only for black and white, somewhat less for "mid-range" colors like yellows and greens, and much less for colors on the end of the spectrum, reds and blues.
Using this knowledge allowed
RCA to develop a system in which they discarded most of the blue signal after it comes from the camera, keeping most of the green and only some of the red; this is
chroma subsampling in the
YIQ
YIQ is the color space used by the analog NTSC color TV system, employed mainly in North and Central America, and Japan.
''I'' stands for ''in-phase'', while ''Q'' stands for ''quadrature'', referring to the components used in quadrature amplitud ...
color space
A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represen ...
.
The result is a signal with considerably less content, one that would fit within existing 6 MHz black-and-white signals as a phase modulated differential signal. The average TV displays the equivalent of 350 pixels on a line, but the TV signal contains enough information for only about 50 pixels of blue and perhaps 150 of red. This is not apparent to the viewer in most cases, as the eye makes little use of the "missing" information anyway.
PAL and SECAM
The PAL and SECAM systems use nearly identical or very similar methods to transmit colour. In any case both systems are subsampled.
Digital
The term is much more commonly used in
digital media and
digital signal processing. The most widely used transform coding technique in this regard is the
discrete cosine transform (DCT),
proposed by
Nasir Ahmed in 1972,
and presented by Ahmed with T. Natarajan and
K. R. Rao in 1974.
This DCT, in the context of the family of discrete cosine transforms, is the DCT-II. It is the basis for the common
JPEG
JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...
image compression standard,
which examines small blocks of the image and transforms them to the
frequency domain for more efficient quantization (lossy) and
data compression. In
video coding, the
H.26x and
MPEG standards modify this DCT image compression technique across frames in a motion image using
motion compensation
Motion compensation in computing, is an algorithmic technique used to predict a frame in a video, given the previous and/or future frames by accounting for motion of the camera and/or objects in the video. It is employed in the encoding of video d ...
, further reducing the size compared to a series of JPEGs.
In
audio coding, MPEG audio compression analyzes the transformed data according to a
psychoacoustic model that describes the human ear's sensitivity to parts of the signal, similar to the TV model.
MP3 uses a hybrid coding algorithm, combining the
modified discrete cosine transform (MDCT) and
fast Fourier transform (FFT).
It was succeeded by
Advanced Audio Coding
Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. Designed to be the successor of the MP3 format, AAC generally achieves higher sound quality than MP3 encoders at the same bit rate.
AAC has been stan ...
(AAC), which uses a pure MDCT algorithm to significantly improve compression efficiency.
The basic process of
digitizing
DigitizationTech Target. (2011, April). Definition: digitization. ''WhatIs.com''. Retrieved December 15, 2021, from https://whatis.techtarget.com/definition/digitization is the process of converting information into a digital (i.e. computer- ...
an analog signal is a kind of transform coding that uses
sampling in one or more domains as its transform.
See also
*
Karhunen–Loève theorem
*
Transformation (function)
*
Wavelet transform
References
{{DEFAULTSORT:Transform Coding
Lossy compression algorithms