TRANSRATING EFFICIENCY — BIT-RATE REDUCTION METHODS TO ENABLE MORE SERVICES OVER EXISTING BANDWIDTH L. Lastein and B. Reul BarcoNet – a Scientific Atlanta Company, Denmark ABSTRACT This paper explains the efficiency of three transrating and statistical re- multiplexing techniques, examines how they work and describes the combinations most suitable for different real-life applications in Digital Headends, Play-outs and regional hubs in Broadcast, Terrestrial, Satellite and Cable Networks. The three techniques are known as: • Time-shifting of MPEG-2 packets • Open loop transrating • Closed loop transratingINTRODUCTIONThe introduction of MPEG-2 audio and video encoding has created a multitude of newpotential revenue streams. One of the major benefits for a network operator is thecompression of both video and audio to fit into a significantly lower bandwidth. The higherthe cost of the accessible bandwidth, the greater the benefit gain from compression. But thetechnology has also meant new challenges for Digital TV (DTV) networks, be it on cable,satellite or wireless platforms. The parameters used to encode services by the originatormay not fit the business model of the DTV operator. Also, technical issues arising from moreefficient encoding technologies (such as statistical multiplexing) create a demand forcontrolling the most important MPEG-2 parameter of them all: The video encoding rates.What is the problem?As MPEG-2 compression technology has matured and improved its bandwidth usageefficiency, one of the major breakthroughs for encoding has been the deployment ofstatistical multiplexing. Video programs can now be encoded using a variable bit-rateaccording to the actual needs of the specific program compared to other programs in thesame multiplex. When combined with the typical business scenario from a DTV-operator, aproblem arises. The operator has to aggregate existing content to a digital network, reachingthe subscribers. When this content is acquired from different networks and re-multiplexed tonew service bouquets, the total bit-rate of the newly created multiplexes are out of control.The operator needs to build in a margin to allow for the fluctuating bit-rate and avoid anetwork breakdown in the event that the total bandwidth capacity is exceeded.A relatively new group of players in this market are broadband operators, who targetalternative broadband access methods to the home. Very rapid deployments of xDSLconnections in different regions of the world have enabled new players to enter the marketof other network operators. To increase the number of potential customers they are offeringvideo-services to be delivered on a broadband connection. However, the bandwidthlimitations of the “last-mile” to the subscriber is a challenge – only single services at a time
can be streamed and connected to the subscriber PC or Ethernet-based Set Top Box (STB).Often the access-network allows only a limited bandwidth, meaning that re-compression ofthe individual program stream is necessary. The purpose is to limit all programs to a fixed,defined bit-rate that may have no relation to the bit-rate used when these services arepresented to the broadband provider.Optimize the revenue on the existing bandwidthNetwork operators offer different content based on unique business models. Theprioritization of the scarce bandwidth does not necessarily match the priorities alreadyallocated by the content providers using MPEG-2 compression for their distribution. Theoperator must allocate the bandwidth resource to the services where he makes money. Thishas particular relevance for bandwidth-scarce applications like DVB-T and DVB-S whereclear prioritizations are needed. For high-revenue earning services, the perceived quality bythe subscriber should be high, and for the low-revenue earners there must be compromiseswhen selecting the quality level.Using Transrating to create the optimized Business-model for digital servicesControlling a video rate is conceptually simple. It can be achieved by, first decoding, andthen again re-encoding each service. The encoding can be set at a lower rate.. A decoderand encoder for each service can quickly runs up to a large investment.In addition, re-encoding does not always result in the best video quality. MPEG-2 encodingis based on reducing the details in the picture that are less visible to the human eye.However, when this technology is re-applied on a previously encoded video, artifactsintroduced to the video will be enhanced. Re-encoding will, in these applications, not be in aposition to take advantage of information about previous encoding processes in thetransmission chain.A solution to this problem is the technology of transrating. This technology results in acontinued high video quality level by effectively re-using information about previousencoding whilst at the same time delivering a more cost effective solution to the problem. .The following types of applications may benefit from using transrating. - Re-multiplexing VBR streams: Relevant for both DVB-T, DVB-S and cable environments. Single MPEG-2 programs encoded as a part of a statistical multiplex may vary from 2 to 10 Mbit/s. Transrating will ensure that the operator does not have to reserve expensive mainly unused overhead in the downstream network. - Reducing Constant Bit Rate (CBR) streams. Still some services are distributed in high CBR-rates that will not be cost-efficient to aggregate and re-transmit in a digital network without lowering the rates. Relevant for all DTV applications. - A mix of the two applications mentioned above (all applications), where VBR-services are re-multiplexed with CBR-services. All rates are reduced and statistically multiplexed in order to achieve better network utilization. - IP-Streaming of MPEG-2 services: Rate-limiting for single service, enabling a CBR on a specified bit-rate, typically significantly lower than both average rates of incoming CBR or VBR-programs. - Ensuring compliance with Service Level Agreements between network operators and content providers by limiting/controlling the rates of single programs re-transmitted over a DTV-network.
THE TECHNOLOGIES OF TRANSRATINGTransrating focuses on reducing the rate of a single service and/or a multiplex of services. Aservice consists of many elements, such as SI/PSI data, audio, VBI, other data and finallyvideo. Video is seen from a bit-rate perspective as the largest consumer of bandwidth.Reducing video-rates is the only real alternative that will have a significant effect on the totalservice bandwidth. Transrating works specifically by reducing the video encoding rates andit does affect the rest of the components in a service. A transrating device typically makesuse of one of two core processes. The first one, “time-shifting of MPEG-2 packets,” involvesthe “smoothing” of the total Transport Stream rate without altering the actual video encodingrates of the individual services. The second process is where rate reduction occurs forindividual services.Time-shifting of MPEG-2 data packetsOne of the main problems faced is the re-multiplexing ofservices, previously encoded using Variable Bit-Rates (VBR). Arelatively easy way around this is to advance and delay MPEG-2 packets in order to “equalize” the bit-rate level of the totalstream. Figure 1 shows five different services, encoded usingvariable bit-rates. Horizontal arrows show how MPEG-2packets can be advanced or delayed in the time-domain todecrease the peaks.Of course, there are limits as to how much such a process canreduce the rate-peaks. These limits are defined by the VirtualBuffer Verifier specifications (VBV) (1) in the MPEG-2compression standard. The VBV specifies how much data the Figure 1 - Time-shifting ofdecoder of the programs must be able to store in the local data-packets in anmemory before it converts the data into base band video. It is MPEG-2 VBR streambased on the fact that data must arrive at the decoder before ithas to be removed from the buffer for decoding.This type of technology is only really effective in two types of applications. Single programsmust be VBR encoded, otherwise there is no point in shuffling the data packets. If a numberof programs use high rates simultaneously, shifting does not help. Time shifting only lowersthe maximum rate, not the average rate. Because the transrater must have finite delay andcannot send packets it has not received, the maximum time that a packet can be advancedis limited.Time-shifting technology does not guarantee any output bandwidth. Also it does not help theoperator to adjust the bandwidth-consumption of the services to where the potential profit ismost likely to be found. Nevertheless it works well for the preparation of VBR-feeds for theprocess of transrating, which will be explained next.Transrating – Open LoopThe tools of rate-reduction are revealed when looking into the fundamentals of the encodingprocess. A typical encoder can compress the video to a digital bit-rate specified by theoperator. The same tools can be used for transrating and therefore the basic processes ofencoding need to be explained.
The basics of MPEG-2 video encodingThe major steps in MPEG-2 encoding are: - Dividing the pixels into macro-blocks of 8x8 pixels - Perform motion compensation by identifying temporal redundancy. - Find spatial redundancy with DCT-encoding. Until now no reduction of the video information has been made and the process is therefore reversible. - The reduction of the information so far generated is done with the quantization. This process removes details in the picture, which is often in the high frequencies. In areas with high spatial activity (pencil drawings, letters in a book or other high contrast areas) the details are both high and low frequency, and low frequencies will be reduced. The logic of this is based on the limited perception of the human brain, which filters out this information. The quantification removes content in the picture, which will give the user none or limited perception of degradation anyway. - Variable Length Encoding is the process of reducing the mathematical redundancy.The output of the variable length encoding is the core content ofthe video in the transport stream. In addition to these principles, Figure 2 - The MPEG-2each picture is treated as either an I, P or B-frame which will be Encoding processexplained below.The schematics of open loop transratingThe obvious way to obtain actual rate-reductions on a video-service is to perform re-quantization. The process of quantization is (as seen in Figure 2) integral to MPEG-2encoding. When a transrater device performs this process, it partially inverse encodes,including the process of inverse quantization, and then decides on an increased level ofquantization, resulting in higher compression rates. On a flow chart this can be explainedlike Figure 3 below. Figure 3 - The Open Loop AlgorithmSuch a transrating process reduces the bit-rate of the video, but not without trade-offs in thevideo quality. The problem is this relatively simple algorithm works in an “open loop” withoutany information about what kind of effect it has on the image while transrating the individualpictures in the stream. When making a transrater device using this algorithm, a defensiverate-reduction strategy must be followed to avoid a picture breakdown from visual artifacts inthe video. This will mean limitations to the device, since all streams carry different content
encoded by different encoders. It will only be capable of a certain degree of rate-reduction.The consequences for exceeding the maximum level for quantification will be very easy tosee for the end-consumer – the subscriber. The complex referenced structure of the MPEGstream may cause programs to break up and show severe artifacts for up to a half second.The risks when reducing the ratesThe problems of open loop transrating can be explained based on the basic terminology of atransport stream, the so-called Group of Pictures (GOP). The stream consists of I, B and P-pictures, typically in a syntax as seen below on Figure 4, showing a 15,2 GOP (15 picturesin total, 2 B-pictures between each P or I frame). The arrows indicate how the P-pictures arerelated to either the previous P-picture or the I-picture. The B-pictures are related to the I orP picture before it and, if present, the P-picture after. Figure 4 - The structure of the GOP-sequence anchor picture referencesAs mentioned previously, the re-quantification removes information and compromises thepicture quality. This will be referred to as introducing an error. Depending on the GOP-sequence, there is a difference where this error is introduced. Errors introduced on B-pictures will have little effect. It will only be shown for 1 or 2 frames, out of 25 frames duringone second. If the error is a block-artifact, the visual impact will be a short pulse. Thesituation is however more serious if errors are applied on the P or I-pictures.The I and the P-pictures are the anchor-pictures of the GOP, since the following P-picturesalways refer to the previous I or P-picture. So, if there is an error applied to one of the earlyP-pictures in the GOP – or even the I-picture, which is starting the GOP – the errorcontinues throughout the GOP-sequence. If introduced on an I-picture, the block artifact willbe present in the video for approximate half a second, depending on the length of the GOP.This problem for the video quality, can also been seen in other ways.If an error on the I-picture is introduced due to re-quantization, it is easy to imagine thaterrors also can be applied to the following P-pictures. The result is shown in Figure 5. Figure 5 - The development of the error throughout the GOP (error-drifting)Simply put, one bad decision at the start of the GOP is compounded by another one. Thesum of these errors, “breathing”, is perceived as degradation of picture quality throughoutthe GOP, until a new GOP starts with an I-frame. The cycle repeats every half a second.Other factors that influence the quality level of Open Loop Transrating are mainly related tohow the actual decisions on the level of re-quantification are made, mainly on the B-pictures.
How to apply open loop transratingOpen loop transrating is a method of actually lowering the bit-rate of video services, thatshould be followed by the time-shifting of the MPEG-2 data packets.Since I and P pictures are risky to re-quantify, the transrater device could limit its processingrange to only include B-pictures. That will prevent any major introduction of artifacts in thedecoded video. It will have a rate-reducing effect since most of the pictures in the stream areB-pictures. On the other hand, the biggest pictures by far in the GOP – the ones carryingmost information – are the I and P pictures. Only very limited re-quantification can be doneon these pictures in order to avoid severe block artifacts in the decoded video.In conclusion, the typical rate-reduction made by transrater devices utilizing open looptransrating is app 10-20%, depending on how the GOP previously has been encoded. As anexample, better quality encoders have better motion-estimation performance, meaning thatthe B-pictures already will be small in size. In that case, the benefit of transrating is reduced.Since this does not fulfill all the business-objectives mentioned in the introduction, anothertype of transrating can be applied, solving these problems by enabling a higher level of rate-reduction. This is in this paper referred to as “Closed Loop Transrating”.TRANSRATING - CLOSED LOOPThe name of this kind of transrating indicates that the main differentiator from the open looptype is a “learning” loop. This algorithm aims to overcome the problems of the Open LoopTransrating, which in effect means that the total rate-reductions of a stream will be greater.The schematics of Closed Loop TransratingClosed Loop is built on the Open Loop schematics, but it has two “learning” loops added: - First of all, the re-quantification of the single macro-blocks is done based on measurements of the output picture quality for the single picture. In order to optimize the level of re-quantification, the first step is to actually measure the error applied.Secondly, the error detection must have a valid basis. It is very difficult to determine. Figure 6 - The Closed Loop Transrating algorithm. Based on Assuncao et al (2)
Compared to the Open Loop Transrating algorithm it can be seen in Figure 6 that the basicprocess is unchanged, but that several loops have been added. The box named “REF” isthe reference picture comparison between the previous I or P picture and the picture inquestion. The function of REF is to store the error, which is feed back into the transratingprocess for the next anchor picture.Note that this process also makes use of both DCT encoding and decoding (DCT encodingand inverse DCT encoding). According to the physics of the video encoding schemedescribed previously, this takes the MPEG-2 stream almost back to the pixel-domain. This isdone to ensure use of the reference-picture quality measurements. The error applied on theactual anchor picture is known and transferred into the process of re-quantifying the nextanchor picture. In this way the re-quantification of the anchor picture will be able tocompensate for the error.Although this is more complicated, it does solve some of the problems occurring when OpenLoop Transrating is applied. It cannot change the fact, that reducing the picture size willreduce the quality. This process is however capable of, first of all, knowing the exact level oferror that has been introduced on each frame and second , compensating for this error inthe next anchor picture.Figure 7 shows how the Closed Loop Transrating algorithm overcomes the risk of breathing,since it monitors the error included on a frame-by-frame reference. Figure 7 - Eliminating the error developing throughout the GOPEven though errors still are introduced in each picture, the error will be compensated for inthe next anchor picture. As a consequence, the total error throughout the GOP-sequencewill not exceed the error introduced in the first anchor-picture. That has basically beendetermined by the rate-reductions requested by the operator.In short, the advantage of the Closed Loop Transrating is built on these two processes: - The level of re-quantifications on the individual anchor picture is being applied while monitoring the actual errors applied on the picture. - All errors applied as a natural consequence of the re-quantification are eliminated in the next anchor-picture, thereby eliminating the risk of an exponentially developing error throughout the GOP, which could cause video quality problems.How to apply Closed Loop TransratingThe main advantage of this algorithm compared to the Open Loop Transrating is that itallows significant rate-reductions of the I and P pictures, without risking severe impairmentsin the video quality. The algorithm will, based on the inputs from the user itself, determinehow large the rate-reductions on the I and P pictures can be.Based on simulations with different types of stream, the Closed Loop Algorithm has shownup to 50% video bit-rate reductions, without causing significant blocking artifacts on the
decoded video image. Compared to the Open Loop algorithm, the further reduction isachieved on the I and P pictures, while the B pictures remain as compressed as they wouldbe using conventional Open Loop transrating.A particular benefit of the Closed Loop algorithm is that it will be able to achieve significantrate-reductions also on high-quality encoders, which outputs smaller B pictures thanconventional encoders.It is also clear that the complexity of the algorithm is greater than Open Loop. From animplementation standpoint that means that there is a need for more powerful hardware. The“learning” loops require extra processing compared to the Open Loop Transrating. Testshave shown that if a Closed Loop Transrating process carried out on I and P pictures takes100% processing power, the Open Loop needs approximately 40-50%. In effect, ClosedLoop Transrating requires twice as much processing hardware as the Open LoopTransrating algorithm.CONCLUSIONThis paper has presented three different processes relevant to transrating. The conclusion isas follows: - Time-shifting of MPEG-2 data packets is a necessary tool for preparing all transrating processes, as long as the incoming video-streams are in Variable Bit-Rate mode (VBR). This will reduce the potential peaks when re-multiplexing independently encoded VBR programs, but it will not guarantee a specific bit-rate and will still require a margin of error to prevent overflow in the streams coming out of the headend. - Open Loop Transrating is based on re-quantification of the video and results in a rate-reduction of the encoded video streams. Due to the linear process of re- quantification, the process can only output a certain bit-rate reduction, varying from 10-20% max on common streams. - Closed Loop Transrating is similar to the Open Loop transrating, but it uses a double “learning” loop, based on inputs from measurement of the video quality of the specific, transrated frame and the frames before it. This complex algorithm enables up to 50% video bit-rate reductions.Transrating is a strong alternative to decoding/re-encoding programs, which offers cost-effective rate-reductions with a minimum loss of video quality, especially when Closed LoopTransrating is applied.REFERENCES1. ISO/IEC 13818-2 (1995) Generic Coding of Moving Pictures and Associated AudioInformation: Video. Appendix C: Video Buffering Verifier2. A.A. Assuncao, Pedro and Ghanbari, Mohammed; Transcoding of MPEG-2 video in thefrequency domain; Department of Electronic Systems Engineering, University of Essex,IEEE 1997