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8.3 PRODUCT FEATURES
8.3.1 What is the single, most important reason to buy a DVC camera vs. the
"competition"?
That's easy ! With DVC Cameras you get a terrific combination; high signal-to-noise ratio without
sacrificing sensitivity or real-time operation. DVC's real time (30 frames/sec) cameras provide
>62 dB SNR at a sensitivity of 0.5 lux. Most high-end commercial cameras are much noisier
(lower SNRs), at 0.5 lux OR they just cannot match the sensitivity; low-end cameras produce
unusable, noisy video at "max" gain if they try to reach a sensitivity of 0.5 lux !
8.3.2 Can you define the term signal-to-noise ratio ?
The term signal-to-noise ratio of a camera is best described by the following:
Let's say that it takes 0.5 lux of light at the CCD of a camera to produce 100 IRE of video (for the
purposes of this discussion, 100 IRE is 1.0 Volts of video signal). If you take away the source of
light (by capping the lens), you expect to get 0 IRE of signal, right ?
Sorry, but that's true only in an ideal world !
In the real world, you'll get, for a typical competitor's high-end camera, 0.32 IRE (peak) of
randomly varying video signal, (called noise) with the lens capped. That's because, a few pesky
electrons called dark current electrons flow in the CCD, even without any light. It's analogous to
a leaky faucet, no matter how tightly you close the faucet, a few drops always leak through and
get collected in the bucket (the CCD charge site). The random variation in the dark current
electrons contribute significantly to the noise in the video signal - this shows up as "snow" in the
image on a monitor !
The ratio of the 100 IRE signal and the 0.32 IRE noise is the signal-to-noise ratio.
In this example (not a DVC Camera) the SNR is 100 IRE / 0.32 IRE = 316
The SNR is usually expressed in decibels (dB). SNR (in dB) = 20*log
10
(Signal/Noise)
In our example, the SNR (in dB) is 20*log
10
(316) = 50 dB (approximately)
The above example is typical of a so-called "security and surveillance" camera. Quite frankly,
most of them can't even do 50 dB at a 0.5 lux sensitivity ! To be that sensitive, they have to
crank up the gain - but that degrades the SNR much, much below 50 dB !
Beware of camera specifications that use terms like SNR and sensitivity without linking them
together ! Any camera can claim to have "super sensitivity". The question is that "What signal-to-
noise ratio (how "snowy" a picture) do you have to live with to achieve that "super sensitivity".
The converse is also true; a camera can claim to have "low, low noise" - but it may have to be
virtually "blind" in terms of sensitivity to achieve that "low SNR" specification. By de-linking the
terms SNR and sensitivity, some camera manufacturers can be awfully creative !
Another trick is to use words such as "usable video" as part of the sensitivity specification. It is
more scientific to specify sensitivity as the light required for 100 IRE (full video). Since CCDs are
Summary of Contents for DigitEyes Series
Page 14: ...9 Figure 4 3 TC 245 Gate Level Drawing Texas Instruments 1994 ...
Page 61: ...56 12 Appendix D Camera Mechanical Drawings Figure 12 1 Camera Mechanical Drawings ...
Page 63: ...58 Figure 13 2 Camera Noise Spectrum Min Gain Bandwidth 10kHz to 4 2MHz ...
Page 64: ...59 Figure 13 3 Camera Noise Spectrum Max Gain Bandwidth 100kHz to 4 2Mhz ...
Page 65: ...60 Figure 13 4 Camera Noise Spectrum Max Gain Bandwidth 10kHz to full ...