Variable Bitrate Stream in Set top Box device

Transcription

1 Variable Bitrate Stream in Set top Box device Preeti Chourasia Student M.Tech (CS) United Institute of Technology And Research Greater Noida (UP) Priyank Chourasia MCA (MITS Gwalior) ABSTRACT Video processing systems of set top boxes require high performance processing elements and an efficien t memory hierarchy design to reach real-time performance in the decoding of high-definition video sequences and display. Dedicated high performance modules are integrated into a single system which decodes the incoming bit stream and produces the output video images. As the video is delivered, the client in the set-top box maintains a communication channel with the streaming origin or edge cache. The client downloads segments as individual files, which are buffered by the set-top box, decoded and displayed. During the playback session, the client monitors the rate at which the client buffer is filling and the speed at which the current segment is downloading, thereby inferring the performance of the network. If there is degradation in network performance, the client can request that segments be delivered from one of the lower bitrates files. Likewise, if the client detects an improvement in performance, it can request segments from one of the higher bitrates. This is all seamless to the viewer since each source file is segmented and time-stamped in the same, very precise intervals so there is no visible interruption or hesitation when switching to a different bit-rate profile. Starting from video sources it is common knowledge that variable bit rate (VBR) encoding of video is significantly more efficient than constant bit rate (CBR) encoding. In MPEG video encoding while the CBR video keeps the bit rate constant. The VBR video attempts to keep the video quality constant. The nature of MPEG video encoding allo ws encoders to use the fewer bits for simple scenes and more bits for complicated and motion rich scenes. With comparable video quality, VBR can yield 40 percent or more bandwidth saving over CBR.As a result of its coding efficiency, VBR video is widel y used in DVD and in broadcast video applications such as digital satellite and cable.vbr streaming of incoming video to the set top box need some special mechanism for buffering at the client side as the fixed buffer size doesn t quite well we have to introduce some dynamic buffer size mechanism to accommodate the data of variable bit rates which is fluctuating according to the user s bandwidth and resourses.we need to keep picture quality constant instead of keeping buffer size constant. In other words we need to keep the size of buffer constant in terms of duration.so that it is very important to devise some algorithm which efficiently changes the buffer size dynamically according to change in variable bit rate due to change in bandwidth.we will carry out work to improve the buffer size management dynamically so that change in bit rate is seamless and picture quality always remains constant throughout the reception of the picture at the set top box. 1. INTRODUCTION Video-on-Demand (VOD) systems either stream content through a set-top box, allowing viewing in real time, or download it to a device such as a computer, digital video recorder, personal video recorder or portable media player for viewing at any time. The majority of cable and telephone company based television providers offer both VOD streaming, such as pay-per-view, whereby a user buys or selects a movie or television program and it begins to play on the television set almost instantaneously, or downloading to a digital video recorder rented from the provider, for viewing in the future. Download and streaming video on demand systems provide the user with a large subset of VCR functionality including pause, fast forward, fast rewind, slow forward, slow rewind, jump to previous/future frame etc. All these devices uses HTTP Adaptive Bitrate Streaming technology Adaptive Bitrate Streaming HTTP Live Streaming Conceptually, HTTP Live Streaming consists of three parts: The server component, the distribution component, and the client component. The server component is responsible for taking input streams of media and encoding them digitally, encapsulating them in a format suitable for delivery, and preparing the encapsulated media for distribution. The distribution component consists of standard web servers. They are responsible for accepting client requests and delivering prepared 92

2 media and associated resources to the client. For large-scale distribution, edge networks or other content delivery networks can also be used. The client software is responsible for determining the appropriate media to request, downloading those resources, and then reassembling them so that the media can be presented to the user in a continuous stream. In a typical configuration, a hardware encoder takes audio-video input, encodes it as H.264 video and AAC audio, and outputs it in an MPEG-2 Transport Stream, which is then broken into a series of short media files by a software stream segmenter. These files are placed on a web server. The segmenter also creates and maintains an index file containing a list of the media files. The URL of the index file is published on the web server. Client software reads the index, then requests the listed media files in order and displays them without any pauses or gaps between segments. Some mechanism of set top box set up and network distribution and whole system is explained with below diagrams. 1. Schematic diagram of set top box 2. Network diagram 3. Generic set top box diagram. Fig.1 Schematic diagram of set top box 93 Fig.2 Network Diagram

3 Fig.3 Generic set Top Box 2. PROBLEM FORMULATION Starting from video sources it is common knowledge that variable bit rate (VBR) encoding of video is significantly more efficient than constant bit rate (CBR) encoding. In MPEG video encoding while the CBR video keeps the bit rate constant. The VBR video attempts to keep the video quality constant. The nature of MPEG video encoding allows encoders to use the fewer bits for simple scenes and more bits for complicated and motion rich scenes. With comparable video quality, VBR can yield 40 percent or more bandwidth saving over CBR.As a result of its coding efficiency, VBR video is widely used in DVD and in broadcast video applications such as digital satellite and cable. VBR streaming of incoming video to the set top box need some special mechanism for buffering at the client side as the fixed buffer size doesn t quite well we have to introduce some dynamic buffer size mechanism to accommodate the data of variable bit rates which is fluctuating according to the user s bandwidth and resourses.we need to keep picture quality constant instead of keeping buffer size constant. In other words we need to keep the size of buffer constant in terms of duration. So that it is very important to devise some algorithm which efficiently changes the buffer size dynamically according to change in variable bit rate due to change in bandwidth. We will carry out work to improve the buffer size management dynamically so that change in bit rate is seamless and picture quality always remains constant throughout the reception of the picture at the set top box. 3. METHODOLOGY Programs transmitted by operators are in transport stream container which typically have variable bit rate. The variability of bit rate is dependent on the nature and resolution of the content transmitted. With change in bit rate the buffer size should dynamically increase /decrease so that review buffer duration remains constant.to the user user it should be transparent but internally it poses two main problems or we can say challenges. First is Not to overwrite old data that is still valid 94

4 Pictorial view of instances of variable bit rate streaming on constant buffer size is shown below. We explain the problem of valid data overwrite at the instance 2 where the bit rate changes and due to constant buffer size, the increased bit rate cannot accommodate in the buffer and some valid data which is yet to be displayed is overwritten. And we lost the picture quality due to this overwritten data. This is the problem we encountered when using this constant buffer size in spite of having variable bit rate. The second main problem of having constant buffer size in variable bit rate transmission is to Maintain duration (in time) of Review buffer with change in size (in bytes)of Review buffer.in this problem we are having noncontiguous disk space in buffer which remains unused throughout the transmission.hence wasting the valuable disk space and having data overwritten inspite of having space. Pictorial view of this problem is depicted in instance5 of below diagram. Instance1 Stream1:2mbps 30 min RB After 20 min Instance2 Useful data overwrite 26 min RB stream2:3mbps Instance3 20 min RB After 27 min After 40 min Instance4 20 min RB Instance5 40 min RB After 50 min Stream3:1mbps After 90 min 95 Fig.4: Instance Diagram of View Buffer Objective of this research paper is to devise such an algorithm which address all the above mentioned problems.we need such an algorithm that will translate the discontinuous disk space into logical contiguous view(review buffer) and have fixed duration buffer for variable bit rate data with minimal wastage of disk space. We can suggest an algorithm with a Pointer management technology,because pointer can handle disk space dynamically and we require dynamic management of disk space in set top boxes and similar devices. 1. Create the x mins review buffer at the incoming bit rate. 2. Create an entry in segment table showing the start of this segment. 3. Monitor the incoming rates continuously while data is received If incoming rate<previous rate Create new segment starting from the last segment s stop value with the new rate. If incoming rate >previous rate

5 Fid/create a free segment to accommodate the x min review buffer at the new rate. Seek to new position. 4. Before writing to buffer check if segment boundary hit. Write available data to buffer. Find/create new segment with same end position as last one and seek to new position. Write rest of the data at new position. Above algorithm is proposed algorithm or pseudo code. 5. CONCLUSION The main problem identified in fixed duration review buffer for variable bit rate bit rate stream is to keep the duration fixed in time while the size of the buffer may vary depending on the variable bit rate. With simple organization of review buffer into segments of different lengths and manipulation of the pointer within the buffer we can always have a fixed review buffer duration (in time) for variable bitrate streams. If the suggested algorithm can be implemented successfully then two main problem in set top boxes can be resolved regarding to picture quality. In future we will try to implement the suggested algorithm with al resource availability and feasibility. REFERENCES [1] [2] en.wikipedia.org/wiki/adaptive_bitrate_streaming [3] Video compression for flash and apple devices. [4] Alex Zambelli, IIS Smooth Streaming Technical Overview, bit.ly/smoothstreamzambelli [5] Overview, bit.ly/smoothstreamzambelli [6] A. Balachandran, V. Sekar, A. Akella, S. Seshan,I. Stoica, and H. Zhang. A quest for an internet video Quality-of experience metric. In ACM HotNets Workshop, [7] R. Houdaille and S. Gouache. Shaping http adaptive Streams for a better user experience. ACM MMSys, [8] T.Y. Huang, N. Handigol, B. Heller, N. McKeown, and R. Johari. Confused, timid, and unstable: picking a Video streaming rate is hard. In ACM IMC, [9] 96