AC coefficient
Any
DCT
coefficient for which the frequency in one or both dimensions is
non-zero.
ADPCM (Adaptive Differential Pulse Code Modulation)
It is a compression technique which encodes the predictive residual
instead of the original waveform signal so that the compression efficiency
is improved by a predictive gain.
Rather than transmitting
PCM
samples directly, the difference between the estimate of the next sample
and the actual sample is transmitted.
This difference is usually small and can thus be encoded in fewer bits than
the sample itself.
ATV (Advanced TV)
Although sometimes used interchangeably, advanced and high-definition
television (HDTV)
are not one and the same. Advanced television (ATV) would
distribute wide-screen television signals with resolution substantially
better than current systems. It requires changes to current emission
regulations, including transmission standards. In addition, ATV would offer
at least two-channel, CD-quality audio.
A:B:C notation
The a:b:c notation for sampling ratios, as found in the
CCIR-601
specifications, has the following meaning :
- 4:2:2 means 2:1 horizontal downsampling, no vertical downsampling.
(Think 4 Y samples for every 2 Cb and 2 Cr samples in a scanline.)
- 4:1:1 ought to mean 4:1 horizontal downsampling, no vertical.
(Think 4 Y samples for every 1 Cb and 1 Cr samples in a scanline.)
It is often misused to mean the same as 4:2:0 .
- 4:2:0 means 2:1 horizontal and 2:1 vertical downsampling.
(Think 4 Y samples for every Cb and Cr samples in a scanline.)
Not only is this notation not internally consistent, but it is incapable of
being extended to represent any unusual sampling ratios, eg different
ratios for the Cb and Cr channels.
Arithmetic Coding
Perhaps the major drawback to each of the
Huffman
encoding techniques is their poor performance when processing texts where one
symbol has a probability of occurrence approaching unity. Although the
entropy
associated with such symbols is extremely low, each symbol must still
be encoded as a discrete value.
Arithmetic coding removes this restriction by representing messages as
intervals of the real numbers between 0 and 1. Initially, the range of
values for coding a text is the entire interval [0, 1]. As encoding
proceeds, this range narrows while the number of bits required to represent
it expands. Frequently occurring characters reduce the range less than
characters occurring infrequently, and thus add fewer bits to the length of
an encoded message.
ATM
ATM (Asynchronous Transfer Mode) is a
switching/transmission technique
where data is transmitted in small, fixed sized cells (5 byte
header,
48 byte payload). The cells lend themselves both to the
time-division-multiplexing characteristics of the transmission media,
and the packet
switching characteristics desired of data networks. At each switching
node, the ATM
header
identifies a virtual path or virtual circuit
that the cell contains data for, enabling the switch to forward the
cell to the correct next-hop trunk. The virtual path is set up
through the involved switches when two endpoints wish to communicate.
This type of switching can be implemented in hardware, almost essential
when trunk speed range from 45Mb/s to 1Gb/s.
The ATM Forum, a worldwide organization,
aimed at promoting ATM within the industry and the end user community was
formed in October 1991 and currently includes more than 500 companies
representing all sectors of the communications and computer industries, as
well as a number of government agencies, research organizations and users.