117 Who uses constrained parameters bitstreams? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
117 Who uses constrained parameters bitstreams? (MPEG-2)
Principal CPB applications are Compact Disc video (White Book or
CD-I) and desktop video. Set-top TV decoders fall into a higher
sampling rate category known as "CCIR 601" or "Broadcast rate," which
as a rule of thumb, has sampling dimensions and bandwidth 4 times
that of SIF (Constrained Parameter sample rate limit).
Are there ways of circumventing constrained parameters bitstreams for
SIF class applications and decoders ?
A. Yes, some. Remember that CPB limits pictures by macroblock count
(or pixels/frame). 416 x 240 x 24 Hz sampling rates are still within
these constraints. Deviating from 352 samples/line could throw off many
decoder implementations which possess limited horizontal sample rate
conversion abilities. Some decoders do in fact include a few rate
conversion modes, with a filter usually implemented via binary taps
(shifts and adds). Likewise, the target sample rates are usually
limited or ratios (e.g. 640, 540, 480 pixels/line, etc.). Future MPEG
decoders will likely include on-chip arbitrary sample rate converters,
perhaps capable of operating in the vertical direction (although there
is little need of this in applications using standard TV monitors where
line count is constant, with the possible exception of windowing in
cable box graphical user interfaces).
Also, many CD videos are letterboxed at the 16:9 aspect ratio. The
actual coded and display sampling dimensions are 384 x 216 (note
384/216 = 16/9). These programs are typically movies coded at the more
manageable 24 frames/sec.
Are there any other conformance points like CPB for MPEG-1?
A. Undocumented ones, yes. A second generation of decoder chips
emerged on the market about 1 year after the first wave of SIF-class
decoders. Both LSI Logic and SGS-Thomson introduced CCIR 601 class
MPEG-1 video decoders to fill in the gap between canonical MPEG-1 (SIF)
and the emergence of Main Profile at Main Level (CCIR 601) MPEG-2
decoders. Under non-disclosure agreement, C-Cube had the CL- 950,
although since Q2'94, the CL-9100 is now the full MPEG-2 successor in
production. MPEG-1 decoders in the CCIR 601 class, or Main Level, were
all too often called MPEG-1.5 or MPEG-1++ decoders. For the first year
of operation, the Direct Broadcasting Satellite service in the United
States (Hughes Direct TV and Hubbards USSB) called only upon MPEG-1
syntax to represent interlaced video before switching to full MPEG-2
syntax.
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