fig1 shows a known encoding apparatus 100 . an encoder 110 receives video information and a bit rate allocation . the bit rate allocation is received from a bit rate controller . the encoder 110 encodes the received video information and passes this to a buffer 120 . although encoded according to the allocated bit rate , the encoded video information stream is bursty in nature , meaning that the bit rate changes abruptly . this can be due to the encoding standard used . for example , a video frame encoded according to mpeg - 2 can be encoded as an i frame , p frame or b frame . an i frame requires a relatively large amount of data to encode , a p frame requires somewhat less data and a b frame requires less data still . the encoding process will output different types of frames , and this will affect the instantaneous bit rate output by the encoder 110 . further , the amount of detail or complexity in the frame that is encoded will also impact the instantaneous bit rate output by the encoder 110 . the encoded video information from the encoder 110 is passed to a buffer 120 , which smoothes the received encoded video information stream and outputs it at a bit rate according to the bit rate allocation . the smoothed encoded video information is then passed to a packetizer 130 . the packetizer 130 packetizes the received stream to generate a transport stream suitable for multiplexing . during packetization , a timer 140 is used to monitor a pcr time interval and generate an insert program clock reference ( pcr ) instruction which is sent to the packetizer 130 at the appropriate time . pcrs may thus be inserted into the transport stream at regular time intervals according to a pcr time interval received by the timer 140 . fig2 shows a encoding apparatus 200 as disclosed herein . a difference between encoding apparatus 100 and encoding apparatus 200 , is that in the latter the packetizer 230 comes before the buffer 220 , as opposed to the former where packetizer 130 follows buffer 120 . as explained above , packetization pre - buffer is efficient because the output bit rate from the buffer 220 is equal to the bit rate allocated to that stream by a bit rate controller . therefore when a multiplexer aggregates all components from multiple streams to form a single multiplex , the sum equals an expected value with no variation . because of this exactness , the bit rate controller can allocate all available bandwidth to the streams , without leaving a margin for variation , thus having a positive impact on picture quality . however , when ts packetization occurs before the buffer 220 the process for inserting pcrs is more complicated . this is because the pcr insertion must now take account of the delay of the encoded video data through the buffer 220 . the apparatus shown in fig2 provides a solution to this problem . an encoder 210 receives video information and a bit rate allocation . the bit rate allocation is received from a bit rate controller . the encoder 210 encodes the received video information and passes this to a packetizer 230 . the packetizer 230 packetizes the encoded stream to generate a transport stream . the encoded video information is bursty and so the packetized transport stream output by the packetizer 230 is also bursty , and so not suitable for multiplexing . the packetized transport stream is output from the packetizer 230 and passed to a buffer 220 , which smoothes the bit rate of the transport stream and outputs it to a multiplexer at a bit rate according to the bit rate allocation . during packetization , program clock references ( pcrs ) are inserted into the transport stream . in addition to the encoder 210 , packetizer 230 and buffer 220 , there is provided a calculate number of bits process block 240 , a fifo 250 and a pcr instruction generator 260 . the calculate number of bits process block 240 includes a memory 245 . the calculate number of bits process block 240 receives a pcr time interval from the bit rate controller or another component , and also receives from encoder 210 an indication of the encoding bit rate and the duration of each encoding bit rate . when the encoder 210 is working in variable bit rate mode ( for example in a multi - channel system that implements statistical multiplexing ) the bit rate that the multiplexer is able to receive from the buffer 220 varies with time . this variability affects the pcr spacing and so this must be managed . the required time between pcr fields in the transport stream is expressed as the parameter pcr time interval . all bit rate values , br n , that the encoder uses , along with the duration dur n of each valid bit rate value , are stored in the memory 245 by the calculate number of bits process block 240 . these values are stored as shown in the following table and are used to calculate the number of bits between pcrs , referred to as a pcr bit interval . the value of the running total of the duration values is also stored . a calculation is performed which converts the bit rates and pcr interval values into a series of pcr bit interval values which are stored in the fifo 250 until they are needed . the calculate number of bits process block 240 performs the following calculation when the running total of durations meets or exceeds the pcr time interval : each calculated pcr bit interval is made available to the pcr instruction generator 260 at the appropriate time by the fifo 250 . the fifo 250 is a first - in first - out memory used to buffer the calculated pcr bit intervals ready for when they are required by the pcr instruction generator 260 . there is also feedback ( not shown in fig2 ) of the fill level from the fifo 250 to the calculate number of bits process block 240 which is used to ensure that the fill level of fifo 250 is maintained between empty and full . the pcr instruction generator 260 counts the valid ts bits coming out of the packetizer 230 and when the count reaches the value of the oldest calculated pcr bit interval an “ insert pcr ” signal is generated and sent to the packetizer 230 , then the next oldest pcr bit interval value is fetched from the fifo 250 and a new count begins . when the oldest value in fifo 250 is read by pcr instruction generator 260 , that value is deleted from fifo 250 . upon receipt of the insert pcr signal from the pcr generator 260 , the packetizer 230 inserts a pcr into the transport stream . fig3 a , b and c show the relationship between the bit rate allocation , encoder output bit rate and buffer output bit rate respectively . bit rate is shown in a vertical axis and time on a horizontal axis . fig3 a shows the first four discrete allocated bit rate values br n of a series together with their respective durations dur n . fig3 b shows the corresponding encoder output 380 , which is bursty and loosely follows the allocated bit rate 370 . fig3 c shows the buffer output bit rate 390 which exactly follows the bit rate allocation 370 . fig4 illustrates the method 400 of converting a clock reference time interval to a clock reference data interval as used in the clock reference insertion method and encoding apparatus disclosed herein . the method starts 410 and waits 420 for a new bit rate value ( br ) and duration value ( dur ) to be determined . upon receipt of a new bit rate and duration pair , the value of the duration ( dur ) is added 422 to a running total ( rt ). it is then determined 424 whether the running total ( rt ) of durations ( dur ) exceeds the pcr time interval . if it does not , then the received bit rate value ( br ) and duration value ( dur ) are stored 426 in a table , and the process waits 420 for a further bit rate value ( br ) and duration value ( dur ) pair . if it is determined 424 that the running total ( rt ) of durations is greater than the pcr time interval , then calculations are made 430 to correctly apportion bit rate and bit rate duration values to the appropriate pcr time interval . a duration remainder dur ( rem ) is calculated as the final running total ( rt ) less the pcr time interval ; the last duration value for the table dur ( last ) is calculated as the final received duration value less the duration remainder dur ( rem ). a new running total ( rt ) is equal to dur ( rem ). the bit rate for the remainder portion br ( rem ) is equal to the bit rate for the last portion br ( last ) which are both equal to the final received bit rate value . the last duration value for the table dur ( last ) and the associated bit rate value br ( last ) are stored 432 in the table . a pcr bit interval is calculated 434 as the sum of the products of each pair of bit rate ( br ) and duration ( dur ) values stored in the table . the calculated pcr bit interval is output 436 and stored in a fifo 250 as explained above . after the calculation 434 and output 436 of a pcr bit interval , the table is cleared 440 . then , the remainder values for bit rate br ( rem ) and duration dur ( rem ) are stored 442 in the table . a determination 444 is made as to whether the new running total ( equal to dur ( rem )) is greater than the pcr time interval . this is possible if the last received bit rate duration is longer than the pcr time interval . if the new running total is greater than the pcr time interval then the process repeats the series of calculations 430 and subsequent processes 432 to 442 . if the new running total is not greater than the pcr time interval then the process again waits 420 for a new bit rate value ( br ) and duration value ( dur ) to be determined . the disclosed system enables the insertion of pcrs into a transport stream at accurately controlled intervals in a system where the buffer is arranged after the transport stream packetization stage . accurate pcr intervals are important since they ensure that the transport streams are compliant with appropriate standards while minimizing the number of bits used for pcr fields and hence maximizing the bandwidth available for providing improved video quality . the disclosed method and apparatus provide a solution which is able to insert pcrs into a transport stream so they are at regular time intervals in the output and at the correct rate even when the packetization is performed pre - buffer . the method , when implemented in practical encoder products , can easily be detected by analysing recorded transport streams at an output post - buffer , which is generally easily accessible . the transport stream may be analysed with an mpeg test instrument such as the dektek streamxpert or wwg dts - 300 . in a recording of a transport stream that contains pcrs inserted according to the disclosed method , the interval between pcrs varies slightly depending on the configuration of the encoder and the type of material fed into the encoder . this slight variation in pcr interval is presently unique . the cause of this phenomenon is the variation in the packetization process . in a traditional system ( with post - buffer packetization ) there is up to +/− 3 % variation in the bit rate of the output transport stream . the disclosed method provides a transport stream having an accurately controlled bit rate , but due to the packetization process the position of pcrs in the output stream is subject to around a +/− 3 % variation . this means that the allocated pcr time interval must be reduced slightly to ensure that any requirement for a maximum pcr time interval is met . however , because the output bit rate of the transport stream is accurately controlled by the buffer , no margin for variation is required in the bit rates allocated to the encoder of each information stream . it should be noted that the drawbacks of the requirement for a slightly higher pcr frequency to accommodate this variation are significantly outweighed by the ability of a statistically multiplexed system to distribute all bandwidth to available streams , without leaving a 3 % margin for each stream . it will be apparent to the skilled person that the exact order and content of the actions carried out in the method described herein may be altered according to the requirements of a particular set of execution parameters , such as speed of encoding , accuracy of detection , resolution of video sources , type of compression standards in use with the statistical multiplexing method , and the like . accordingly , the order in which actions are described and / or claimed is not to be construed as a strict limitation on order in which actions are to be performed . further , while the examples described herein are explained in the context of video encoding , the described methods and apparatus may be used in the encoding of any data , such as , for example , audio information .