Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1998-11-06
2001-04-03
Kelley, Chris (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06212233
ABSTRACT:
The invention relates to the coding of audio, video or auxiliary digital data.
It relates more particularly to systems for variable bit rate transmission and for storage which utilize compression of audio and video digital data. It is compatible with the MPEG 2 standard.
The role of video compression systems is to obtain the best image quality for as low a transmission bit rate as possible. This quality of coding is related among other things to the choice of the motion estimator and of the coder regulating algorithm. It depends directly on the optimality criteria adopted.
In respect of motion estimation, the capacity for searching in the reference image is an important characteristic. In respect of regulation, the major objective is to maintain stable quality of the coded images. The “padding” factor employed in order to adjust the coding to the output bit rate (in general an insertion of “0” into the data stream making it possible to preserve the bit rate setting) must be minimized for optimal coding.
The cost of coding each image is variable. It is the role of the regulating loop to correct the flow variations due to the variations in the complexity of the image (defined hereafter) and to transmit as output a data flow corresponding to a data rate setting. This regulation generally acts on the quantization interval which is dependent on the fill level of the buffer memory. This buffer memory at the output of the coder makes it possible to absorb the deviations in bit rate which are due to the response time of the regulating loop, and to which the capacity of the buffer is related.
The data flow at the output of a coder of a video source can be variable. It may indeed be beneficial, for example in the case of transmitting several video sources over a single channel, to distribute the available “global” bit rate of this channel between the various sources dynamically, that is to say as a function of the change in the complexity of the images to be coded from each video source. This process makes it possible to improve the global quality of the transmitted images, relative to existing processes, for example those which assign a bit rate to the video sources as a function of the type of programme transmitted.
FIG. 1
represents a set of a transmission using such a process. This involves transmitting from several coded video sources over a single transmission channel with a constant global bit rate. This bit rate corresponds to its capacity, for example that of the channel of a transponder. The coding circuits of the video sources
13
i
are linked to the input of a multiplexer
11
whose output supplies the data stream to be transmitted over the channel. They are also linked to an interface circuit
14
which transmits the data relating to the complexity of the video sources to a bit rate allocator
12
and receives the bit rates allotted to each source by this bit rate allocator
12
. The latter, whose role is therefore to allocate a bit rate to each source
13
i
, controls the multiplexing circuit or multiplexer
11
. The information is exchanged by way of a fast bus hooked up to the bit rate allocator
12
and the interface circuit
14
. A supervisory circuit
15
is linked to this bus to manage the transmission set.
Multiplexing is performed for example at a GOP speed, the GOP being an image group defined hereafter, as a function of the bit rate allocated by the bit rate allocator to each video source transmitted over the same channel. This bit rate depends on a coefficient of complexity of the video source which takes into account the volume of information generated for a given quantization interval. This coefficient is measured for each GOP of each video source. It corresponds for example to the number of bits used to code a GOP, the quantization interval being constant or alternatively to a standardized counting of bits corresponding to the number of bits multiplied by the quantization interval if this step is variable. Since the flow is constant over the duration of a GOP, the output buffer of the corresponding source must absorb the bit rate spikes over the duration of a GOP.
The utilization of variable bit rate encoders associated with statistical multiplexing which depends on the bit rate of each source, as described above, is known and is a means of optimizing the capacity of the transponder channel. The drawbacks of these regulating processes are also known. These are principally the sizes of the buffer memories at the decoders which need to be large in order to be capable of absorbing the bit rate variations of the corresponding video sources without the memory drying up or saturating. The buffer of the decoder must neither overflow (loss of data) nor dry up. The cost of the decoder is thus increased. On the other hand, the necessary memory capacity is no longer generally compatible with the MPEG2 standard.
The instantaneous variation in bit rate, that is to say at image level, generally demands even larger buffer memory capacities, whether this be at coder level or decoder level or alternatively has to be limited as a function of these memory capacities.
The aim of the present invention is to remedy the aforesaid drawbacks.
Its subject is a process for coding digital data of a video image sequence effecting image compression of this data and regulation of the bit rate at the output of the coder by way of a regulating loop acting on a quantization interval for the data to be coded, storage in a buffer of the data output by the coder for its transmission over a channel at a variable bit rate to a buffer of a decoder, a tagging of this data so as to guarantee a constant time &tgr; between the instant of storage of an image in the buffer of the coder and the instant of output of this image from the buffer of the decoder, characterized in that the bit rate regulation in respect of the image at the instant n is performed as a function of a bit rate prediction in respect of the transmission channel for the instant n+&tgr;, and in that the regulation is performed within the limits of a virtual buffer defined by a top address and a bottom address in the buffer of the coder, the top address or offset of the virtual buffer changing with respect to the top address of the buffer of the coder, starting from a time &tgr; before the variation of the transmission bit rate at the output of the buffer of the coder, depending on a value corresponding to the bit rate variation multiplied by the time &tgr;.
Its subject is also a process for transmitting data from several video sources effecting for each source a coding of this data in order to transmit it at a variable bit rate over a given transmission channel, characterized in that it effects, for each source, a bit rate allocation &tgr; instant before the actual bit rate of this source over the channel, depending on the channel available bit rate and on measurements of complexity of the previous images from the set of video sources.
The characteristics and advantages of the present invention will emerge better from the following description given by way of example and reference to the appended figures in which:
FIG. 1
represents a known set of transmissions from the prior art;
FIG. 2
represents a device for coding and for decoding according to the MPEG2 standard;
FIG. 3
represents, in a schematic manner, the filling of the buffer memories at the coder and decoder as a function of the bit rates;
FIG. 4
represents an example of the change in size and position of the virtual buffer as a function of time;
FIG. 5
represents the size and position of the virtual buffer in the physical buffer of the coder as a function of the bit rate;
FIG. 6
represents the size of the virtual buffer in the physical buffer of the coder as a function of the bit rate;
FIG. 7
represents the change in the parameter &agr; as a function of the bit rate;
FIG. 8
represents a device for coding according to the invention;
FIG. 9
represents a device for regulating according to the invention;
FIG. 10
represents a flowchart of the regulating process acc
Alexandre Patrice
Noblet Ludovic
Perron Claude
Kelley Chris
Kurdyla Ronald H.
Thomson Licensing S.A.
Tripoli Joseph S.
Wong Allen
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