This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
VHS picture quality can be achieved for film source video at about 1
million bits per second (with careful application of proprietary
encoding methods). Objective comparison of MPEG to VHS is complex.
The luminance response curve of VHS places -3 dB (50% response, the
common definition of bandlimit) at around analog 2 MHz (digital
equivalent to 200 samples/line). VHS chroma is considerably less dense
in the horizontal direction than MPEG's 4:2:0 signal (compare 80
samples/line equivalent to 176 !!). From a sampling density
perspective, VHS is superior only in the vertical direction (480
luminance lines compared to 240). When other analog factors are taken
into account, such as interfield crosstalk and the TV monitor Kell
factor, the perceptual vertical advantage becomes much less than 2:1.
VHS is also prone to such inconveniences as timing errors (an annoyance
addressed by time base correctors), whereas digital video is fully
discretized. Duplication processes for pre-recorded VHS tapes at high
speeds (5 to 15 times real time playback speed) introduces additional
handicaps. In gist, MPEG-1 at its nominal parameters can match VHSs
sexy low-pass-filtered look, but for critical sequences, is probably
overall inferior to a well mastered, well duplicated VHS tape.
With careful coding schemes, broadcast NTSC quality can be approximated
at about 3 Mbit/sec, and PAL quality at about 4 Mbit/sec for film
source video. Of course, sports sequences with complex spatial-
temporal activity should be treated with higher bit rates, in the
neighborhood of 5 and 6 Mbit/sec. Laserdisc is perhaps the most
difficult medium to make comparisons with.
First, the video signal encoded onto a laserdisc is composite, which
lends the signal to the familiar set of artifacts (reduced color
accuracy of YIQ, moirse patterns, crosstalk, etc). The medium's
bandlimited signal is often defined by laserdisc player manufacturers
and main stream publications as capable of rendering up to 425 TVL (or
frequencies with Nyquist at 567 samples/line). An equivalent component
digital representation would therefore have sampling dimensions of 567
x 480 x 30 Hz. The carrier-to-noise ratio of a laserdisc video signal
is typically better than 48 dB. Timing accuracy is excellent,
certainly better than VHS. Yet some of the clean characteristics of
laserdisc can be simulated with MPEG-1 signals as low as 1.15 Mbit/sec
(SIF rates), especially for those areas of medium detail (low spatial
activity) in the presence of uniform motion (affine motion vector
fields). The appearance of laserdisc or Super VHS quality can therefore
be obtained for many video sequences with low bit rates, but for the
more general class of images sequences, a bit rate ranging from 3 to 6
Mbit/sec is necessary.
 
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