[opendtv] Re: Digital vs. Analog Quality
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 9 Jul 2008 08:41:12 -0400
At 1:56 PM -0400 7/8/08, Cliff Benham wrote:
Craig Birkmaier wrote:
...baseband NTSC could be
far better than transmitted NTSC. True for the luminance, but not
true for the color difference signals.
I think there are several examples of wideband color difference
signals being used in NTSC. As far back as the late 1970s, baseband
NTSC has been capable of 120 lines of chroma resolution. This
includes Laserdisk and later in the early 1980s, analog component
recording formats, notably Betacam.
Neither of these are valid examples.
Laserdisc is an analog format with extended frequency response for
the luminance component - in the range of 4.9 to 5.4 MHz. Laserdis
uses the same subcarrier based system for the carriage of the color
components. I found one reference that suggests that the internal
recorded bandwidth for the subcarrier signal is 1.5 MHz, qwhich could
allow for improved color recording in the Q channel. The system is
said to provide 120 lines of color resolution, which is the spec
cited for NTSC (but not for all colors because of the 0.5 MHz
bandpass for Q). It may have had a bit more resolution internally,
but the outputs are baseband are limited by the NTSC specification.
see: http://home.online.no/~espen-b/ld/
Betacam is an analog component format with R-Y and B-Y sampled at
half the bandwidth for the luminance channel - i.e. 4:2:2. It
provided a HUGE improvement over composite video recording formats
for several reasons:
1. NO mixing of the color and luma components as with
subcarrier-based systems. The color components are completely
recoverable without the problems of separating luma and subcarrier
via comb filtering techniques.
2. The R-Y and B-Y color difference signals are sampled at 1/2 the
bandwidth of the luminance signal This is more than double the color
information in NTSC (about 8x the bandwidth of the NTSC Q component.
Alas, when a Betacam machine encodes the analog component signals
into a baseband NTSC output, the color signals are bandpass limited
to NTSC specs. The good news is that the color difference signals are
"oversampled" relative to NTSC and can be recovered accurately - thus
the composite output is MUCH cleaner than for any heterodyne color
recording system.
The BREAKTHRU with Betacam was the availability of analog component
outputs, which could be used to feed an analog component video mixer
that did not suffer from the limitations of composite video
processing. I am reasonably knowledgeable about this, having built a
prototype analog component mixer for Sony for the 1983 NAB. This was
followed by an RGB 1600 series mixer, which both Sony and Grass
Valley used at NAB in 1984 - This was the year we had two Eiodophor
projectors displaying analog component video on huge screens above
the NAB booth. It was also the year that we introduce the Model 100.
After building the 1600 series mixer using RGB processing we realized
that these were the wrong components to process - it was difficult to
get three mixers to track in a perfectly linear fashion - any
non-linearities showed up as shifts in the luminance level. So when
we built the first commercial analog component switcher - the Model
100 CV - we change the components to Y, R-Y, B-Y. This allowed for
excellent linearity in the luminance component - any tracking errors
were shifted to the color difference components and were far less
noticeable.
I might add that there were a number of improved NTSC encoders and
NTSC enhancements developed in the same time frame as Betacam.
Central Dynamics developed an excellent NTSC encoder that carefully
limited the energy from the subcarrier in three dimensional space so
that it did not overlap with the energy from the luminance signal.
This allowed decoders to produce improved color components largely
free from the luma/cross color issues in most NTSC decoders.
And Yve Farudja came up with several excellent NTSC enhancements with
significantly improved performance, but the world was more interested
in moving to HD.
[Betacam]"provides a crisp, true broadcast quality product with 300
lines of horizontal luma resolution, and 120 lines chroma resolution
(versus ~30 for Betamax/VHS)."
http://en.wikipedia.org/wiki/Betacam
Luma - 300 lines, which is better than the 240 line (typical) from NTSC
Chroma- 120 lines - this is the same as NTSC, but is misleading, as
only certain colors actually have this resolution. Any color based on
the ! component has far less resolution
Betamax/VHS - really poor color performance due to heterodyne color
recording - FAR LESS color information and far more color distortion
than a clean NTSC baseband signal.
Regards
Craig
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