JP2004530357A - Data packet transfer method and apparatus in communication network - Google Patents

Abstract

A method and transcoder for transcoding data communicated between a transmitter (100) and a receiver (102). A data unit (202, 208) containing information in a first format (F, IF) is received and extracted based on a current coding state (302, 502, 706) extracted from previous coded data. Encoded into new data units (208, 210) in two formats (IF, F '). A new current encoding state is extracted each time a data unit (202, 500, 700) is encoded. If the received data units are in the wrong sequence order, the current coding state is saved and the new pseudo-coding state (DS2 ', ES2', CS2 ') codes the next received data unit. Extracted to make The finally received missing data (F2, S4) may be coded into a second format (IF, F ') based on the stored coding state and then used as valid data. In this way, the delay is minimized while still maintaining high quality, and no jitter buffer is required.
[Selection diagram] FIG.

Description

【Technical field】
[0001]
The present invention relates to a method and an apparatus for transferring a data unit from a transmitter to a receiver. In particular, the invention relates to compensating for missing or out-of-order data units received by the transcoder.
[Background Art]
[0002]
Recently, systems and solutions have evolved to provide packet-based transmission of digitally encoded information for various services such as telephones, videophones, television video distributors, as well as Internet surfing and e-mail. I have. These services are susceptible to delays in various ranges and are therefore often classified to select an appropriate transmission mechanism. The most delay-sensitive applications, such as telephones and videophones, are sometimes referred to as real-time services, which require a total transmission time of less than about 200 milliseconds from the transmitter to the receiver. Applications that are not delay sensitive, such as downloading home page data from the Internet, are referred to as best effort services, and transmission times of a few seconds are usually acceptable. In this way, when transmission means are shared, real-time data is generally given priority over best-effort data.
[0003]
The information to be transmitted is digitally encoded on the transmitter side and is tailored to data units of a particular format, for example data packets or frames, according to a coding scheme. Data units are treated as transmission units along a transmission path, including, for example, various networks, switches, gateways, and routers. The encoded information is displayed or played back to the user at the receiving end, so that when it is received at the destination, it is finally decoded. However, due to mismatching of the coding criteria, the transmitting and receiving devices can often only use specific coding schemes which are different from each other, so that the information encoded by the transmitter is Need to be re-coded at the location. This operation is often called transcoding.
[0004]
Moreover, different networks and / or switching nodes along the transmission line also use different standard formats and coding schemes, resulting in a further need for transcoding. In this way, the transmitted data may have to be recoded several times before reaching the final receiver. For real-time services and also for some media streaming services, it is desirable to minimize delays due to transcoding activity within nodes on all transmission paths. For example, if a delay of more than 200 milliseconds is introduced in a voice call between two parties, the flow of the dialogue will be significantly hindered.
[0005]
FIG. 1 illustrates a typical communication scheme for transferring data between a transmitter 100 and a receiver 102. In this simplified embodiment, an asymmetric transmission in only one direction at a time is considered. With symmetric transmission, it is of course possible for each party 100 and 102 to be both a transmitter and a receiver, such as a voice call. The transmitter 100 is connected to an access network 104, which may be a local area network (LAN), a wireless network, or the like. Receiver 102 is similarly connected to another access network 106. Each access network 104, 106 includes one or more public networks, such as the Internet or the Public Switched Telephone Network (PSTN), and further connects to respective gateway switches 108, 110 for communicating with the intermediate communication system 112. Is done.
[0006]
The transmitter 100 can use the first coding scheme to encode the data to be transmitted. On the other hand, receiver 102 can use a second coding scheme to decode the received data, where the second coding scheme is different from the first. Further, the intermediate communication system 112 may use one or more additional coding schemes where the coding schemes are different to transfer the communicated data. Thus, it is clear that each time the communicated data enters a new domain that uses a coding scheme different from the coding scheme used previously, it must be re-coded.
[0007]
For example, transcoding must occur between a packet-switched network and a circuit-switched network using, for example, time division multiplexing (TDM). In this way, data is transferred by one or more transcoders to create new data units from previously received data units before being further transmitted in the transmission path. The gateways 108, 110 and other switching nodes in the intermediate communication system 112 of FIG. 1 may generally include such transcoders.
[0008]
Furthermore, if one or more networks 104, 112, 106 use packet-based transmission, there is a potential risk that data packets will be received at the transcoder in the wrong order. With packet-based transmission, each data packet can be processed individually and take a separate transmission path and / or experience different delays at intermediate nodes. Thus, buffers are often located in transcoders and receivers to compensate for packets received with different delays, sometimes referred to as jitter buffers. The jitter buffer allows the reception of a plurality of data packets that are ordered and arranged before being transcoded or decoded. The packets are then reordered if necessary. Another approach is to simply discard the packets received out of order, but with the consequence of quality degradation. The receiver side of the intermediate node between the packet domain and the TDM domain generally includes a jitter buffer.
[0009]
Jitter buffer technology has the disadvantage of introducing unnecessary delay in transmission. It also requires a clock timing reference when TDM based transmission is involved. Further, the final receiver 102 generally includes a jitter buffer or similar decoding function to increase the total latency of the transmitted data. The transcoding function should not add more latency than is necessary to process the data and perform the transcoding. However, if every data packet is transcoded sequentially as it arrives, the resulting quality will degrade when the data packets arrive out of order. In particular, many coding schemes rely on historical data when decoding special large amounts of data. That is, information from previously decoded data is used to decode the next data. Therefore, all data must be decoded in the same sequence as that output from the transmitting encoder in order to obtain high quality at the receiving end.
[0010]
It is an object of the present invention to overcome the drawbacks outlined above and to achieve high quality of the received encoded data when decoded at the receiving end, while at the same time minimizing the transmission delay in the intermediate transcoder. It is.
DISCLOSURE OF THE INVENTION
[Means for Solving the Problems]
[0011]
These objectives are achieved by a simple solution to minimize the delay in the transcoder and eliminate the need for a jitter buffer, which ensures that data units arriving in the wrong sequence order , By recovering the data that arrives late.
[0012]
A method and a transcoder unit are provided for transcoding communication data in a transmission path between a transmitter and a final receiver. A data unit containing coded information in the first format is received and coded in the second format into a new data unit based on the current coding state extracted from the previous coded data. The new current encoding state is extracted each time a data unit is encoded.
[0013]
Upon receiving each data unit, it is detected whether the received data is in the wrong sequence order, which can be done by reading information indicating the sequence order of the received data. If this is the case, the coded state extracted from the previous coded data is saved and a new pseudo coded state is extracted to code the next received data unit.
[0014]
If the missing data is finally received, it is coded into a second format based on the saved coding state, and the coded missing data can be used as valid data. The saved coding state can then be deleted. If the corresponding missing data is not received within a preset time limit, the saved coding state may also be deleted, but it may be based on data flow characteristics. If the number of saved decoding states reaches a preset threshold, the saved coding states can also be deleted, but also based on data flow characteristics.
[0015]
The inventive procedure can be used to decode the received data unit, where the encoding state is the decoding state and the second format is the intermediate format. The received data unit may then be decoded into a stream of intermediate data that is split into intermediate data segments that are encoded into new data units.
[0016]
The inventive procedure can also be used to encode received data units, where the encoding state is the encoding state and the first format is the intermediate format. The received data unit may then include the decoded intermediate data encoded into a new data unit.
[0017]
If the received data is in the wrong sequence order, error correction data can be derived from the previously received data unit and the current decoding state, where the new pseudo-coding state is based on the derived error correction data. Can be extracted.
[0018]
The inventive procedure can be implemented in a transcoder unit located in the transmission path between the transmitter and the final receiver and having means for performing the procedure.
[0019]
The inventive procedure may also be embodied in a computer program product loadable directly on a computer in the transcoder unit, including software code means for performing the procedure.
[0020]
The inventive procedures may alternatively be embodied in a computer program product stored on a computer usable medium, including a readable program for causing a computer in a transcoder unit to perform the procedures.
[0021]
The invention will now be described in more detail with reference to the drawings.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022]
FIG. 1 illustrates the exemplary communication system described above for transferring data between a transmitter 100 and a receiver 102 in which the present invention may be implemented. Of course, the present invention may be used for many other types of communication systems.
[0023]
FIG. 2 is a schematic diagram of the data flow in an exemplary transcoder unit 200 for transcoding data according to the present invention. Data 202 encoded according to a first coding scheme and having a first format F is received by a transcoder unit 200 that transcodes the data into a second coding scheme having a second format F ′. The transcoding method is divided in this embodiment into two operations, a decode operation 204 and an encode operation 206. Between the two operations, the data is in the decoded intermediate format IF 208. However, it is possible within the scope of the invention to recode or transcode the input data 202 of the first format F directly into the second format F 'without using any intermediate format. After being encoded in the second format F ′, the data 210 is further transmitted to the next node (not shown) in the transmission path. In addition, the transcoder includes a memory 212 for storing coding states, described in more detail below.
[0024]
Generally, input data is grouped into data units, eg, packets or frames, that are encoded according to a first format F. Input data can be packet-switched or circuit-switched. Each input data unit includes a field or the like, which is read by the receiving transcoder unit 200 and has information indicating a sequence order such as a sequence number or a time stamp. Normally, if the received data units are received in the correct order as transmitted first and without data loss, the data units are sequentially decoded in operation 204 to the next format IF or F '.
[0025]
This embodiment is further illustrated in FIG. 3 with further reference to FIG. 2, where F1, F2, F3. . . Is a first format F data packet 202 received in the correct order by the transcoder unit 200. Data packet 202 is separately decoded in operation 204 into an intermediate data unit 300 having an intermediate format IF. When decoding each data packet 202, information from the previous decode result shown in FIG. 3 is used as a series of current history decode states DS302. The decode state includes, besides the actual data, variables or parameters that are extracted from the performed decode operation and reflect some characteristic of the data.
[0026]
In this manner, the received data packet F1 is decoded 204 into the intermediate data unit IF1 based on the decoding result of the previously decoded data packet, not shown, but based on the current decoding state DS0 extracted from the data packet. After using the decode state DS0, a new decode state DS1 is extracted from the latest decode of F1. The next data packet F2 is then decoded 204 to IF2 based on the last decoding state DS1 and the like.
[0027]
In the above description, it is generally assumed that all input data packets have the same size, although in reality the packet lengths may vary. In that case, the data may be reordered into equally sized units, but the transcoding method may be handled using the same principles as simplifying equal packet lengths.
[0028]
The decoding method illustrated in FIG. 3 involves data packets received in the correct sequence order. However, if a data packet is received with a higher sequence number increased by one from the previous sequence number, it may be assumed that the missing packet has been dropped or delayed. Alternatively, the packet order may be indicated by a time stamp or any other order that indicates information. If data packets are short in this way, an error correction mechanism may be invoked to minimize the resulting quality degradation at the receiving end. The error correction mechanism may derive error correction data based on the previous data packet and the latest decoding state extracted from the decoding of the previous data packet. Error correction is a well-known procedure that attempts to create the most likely data when the actual data is not available. In general, error correction is performed according to predetermined criteria of the data format, or if no criteria exist, a proprietary mechanism may be used.
[0029]
According to the present invention, when data is received in the wrong sequence order, an error correction mechanism is invoked, and if the missing data packets are in the wrong order but eventually come out, the error correction The previous most recent decode state is saved for later use. In this case, the missing data packet is decoded according to the invention based on the stored decoding state.
[0030]
FIG. 4 illustrates an exemplary decoding procedure according to the present invention when data packets are received in the wrong sequence order. First, the data packet F1 is decoded 204 into an intermediate data unit IF1 based on the latest decoding state DS0, where a new decoding state DS1 is extracted as in the previous embodiment of FIG. However, next comes a data packet F3 having a high sequence number 3 which is increased by one from the previous sequence number 1. Upon detecting that the sequence number of the received data packet F3 is erroneous, that is, higher than expected, the error correction mechanism 400 derives error correction data 402 in an intermediate format based on the previous decoding state DS1, and generates a new Called to extract the pseudo decode state DS2 '. Since the error correction is a decoder function whose decoding state can be extracted irrespective of whether or not actual data is available, the pseudo decoding state DS2 'can be extracted similarly to other decoding states.
[0031]
Next, the data packet F3 is decoded 204 into an intermediate data unit IF3 based on the new pseudo decode state DS2 '. Further, the previous decoding state DS1 is stored in the memory 212 in the transcoder unit 200. The new decode state DS3 is also extracted, but not shown, from the decode 204 of the data packet F3 for later use when decoding the next sequence of data packets.
[0032]
In this embodiment, the missing data packet F2 may be decoded 204 into an intermediate data unit IF2 based on the previous received and stored previous decoding state DS1. The used saved decoding state DS1 may be subsequently deleted from the memory 212 in the transcoder unit 200. Finally, the recovered intermediate data unit IF2 replaces the previously derived error correction data 402 and is inserted between IF1 and IF3 in the correct sequence order. However, if the missing data packet F2 is not received, for example, within a preset time period, when the data in the intermediate format 208 is encoded 206 into the second format F ′ in the transcoder unit 200, the error correction data 402 May remain valid.
[0033]
In this way, even if one or more data units in the first format F are received by the transcoder in the wrong order, the effectively decoded intermediate data unit 300 will again be in the second format F '. They can be placed in the correct sequence order before being encoded. Furthermore, this procedure is performed with minimal delay while maintaining the highest possible quality.
[0034]
The embodiment of FIG. 4 described above involved one data packet received out of sequence. If multiple data packets are received out of sequence using multiple stored decoding states, a corresponding procedure may be performed. In addition, the saved decode state may be deleted from memory 212 if left unused for a preset time period. The preset deletion time limit may be further configurable or adaptive depending on data flow characteristics such as the number of data packets in the wrong sequence that changes over time. Alternatively, the number of simultaneously stored decode states can be similarly set depending on the data flow characteristics, or can be maximized by a preset threshold that is adaptive. If the number of saved decode states reaches a preset threshold, a mechanism may be invoked to delete individual saved decode states from memory 212.
[0035]
After the decoding operation 204, the intermediate data unit 300 is arranged in a continuous stream of data 208 input to the encoding operation 206. See FIG. The transcoder unit 200 then converts the intermediate data into new data units, such as packets, frames or blocks, ie data parts of a predetermined size, according to a new second coding scheme or format F ′, depending on the availability of the data. Encode 208. If the intermediate data 208 is not currently available, the encoding method may simply be deferred until it becomes available. The new data unit in the second format F 'is finally transmitted to the next node in the transmission path.
[0036]
FIG. 5 shows the intermediate data units IF1, IF2, IF3. . . 5 illustrates an encoding procedure according to the invention for an intermediate data stream 208 comprising First, new intermediate data segments S1, S2, S3... Are required, if necessary, to be received and encoded in a second format F '. . . 500. In the figure, the size of the new intermediate data segment 500, indicated by the vertical dashed line, differs in this embodiment from the size of the intermediate data unit 300 resulting from the decoding operation 204. Each new intermediate data segment or unit 500 is then encoded 206 into a new data unit 506 in the second format F ′ based on the current encoding state ES 502 extracted from the previous encoding operation. The encoding state includes, besides the actual data, variables or parameters extracted from the encoding operation to be performed and used to encode the data.
[0037]
In this way, the intermediate data segment S1 is encoded 206 into a new data unit F′1 based on the last encoding state ES0 extracted from the previously encoded new data packet, not shown. After using the encoding state ES0, a new encoding state ES1 is extracted from the latest encoding of F′1. The next intermediate data segment or unit S2 is then encoded 206 into F'2 based on the last decoding state DS1 etc.
[0038]
When an input data packet or unit in the correct sequence order is not available, the intermediate data unit 300 is such that "good data" is created directly from the payload of the input data packet or unit, and "error correction data" is the error correction mechanism. By using 400, it can be classified according to the nature of its content, as derived from the latest decoding state. In the embodiment of FIG. 5, IF1 and IF2 include "good data", while IF3 includes "error correction data" in the hatched diagram. Thus, segments S1 and S2 contain only "good data", while segments S3 and S4 contain "good data" and "good data" as a result of the difference in size between intermediate data unit 300 and intermediate data segment 500. Error correction data ".
[0039]
According to one aspect of the invention, intermediate data segment 500 may be treated differently depending on the classification of the data. Thus, if segment 500 contains at least some "good data", the segment is encoded and the new encoding state is extracted accordingly. This is true for all segments S1-S4 shown in FIG.
[0040]
However, if the entire segment does not contain "good data", i.e., only contains "error correction data", then the segment is simply discarded and at least partially extracted from the "good data". Is saved. This is illustrated in FIG. 6a, where the segment S1 containing "good data" is encoded into a new data unit F'1 and a new encoding state ES1 is extracted. The next segment S2 contains "error correction data" and does not contain "good data", which is indicated by the diagonal lines in the figure, but is therefore discarded 600. One new data unit is As a result, it is considered lost. The latest encoding state ES1 is saved for later use if the missing data finally arrives.
[0041]
Referring to FIG. 6b, a segment S1 containing "good data" is encoded into a new data unit F'1, and a new encoding state ES1 is extracted, just as in the embodiment of FIG. 6a. The next arriving segment S2 contains "error correction data" and does not contain "good data" and is again shown by the hatched lines in the figure. According to another embodiment, based on the available error correction data, segment S2 is first encoded into pseudo data unit 602, which is then discarded 600. In this case, a new pseudo-encoding state ES2 ', not shown, is extracted from the encoding of S2 and used to encode the next segment. In this embodiment, the missing data is finally encoded into a new valid data unit F′2 based on the previously saved encoding state ES1, arriving later at segment S4. It should be noted that the sequence order of the segments S1-S4 was selected from the division of the intermediate data stream for the purpose of explanation and should not be confused with the sequence order of the actual data. Thus, in this example, segment S4 contains the data that should have been included in segment S2.
[0042]
Next, the restored new data unit F'2 can be transmitted to the next node in the transmission path. Furthermore, the final receiver can often restore the correct data unit order before decoding, so that F'2 can be transmitted after the wrong sequence position, in this case F'3. .
[0043]
The procedure described above in connection with FIG. 6b may also be performed for multiple segments arriving out of sequence using multiple stored encoding states. Moreover, the same mechanism for storing multiple decoding states as described above may be generally applied for storing multiple encoding states.
[0044]
The embodiment described above involves two separate operations of first decoding the received data from the first format to the intermediate format and then encoding the intermediate data to the second format for transmission. However, the invention can also be used to recode input data of a first format directly into a second format without using an intermediate format.
[0045]
FIG. 7 illustrates the most general case according to the invention of a recoding or transcoding procedure when data units are received in the wrong sequence order. Generally, F1, F2, F3. . . Indicates an input data unit 700 having a first format F. The data unit 700 has a second format F ', generally F'1, F'2, F'3. . . , Are separately coded in operation 702 into a new data unit 704. The expression "coding" is used herein to indicate either decoding, transcoding or encoding. When encoding each input data unit 700, information from the previous encoding result shown in FIG. 7 is used as a series of current history encoding states CS706.
[0046]
In this way, the data unit F1 of the first format F is coded 702 into a new data unit F'1 of the second format F 'based on the current coding state CS0, where the new coding state CS1 Is extracted. The next received data unit F3 contains a higher sequence number 3 by one than the previous sequence number 1. Upon detecting that the sequence number of the received data unit F3 is higher than expected, the pseudo data 708 in the second format F 'is based on the current coding state CS1 to extract a new pseudo coding state CS2'. Created. Next, the data unit F3 is encoded 702 into a new data unit F'3 based on the pseudo-coded state CS2 '. Further, the previous coding state CS1 is stored in the memory 212. The new coding state CS3 is also extracted from the coding 702 of the data unit F3 into a new data unit F'3.
[0047]
The missing data unit F2 may then be encoded 702 into a new data unit F′2 based on the previous received and saved encoding state CS1. The used saved coding state CS1 can be subsequently deleted from the memory 212. Finally, the restored data unit F'2 is used as valid data, and the previously created pseudo data 708 is discarded. However, if missing data unit F2 is not received within a preset time period, pseudo data 708 may be used as valid data or discarded, depending on its implementation.
[0048]
By using the present invention as described above, the delay in the transcoder is minimized and the need for a jitter buffer is eliminated, while maintaining high quality of the transmitted data. Specifically, data units arriving in the wrong sequence order are compensated for by recovering late arriving data in accordance with the present invention. The invention is primarily aimed at being used for voice transcoding, but could also be used for video signals based on, for example, fax transcoding, modem data or packets.
[0049]
The inventive method may be implemented in software code contained in a computer program product that can be directly loaded on a computer in transcoder unit 200 or stored on a computer usable medium.
[0050]
Although the present invention has been described with reference to certain exemplary embodiments, the description is intended only to illustrate the concepts of the invention and should be taken to limit the scope of the invention. Not. Various alternatives, modifications and equivalents may be used without departing from the spirit of the invention, as defined by the appended claims.
[Brief description of the drawings]
[0051]
FIG. 1 is a schematic diagram of an exemplary communication system for transferring data.
FIG. 2 is a schematic diagram of a data flow in a transcoder.
FIG. 3 is a logic diagram illustrating the decoding of data packets arriving in the correct order.
FIG. 4 is a logic diagram illustrating decoding of data when data packets arrive out of order.
FIG. 5 is a logic diagram illustrating the encoding of intermediate data.
FIG. 6A is a logic diagram illustrating the encoding of data when a data unit is dropped, and FIG. 6B is a logic diagram illustrating encoding of data when missing data arrives late.
FIG. 7 is a logic diagram generally illustrating the transcoding of data when data units arrive out of order.
[Explanation of symbols]
[0052]
100 transmitter
102 Final receiver
200 transcoder unit
202 data units
204 coding
206 Coding
208 Intermediate data
300 data units
302 Encoding state
402 Error correction data
500 data units
502 Encoding state
506 data unit
700 data units
702 coding
704 data unit
706 coded state
CS2 'pseudo-coded state
DS1 Current decoding status
DS2 'pseudo-coded state
ES2 'pseudo-coded state
F 1st format
F '2nd format
F2 missing data
F'2 coded missing data
F3 Receive data unit
IF intermediate format
IF2 coded missing data
S3 Receive data unit
S4 Missing data

Claims (18)

-A) receiving a data unit (202, 500, 700) including coding information in a first format (F, IF);
-B) the received data units (202, 500, 700) in a second format (IF, F ') based on the current coding state (302, 502, 706) extracted from the previous coded data Encoding (204, 206, 702) into new data units (300, 506, 704);
-C) extracting a new current coding state (302, 502, 706) each time the data unit (202, 500, 700) is coded; Transcoding communication data in a transmission path between the devices (102),
-D) detecting whether the received data is in the wrong sequence order; and
-E) a new pseudo-coding to save the coding state (302, 502, 706) extracted from said previous coded data and to code the next received data unit (202, 500, 700) Extracting the states (DS2 ', ES2', CS2 '). -F) receiving missing data (F2, S4);
-G) coding the missing data (F2, S4) into the second format (IF, F ') based on the stored coding state (302, 502, 706);
-H) using the coded missing data (IF2, F'2) as valid data. The method of claim 2, wherein the saved coded state (302, 502, 706) is deleted after step G). The method of claim 1, wherein the stored coded state (302, 502, 706) is deleted if the corresponding missing data is not received within a preset time period. . The method of claim 4, wherein the preset time period is based on data flow characteristics. The method of claim 1, wherein the stored coding states (302, 502, 706) are deleted if a number of stored decoding states reaches a preset threshold. Method. The method of claim 6, wherein the preset threshold is based on data flow characteristics. 2. The received data unit (202) is decoded in step B), the coding state is a decoding state (302), and the second format is an intermediate format (IF). 8. The method according to any one of 7. The method according to claim 8, wherein the received data unit (202) is decoded in step B) into a stream of intermediate data (208) that is divided into intermediate data segments (500). The method of claim 9, wherein the intermediate data segment (500) is encoded into a new data unit (506). 2. The received data unit (500) is encoded in step B), the encoding state is an encoding state (502), and the first format is an intermediate format (IF). 8. The method according to any one of 7. The method of claim 11, wherein the received data unit (500) comprises decoded intermediate data (208) encoded in step B) into a new data unit (506). The new pseudo-coded state (DS2 ', ES2', CS2 ') extracted in step E) is used for coding the next received data unit (F3, S3). A method according to any of the preceding claims. 14. The method according to claim 1, wherein step A) comprises reading information indicating a sequence order of the received data (202, 500, 700, F2, S4). If the received data is in the wrong sequence order, error correction data (402) is derived from the previous received data unit and the current decode state (302, DS1) and the new pseudo-coded state (DS2). 15. The method according to any of the preceding claims, wherein ', ES2', CS2 ') are extracted in step E) based on the derived error correction data (402). A transcoder unit (200) located in the transmission path between the transmitter (100) and the final receiver (102) and having means for performing the method of any of the preceding claims. Computer program product directly loadable on a computer in a transcoder unit (200), comprising software code means for performing the method of any of claims 1 to 15. A computer program product stored on a computer usable medium comprising a readable program for causing a computer in a transcoder unit (200) to perform the method of any of claims 1 to 15.

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