Frequently Asked Questions (FAQs)
Why does Scientific Atlanta favor Variable Bit Rate (VBR)
encoding over Constant Bit Rate (CBR)?
Using CBR, the number of bits used for
encoding each segment of a video program is the same. For many video
broadcasting or transmission applications, a fixed, higher bit rate is usually
used to guarantee the acceptable video quality for the most complex video
content segments. But, because the bit rate is fixed, bits are wasted for simple
video content segments.
In comparison, VBR coding encodes the video
content as much as possible to a constant quality. The number of bits used for
coding each frame is a function of frame content complexity. This results in a
variable bit rate bit stream. The bit saving from the VBR encoder and the
multiplexing efficiency improvement from the intelligent multiplexer can be used
to increase the number of video programs per transmission bandwidth.
What's the difference between 4:2:2 and
4:2:0 encoding?
Both terms refer to the ratio of video
sampling frequencies which have an impact on the quality of the video delivered
by a digital encoder. More video sampling is done in 4:2:2 so it offers higher
video quality than 4:2:0.
4:2:2 is a commonly used term for a component
digital video format. A ratio of sampling frequencies used to digitize the
luminance and color difference components (Y, R-Y, B-Y) of a video signal. It is
generally used as shorthand for ITUL-R 601. The term 4:2:2 describes that for
every four samples of Y, there are two samples each of R-Y and B-Y, giving more
chrominance bandwidth in relation to luminance compared to 4:1:1 sampling.
4:2:0 is a sampling system used to digitize the
luminance and color difference components (Y, R-Y, B-Y) of a video signal. The
four represents the 13.5 MHz sampling frequency of Y, while the R-Y and B-Y are
sampled at 6.75 MHz-effectively between every other line only (one line is
sampled at 4:0:0, luminance only, and the next at 4:2:2).
What's the difference in Dolby AC-3 and
the Dolby Digital term used today?
They are one and the same. Dolby Digital is
the approved 5.1 channel (surround-sound) audio standard for ATSC digital
television, using approximately 13:1 compression. Six discreet audio channels
are used: Left, Center, Right, Left Rear (or side) Surround, Right Rear (or
side) Surround, and a subwoofer (considered the ".1" as it is limited
in bandwidth). The bit rate can range from 56 Kbps to 640 Kbps.
When moving from analog recording to a digital
recording medium, one finds that the digital audio coding used yields an amount
of data often too immense to store or transmit economically, especially when
multiple channels are required. As a result, new forms of digital audio
coding-often known as "perceptual coding" -have been developed to
allow the use of lower data rates with a minimum of perceived degradation of
sound quality.
Dolby's third generation audio coding algorithm
(originally called AC-3) is such a coder.
This coder has been designed to take maximum
advantage of human auditory masking in that it divides the audio spectrum of
each channel into narrow frequency bands of different sizes, optimized with
respect to the frequency selectivity of human hearing. This makes it possible to
sharply filter coding noise so that it is forced to stay very close in frequency
to the frequency components of the audio signal. By reducing or eliminating
coding noise wherever there are no audio signals to mask it, the sound quality
of the original signal can be subjectively preserved. In this key respect, a
coding system like Dolby Digital is essentially a form of very selective and
powerful noise reduction.
What are motion compensation and motion
estimation?
Motion compensation is the use of motion
vectors to improve the efficiency of the prediction of pixel values. The
prediction uses motion vectors to provide offsets into past and/or future
reference frames containing previously decoded pixels that are used to form the
prediction and the error difference signal. Motion estimation is an image
compression technique that achieves compression by describing only the motion
differences between adjacent frames, thus eliminating the need to convey
redundant static picture information from frame to frame. Used in the MPEG
standards. |
