8.2 Level and profile

MPEG-2 is intended to be generic in the sense that it serves a wide range of applications, bit rates, resolutions, qualities and services. Applications should cover, among other things, digital storage media, television broadcasting and communications. In the course of the development, various requirements from typical applications were considered, and they were integrated into a single syntax. Hence MPEG-2 is expected to facilitate the interchange of bit streams among different applications. Considering the practicality of implementing the full syntax of the bit stream, however, a limited number of subsets of the syntax are also stipulated by means of profile and level [6].

A profile is a subset of the entire bit stream syntax that is defined by the MPEG-2 specification. Within the bounds imposed by the syntax of a given profile it is still possible to encompass very large variations in the performance of encoders and decoders depending upon the values taken by parameters in the bit stream. For instance, it is possible to specify frame sizes as large as (approximately) 2¹⁴ samples wide by 2¹⁴ lines high. It is currently neither practical nor economical to implement a decoder capable of dealing with all possible frame sizes. In order to deal with this problem levels are defined within each profile. A level is a defined set of constraints imposed on parameters in the bit stream. These constraints may be simple limits on numbers. Alternatively, they may take the form of constraints on arithmetic combinations of the parameters (e.g. frame width multiplied by frame height multiplied by frame rate). Both profiles and levels have a hierarchical relationship, and the syntax supported by a higher profile or level must also support all the syntactical elements of the lower profiles or levels.

Bit streams complying with the MPEG-2 specification use a common syntax. In order to achieve a subset of the complete syntax, flags and parameters are included in the bit stream which signal the presence or otherwise of syntactic elements that occur later in the bit stream. Then to specify constraints on the syntax (and hence define a profile) it is only necessary to constrain the values of these flags and parameters that specify the presence of later syntactic elements.

In order to parse the bit stream into specific applications, they are ordered into layers. If there is only one layer, the coded video data is called a nonscalable video bit stream. For two or more layers, the bit stream is called a scalable hierarchy. In the scalable mode, the first layer, called the base layer, is always decoded independently. Other layers are called enhancement layers, and can only be decoded together with the lower layers.

Before describing how various scalabilities are introduced in MPEG-2, let us see how levels and profiles are defined. MPEG-2 initially defined five hierarchical structure profiles and later on added two profiles that do not fit the hierarchical structure. The profile mainly deals with the supporting tool for coding, such as the group of picture structure, the picture format and scalability. The seven known profiles and their current applications are summarised in Table 8.1.

Table 8.1: Various profiles defined for MPEG-2
Type	Supporting tools	Application
Simple	I & P picture, 4:2:0 format; nonscalable	currently not used
Main	simple profile + B-pictures	broadcast TV
SNR scalable	main profile + SNR scalability	currently not used
Spatial	SNR profile + Spatial scalability	currently not used
High	spatial profile + 4: 2: 2 format	currently not used
4:2:2	IBIBIB . . . pictures, extension of main profile to high bit rates	studio postproduction; high quality video for storage (VTR) & Video distribution
Multiview	main profile + temporal scalability	several video streams; stereo presentation

The level deals with the picture resolutions such as the number of pixels per line, lines per frame, frame per seconds (fps) and bits per second or the bit rate (e.g. Mbps). Table 8.2 summarises the levels that are most suitable for each profile.

Table 8.2: The levels defined for each profile
Level	resolutions	Simple I,P 4:2:0	Main I,P,B 4:2:0	SNR I,P,B 4: 2:0	Spatial I,P,B 4:2:0	High I,P,B 4:2:0 4:2:2	4:2:2 I,P,B 4:2:0 4:2:2	Multiview I,P,B 4: 2: 0
Low	pel/line			352	352			352
	line/fr			288	288			288
	fps			30/25	30/15			30/25
	Mbps			4	4			8
Main	pel/line	720	720	720		720	720	720
	line/fr	576	576	576		576	512/608	576
	fps	30/25	30/25	30/25		30/25	30/25	30/25
	Mbps	15	15	15		20	50	25
High 1440	pel/line	1440		1440	1440	1440
	line/fr	1152		1152	1152	1152
	fps	60		60	60	60
	Mbps	60		60	80	100
High	pel/line	1920			1920	1920		1920
	line/fr	1152			1152	1152		1152
	fps	60			60	60		60
	Mbps	80			100	130		300

The main profile main line (MP@ML) is the most widely used pair for broadcast TV and the 4:2:2 profile and main line (4:2:2@ML) is for studio video production and recording.