Transcoder

Details Transcoder Transcoder

1999-04-20

2003-02-25

Britton, Howard (Department: 2613)

Pulse or digital communications

Bandwidth reduction or expansion

Television or motion video signal

C375S240160, C375S240200

Reexamination Certificate

active

06526099

ABSTRACT:

TECHNICAL FIELD
The present invention relates to transcoders for conversion of signals between a first and a second coding scheme. The transcoders according to the invention are particularly suitable for converting two- and three-dimensional images and video signals.
BACKGROUND OF THE INVENTION AND PRIOR ART
There are many occasions when it is necessary to transmit moving picture television over long distances via a transmission link. Broadcast quality television requires an excess of 100 Mbit/s when transmitted in digital form which is expensive to transmit and requires links of high bandwidth. An acceptable degree of degradation in the picture quality can be introduced in order to reduce the information content being transmitted. Additionally or alternatively, compression coding techniques may be used which take advantage of the high degree of spatial and temporal redundancies in the video signals being encoded.
There are also other applications where compression coding techniques are used. So, for example, for video conference applications a compression down to a bit rate of a few hundred kbit/s is possible whereas videophone-quality pictures including sound can be compressed down to less than 64 kbit/s.
Redundancy reduction techniques assume that there is a spatial and/or temporal correlation between neighbouring pixels or blocks of pixels. The details of correlation are encoded as well as the differences between the assumptions and the actual pixels or blocks. Typically each frame of an image to be coded comprises an array of picture elements (pixels) which are divided into blocks of N×M pixels.
Predictive coding exploits the assumption that a value within a frame is related to some neighbouring values, in the same or a different frame, and the value may therefore be calculated at the receiver instead of being transmitted. It is then only necessary to transmit the prediction error arising from such an assumption. For instance the first pixel of a frame may be transmitted exactly whilst each subsequent pixel is transmitted as a difference from its predecessor. In more complex schemes the prediction may be found by a combination of a number of pixels.
Transform coding exploits the correlation of pixel magnitudes within a frame by transforming the magnitudes into another set of values, many of which are expected to be relatively small and which can therefore be coded using fewer bits. The most common form of transform coding uses the Discrete Cosine Transform (DCT). A block of N×M pixels is transformed into an array of N×M transform coefficients. The resulting array of coefficients is then quantised by dividing each coefficient by a quantisation factor. The quantised coefficients may be coded by a variable length code, for instance a Huffman code.
Another coding technique is motion compensation in which a picture is divided into blocks of pixels and each block of the current frame is compared with the corresponding block of a reference frame, which may be a previous and/or a subsequent frame, and with regions shifted in position from that block, and that region of the reference frame which the block most closely resembles is identified.
The vector difference in position between the identified region and the block in question is termed a motion vector and is used to shift the identified region of the reference frame into the position of the relevant block in the current frame. Motion vectors are generated for all the blocks of the current frame and these are used to derive a predicted frame from the reference frame. The differences between the current and predicted frame are, on an average, smaller than those between the current and reference frame and can be encoded using fewer bits. A decoder which already has the reference frame stored can thus reproduce the current frame using the motion vectors and the difference values. A signal may be coded using any of the aforementioned coding techniques, either separately or in combination.
Furthermore, it is reasonable to expect that in the future a wide range of quality video services like HDTV, etc. will be available together with the lower quality video services such as the video-phone and video-conference services. Multimedia documents containing video will most probably not only be retrieved over computer networks, but also over telephone lines, ISDN, ATM, or even mobile networks. The transmission over several types of links or networks with different bit rates and varying traffic load will require an adaptation of the bit rate to the available channel capacity. A main constraint on the systems is that the decoding of any level below the one associated with the transmitted format should not need the complete decoding of the transmitted source.
To maximise the integration of these various quality video services, a single coding scheme which can provide an unlimited range of video services is desirable. Such a coding scheme would enable users of different qualities to communicate with each other. For example, a subscriber to only a lower quality video service should be capable of decoding and reconstructing a digitally transmitted higher quality video signal, albeit at the lower quality service level to which he subscribes. Similarly, a higher quality service subscriber should be capable of decoding and reconstructing a digitally transmitted lower quality video although of course its subjective quality will be no better than its transmitted quality.
The problem therefore is associated with the way in which video will be transmitted to subscribers with different requirements (picture quality, processing power, memory requirements, resolution, bandwidth, frame rate, etc.). The following points summarise the requirements:
satisfy users having different bandwidth requirements,
satisfy users having different computational power,
adapt frame rate, resolution and compression ratio according to user preferences and available bandwidth,
adapt frame rate, resolution and compression ratio according to network abilities,
short delay, and
conform with standards, if required.
One solution to the problem of satisfying the different requirements of the receivers is the design of scalable bitstreams. In this form of scalability, there is usually no direct interaction between transmitter and receiver. Usually, the transmitter is able to make a bit stream which consists of various layers which can be used by receivers with different requirements in resolution, bandwidth, frame rate, memory or computational complexity. If new receivers are added which do not have the same requirements as the previous ones, then the transmitter has to be re-programmed to accommodate the requirements of the new receivers. Briefly, in bit stream scalability, the abilities of the decoders must be known in advance.
Furthermore, the design of a scalable bitstream can result in a higher number of bits compared to a single bit-stream for achieving a similar quality. A scalable bit stream also requires very computationally powerful coders, which may consist of a number of coders equal to the number of different receivers.
A different solution to the problem is the use of transcoders. A transcoder accepts a received data stream encoded according to a first coding scheme and outputs an encoded data stream encoded according to a second coding scheme. If one had a decoder which operated according to a second coding scheme then such a transcoder would allow reception of the transmitted signal encoded according to the first coding scheme without modifying the original encoder. For example, the transcoder could be used to convert a 128 kbit/s video signal conforming to ITU-T standard H.261, from an ISDN video terminal for transmission to a 28.8 Kbit/s signal over a telephone line using ITU-T standard H.263.
Most of the known transcoders decode video signals according to a first coding scheme into an uncompressed video signal which is then encoded by an encoder according to a second coding scheme to output a new compressed data stream. Thus a full decoding operat

Affiliated with

Also associated with

Rating

Transcoder does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Transcoder, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Transcoder will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3163249