Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-02-17
2003-08-26
Le, Vu (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240250
Reexamination Certificate
active
06611561
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to video coding.
One of the recent targets in mobile telecommunications has been to increase the speed of the data transmission in order to enable multimedia services via radio networks. One of the key components of multimedia is digital video. Digital video offers a great many advantages over traditional analogue systems, supporting services such as video telephony and multi-media applications. However, a key problem of digital video when compared with analogue systems is the demand it places on communications and storage resources. For example, a bandwidth of approximately 160 Mbps is required in order to transmit broadcast quality video, which compares with a bandwidth of approximately 5 MHz for comparable quality analogue video. Thus, to be able to use digital video the digital signal requires reduction of the quantity of data.
Transmission of video comprises a continuous traffic of data representing moving pictures. As is generally known, the amount of data needed to transfer pictures is high compared to many other types of media, and so far usage of video in low bit-rate terminals has been negligible. However, significant progress has been achieved in the area of low bit-rate video compression. Acceptable video quality can be obtained at bit-rates around 20 kilo bits per second.
As a result of this progressive reduction in bit-rate, it is expected that video is shortly going to become a viable service to offer limited bandwidth networks such as public switched telephone networks (PSTNs) and mobile telecommunications networks. In videophone applications using fixed networks, errors are typically overcome by re-transmitting data. However mobile telephony is prone to higher error rates than the PSTN and has longer round-trip delays. These longer delays make it impracticable to use retransmission with real-time mobile videophone applications. Retransmission is also ineffective in high error rate situations.
A video sequence consists of a series of still images or frames. Data reduction is achieved by using compression techniques to remove redundant data while still retaining sufficient information to allow the original image to be reproduced with an acceptable quality. There are two main types of redundancy in video signals: spatial and temporal. For the coding of images, techniques which exploit spatial redundancy only are termed intra-frame or I frames (i.e. they treat each frame separately), while those which exploit temporal redundancy are termed inter-frame or P frames (i.e. they exploit similarities between frames). The latter invariably also exploit spatial redundancy e.g. by generating motion compensation data which describes the motion (i.e. displacement) between similar areas of the current and a previous image. In the inter frame case, the predicted (motion-compensated) image is rarely precise enough and therefore a spatially compressed prediction error image is also associated with each inter frame.
However, sufficient compression cannot usually be achieved by just reducing the redundancy of the sequence. Thus, video encoders try to reduce the quality of those parts of the video sequence which are subjectively the least important. In addition, the redundancy of the encoded bitstream is reduced by means of efficient lossless coding of compression parameters and coefficients. The main technique is to use variable length codes in which each value is coded using a unique codeword. The shortest codewords are allocated to those values, which statistically occur most often.
Several video coding techniques have been developed. These include run length coding, conditional replenishment, transform coding, Huffman coding and differential phase code modulation (DPCM). Many of these are utilised in key standards such as ITU-T Recommendations JPEG, MPEG-1 and MPEG-2, and H.261/H.263. JPEG defines the form of compressed data streams for still images; MPEG/MPEG2 are for compression of moving pictures; H.261/H.263 have primarily been defined for video telephony applications employing low bit rate communications links (of the order of tens of kbit/s). Current video telephony systems have primarily been designed for use in PSTN or packet networks, and are governed by ITU-T recommendations H.324, which covers low bit rate multimedia communication, H.245 which covers transmission protocols, H.233 which relates to multiplexing and H.323, which covers video conferencing over traditional shared media local area networks. The first mobile videophones will be based on H.324.
Compressed video is easily corrupted by transmission errors, mainly for two reasons. Firstly, due to utilisation of temporal predictive differential coding (inter frames), an error is propagated both spatially and temporally. In practice, this means that once an error occurs, it is easily visible to the human eye for a relatively long time. Especially susceptible are transmissions at low bit-rates where there are only a few intra-coded frames (the transmission of intra-coded frames would stop the temporal error propagation). Secondly, the use of variable length codes increases the susceptibility to errors. When a bit error alters the codeword, the decoder will lose codeword synchronisation and also decode subsequent error-free codewords (comprising several bits) incorrectly until the next synchronisation (or start) code. A synchronisation code is a bit pattern which cannot be generated from any legal combination of other codewords and such codes are added to the bit stream at intervals to enable re-synchronisation.
Every bit in a compressed video bitstream does not have an equal importance to the decompressed images. Some bits belong to segments defining vital information such as picture type (e.g. intra or inter), quantiser value and optional coding modes that have been used. In H.263, the most vital information is gathered in the picture header. A transmission error in the picture header typically causes a total misinterpretation of the subsequent bits defining the picture content. Due to utilisation of temporal predictive differential coding (inter frames), the error is propagated both spatially and temporally. Thus, when a decoder detects a corrupted picture header, a typical approach is to freeze the previous picture on the screen, to send an intra picture request to the transmitting terminal and to wait for the requested intra frame. This causes an annoying pause in the received video.
Transmission errors have a different nature depending on the underlying network. In packet-switched networks, transmission errors are typically packet losses (due to congestion in network elements). In circuit-switched networks, transmission errors are typically bit errors where a ‘1’ is corrupted to ‘0’ or vice versa and, in radio communications networks, errors may occur in bursts making the situation even more difficult.
To impede degradations in images introduced by transmission errors, retransmissions can be used (as described above), error detection (e.g. Cyclic Redundancy Checking (CRC)) and/or error correction methods can be applied, and/or effects from the received corrupted data can be concealed. In fixed networks retransmission provides a reasonable way to protect video data streams from errors since Bit Error Rates (BER) are typically in the region of 10
−6
. However large round-trip delays associated with low bit-rate radio transmission and moderate or high error rates (e.g. 10
−4
to 10
−3
) make it impracticable to use retransmission, especially with real-time radio videophone applications. Error detection and correction methods usually require large overheads in terms of the data that must be transmitted and memory/processing capability required. Consequently, for low bit-rate applications, error concealment may be considered the preferred way to protect and recover images from transmission errors. Video error concealment methods are typically applicable to transmission errors occurring through packet loss and bit corruption.
H.263 is an ITU-T re
Hannuksela Miska
Hourunranta Ari
Antonelli Terry Stout & Kraus LLP
Le Vu
Nokia Mobile Phones Limited
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