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Teracom Tutorial: MPEG

Note: This is an archived article that appeared in the Teracom newsletter in 2004, and this article has not been updated to reflect technology developments since then (H.264 was standardized as part 10 of the MPEG-4 standard, and the 4K Ultra HD standard).

Please be assured that our training courses have been updated since the time of this article!

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4.23  Video Applications (MPEG)
 
Picture Size
720x480 (Standard Definition, US)
1280x720 (High Definition)
1920x1080 (High Definition)
Codecs: Compression and Processing
MPEG-1: 1.5 Mb/s: CDs, VOD trials
MPEG-2: 4+ Mb/s: DVD, DTH video
H.264: new, replacing MPEG-2
1/3 the bitrate, better error recovery.
Specified in MPEG-4 Part 10.
Network Evolution
Analog or DS0 channels → IP packets
Connection Control
Dedicated → ISDN → H.323/H.225 → SIP
Digital video communication is becoming more common – and will come to the forefront of discussions of telecommunications technologies for a while as the telephone companies go into the “cable TV” business with television in IP over VDSL, and to a lesser extent, when videophones become as popular as telephones.
A number of factors affect the perceived or subjective quality of the images on the far-end user’s screen.  Aside from network issues like transmission error rate and variability of delay, the main factors are picture size and refresh rate, the number of bits per second required and the number of processing operations that must be performed per second to implement the coding and decoding in real time. The objective is to transmit high-definition images using a low number of bits per second and achieving reconstructed picture quality people will be willing to use. However, these factors are often in conflict: for example, high compression requires intensive processing, and large picture size means a higher number of bits per second.  It is one thing to optimize two of the three factors; it is another to optimize all three at the same time.
Definition, measured in pels (pixels) is the correct terminology for picture size.  The term “resolution” for picture size is not correct, as it refers to the quality of the image after reconstruction.  Videophones and desktop videoconferencing systems have in the past supported the Common Interface Format (CIF) at  352x258 pels… though that sounds antiquated compared to a 1280 x 1024 pixel display.  Standard Definition (SD) is 720x480 in the US, refreshed 30 times per second.  For historical reasons, the refresh is done in two passes, every odd-numbered line then every even-numbered line. This is referred to as 60 Hz interlaced. Also for historical reasons, there is enough time to draw 525 lines even though only 480 are drawn on the screen.
This gave rise to a secret code to refer to US SD: SD/525 60i.  In the rest of the world, the definition is 720x576 since the screen is only refreshed 25 times per second (the number of lines per second is the same).  The secret code: SD/625 50i. There are a number of flavors of High Definition (HD), the two most popular currently being 1280x720 non-interlaced (progressive) “720p” and 1920x1080 either interlaced or progressive “1080i” or “1080p”.  Either can be refreshed 50 or 60 times per second.
Compression is required to transmit or even store these images. 720x480 at 30 Hz, with one byte each for red, green and blue is 250 Mb/s. 1920x1080 at 60Hz is 3 Gb/s. Compression is performed by an algorithm called a coder/decoder or codec. To operate in real time (at playing speed), codecs are usually implemented as highly optimized machine code on custom-built chips containing multiple Digital Signal Processors (DSPs).  Standards are required for interoperability. The Moving Picture Experts Group (MPEG) and the ITU establish standards in this area. MPEG-1 was for video on CDs, with the video coded at 1.15 Mb/s.  This was replaced with MPEG-2, which offers a wide range of coding and compression options, grouped in profiles.  Each profile supports a certain picture size, definition, refresh rate and image quality, and results in a different average bit rate. MPEG-2 is currently used as the basis for video stored on Digital Versatile Disks (DVDs) and transmitted via Direct to Home (DTH) video services.  Recently, a new coding algorithm standardized as ITU H.264 has emerged.  It provides the same quality as MPEG-2 at 1/3 the bit rate with better tolerance to transmission errors.  It appears that this will replace MPEG-2 on discs and broadcasts in the future. H.264 is specified in Part 10 of the MPEG-4 standard.
Network services used for video are changing from dedicated channels to IP packets.  For videoconferencing and videophones, the call setup standards have evolved from DS0 channels (ISDN) to H.323’s H.225/245 (NetMeeting) to the Session Initiation Protocol (SIP).  SIP appears to have gained critical mass, and is covered in detail in Courses 130 and 110.
Source: Teracom Course 101, Telecom, Datacom and Networking for Non-Engineers,
Module 1: Fundamentals of Telecommunications, slide 4.23
P.S. I did my Master's degree in Electrical Engineering on digital video! - EC
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