MPEG-2 Decoding Methods

Video engineers have been studying the process of optimal image display on screen, but the best method used often depends on the source material.

The second artifact is often detected when there is motion in the image, often detected as jagged edges in on diagonal lines. There are two distinct types of formats available today on DVD disks.

Video - Derived from video sources which are captured as interlaced images at 30 frames per second.  This is the source often used for television studio cameras and hand held camcorders. 

Film - Derived from 24 frames per second sources, which must be converted to 30 frames per second, using a technique called 3:2 pulldown, or converting the  24 fps. material into 30 fps.

MPEG-2 decoders in the PC uses two different methods for decoding these distinct sources, BOB and WEAVE.  BOB is used to line double video sources, while WEAVE is used to re-create original film frame.  The HTPC is exceptional with the WEAVE method due to it's use of buffering prior to it's output, and it's use of refreshing the image at specific intervals. 

The refreshing or optimally using the vertical frequency for 24fps. sources is done in a reverse telecine method by using WEAVE and displaying the resulting image specific times, the simplest illustrated below:

24fps. shown 3 times = 72Hz
24fps. shown 4 times = 96Hz
24fps. shown 5 times = 120Hz

Bear in mind that not many displays can refresh faster than 60Hz which is typical in most consumer digital TVs.  The higher refresh rate pays off with high-performance displays such as Presentation Monitors and Front Projectors. 

The other benefit of a being able to obtain full original frame from source is the ease of interpolating the sampled image at higher resolution, area where the HTPC excels.  Using the same type of interpolating done in image editing programs such as PhotoShop, the DVD information is upconverted with perfection to a higher resolution video, ideal for displaying to a big screen and digital displays (LCD, DLP, D-ILA) requiring pixel perfect resolution for optimal image presentation.

The advent of the High-Definition TV has given the videophile the minimum requirement for higher-frequency image.  The ability to mimic a PC's 640x480 (480p) as well as the equivalent of 960x540 (1080i) resolutions provides the ability capability of connecting the HTPC to today's Digital TVs. 

MPEG-2 is has 8 current or proposed components or parts. The 1-3 of MPEG-2 have reached International Standard acceptance, the other parts are at different levels of completion.

Part 1 of MPEG-2 addresses the combining of one or more elementary streams of video and audio, as well as, other data into single or multiple streams which are suitable for storage or transmission. This is specified in two forms: the Program Stream and the Transport Stream. Each is optimized for a different set of applications. A model is given in Figure 1 below.

Figure 1 -- Model for MPEG-2 Systems

The Program Stream is similar to MPEG-1 Systems Multiplex. It results from combining one or more Packetized Elementary Streams (PES), which have a common time base, into a single stream. The Program Stream is designed for use in relatively error-free environments and is suitable for applications which may involve software processing. Program stream packets may be of variable and relatively great length.

The Transport Stream combines one or more Packetized Elementary Streams (PES) with one or more independent time bases into a single stream. Elementary streams sharing a common timebase form a program. The Transport Stream is designed for use in environments where errors are likely, such as storage or transmission in lossy or noisy media. Transport stream packets are 188 bytes long.

Part 2 of MPEG-2 builds on the powerful video compression capabilities of the MPEG-1 standard to offer a wide range of coding tools. These have been grouped in profiles to offer different functionalities. Only the combinations marked with an "X" are recognized by the standard.

Tab. 1 - MPEG-2 Video profiles

Since the final approval of MPEG-2 Video in November 1994, one additional profile has been developed. This uses existing coding tools of MPEG-2 Video but has the capability to deal with pictures having a color resolution of 4:2:2 and a higher bitrate. Even though MPEG-2 Video was not developed having in mind studio applications, a set of comparison tests carried out by MPEG confirmed that MPEG-2 Video was at least good, and in many cases even better than standards or specifications developed for high bitrate or studio applications.

The 4:2:2 profile was approved in January 1996 and is now an integral part of MPEG-2 Video.

The Multiview Profile (MVP) is an additional profile currently being developed. By using existing MPEG-2 Video coding tools it is possible to encode in an efficient way video sequences issued from two cameras shooting the same scene with a small angle between them. 

Part 3 of MPEG-2 is a backwards-compatible multichannel extension of the MPEG-1 Audio standard. Fig. 2 below gives the structure of an MPEG-2 Audio block of data showing this property.

Figure 2 -- Structure of an MPEG-2 Audio block of data

Part 4 and 5 of MPEG-2 correspond to part 4 and 5 of MPEG-1 approved in March 1996.

Part 6 of MPEG-2 - Digital Storage Media Command and Control (DSM-CC) is the specification of a set of protocols which provides the control functions and operations specific to managing MPEG-1 and MPEG-2 bitstreams. These protocols may be used to support applications in both stand-alone and heterogeneous network environments. In the DSM-CC model, a stream is sourced by a Server and delivered to a Client. Both the Server and the Client are considered to be Users of the DSM-CC network. DSM-CC defines a logical entity called the Session and Resource Manager (SRM) which provides a (logically) centralized management of the DSM-CC Sessions and Resources (see Figure 3).

Figure 3 - DSM-CC Reference Model

Part 6 was approved as an International Standard in July 1996.

Part 7 of MPEG-2 will be the specification of a multichannel audio coding algorithm not constrained to be backwards-compatible with MPEG-1 Audio. This standard was approved in April 1997.

Part 8 of MPEG-2 was originally planned to be coding of video when input samples are 10 bits. Work on this part was discontinued when it became apparent that there was insufficient interest from industry for such a standard.

Part 9 of MPEG-2 is the specification of the Real-time Interface (RTI) to Transport Stream decoders which may be utilized for adaptation to all appropriate networks carrying Transport Streams (see Figure 4).

Figure 4 - Reference configuration for the Real-Time Interface

Part 9 was approved as an International Standard in July 1996.

Part 10 is the conformance testing part of DSM-CC, under development.