A video streaming technique, in which video content is played back while the video data is being downloaded from an origin server, is useful for the

Transcription

1 Video Transcoding

2 Introduction At present, mobile phones can also process video data and mobile users can enjoy video content on mobile phones. In current services for video content distribution to mobile phones, users can playback video content after the video data is downloaded completely to their mobile phones. In these services, the video content data size is limited according to the mobile phone s memory for processing video data and that of the total download time of the video data.

3 A video streaming technique, in which video content is played back while the video data is being downloaded from an origin server, is useful for the distribution of higher quality and longer video content in a mobile environment.

4 In the mobile environment, however, constant end to end QoS is not guaranteed. Therefore, to meet the performance requirement for viewing multimedia content, adaptations for a variety of mobile devices and for QoS fluctuations over a wireless link are required.

5 ADAPTATION TECHIQUES Basic Architecture of Transcoding Systems

6 ADAPTATION TECHNIQUES Format Conversions Format conversions convert one data format into another format that the device can support. WML is a Web content markup language for mobile phones. To process Web content distributed for Internet connected PCs on mobile phones, a format conversion from HTML to WML is needed.

7 Web page rearrangement Web page rearrangement refers to techniques thatchangewebpagelayoutinordertofitthe display size of a mobile device. Page size reduction is useful for PDAs, where a Web page layout is reduced by changing the size properties of Web page objects (e.g., frames, tables, images) while maintaining the same proportions. With this technique, mobile users can view a Web page with the look and feel of a PC

8 Issues of Viewing a Video Stream in a Mobile Environment Lack of mobile device s capability: Receiving video streaming data that exceeds a mobile device s capabilities will cause a lack of video frames and deterioration of the picture quality. Insufficient and unstable quality of wireless links: Insufficient quality of wireless links would cause a lack/delay of video frames and deterioration of the picture quality.

9 Adaptation Techniques for Video Streaming Adaptation Approaches Simulcast a sender of video data prepares a small number of video streams carrying the same video data but targeted at receivers with different capabilities. Layered Coding video data is decomposed into multiple video streams. Transcoding converts a high rate and high quality video stream to another low rate and low quality stream depending on changes in the reception quality in real time. The transcoder converts video formats corresponding to receiver capabilities.

10 Unicast Server Client Client Client

11 Multicast Server Client Client Client

12 Types of Transcoders Frame dropping reduces the output data rate by dropping video frames. Requantization reduces the output data rate by reducing the number of bits used to encode pixels. Video format conversion is used for swapping between different video formats. (frame size, frame rate, quantizer parameter are adjusted to fit the receiver capabilities and available bandwidth)

13 BASIC CONCEPT OF THE SYSTEM Video Transcoding Proxy is based on the following concepts. 1. Adaptations for long term bandwidth fluctuations: The quality fluctuations of the wireless link can be classified into two categories. Short term fluctuations transmission characteristics of the air interface such as fading, interference, and so on. Long term fluctuations Handoffs of mobile devices initiations/ terminations of streaming.

14 2. Adjust an output data rate to throughputs: VTP tries to adjust the output data rate for a mobile device to the throughput reported from the mobile device. 3. Coping with capability variation in mobile devices: There are capability gaps between Notebook PCs and mobile phones (e.g., display size, CPU power, and battery life), even in future mobile communication systems.

15 4. Reducing the cost of implementation: It is supposed that two symmetric proxies are added to both a mobile device [a client proxy (CP)] and a mobile network (a VTP). The VTP collects information about reception quality from the CP. To minimize the feedback delay, the VTP should be placed at the nearest node in the mobile communications network to the CP (e.g., the BS).

16 Network Architecture

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18 Control Mechanism The control module decides the following encoding parameters by evaluating the profiles reported from the CP. 1. Frame format: The control module determines the frame format for the video stream to be fitted to the display size of the mobile device. 2. Maximum frame rate: The control module determines the maximum framerate(fmax) for the video stream based on the mobile device s processing capability. The following equations show the method of Fmax calculation (F and Fmax 30; A: constant value): P d = A. F. S P i F max = (P t / A ). (1 / S)

19 where Pd and Pt indicate the amount of processing that is required for decoding in real time and which the client device can perform, respectively. F is the frame rate of the video stream S is the frame size of the video stream and is derived by multiplying width and height.

20 3. Encoding bit rate: The control module determines the encoding bit rate of a video stream from the VTP to the CP based on the transmission profile

21 MPEG Transcoder Transcoders decode the original stream and then re encode the decoded media using a different encoding setting. Frame rate conversion: It reduces the frame rate compared to the original stream. In MPEG video encoding, there are three picture coding types: I pic (INTRA coded pictures), P pic (INTER frame predictive coded pictures), and B pic (bidirectionally predicted pictures).

22 Imagesizeconversion: It reduces the size of the image from the original stream. Format conversion: It converts the coding format from MPEG 2toMPEG 4, which will be commonly used in the Internet and mobile environment.