NO994939L - System for transmitting MPEG-2 encoded video in ATM networks, as well as methods and apparatus for the same - Google Patents

Abstract

There is disclosed a telecommunication transmission system adapted for transmitting MPEG-2 data streams with a variable bit rate including an encoder and a decoder. The encoder includes a system layer multiplexer adapted to Eor to tame a video encoder buffer by reading data blocks therefrom, at a bit rate constant within a block. The encoder provides a way to implement VBR in an MPEG-2 system layer multiplexer that has been adapted for ATM, and represents a particularly simple implementation of a VBR MPEG-2 encoder having good jitter control. The encoder eliminates the need for a special "cell forming" function to fulfill a traffic contract. To recover bit-timing in the MPEG-2 decoder, data is buffered in a receiver buffer. By continuously reading the deviation from the buffer level, i.e. the level to which the buffer is filled with data, from its nominal value, in this buffer, the deviation for the discharge rate, from the nominal bit rate, can be controlled with a phase-locked loop (PLL) or similar arrangement.

Description

The present invention relates to variable bit rate MPEG-2 encoders and decoders, particularly encoders and decoders adapted for use with ATM, together with methods for encoding and decoding MPEG-2 data having a variable bit rate, and systems for transmitting MPEG-2 data at a variable bit rate.

Systems for the transmission of MPEG-2 coded video data over ATM networks are known, see e.g. "The AMPEC System: A Broadband Multimedia Terminal Supporting High Definition Audiovisual Communications" -Bo Burman, Session 1.4 of the 7th World Telecommunications Forum, Geneva 3 - 11 October 1995.

MPEG-2, or ISO/IEC IS 13818, which is the official name for the MPEG-2 standard, is a key standard for high-quality audiovisual communication and distribution. -A number of other standards/recommendations use MPEG-2 for coding and multiplexing, e.g. the new ITU-T Ree. H.310 which specifies a terminal specification for broadband audiovisual communication over ATM.

Currently, standards such as H.310 and the ATM Forum<1>'s on-demand video specification, etc, only support MPEG-2 streams that have a constant bit rate, CBR. However, both MPEG-2 and ATM have the potential to operate with variable bit rates, VBR. VBRs can be used, with advantage, for coding video signals. It is worth noting that the new DVD discs, which will replace CDs, will be able to store MPEG-2 encoded streams with VBR. Future versions of H.310, and other standard specifications, are expected to include VBR support.

VBR, which must comply with a traffic contract, cannot be easily implemented in an MPEG-2 encoder. There are three main problems that must be overcome if a VBR MPEG-2 encoder is to be realized, namely:

- reading video buffer,

- guarantee that traffic characteristics do not change during subsequent network adaptations, e.g. ATM Adaptation Layer, and

- avoid introducing a self-induced jitter.

The present invention provides, inter alia, a way to realize VBR in an MPEG-2 system layer multiplexer that has been adapted for ATM. This can be used to provide a particularly simple implementation of a VBR MPEG-2 encoder that has good jitter control and, at the same time, eliminates the need for a special "cell shaping" function to fulfill a traffic contract.

If an MPEG-2 stream has been transmitted over an ATM network that introduces significant delay variation, an MPEG-2 decoder used to decode the bit stream must include a device that recovers the transmitted bit rate. The requirements of variable bit rate (VBR) with respect to the "timing" of recovery are different from those of constant bit rate (CBR). It should be particularly noted that a VBR traffic class is expected to introduce higher delay variation, or cell jitter, than is typically introduced by CBR traffic class, due to the fact that statistical multiplexing requires relatively large buffers in the ATM exchanges.

To recover bit timing in an MPEG-2 decoder, data is buffered in a receiver buffer. By continuously reading the deviation of the buffer level, i.e. the level to which the buffer is filled with data, from its nominal value, in this buffer, the deviation of the emptying rate for the buffer, from the nominal bit rate, can be regulated with a phase-locking cycle ("phase lock loop") (PLL), or equivalent arrangement. This is the principle used in the "Adaptive clock" technique, which is specified in ITU-T Ree. 1,363 (AAL1). However, the "Adaptive clock" principle is only able to handle CBR. In a variable bit rate system, timing recovery is more difficult to achieve because: - the nominal buffer level must be allowed to vary in time as the bit rates vary to keep the delay constant in the receiver, and

- the nominal bit rate is not known.

The present invention can be used in encoders, or terminals, for audiovisual communication or distribution, based on MPEG-2 over ATM. Although this technique is primarily intended for use with MPEG-2 decoders capable of managing variable bit rates, eg, ATM-compatible MPEG-2 decoders operating with VBR. However, the technique is not limited to ATM transmission technology, but can be used for any transmission technology that supports VBR.

Proposals related to ETSI NA5 for an AAL2 that can handle dejittering in a VBR data stream have been made. However, these proposals are significantly more complicated than the technique described here for digitizing a VBR data stream, e.g. they require the use of special timestamps which significantly increase the complexity of any practical implementation.

In accordance with a first aspect of the present invention, there is provided a telecommunications transmission system adapted for the transmission of MPEG-2 data streams, over a transmission network, with a variable bit rate including an encoder and decoder, characterized in that said encoders have data reading means, in order to read data blocks from a video buffer, and that a bit rate within a block is constant, and that time tracks between the blocks have a variable duration.

Said decoder may have a receive buffer for receiving incoming MPEG-2 data, time recovery means arranged to extract a current bit rate from an MPEG-2 protocol system layer, calculation means for calculating a nominal buffer level, and recovery means for adaptively recovering deviation from said nominal bit rate.

In accordance with a second aspect of the present invention, a telecommunications transmission system is provided adapted for the transmission of MPEG-2 data streams, over a transmission network, with a variable bit rate including an encoder and decoder, characterized in that said decoder has a reception buffer for receiving incoming MPEG-2 data, timing recovery means arranged to extract a current bit rate from an MPEG-2 protocol system layer, calculation means for calculating a nominal buffer level for said receive buffer, and recovery means for adaptively recovering deviations from said nominal bit- speed.

Said codes may have data reading means, for reading data blocks from a video buffer, and a bit rate within a block which is constant, and time tracks between the blocks which have a variable duration.

Said transmission network may be an ATM transmission network, and said encoders and decoders may be adapted to operate with ATM.

In accordance with a third aspect of the present invention, there is provided an encoder adapted to encode MPEG-2 data for transmission over an ATM network at a variable bit rate, characterized in that data reading means are provided for reading data blocks from a video buffer, that a bit rate within a block that is constant, and that time slots between blocks that have a variable duration.

In accordance with a fourth aspect of the present invention, there is provided an encoder adapted to encode MPEG-2 data for transmission over an ATM network at a variable bit rate, characterized in that said encoder includes a system layer multiplexer adapted to dump a video encoder buffer by reading the data blocks from there, with a bit rate that is constant within a block.

Said blocks can have a size B, where B = Bg - H, where B0 is equal to a SAR-PDU "payload", and H is the number of header bytes contained in a block, and that H = 0 for most, but not all of said blocks.

Said system layer multiplexer may be adapted to regulate time intervals between said blocks in response to signals transmitted thereto by a regulator means.

Said system layer multiplexer may generate an AAL payload stream that meets the criteria specified for a traffic contract.

Said regulator means can select time intervals between said blocks in dependence on an input signal which is indicative of said video data buffer's content.

Said system layer multiplexer may be adapted to locate PCR fields early in said blocks, thereby maintaining alignment between transport packets and data blocks.

Said bit rate can essentially be equal to one eighth of a network bit clock over which said MPEG-2 data is to be transmitted.

Said codes may include a programmable counter adapted to count down from a value, corresponding to the separation between two consecutive cells, to zero, and, in a final part of a counting cycle, a cell may be transferred.

Said regulator means may be adapted to measure network load and bandwidth requirements for a given application, and to determine the separation of two consecutive cells depending thereon.

Said regulator means can be adapted to negotiate available bandwidth with a network over which said MPEG-2 data is to be transmitted.

ATM cells can be transmitted in accordance with AAL1, mentioned

programmable counters may be adapted to count from 52 to zero, a header may be transmitted with said counter having values between 52 and 47, and MPEG-2 data may be transmitted with said counter having values between 46 and zero.

Flag means may be provided and arranged to set a flag, said counter being zero, and re-set said flag before reading the cells.

In accordance with a fifth aspect of the present invention, a method is provided for encoding MPEG-2 data for transmission over an ATM network with a variable bit rate, characterized by reading data blocks from a video buffer, that a bit rate within a block is constant, and that time slots between blocks have a variable duration.

A video encoder buffer can be emptied by reading the data blocks from it at a bit rate that is constant within a block. Said blocks can have a size B, where B = Bq - H, where Bq is equal to a SAR-PDU "payload", and H is the number of header bytes contained in a block, H can be zero for most, but not for all, of said blocks.

The time interval between said blocks can be regulated in response to signals transmitted by a regulator means.

An AAL "payload" stream can be generated that meets

the criteria specified by a traffic contract.

Time intervals between said blocks can be selected in

dependence on said video data buffer's contents.

PCR fields can be located early in said blocks, thus maintaining alignment between transport packets and data blocks.

Said bit rate can be set essentially equal to one eighth of the network bit clock for a network over which said MPEG-2 data is to be transmitted.

Network load and bandwidth requirements can be measured for a given application, and the separation of two consecutive cells can be determined accordingly.

Available bandwidth can be negotiated with a network

over which said MPEG-2 data is to be transferred.

ATM cells can be transmitted in accordance with AAL1.

In accordance with a sixth aspect of the present invention, there is provided a decoder adapted to decode MPEG-2 data transmitted at a variable bit rate, characterized in that said decoder has a receive buffer for receiving incoming MPEG-2 data, time recovery means arranged for extracting a current bit rate from an MPEG-2 protocol system layer, calculation means for calculating a nominal buffer level and recovery means for adaptively recovering deviations from said nominal bit rate.

Said MPEG-2 data may have been transmitted over an ATM network, and said decoder may be adapted to operate with ATM.

Said deviation, from a nominal bit rate, of the rate of emptying said buffer can be regulated by a phase-locked loop.

Said nominal buffer level can be allowed to vary in time as said variable bit rate varies.

Said calculating means may operate at a bit rate immediately preceding said current bit rate, and said current bit rate, to calculate a nominal buffer level.

Said nominal buffer level can be used as a reference to adaptively recover deviations from a current nominal bit rate by means of said phase-locked loop.

Said nominal buffer level can vary, in a period where said nominal buffer level changes from an old to a new value, in the form of a damper, either up or down, between periods where said nominal buffer level is essentially constant.

Bit rate information can be extracted from outgoing data from said buffer, and said bit rate information can be used to determine when said nominal bit rate is changed.

In accordance with a seventh aspect of the present invention, a method is provided for decoding MPEG-2 data for transmission with a variable bit rate, characterized by: - extracting a current bit rate from an MPEG-2 protocol system layer , - calculate a nominal buffer level for a reception buffer adapted to receive incoming MPEG-2 data, and - adaptively recover deviations from said nominal bit rate.

A phase-locked loop can be used to regulate said deviation, from a nominal value, of the buffer level for said reception buffer.

A nominal buffer level can be calculated from an immediately preceding bit rate, and said current bit rate.

Bit rate information can be extracted from outgoing data from said buffer, and said bit rate information can be used to determine when said nominal bit rate is changed.

In accordance with an eighth aspect of the present invention, a system is provided for transmitting MPEG-2 data with a variable bit rate, including an encoder, a decoder and an ATM network, characterized in that: - said encoder is an encoder as described in the paragraphs above, and/or - said codes operate in accordance with a method as described in one of the paragraphs above, and/or - said decoder is a decoder as described in one of the paragraphs above, and/or - said decoder operates in accordance with a procedure as described in one of the sections above.

According to a ninth aspect of the present invention, an MPEG-2 terminal is provided for sending/receiving MPEG-2 data with a variable bit rate, including an encoder, a decoder and an ATM network, characterized in that: - said encoder is an encoder as described in the paragraphs above, and/or - said encoder operates in accordance with a method as described in one of the paragraphs above, and/or - said decoder is a decoder as described in one of the paragraphs above, and/ or - said - decoder operates in accordance with a method as described in one of the paragraphs above.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying figures, in which: Fig. 1 illustrates a process for reading data, block-by-block, from a video buffer with a variable bit rate . Fig. 2 illustrates the variation with time of the contents of a programmable counter with constant block and cell spacing. Fig. 3 illustrates the variation with time of the content of a programmable counter with variable block and cell spacing.

ATM for broadband transmission has the ability to support traffic sources with variable data rates, VBR. Special traffic classes are defined for VBR ATM transmission, namely:

- real-time traffic, and

- other data traffic without special requirements for real time.

The MPEG-2 video coding technique supports VBR. It is thus possible to construct an MPEG-2 adapted encoder that has the advantages that apply to VBR, so that a more uniform image quality and shorter delay is achieved. It should also be noted that the MPEG-2 specifications defining packing, multiplexing and synchronization include support for

VBR.

An encoder operating with VBR requires a control mechanism to continuously calculate a current data rate based on the number of different input signals. Ideally, the data rate is a continuously variable function of time, and may be different for each bit produced by the encoder. The ideal solution would be difficult to implement in a real encoder since the bit clock, which empties the video buffer, would in principle have to vary continuously.

However, a VBR implementation of MPEG-2 encoders can be substantially simplified by using a fixed clock frequency, selected depending on the highest data rate processed by the encoder, instead of continuously varying the transmission clock. The data is then read out of the video buffers, in blocks, at the fixed clock frequency. The variation in bit rate is achieved by varying the intervals between the data blocks. In other words, the instantaneous rate will be "burst"-like with varying length time slots between the bursts, in which the bit rate is zero. This process is illustrated in fig. 1. It appears that the bit rate can be varied from a maximum value, to the right in fig. 1, where the bursts are continuous, i.e. no intermediate time slots between the data blocks, to a much lower bit rate on the left in fig. 1 in which there is a substantial time track between the data blocks. When averaging over the time from the start of a data block to the start of the next block, it can be seen that a constant data rate is achieved within a data block, which can be varied from data block to data block. If the size of the data blocks is kept relatively small, e.g. corresponding to the size of an ATM cell, there are no limitations on the use of a stepwise block-to-block variation in bit rate compared to a continuous variation in bit rate. In practice, the use of stepwise variation in bit rate, based on data blocks corresponding to the size of ATM cells, fits very well with the implementation of VBR in an MPEG-2 encoder adapted for use with ATM.

MPEG-2 supports VBR in the form of a "piecewise constant rate". This means that the byte stream has a constant byte rate between either: - two consecutive PCR fields (the transport stream case), or - two consecutive program mux rates (the program stream case),

see IS 13818-1 for more information.

When an ATM network carries a cell stream with VBR, the transmission of the cell stream is such that the cell intervals are allowed to vary. A traffic contract is established when a VC (Virtual Channel) is established, and such a traffic contract can be modulated by "leaky bucket", see e.g. ATM Forum's description of VBR. In order to comply with the traffic contract, the coder must take care of the following three parameters:

- peak cell speed,

- sustained cell speed, and

- burst size.

These three parameters must not be allowed to exceed the maximum value specified in the traffic contract if the network is to guarantee transmission of the cell stream.

The present invention provides VBR in an MPEG-2 encoder through the action of the system layer multiplexer which flushes video encoder buffers block-by-block at a bit rate R. The block size B is selected according to the size of the AAL payload currently available . For most blocks, B is a constant B0, but for certain blocks the value of B must be reduced, i.e. B < Bq. It is at these points that other information is multiplexed into the bit stream, e.g. PES headers etc. In other words:

where H indicates the number of header bytes that are multiplexed into the data stream.

If AAL1 is used for VBR, then Bq is selected to be equal to 4 7 bytes, since this is the SAR-PDU (Segmentation and Reassembly Protocol Data Unit) payload size in the simplest case.

The intervals between two consecutive blocks are controlled by a multiplexer which, then, receives information from a

regulator. The multiplexer thus becomes the "rate master" in the system, and other elements in the system adapt their processing speed to that set by the multiplexer. The multiplexer can thus, while the video buffer is being emptied, create a cell stream, strictly speaking an AAL payload stream, which meets the parameter values specified in the traffic contract. No subsequent "cell shaping" is necessary if the sequential steps in AAL are implemented as pipelines with fixed delays.

R is preferably chosen to have a relatively high value because this provides a low self-induced jitter in the case where a plurality of different channels are multiplexed together, each with its own VCI.

If the control mechanism, used to select

the data rate, uses the degree of filling in the video buffer as an input signal, rapid changes of the signals must be allowed during the short period when a block of data is read out of the video buffer. The gradient in buffer filling, as a

function of time increases as the selected value of R increases. This does not lead to any practical difficulties, but it has implications for the design of the regulator.

Internally, in the encoder, multiplexer and AAL, etc., data is transferred in the form of blocks that always have an internal bit rate R. This need not cause jitter problems. If alignment between transport packets and data blocks is maintained, i.e. so that the PCR fields always occur early in the blocks, the self-induced jitter amplitude can be kept to an acceptably low level.

Using the method described above, the multiplexer generates a "piecewise constant cell rate", which is referred to as a PCCR in this patent specification. Implementation of the PCCR control mechanism in the multiplexer will now be considered.

The control mechanism for PCCR operates with a clock frequency which is preferably, but not necessarily, selected as the bit clock of the network divided by eight, i.e. the clock rate corresponds to the network "byte clock". The rate is used to select the number of network clock cycles that will separate each cell. This mechanism includes a programmable counter that counts from the "cell distance" value, see fig. 2, to zero. The "cell off-stand" is the distance between two consecutive cell starts. During the last part of the counting cycle, the cell is placed on the channel for transmission.

By using a programmable counter, the cell distance value can be changed, and this can thus mean that the distance between the cells can be varied. The value of the cell spacing is determined in a separate regulator that can measure the load on the network and the bandwidth requirements of the application to be used. On the basis of this data, the regulator negotiates with the network about the bandwidth that will be made available - traffic contract. The case with variable block and cell spacing is illustrated in fig. 3.

The number of bits that the counter contains depends on how low the bit rate, i.e. the size of the maximum distance between cells, can be achieved. E.g. if the maximum channel speed is 155 Mbit/s, and the encoder needs to use 1.5 Mbit/s, then the distance will be: cell distance = (cell size x 155)/l.5, expressed as a number of network clock cycles (bytes -oriented transmission).

When this method is used, it is advantageous to adapt the reading out of a cell to the counter. This means that if ATM cells are transmitted in accordance with AAL1, the header will be transmitted when the counter has values between 52 and 47, inclusive, so that data can be transmitted between counter values of 46 to 0, inclusive, (byte-oriented transmission). It should be noted that it is not allowed to load the counter with values smaller than the cell size, because this will inevitably result in defective reading. By setting a "flag" when the counter reaches zero, which is reset before the start of reading the cells, it is possible to provide protection against faulty operation. If the counter value, when the flag is reset, has not been passed, the cell readout process will end.

To recover bit timing in an MPEG-2 decoder, data is buffered in a receiver buffer. By continuously reading the deviation of the level, from its nominal value, in this buffer, the deviation of the emptying rate of the buffer, from the nominal bit rate, can be controlled with a phase-locked loop (PLL) or similar arrangement. This is the principle used in the "Adaptive clock" technique, which is specified in ITU-T Ree 1.363 (AAL1). However, the "Adaptive clock" principle is only able to handle CBR. In a variable bit rate system, timing recovery is more difficult to achieve because: - the nominal buffer level must be allowed to vary in time as the bit rates vary to maintain the delay constant in the receiver, and

- the nominal bit rate is not known.

A buffer in the receiver is filled with incoming MPEG-2 data. Before the data enters the buffer, the piece-by-piece constant bit rate is extracted from the incoming data at the system layer of the MPEG-2 protocol. This can be done e.g. using the program_mux_ speed parameter, see ISO/IEC IS13818-1. The nominal buffer level is calculated on the basis of this information and the previous bit rate. The nominal buffer level will typically adopt the form of a "ramp" up, or down, which changes to a constant level when all the data transferred at the old rate has left the buffer.

The calculated nominal level can then be used as a reference for the adaptive recovery of the deviation from the nominal bit rate by means of a PLL, or equivalent arrangement. To determine when the nominal bit-rate should be changed, the bit-rate sinf ormas j on is also extracted from the output data.

This technique can be considered an extension of the "Adaptive Clock", which enables it to work with VBR, as well as CBR. An implementation of this technique in an ATM-adapted MPEG-2 decoder can therefore use the same hardware as that used for the AAL1 Adaptive Clock. This means that separate AAL1 implementations for VBR and CBR are not required.

Claims (47)

1. Telecommunications transmission system adapted for transmission of MPEG-2 data streams, over a transmission network, with a variable bit rate including an encoder and decoder, characterized in that said encoders have data reading means, to read data blocks from a video buffer, that a bit rate within a block is constant, and that time slots between the blocks have a variable duration. 2. Telecommunications transmission system adapted for transmission of MPEG-2 data streams, over a transmission network, with a variable bit rate including an encoder and decoder, characterized in that said decoder has a reception buffer for receiving incoming MPEG-2 data, time recovery means arranged to extract a current bit rate from an MPEG-2 protocol system layer, calculation means for calculating a nominal buffer level for said receive buffer, and recovery means for adaptively recovering deviations from said nominal bit rate. 3. Telecommunication transmission system according to claim 1, characterized in that said decoder has a receive buffer for receiving incoming MPEG-2 data, time recovery means arranged to extract a current bit rate from an MPEG-2 protocol system layer, calculation means for calculating a nominal buffer level for said receive buffer and recovery means for adaptively recovering deviations from said nominal bit rate. 4. Telecommunications transmission system in accordance with claim 2, characterized in that said codes have data reading means, to read data blocks from a video buffer, that a bit rate within a block is constant, and that time tracks between the blocks have a variable duration. 5. Telecommunications transmission system in accordance with one of claims 1-4, characterized in that said transmission network is an ATM transmission network, and that said codes and decoders are adapted to operate with ATM. 6. Encoders adapted to encode MPEG-2 data for transmission over an ATM network at a variable bit rate, characterized in that data read memory is provided for reading blocks of data from a video buffer, that a bit rate within a block is constant , and that the time slot between the blocks has a variable duration. 7. Encoders adapted to encode MPEG-2 data for transmission over an ATM network at a variable bit rate, characterized in that said encoders include a system layer multiplexer adapted to empty a video encoder buffer by reading data blocks therefrom, at a bit rate that is constant within a block. 8. Codes in accordance with claim 7, characterized in that said blocks have a size B, where B = Bq - H, where Bq is equal to a SAR-PDU payload, and H is the number of header bytes contained in a block, where H is equal to zero for most, but not all, of the mentioned blocks. 9. Codes in accordance with one of claims 7 or 8, characterized in that said system layer multiplexer is adapted to regulate time intervals between said blocks in response to signals transmitted thereto by a regulator means. 10. Codes in accordance with claim 9, characterized in that said system layer multiplexer generates an AAL payload stream that meets the criteria specified by the traffic contract. 11. Codes in accordance with one of claims 9 or 10, characterized in that said regulator means selects time intervals between said blocks in dependence on an input signal which is indicative of said video data buffer's content. 12. Codes in accordance with one of claims 7-11, characterized in that said system layer multiplexer is adapted to locate PCR fields early in said blocks, and thus maintain alignment between transport packets and data blocks. 13. Codes in accordance with one of claims 7-12, characterized in that said bit rate is essentially equal to one eighth of a network bit clock over which said MPEG-2 data is to be transmitted. 14. Codes in accordance with one of claims 7-13, characterized in that said codes include a programmable counter adapted to count down from a value, corresponding to the separation of two consecutive cells, to zero, and that a cell is transmitted in a final part of a counting cycle. 15. Codes in accordance with one of claims 9-14, characterized in that said regulator means is adapted to measure network load and bandwidth requirements for a given application, and to determine the distance between two consecutive cells depending thereon. 16. Codes in accordance with claim 15, characterized in that said regulator means is adapted to negotiate available bandwidth with a network over which said MPEG-2 data is to be transmitted. 17. Codes in accordance with one of claims 9-16, characterized in that ATM cells are transmitted in accordance with AAL1, in that said programmable counter is adapted to count from 52 to zero, in that a header is transmitted when said counter has values between 52 and 47, and that MPEG-2 data is transmitted when said counter has values between 4 6 and zero. 18. Codes in accordance with claim 17, characterized in that flag means are provided and arranged to set a flag, when said counter is zero, and reset said flag before reading the cells. 19. Method for encoding MPEG-2 data for transmission over an ATM network with a variable bit rate, characterized by reading data blocks from a video buffer, that a bit rate within a block is constant, and that time tracks between the blocks has a variable duration. 20. Method for encoding MPEG-2 data for transmission over an ATM network with a variable bit rate, characterized by emptying a video code buffer by reading data blocks therefrom at a bit rate that is constant within a block. 21. Method in accordance with claim 20, characterized in that said blocks have a size B, where B = Bq - H, where Bq is equal to the SAR-PDU payload, and H is the number of header bytes contained in a block, and that H = 0 for most, but not all, of said blocks. 22. Method in accordance with one of claims 20 or 21, characterized in that time intervals between said blocks are regulated in response to signals transmitted by a regulator means. 23. Method in accordance with claim 22, characterized in that an AAL payload stream that meets the criteria specified by a traffic contract is generated. 24. Method in accordance with one of claims 22 or 23, characterized in that the time intervals between said blocks are selected depending on said video data buffer's content. 25. Method in accordance with one of claims 20-24, characterized in that PCR fields are located early in said blocks, thus maintaining alignment between transport packets and data blocks. 26. Method in accordance with one of claims 20-25, characterized in that said bit rate is set substantially equal to one eighth of a network bit clock for a network over which said MPEG-2 data is to be transmitted. 27. Method in accordance with one of the claims 22-26, characterized by measuring the network load and bandwidth requirements for a given application, and determining the distance of two consecutive cells depending thereon. 28. Method in accordance with claim 27, characterized in that available bandwidth is negotiated with a network over which said MPEG-2 data is to be transmitted. 29. Method in accordance with one of claims 22-27, characterized in that ATM cells are transmitted in accordance with AAL1. 30. Decoder, adapted to decode MPEG-2 data transmitted at a variable bit rate, characterized in that said decoder has a receiver buffer for receiving incoming MPEG-2 data, time recovery means arranged to extract a current bit rate from an MPEG-2 protocol system layer, calculation means for calculating a nominal buffer level for said receive buffer, and recovery means for adaptively recovering deviations from said nominal bit rate. 31. Decoder in accordance with claim 30, characterized in that said MPEG-2 data has been transmitted over an ATM network, and that said decoder is adapted to operate with ATM. 32. Decoder in accordance with one of claims 30-31, characterized in that said deviation, from a nominal bit rate, is regulated by a phase-locked loop. IN 33. Decoder in accordance with one of claims 30-32, characterized in that said nominal buffer level is allowed to vary in time as said variable bit rate varies. 34. Decoder in accordance with one of claims 30-33, characterized in that calculation means operates at a bit rate immediately before said current bit rate, and said current bit rate, to calculate a nominal buffer level. 35. Decoder in accordance with one of claims 32-34, characterized in that said nominal buffer level is used as a reference to adaptively recover deviations from a current nominal bit rate by means of said phase-locked loop. 36. Decoder in accordance with one of claims 30-35, characterized in that said nominal buffer level varies, during a period when said nominal buffer level level varies from an old to a new value, in the form of a "ramp", either up or down, between periods where said nominal buffer level is essentially constant. 37. Decoder in accordance with one of claims 30-36, characterized in that bit rate information is extracted from outgoing data from. said buffer, and said bit rate information is used to determine when said nominal bit rate is changed. 38. Method for decoding MPEG-2 data for transmission with a variable bit rate, characterized in that the method comprises - to extract a current bit rate from an MPEG-2 protocol system layer, - calculate a nominal buffer level for a receive buffer adapted to receive incoming MPEG-2 data, and - adaptive recovery of deviations from said nominal bit rate. 39. Method in accordance with claim 38, characterized in that said MPEG-2 data has been transmitted over an ATM network. 40. Method in accordance with claim 38 or 39, characterized in that a phase-locked loop is used to regulate said deviation, from a nominal value, of the buffer level for said receiving buf fer . 41. Method in accordance with one of claims 3 8-40, characterized in that nominal buffer levels are allowed to vary in time as said variable Å) bit rate varies. 4 2. Method in accordance with one of the claims 38-41, characterized by the calculation of the nominal buffer level from an immediately preceding bit rate and said current bit rate. 43. Method according to one of claims 40-42, characterized by using said nominal buffer level as a reference to adaptively recover deviations from a current nominal bit rate by means of said phase-locked loop. 44. Method in accordance with one of claims 38-43, characterized in that said nominal buffer level varies, during a period where said nominal buffer level changes from an old to a new value, in the form of a ramp, either "up or down. 45. Method in accordance with one of claims 3 8-44, characterized by extracting bit rate information from outgoing data from said buffer, and using said bit rate information to determine when said nominal bit rate is changed. 46. System for transmitting MPEG-2 data with a variable bit rate, including an encoder, a decoder and an ATM network, characterized in that: - said coder is a coder in accordance with one of claims 6-18, and/or - said codes operate in accordance with the method in accordance with one of claims 19-29, and/or - said decoder is a decoder in accordance with one of claims 3 0-37, and/or - said decoder operates in accordance with the method in accordance with one of claims 38-45. 47. MPEG-2 terminal for sending/receiving MPEG-2 data with a variable bit rate, including an encoder, a decoder and an ATM network, characterized in that: - said code is a code in accordance with one of claims 6-18, and/or - said codes operate in accordance with the method in accordance with one of claims 19-29, and/or - said decoder is a decoder in accordance with one of claims 30-37, and/or - said decoder operates in accordance with the method in accordance with one of claims 38-45.