kerabala University- Science College-Computer Science Some simple methods of data compression are:

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1 Video Compression Introduction Once a video signal is digital, it requires a large amount of storage space and transmission bandwidth. To reduce the amount of data, several strategies are employed that compress the information without negatively affecting the quality of the image. Some methods are lossless, meaning that no data is lost, but most are lossy, meaning that information is thrown away that can t be retrieved. Some simple methods of data compression are: Throw away pixels at regular intervals: This essentially scales the image, or makes it blockier. Average several pixel values together (subsampling): This involves taking several adjacent pixel values and averaging them together, resulting in a single rectangular pixel that approximates the value of several. For more information, Examples JPEG Image Compression MPEG-1 Video Compression 1 P age

2 MPEG-2 Video Compression MPEG-4 Video Compression Containers: (MP4, MOV, AVI) Audio Compression (AAC, MP3) Lossless Compression Run Length Encoding (RLE): aaaaaaabbbb 7a4b abababababa 1a1b1a1b1a1b1a1b1a1b1a Lossless compression relies on input being non-random to achieve compression. Lossy Compression Removes information Does so intelligently 2 P age

3 For media files, remove what is least noticeable by the senses. Things Image/Video Compression Has Problems With Edges High Motion Transparency (Smoke, Shadow) Subtle gradations of color Small details Quick Cuts Fading In./ Out 3 P age

4 When to use which? MPEG 1 for CD-ROM quality video (1.5Mbps) MPEG 2 for high quality DVD video (3-6 Mbps) MPEG 4 for object-oriented video compression (For web downloading) JPEG Compression Lossy (JPEG-2000 can be lossless) JFIF file format and JPEG compression 8x8 blocks matched to combinations of 64 basic patterns Quality determines how hard it tries to match 4 P age

5 Video Compression Series of compressed images (JPEG) But can make smaller by compressing what s common between frames Only save the information that s changed between frames Match macroblocks to previous and possibly next frames MPEG (Moving Pictures Expert Group) Committee of experts that develops video encoding standards. Until recently, was the only game in town (still the most popular, by far). Suitable for wide range of videos Low resolution to high resolution Slow movement to fast action Can be implemented either in software or hardware 5 P age

6 MPEG-1 3 frame types: I frame - Independent, full image frame P frame - depends on previous frame B frame - depends on last and next frame Group of Pictures (GOP) Pattern of I,P and B frames Implications for editing Implications for errors 6 P age

7 Things Image/Video Compression Has Problems With Edges High Motion Transparency (Smoke, Shadow) Subtle gradations of color Small details Quick Cuts Fading In./ Out MPEG-2 Designed for DVD and Satellite TV Generally higher quality and larger than MPEG-1 No free players Commercial disks are encrypted 7 P age

8 Containers / Files MP4, AVI and MOV are container & file formats They can contain a variety of audio and video and specify the codecs to decode them For instance: A MOV file can contain MPEG-4 video with AAC audio OR A MOV file can contain Sorenson video with MPEG-3 audio OR MP4 (M4V) is MPEG-4 part 14 8 P age

9 MPEG Compression (cont.) MPEG System Streams (cont.) 9 P age

10 MPEG System Streams (cont.) Dealing with packet jitter Adaptive payout delay Objective is to use a value for p-r that tracks the network delay performance as it varies during a transfer. The following formulas are used: di = (1-u)di-1 + u(ri ti) u=0.01 for example νi = (1-u)νi-1 + u ri-ti-di Where 10 P age

11 ti is the timestamp of the ith packet (the time pkt i is sent) ri is the time packet i is received pi is the time packet i is played di is an estimate of the average network delay νi is an estimate of the average deviation of the delay from the estimated average delay Problem: Packet loss Loss is in a broader sense: packet never arrives or arrives later than its scheduled playout time Since retransmission is inappropriate for Real Time applications, FEC or Interleaving are used to reduce loss impact. Recovering from packet loss Piggybacking Lo-fi stream With one redundant low quality chunk per chunk, scheme can recover from single packet losses Recovering from packet loss Interleaving Divide 20 msec of audio data into smaller units of 5 msec each and interleave 11 P age

12 Upon loss, have a set of partially filled chunks Recovering from packet loss Receiver-based Repair The simplest form: Packet repetition Replaces lost packets with copies of the packets that arrived immediately before the loss A more computationally intensive form: Interpolation Uses Audio before and after the loss to interpolate a suitable packet to cover the loss Real Time Protocol (RTP) RTP logically extends UDP User Datagram Protocol : protocols send short packets of data like TCP Sits between UDP and application Implemented as an application library What does it do? 12 P age Framing Multiplexing Synchronization Feedback (RTCP)

13 RTP packet format Payload Type: 7 bits, providing 128 possible different types of encoding; e.g. PCM, MPEG2 video, etc. Sequence Number: 16 bits; used to detect packet loss RTP packet format (cont.) Timestamp: 32 bytes; gives the sampling instant of the first audio/video byte in the packet; used to remove jitter introduced by the network Synchronization Source identifier (SSRC): 32 bits; an id for the source of a stream; assigned randomly by the source Timestamp vs. Sequence No Timestamps relates packets to real time Timestamp value sampled from a media specific clock Sequence number relates packets to other packets 13 P age