Abstract:
This document relates to video transport systems and in particular, though not exclusively, where several compressed or encoded video services are multiplexed into a fixed bit rate transmission stream. There is provided a method of allocating bit-rates for video services in a transport channel. The method comprises receiving user defined weighting parameters for a number of video services ( 410 ); then determining an available bit-rate for a transport channel for the video services ( 420 ). The method then automatically allocates a proportion of the available bit-rate to each video service according to their respective weighting parameters ( 430 ); and multiplexes the video services into the transport channel according to the allocated proportions of the available bit-rate ( 440 ).

Description:
This application claims priority to EP 10175907.4 filed 9 Sep. 2010, the entire contents of each of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     This document relates to video transport systems and in particular, though not exclusively, where several compressed or encoded video services are multiplexed into a fixed bit rate transmission stream. 
     BACKGROUND 
     Video compression systems that implement standards such as MPEG2 or MPEG4/H264 are usually designed to reduce the bit rate of the input video to levels that permit affordable transmission of high quality pictures. It is usual that several video services are multiplexed into a fixed bit rate Transport Stream [TS] that will occupy convenient channels of known transmission systems. Various complex control mechanisms exist for controlling the bit rate of the video services, based in part of their content, and are typically used for direct broadcasting of video services to the home. In some situations, video services are combined and transmitted for the purposes of returning original material to a central studio or processing centre, for example for Digital Satellite News Gathering [DSNG] or so-called “Contribution” or “Back Haul”. In these situations the usual complex method of controlling the total bit rate for home distribution is not appropriate. Typically the number of channels is limited, for example 4 or 5, and the bit rates for each video service are set manually by an experienced on-site operator. 
     SUMMARY 
     The inventors have appreciated a problem of under underutilization of the transport stream when manually setting bit-rates for each video services to be multiplexed into the transport stream. For example the user set bit-rates may not sum to the available bit-rate of the transport stream resulting in NULL bytes which represent a waste of bandwidth. Furthermore, by not using the full bit-rate available, the video services are of a lower quality than they could be. In addition, when a video service is dropped, the level of NULL bytes will increase. 
     There is provided a method of allocating bit-rates for video services in a transport stream or channel. The method receives user defined weighting parameters for the video services and determines an available bit-rate for the transport channel for these video services. The method then automatically allocates a proportion of the available bit-rate to each video service according to their respective weighting parameters. The video services are then multiplexed into the transport channel according to the allocated proportions of the available bit-rate. 
     By automatically allocating bit-rates based on user defined weighting parameters, the method optimises the use of the transport channel or stream by allocating all or substantially all of the available bit-rate. Therefore video service quality is optimised or maximised, and the number of NULL bytes in the transport channel is minimised. 
     The method can also adapt to changes in the number of video services multiplexed into the transport channel. The method can be used in combination with a compression or encoding system and adapt to constraints associated with that system. For example, when used in combination with an MPEG2 or MPEG4 encoding system, the method can automatically distribute excess bit-rate from one video service which has exceeded it&#39;s maximum bit-rate Bmax to the other video services according to their respective weighting parameters. 
     There is also provided a video service multiplexing system and a computer program product comprising code which when executed on a computer carries out the above described methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are described with reference to the attached drawings, by way of example only and without intending to be limiting, in which: 
         FIG. 1  is a diagram illustrating an application of an embodiment; 
         FIG. 2  is a block schematic of the technical features of an embodiment; 
         FIG. 3  is a user interface of an embodiment; and 
         FIG. 4  is a flow-chart of a method according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an application of an embodiment is illustrated in which a sporting event is being recorded and transmitted to a central processing site (not shown) for subsequent broadcast to viewers. The sports arena is indicated by reference  100 , and the action is being recorded by a number of video cameras  110 . This may include footage of players engaged in a football match together with talking head footage of expert commentary for example. A mobile truck  120  includes a video service multiplexing system  150  according to an embodiment. A number of video services  130  are feed to the system  150 , which may encode or compress them. The video services are then multiplexed to generate a transport stream or channel  140  which is transmitted back to the central processing site. 
     A block schematic of the video multiplexing system  150  is shown in  FIG. 2 . This system  150  comprises a number of encoders  210  corresponding to the received video services  130 , a multiplexer  220 , a user interface  230 , and an allocation function  240 . The encoders  210  may compress the video services  130  according to MPEG2, MPEG4 or another video compression system. The compressed video services are then fed to the multiplexer  220  which multiplexes the video services according to respective bit-rate control settings provided by the allocation function  240 . Thus for example a high definition video service may be set at a bit-rate of 20 Mbs, whilst a standard definition video service may be set at a bit-rate of 10 Mbs. The multiplexed video services are then output from the system as a transport channel  140  for example as a transport stream defined by MPEG2 Part 1. 
     The allocation function  240  is typically implemented by a processor running an algorithm which interacts with the user interface  230 . The user interface  230  receives user defined weighting parameters for the video services. These weighting parameters for each channel ( 130 ) are passed to the allocation function  240  which also determines an available bit-rate for the transport channel  140 . The total bit-rate of the transport stream  140  is typically fixed by a user setting and the bandwidth of the transport system. Some of this total bit-rate will be used for overhead and audio data, leaving an available bit-rate for video services. The overheads and audio bit-rate are removed from the total bit-rate to leave an available bit-rate. The removed bit-rate includes all audio, Vertical Blanking interval [VBI] data, Programme Specific Information [PSI] data and video service Bmins [a parameter set by the MPEG specifications]. Typically the bit-rate provided for audio data and other overhead is allocated by a user; the remaining bit-rate being used by the system to allocate automatically to the video services. In an alternative arrangement, the audio and/or overhead bit-rate may be dynamically variable, in which case the system adapts the available bit-rate for video services accordingly. 
       FIG. 3  shows an input screen for the user interface  230  which may be implemented using a display device and appropriate software as would be appreciated by those skilled in the art. The screen shows the total bit-rate for the transport channel  310 , the available bit-rate for the video services  315 , and the various video services  320 . User entered weighting parameters for each video service are shown at  325 , and the proportion of the available bit-rate allocated to each video service is shown at  330 . An example allocation calculation for video service  1  follows:
         Available bit-rate=49.0   Service  1  weighting parameter=7   Total weighting parameters=19   Service  1  bit-rate=7/19×49.0=18.1 Mbs
 
Similar calculations are performed for the other video services in order to allocate a proportion of the available bit-rate.
       

     The service rate of each video service is shown at  335 , and this includes video, audio and overhead bits for each video service  320 . Actual weightings for each video service are also shown at  340 . In some situations these may not correspond exactly with the user entered weighting parameters at  325  due to system constraints. For example where the allocated bit-rate of a video service would be below the minimum defined by the MPEG2 standard, then the allocation function allocates at least this minimum bit-rate to the video service, and re-allocates the remaining available bit-rate to the other video services according to their respective weighting parameters. 
     The packet NULLS shown at  345  are also minimised by the system as can be seen (Service Rate=0.0). These NULL packets are transport stream packets with no video data and their presence effectively reduces the available bandwidth of the transport stream to carry the video services. If the bit-rates of the video services were incorrectly set manually, such that not all of the available bit-rate was utilized by the video services, then NULLS would be used to fill the transport stream. 
     Once the bit-rates  330  are allocated to each video service, these are used to control the respective encoders  210  to fix the bit-rate of each video service received by the multiplexer  220 . The video services are then multiplexed into the transport stream  140  at their fixed bit-rate allocations, together with their audio and other overhead components, in order to generate the full transport stream bit-rate. The exact multiplexing architecture and operation would be understood by those skilled in the art, for example as described in MPEG Part 1. 
     In practice, this embodiment can be used to allocate video bit rate on DSNG and Contribution encoders in a simple way that gives the user flexibility in how it is allocated but makes it difficult not to make efficient use of the available bandwidth. In the field the operators of equipment can be less skilled in their operation and so a relatively foolproof interface is provided. The bit rate allocation, once set, can remain at a static value. However it is also possible for the system to adapt to changing parameters such as a change in the total or available bit-rate, or a change in the number of video services. The system can also allow for the manual fixing of a bit-rate for one or more of the video services, then automatically allocating the remaining bit-rates for the other video services using the user defined weighting parameters. 
     A method according to the embodiment is shown in  FIG. 4 . This method may be implemented for example by the system of  FIG. 2  and the user interface of  FIG. 3 . At step  410 , the method receives weighting parameters for each encoded video service. As discussed above, this may be achieved using a display screen based user interface in which the user directly enters weighting parameters for each video service. In alternative embodiments, the user may enter fixed bit-rates for one or some of the video services, and weighting parameters for others. In this case, the fixed bit-rates will be subtracted from the available bit-rate and the remaining video service bit-rates allocated from the remaining available bit-rate according to their respective weighting parameters. In a further embodiment, fixed bit-rates are entered by the user, and the method then calculates weighting parameters for each video service based on the entered bit-rates. 
     The method then determines the available bit-rate for the video services at step  420 , and as already described above. Once initially determined, the available bit-rate may be stored locally until it changes. Then at step  430 , the method allocates a proportion of the available bit-rate to each video service according to their respective weighting parameters. These allocated bit-rates may also depend on other factors such as meeting a minimum or maximum bit-rate according to system limitations. Therefore at step  460  the method checks whether an system parameters have been exceeded by any of the video services. In an MPEG based system, the proportion of available bit-rate allocated to each video service is checked to determine whether these fall within Bmin and Bmax. If there is no violation of system parameters, the method moves to the multiplexing step  440 . However if there are system parameters exceeded on any of the video services, then at step  470  the method corrects the bit-rate allocation of the affected video service by either increasing or decreasing its bit-rate allocation to meet the system parameters. The available bit-rate is then re-determined by subtracting the bit-rate allocated to the affected video service from the available bit-rate. The method then returns to step  430  where the re-determined available bit-rate is re-allocated to the remaining non-affected video services according to their respective weighting parameters. 
     For example if the weighting parameter for video service x would result in a bit-rate below a minimum predetermined bit-rate Bmin, then the method ensures that at least Bmin is allocated to video service x, and the remaining available bit-rate is then allocated to the other video services according to their respective weighting parameters. Similarly, if a maximum predetermined bit-rate Bmax would be exceeded due to the weighting parameter allocated to video service x, then the method allocates Bmax to video service x, subtracts this Bmax from the available bit-rate and allocates this remaining bit-rate to the other video services according to their respective weighting parameters. Where possible, all of the available bit-rate is allocated to the video services in order to reduce the number of NULL packets in the transport stream. 
     The allocated bit-rates are then used to fix the bit-rates of the respective video services, which are then multiplexed together at step  440  as described previously. At step  450 , the method checks for the addition or deletion of video services. If there is no change in the number of video services, then the method may either terminate or re-run from step  420  as shown. If there is a deletion or addition of video services, the method returns to step  410 . When a video service is deleted, its weighting parameter can be set to zero so that no bit-rate is allocated to it at step  430 . The method then re-allocates the available bit-rate amongst the remaining channels according to their respective weighting parameters. The multiplexing process is then updated at step  440 . When a video service is added, the user is requested to enter a weighting parameter, and the proportion of the available bit-rate allocated to each video service is re-allocated according to the relative weighing parameters of all of the video services, including the new one. 
     Where there are more video services than can be handled by a single multiplex or transport stream, additional systems according to  FIG. 2  and/or methods according to  FIG. 4  can be implemented in parallel in order to generate additional transport streams where the video services are allocated bit-rates dependent on user defined weighting parameters. 
     Modifications and other embodiments of the disclosed invention will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.