On Mon, 20 Jan 2003 03:00:55 +1100, "Hughy"
<antispam@spamkiller.net> wrote:
>The electronics theory I'm fairly confident of - but
when it comes to camera
>optics I'm on more shaky ground, happy to defer to more
expert comment.
>
>The dB figure refers to video amplifier voltage
gain. 0 dB is normally
>taken as a "reference level" and a cameras
video amplifiers could be
>considered to be running "normally" when
operating at this point.
>
>If a video gain of +6 dB is then selected, the voltage
gain of the video
>amplifiers doubles.
From that point, if you increase the gain another 6 dB,
>the voltage gain doubles again. Mathematically dB=20 log (Voltage Gain
>2/Voltage Gain 1).
At 12 dB, the voltage gain has reached four times its
>original value.
At 18 dB the voltage gain has reached eight times its
>original value.
>
>Looking now at f stops, each time the lens is opened
further by an f stop,
>it roughly doubles the light received by the CCD's. All
other things being
>equal, this should produce double the voltage from a CCD
(ignoring any non
>linearitys).
This is equivalent (AFAIK) to a 6dB increase in voltage gain.
>So two f stops additional lens opening would represent
12 dB - making the
>TRV-900 in theory about 2 f stops "worse" than
our XL1.
>
>But it's not quite as simple as this in the performance
stakes. CCD's and
>video amplifiers introduce noise and some have *much*
better noise numbers
>than others. A guy called Boltzmann managed to put it
all into perspective
>with a formulae: for noise =4KTBR, which is a bit
esoteric but almost all
>radio & electronics guys will recognise it
instantly. The K refers to
>degrees Kelvin, which is why they cool radio telescope
preamps with liquid
>helium (or nitrogen or whatever) - down to close to
absolute zero. To reduce
>the noise. But I
diverge (way off).
>
>The TRV-900 is a truly great little cam for the outlay,
but in practice, it
>is *significantly* noisier than an XL1 in a given low
light situation. I
>have no figures (maybe David Ruether's website looks at
this) but for a
>similar noise performance, my own
"guesstimate" would be around 3 f stops
>"worse" (maybe even a little more), not the
kinda vague "2 f stops"
>suggested by gain figures in earlier post. I will gladly defer to those who
>have made careful measurement of the performance of
these two cameras under
>better controlled test conditions!!
>
>Looking now at the VX2000 we were running at the same
concert, our rough
>estimate (and taking noise into account) would be that
the VX2K is around 2
>f stops better than the XL1. The VX2K noise performance is truly superb!
>And so it should be, the CCD's were developed much more
recently than the
>XL1. When the
VX3K arrives, I'll consider being one of the first to buy
>..... a DSR-275 (which on recent history, should electronically
be a
>VX3K/PD175 thrown into a bigger box). But only if 16:9 is available in a
>better form than currently. These numbers *not* real, but meant to indicate
>later models than those currently on sale!
>
>The substantial difference in bit rate mentioned earlier
was caused IMO by
>the much noisier image produced by the TRV-900 at the
low light levels
>present during that particular section of the
concert. It reinforced a
>basic fact known to all broadcast/ENG types, but not
necessarily to many
>guys using prosumer cameras - the higher the video gain,
the more noisy the
>image, the more the bit budget suffers (or it gets
softer as noise reduction
>is employed to reduce the noise).
>
>Finally, maybe a bit of a guess about MPEG2
algorithms. To me personally,
>the tones in very dark video don't *seem* to have as
much detail in them as
>lighter shades.
So although this is irrelevent to the above (the two images
>were pretty much equall in brightness), maybe darker
images do compress
>better - perhaps due to the varying sensitivity of the
human eye??
>Somebody's sure to have good info on this.
>
>That's it. For
those wanting to flame me over some point I've missed or got
>wrong, kindly do it constructively and without
nitpicking too hard.
Interesting....;-)
In practice, given an "ultimate low-light test"
(where the
light is too low for either camera to produce a good image,
but high enough to produce "some" image, the
TRV900 (with
lens wide-open and at +18db gain) and XL-1 (not
"s" - and
with the lens also wide-open and the gain at its lower
maximum of +12) show the following differences that could
account for the differences in compressed file sizes: the
TRV900 image, being at a higher gain, shows more noise;
the TRV900 fails to provide a good black (unlike the XL-1,
but at +12db rather than +18), making the noise more evident
(see samples at "daylight from small window, low
light", at:
www.David-Ruether-Photography.com/camcorder-comparison.htm). Other
samples there indicate that the TRV900 is more than one stop
more sensitive than the XL-1 (perhaps different from the
"s"?), possibly indicating that the predominant
lighting
was lower for the TRV900 in the example in the post above
than for the XL-1(?). I suspect that if both cameras were
shooting the same thing nearly exactly, with brightnesses
equalized, and with both operating with gain near
"0", that
compression differences (though smaller...) would *still*
show and still favor the XL-1 (!) - in many circumstances,
the XL-1 picture is noticeably less sharp than that of the
TRV900, resulting in less data to compress...;-)