Analog Camera
Overview
The CM-206 produces high resolution video and superior low lux performance, providing
users a better quality picture and allowing for easy identification in critical
situations. This high resolution cameras can record at up to 600 lines of resolution
and utilizes Digital Wide Dynamic Range (D-WDR) that allows the analog camera
to balance light in high contrast images. With D-WDR, the camera will brighten
darker areas while brighter areas remain the same. With features such as a low lux
level for nighttime recording, a high TV line resolution, and a tamper resistant
design, the 200-Series domes are a smart choice for indoor surveillance applications.
OpenEye 200-Series cameras are compatible with both 12vDC and 24vAC power making
them easy to add to an existing installation and come with a quick mount adapter–everything
needed for a quick and easy out-of-the-box installation.
Snappy CM-B140FR produces high resolution video and superior low lux performance,
providing users with better quality picture and allowing for easy identification
in critical situations. These high resolution cameras can record at up to 600 lines
of resolution and 30 high-intensity LEDs provide illumination in low light situations.
With features such as a low lux level for nighttime recording, a high TV line resolution,
and an IP-67 rating, the CM-B140R is a smart choice for indoor and outdoor surveillance
applications.
Fortunately, the conversion from actual pixels to “lines” is much simpler than understanding
the motivation for it. Multiply the horizontal resolution in pixels by 3 and divide
by 4 to arrive at number of lines. So for example, if you have 100 pixels in the
horizontal dimension, you only have 75 “lines of resolution” . In this regard, the
rating understates the true resolution of the system. Be sure to not confuse the
term “line” used in analog TV systems from similar term used to describe different
profiles of the high definition TV (HDTV) standard. In that world the pixels are
square so lines is the same as resolution. But confusingly, the rating refers to
vertical resolution rather than horizontal! I know this all may sound confusing.
To keep things straight, just consider “lines” as a metric for analog cameras.
In higher resolution formats such as HDTV and IP Cameras, true pixel resolution
is used so there is no confusion there.
Of course, nothing stops anyone from putting lower resolution sensors in the camera
and indeed, this is often done. Examine the spec and if the line rating is less
than 540, then the resolution is lower than the highest it could be. As they say,
“but wait, there is more!” Turns out even the 540 line spec is grossly overstated.
So far we have been talking about the sensor resolution and compliances of it with
the standard. But there is another part of the standard which deals with transmission
of the same over the air. You might wonder why we would care about that part. After
all, we are sending our video signal over a coax wire. Well, the standard used over
the coax wire in applications is the same as what would be put on air by a network.
To make it easier (and reduce power consumption of the transmitter) the standard
allows that the signal to be reduced in bandwidth. I will not bore you with the
engineering details but there is a handy rule that for every “Megahertz” of bandwidth
for a radio signal, we can carry 80 “lines” of video resolution. So to carry 540
lines of the broadcast TV signal, we would need 540/80 = 6.75 MHz of bandwidth.
If you look into the specifications for NTSC however, you see that the standard
only allows 4.28 MHz. So it goes without saying that we are not able to transmit
540 lines (or 720 pixels). To figure out what resolution we can transmit, we simply
multiply 4.28 MHz by 80 and arrive at a maximum resolution of 340 lines for NTSC
(rounding down for simplicity). Yes, you read that right. The analog cameras
which advertises 540 lines of resolution, cannot achieve more than 340 once you
look at the image that comes out of it over coax.
- Tamper resistant construction
- 3 axis gimbal for 360° positioning
- Dual voltage operation 12vDC/24vAC
- Optically correct bubble
- Varifocal lens & High resolution
What the vendor is advertising is the raw resolution of the sensor used to capture
the video, not what can actually be achieved in a real system when the output is
viewed over that coax wire. That extra bit of resolution cannot be extracted out
of the analog cameras. It is simply lost as soon as the video leaves the
camera. Using the bit of math we have learned so far, we can translate 340 lines
back into pixels. The result is 450 pixels of resolutions (340 x 4 / 3 = 450), again
rounding down. Are we there yet? Can we assume that our total pixel resolution is
450×480 for NTSC and 450×575 for PAL? Well, not quite! We need to re-examine the
vertical resolution because that is not what it seems either! In order to reduce
the amount of data that needs to be transmitted, both NTSC and PAL employ a poor
man’s form of video compression called “interlace.” NTSC updates the picture on
your display 60 times a second (PAL does so 50 times per second). But instead of
sending all of those 450×480 pixels in every instance, the system transmits every
other line in each transmission. These are the fields mentioned earlier.
The actual resolution then in each field is 450×240 for NTSC, sent 60 times a second. At the receiving end, we don’t display each field separately but rather, combine two fields into one frame and display that. In the old analog TVs, this was done by relying on your eye average the two fields being drawn at their respective positions. In case of digital TVs and computer monitors, they are combined in memory and then displayed as a whole. In either case, it is important to note that the transmission occurs at 60 fields of half vertical resolution, not 30 full resolution frames. The same works for PAL except that the field rate is 50. What does this mean in real life? Well, if you mount your analog cameras on a solid mount with zero vibrations and point it at a static scene with nothing whatsoever moving in it (think of a wall), then the maximum resolution of an interlaced system is the same as progressive (where we transmit full frames of video all at once). So the fact that the system is interlaced doesn’t impact us as at all and we have a vertical resolution of 480. Reason is that it doesn’t matter that we captured and transmitted the subject at different times
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