16-05-2012, 04:35 PM
Video Coding at Low Bit Rates
[attachment=22183]
INTRODUCTION
IN the past few years, there has been significant interest
in digital video applications. Consequently, academia and
industry have worked toward developing video compression
techniques [1]–[5], and several successful standards have
emerged, e.g., ITU-T H.261, H.263, ISO/IEC MPEG-1, and
MPEG-2. These standards address a wide range of applications
having different requirements in terms of bit rate, picture
quality, complexity, error resilience, and delay.
While the demand for digital video communication applications
such as videoconferencing, video e-mailing, and
video telephony has increased considerably, transmission rates
over public switched telephone networks (PSTN) and wireless
networks are still very limited. This requires compression
performance and channel error robustness levels that cannot be
achieved by previous block-based video coding standards such
as H.261. Version 1 of the international standard ITU-T H.263,
entitled “Video Coding for Low Bit Rate Communications”
[6], addresses the above requirements and, as a result, becomes
the new low-bit-rate video coding standard.
Optional Modes
In addition to the core encoding and decoding algorithms
described above, H.263 includes four negotiable advanced
coding modes: unrestricted motion vectors, advanced prediction,
PB frames, and syntax-based arithmetic coding. The first
two modes are used to improve inter picture prediction. The
PB-frames mode improves temporal resolution with little bit
rate increase. When the syntax-based arithmetic coding mode
is enabled, arithmetic coding replaces the default VLC coding.
These optional modes allow developers to trade off between
compression performance and complexity. We next provide
a brief description of each of these modes. A more detailed
description of such modes can be found in [11] and [12].
THE ITU-T H.263+ STANDARD
The objective of H.263+ is to broaden the range of applications
and to improve compression efficiency. H.263+, or
H.263 version 2, is backward compatible with H.263. Not only
is this critical due to the large number of video applications
currently using the H.263 standard, but it is also required by
ITU-T rules.
H.263+ offers many improvements over H.263. It allows the
use of a wide range of custom source formats, as opposed to
H.263, wherein only five video source formats defining picture
size, picture shape, and clock frequency can be used. This
added flexibility opens H.263+ to a broader range of video
scenes and applications, such as wide format pictures, resizeable
computer windows, and higher refresh rates. Moreover,
picture size, aspect ratio, and clock frequency can be specified
as part of the H.263+ bit stream.
TEST MODEL RATE CONTROL METHODS
The latest version of the H.263+ Test Model, TMN-8 [14],
describes two rate control algorithms suitable for low delay
videophone applications. Both methods use a buffer regulation
scheme in which a target bit rate is chosen and pictures are
skipped until the buffer reaches a limit below the number
of bits required to transmit the next picture. Since encoding
delays are directly related to buffer fullness, large variations
in buffer content will produce undesirable variable delays.
The rate control methods try to achieve a target bit rate
by changing the macroblock quantizer. The most recent rate
control method, also described in [15], is based on a model
that chooses an “optimal” quantizer for every macroblock
in a given picture. First, the variances of all macroblocks
in the motion-compensated picture are calculated. Based on
these variances, and the remaining bits available for encoding
the current picture, model parameters are updated.
[attachment=22183]
INTRODUCTION
IN the past few years, there has been significant interest
in digital video applications. Consequently, academia and
industry have worked toward developing video compression
techniques [1]–[5], and several successful standards have
emerged, e.g., ITU-T H.261, H.263, ISO/IEC MPEG-1, and
MPEG-2. These standards address a wide range of applications
having different requirements in terms of bit rate, picture
quality, complexity, error resilience, and delay.
While the demand for digital video communication applications
such as videoconferencing, video e-mailing, and
video telephony has increased considerably, transmission rates
over public switched telephone networks (PSTN) and wireless
networks are still very limited. This requires compression
performance and channel error robustness levels that cannot be
achieved by previous block-based video coding standards such
as H.261. Version 1 of the international standard ITU-T H.263,
entitled “Video Coding for Low Bit Rate Communications”
[6], addresses the above requirements and, as a result, becomes
the new low-bit-rate video coding standard.
Optional Modes
In addition to the core encoding and decoding algorithms
described above, H.263 includes four negotiable advanced
coding modes: unrestricted motion vectors, advanced prediction,
PB frames, and syntax-based arithmetic coding. The first
two modes are used to improve inter picture prediction. The
PB-frames mode improves temporal resolution with little bit
rate increase. When the syntax-based arithmetic coding mode
is enabled, arithmetic coding replaces the default VLC coding.
These optional modes allow developers to trade off between
compression performance and complexity. We next provide
a brief description of each of these modes. A more detailed
description of such modes can be found in [11] and [12].
THE ITU-T H.263+ STANDARD
The objective of H.263+ is to broaden the range of applications
and to improve compression efficiency. H.263+, or
H.263 version 2, is backward compatible with H.263. Not only
is this critical due to the large number of video applications
currently using the H.263 standard, but it is also required by
ITU-T rules.
H.263+ offers many improvements over H.263. It allows the
use of a wide range of custom source formats, as opposed to
H.263, wherein only five video source formats defining picture
size, picture shape, and clock frequency can be used. This
added flexibility opens H.263+ to a broader range of video
scenes and applications, such as wide format pictures, resizeable
computer windows, and higher refresh rates. Moreover,
picture size, aspect ratio, and clock frequency can be specified
as part of the H.263+ bit stream.
TEST MODEL RATE CONTROL METHODS
The latest version of the H.263+ Test Model, TMN-8 [14],
describes two rate control algorithms suitable for low delay
videophone applications. Both methods use a buffer regulation
scheme in which a target bit rate is chosen and pictures are
skipped until the buffer reaches a limit below the number
of bits required to transmit the next picture. Since encoding
delays are directly related to buffer fullness, large variations
in buffer content will produce undesirable variable delays.
The rate control methods try to achieve a target bit rate
by changing the macroblock quantizer. The most recent rate
control method, also described in [15], is based on a model
that chooses an “optimal” quantizer for every macroblock
in a given picture. First, the variances of all macroblocks
in the motion-compensated picture are calculated. Based on
these variances, and the remaining bits available for encoding
the current picture, model parameters are updated.