Pulse or digital communications – Bandwidth reduction or expansion
Reexamination Certificate
1998-06-16
2001-07-10
Kelley, Chris (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
C375S240030
Reexamination Certificate
active
06259733
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for bit rate allocation in a multi-channel video data encoder. The invention relates generally to statistical multiplexing, wherein a bit rate (e.g., bandwidth) is allocated to the different channels based on the channels' bit rate needs and the overall available bandwidth.
Statistical multiplexing is the process of encoding a number of signals at variable bit rates and combining the variable-rate bitstreams into a single fixed-rate transport stream so that the bandwidth allotted to each signal is flexible and varies with each signal's bit rate need. Conventionally, an estimate of bit rate need is made based on signal statistics. After a bit rate is allocated based on the need, the data in each signal is compressed and encoded using a specific quantization level. The amount of data that results from the compression is examined in each channel, and the quantization level is adjusted so that channels with more encoded data receive a higher bit rate. Next, the video data is compressed and encoded again using the adjusted quantization level. The process may be repeatedly successively in multiple feedback cycles. Other conventional techniques attempt to equalize a quantization distortion measure across the channels.
However, the conventional techniques have various drawbacks. For example, the use of successive feedback cycles in the compressor can be time-consuming and computationally intensive. Additionally, special bit rate needs for specific types of video scenes may not be considered. Moreover, the equalization of a quantization distortion measure does not reliably translate to an equalization of perceived image quality.
Accordingly, it would be desirable to provide a high-performance dynamic rate allocation system that quickly and accurately allocates bit rate to a plurality of video channels to equalize the overall image quality of all channels at any time instant. The system should provide a pre-processor which measures statistical information of the video data prior to compression and encoding to estimate the relative bit rate required to adequately encode each video scene. The measurements should be made sufficiently early in the encoding process to eliminate undesirable time delays. The system should provide the allocated bit rate to the video compressor from the pre-processor in a feedforward path to avoid undesirable feedback. The system should also provide the capability for feedback processing to fine tune the allocated bit rate before providing it to the compressor.
Furthermore, the system should measure or detect at least some of the following characteristics of each video frame (e.g., picture): spatial activity, temporal activity, image size, frame rate, scene change, brightness, flash, fade, and horizontal pixel resolution. The system should bias the bit rate allocation according to inter-frame distance, whether the average spatial activity level is below a lower threshold, whether the inter-frame distance is above an upper threshold or below a lower threshold, whether the quantization of previous frames is above an upper threshold, the length of the Group of Pictures (GOP), and a user-selectable priority factor.
The system should also allocate any surplus bit rate, if any, among the channels, to avoid having unused bandwidth.
The system should be compatible with progressive or interlaced video, as well as different image shapes and sizes, including Video Object Planes (VOPs).
The system should further be compatible with different video standards including NTSC, PAL, and NTSC detelecine.
The system should provide a pre-processor which can be used with existing commercially available compression circuitry to allow quick and inexpensive retrofitting of such circuitry.
The present invention provides a system having the above and other advantages.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for bit rate allocation in a multi-channel video data encoder.
A method for allocating a bit rate to a plurality of variable rate video channels in a video encoder includes the steps of: processing video data from a current picture (e.g., frame) in each respective channel to determine at least a spatial activity and a temporal activity thereof; and determining a bit rate demand D
i
for each current picture according to the associated spatial activity and temporal activity.
The method is suitable for use with multiplexed channels that are all variable rate, as well as with a combination of fixed rate and variable rate channels.
The method may include the further step of adjusting the bit rate demand D
i
for each current picture according to whether at least one of a scene change, fade and flash is detected for the current picture. Generally, bit allocation is increased if any of these events are detected since such events cannot usually be efficiently coded.
The method may include the further steps of increasing the associated temporal activity of each current picture if the associated spatial activity is below a lower threshold; and adjusting the bit rate demand D
i
for each current picture according to the increasing step. This is done since motion within a scene with a low spatial activity will produce an artificially small inter-frame difference.
The method may include the further steps of increasing the bit rate demand D
i
for each current picture when the associated temporal activity exceeds an upper threshold; and/or decreasing the bit rate demand D
i
for each current picture when the associated temporal activity is less than a lower threshold. This is done since high motion scenes require additional bits to maintain a given image quality while fewer bits are required for low motion scenes.
The method may include the further steps of determining a quantization level of at least one previous picture for each current picture; and increasing the bit rate demand D
i
for each current picture when the quantization level of the at least one previous picture exceeds an upper threshold. This is done to avoid oscillations in the quantization level that may be noticeable to a viewer.
Each current picture may be part of an associated Group Of Pictures (GOP), where each GOP typically includes one or more intra-coded pictures and several inter-coded pictures. In this case, the method may include the further steps of decreasing the bit rate demand D
i
for each current picture when a length of the associated group of pictures exceeds a nominal level; and/or increasing the bit rate demand D
i
for each current picture when a length of the associated group of pictures is less than a nominal level. This is done since fewer bits are required to code a large GOP since there are relatively more inter-coded (e.g., predictive coded pictures), such as B- and P-pictures.
The method may include the further step of reducing or eliminating the increase or decrease of the increasing and decreasing steps, respectively, when the temporal activity of each current picture exceeds an upper threshold. This is done since high motion is likely to result in relatively more intra-coded pictures in a GOP.
The method may include the further step of adjusting the bit rate demand D
i
for each current picture according to a horizontal pixel resolution thereof. This is done since more bits are required to code a higher resolution picture.
The method may include the further steps of determining a brightness level for each current picture; and increasing the bit rate demand D
i
for each current picture when the associated brightness level is less than a lower threshold. Darker scenes should be coded with additional bits to maintain a perceived image quality.
The method may include the further step of adjusting the bit rate demand D
i
for each current picture according to priority factor thereof that indicates a relative importance of each current picture in the multiplexed data stream. Thus, more important channels, such as movies and pay per view events, for example, may be allocate
Haskell Paul E.
Kaye James E.
Manansala Arthur C.
General Instrument Corporation
Kelley Chris
Lipsitz Barry R.
McAllister Douglas M.
Wong Allen
LandOfFree
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