Common misconceptions about the performance of individual parts of the system
Light levels: A salesperson was heard to say recently, "that's a
great camera it's got a good lux level and it's cheap." This is not an
indictment of the salesperson it is a criticism of the level of
training provided. It is also not untypical of the jargon used by
people desperate to create an impression of knowledge. The camera in
question was in a distributors' catalogue and described as 'sensitivity
.6 lux.
Resolution: 580 lines; 750(H) x 580(V); 435,000 pixels. What
do they all mean? The most useful method of specifying resolution is by
the number of lines in this case 580 horizontal. With the increasing
number of CCD cameras it is becoming common to state the number of
horizontal and vertical pixels, in this example 750 x 580. If this is
the only value given it can easily be mistaken for resolution in lines
and appears to be better than it is. The last value is the total number
of pixels in the chip, impressive but no practical use. Apart that is
from the salesperson who needs to impress,-- "their camera only has 580
lines, mine has 435,000 elements!"
An approximation to convert horizontal pixels to equivalent
lines is TV LINES = PIXELS X 0.7. I.e. 750 pixels is approximately 525
lines.
Colour cameras generally have lower resolution than
monochrome. Be careful not to be taken by the specification for some
colour cameras that specify resolution as 450 lines Y/C. These can only
be used in certain situations and the resolution can only be obtained
when using a compatible monitor and associated control equipment. (See
similar note under monitors).
Auto exposure control: Sometimes called electronic iris. This
is a development in CCD cameras and electronically controls the amount
of light reaching the CCD sensor. Several manufacturers claim that this
eliminates the need for an automatic iris lens. Using manual iris
lenses instead can make a significant saving on a system. If this type
of camera is to be used from full daylight to dusk then use caution.
Call the manufacturers' technical department, describe the application
and go by their advice.
Sensor sizes: Early cameras had a circular tube as the sensor
therefore the size was decided by the diameter of the tube, which is
the diagonal measurement of the picture. This is still the case today
so although CCD sensors are flat rectangular chips the nominal size is
the diagonal measurement.
The sensor sizes shown in the diagram must be considered in
relation to the lens selected. This is because lenses are also designed
for a particular size of sensor. See further notes under lenses.
Note: It is a little known fact that the video output from a
colour camera should be 1.2 volts peak-to-peak compared to 1.0 volts
for a monochrome signal.
Lens functions: The CCTV lens performs two main functions. First it
determines the scene that will be shown on the monitor, this is a
function of the focal length. Second it controls the amount of light
reaching the sensor, this is a function of the iris. The focal length
may be fixed-- or variable as on a zoom lens. The iris may be manual--
or automatic and controlled by the camera.
Lens mountings: All CCTV lenses are based on what is known as
the 'C' mount. This is a photographic standard that specifies the
thread type and dimensions. There is now a generation of lens mounts
for CCTV known as the 'CS' mount. The first point to make is that the
thread type and its dimensions are identical for both types of lenses.
Therefore, either type of lens may be screwed to cameras having either
type of mount without causing any damage. The wrong combination will be
impossible to focus but there is no apparent mechanical indication to
an installer that the wrong combination has been used.
The difference between the two types is the optical distance
from the back of the mounting flange to the face of the sensor. This is
known as the 'flange back length.' In the case of the 'CS' lens this
distance is shorter. This allows the use of smaller glass elements in
the make up of the lens and fewer elements to be needed. The result is
a lens that is more compact and cheaper to manufacture. The differences
are shown in the diagram.
The lenses are not totally interchangeable. A 'CS' lens may only
be used on a camera with a 'CS' format mounting. A 'C' mount lens may
be used on a 'CS' mount camera by adding a 5mm-adapter ring.
Lens sizes: It can be very confusing to establish the actual
field of view that will be obtained from a combination of sensor size
and lens specification. Lenses are specified as being designed for a
particular sensor size. A lens designed for one sensor size may be used
on a smaller size but not the reverse. The reason is that the
extremities of the scene will be outside the area of the sensor. Many
people in the CCTV industry have grown up with the 2/3" camera as being
the most popular and are familiar with the fields of view produced.
However the 1/2" and 1/3" cameras are now being extensively used and
therefore there are important factors that must be taken account.
The diagram shows the effect of using one lens on two different
sizes of sensor. The result of using a larger lens format on a smaller
camera format is to create the effect of a longer focal length, that
is, a narrower angle of view.
To summarise then:
- A lens designed for one format may be used on a smaller format camera but will produce a narrower angle of view.
- A lens designed for one format may not be used on a larger format camera.
- Assuming a focal length has been assessed based on a
particular format of camera and lens. It is then decided to use a
smaller format camera. The same field of view will only be obtained if
a shorter focal length lens is used.
- Always check the angle of view for the particular lens and camera combination it is intended to use.
Size: As with camera sensors the size of monitors is the diagonal
measurement of the screen. The distance at which it is to be viewed
generally decides the size of monitor. Typical figures in metres are:
9"-----0.3--0.6
12"----0.6--1.0
15"----1.0--1.3
17"----1.3--1.6
Another consideration is for viewing multiscreen displays. A 15"
monitor is normally the minimum for viewing a quad display. For
multiplex displays that can show up to sixteen pictures then a 17" is
the minimum with a 21" preferred.
Resolution: With monochrome monitors resolution is not
generally the limiting factor. As always specifications need
interpretation for instance most monitor resolution figures are given
as number lines at the centre. A figure of 600 lines at the centre may
only be 400 lines at the edges. The difference is likely to be greater
the larger the monitor for two reasons. First the problem of
maintaining accuracy of the scanning magnets over a larger area.
Second, it is more difficult to produce larger monitors with as fine a
coating as smaller monitors. This is why the picture on a 9" monitor
always looks sharper than when seen on a 17" monitor.
The resolution of colour monitors is less than can be obtained
with monochrome. This is because three spots are needed to make each
point-- red, green and blue. Typical resolutions for colour monitors
are from 280 lines to 350 lines. There is the same fall off towards the
edges of the screen as with monochrome monitors. Some colour monitors
are specified as 450 TVL Y/C. Take care because these monitors only
produce this resolution when using cameras and control equipment that
produce separate chrominance and luminance signals.
A simple video switcher is designed to direct the signal from one of
a series of cameras to a monitor. Most switchers have a control to
enable the monitor to sequence automatically through each camera in
turn. The time between each sequence is generally adjustable. A
switcher should be selected that incorporates what is known as
'vertical interval' switching. This delays the actual moment of
switching until the blanking period of the sync pulse in the composite
video signal. The result of this technique is to prevent picture
'bounce' between successive pictures. One picture simply replaces the
previous one without any rolling caused by waiting for the next sync
pulse. Some switchers can provide output to two monitors. One monitor
can be locked on to a specific camera while the other sequences.
Matrix switchers are now becoming common place in the market
due to the development of microprocessor technology. This type of
switcher can process the signals from a large number of cameras to many
monitors. There can also be many control positions, each of which can
call up any of the cameras. In a railway system for instance it is
possible to have two hundred stations each with twenty cameras. Each
station would have individual control of its own cameras to sequence or
select. All the stations would be connected back to a central location
that could control all four thousand cameras. The central control could
then be divided into say ten regions each with a control and bank of
monitors for its own group of cameras.
Recording of CCTV cameras has had a fairly mixed press over the last
few years. Obviously the failures to identify culprits and lousy
pictures seen on programs like Crimewatch have not helped. There are
though hundreds of systems incorporating video recording that have paid
for themselves time and time over. The most successful systems are
obviously in conditions of good consistent lighting using good quality
colour cameras and a well-maintained video recorder. This then is the
key to successful video recording, the right conditions, the correct
equipment and proper maintenance of the system.
Until recently, one problem has been the limitation of a video
recorder to provide only 240 lines resolution. This is a function of
the maximum bandwidth that can be used on the standard width VHS tape.
It does not really matter what quality of camera and monitor is used.
The limiting resolution is that of the recorder. That is why the superb
pictures seen on the screen during commissioning and handover of a
system are not reproduced when an incident occurs. Reusing the same
tape repeatedly frequently aggravates this. Also, by lack of
maintenance on the recorder. The problems of poor video recordings
could be dramatically reduced if customers insisted that the
manufacturers' recommendations for maintenance and limits of tape use
were strictly followed.
S-VHS: (Super VHS) There is a new generation of video
recorders using the S-VHS system. This provides greater resolution of
up to 500 lines but only in colour. In a composite video signal there
are two elements that make up the colour information. They are known as
the chrominance (C) and the luminance (Y). The chrominance is specific
to colour signals and determines the colour content of the picture. The
luminance is used in both monochrome and colour signals and determines
the brightness. In S-VHS colour signals these two components are
separated at the camera and transmitted as separate signals. Therefore
point one- the camera has to provide separate Y/C components. This
requires two coaxial cables to be run from each camera. The video
recorder must be able to accept the separate Y/C signals as also does
the monitor. The use of this improved quality is limited because at
present there are no switchers or multiplexers that can pass the Y/C
components. Note that there is no improvement with a monochrome signal.
There has been much written about digital recording recently and is
not within the scope of this article to reiterate all the advantages
and loopholes in specifying this type of recording. However, there are
still tens of thousands of analogue recorders still in use and
thousands still being installed. Analogue recorders are still the main
recording medium for many small installations.
Weatherproof housings must be about the most mundane aspect of a
CCTV installation. Or so it seems because many engineers simply
consider the housing as a protection against the elements. However
there are many aspects to consider and many suppliers of housings. It
is about the cheapest element of an external system yet price seems to
be the main factor in selecting which to use. Important considerations
should be:
- Ease of access for pre-assembly in workshop.
- Ease of access during installation.
- Ease of access for future service needs.
- Is the camera mounting plate insulated from the case?
- Can the mechanical focusing screws on the camera be accessed? Some are at the back, some at the side and some on top.
- Can the lens be focused and the peak/average settings adjusted on site?
- Can one man remove the cover and work on the inside?
- If there is a telemetry board fitted can it be accessed without removing the camera?
The most common type of housing is that where the camera is mounted
on a flat bed. A rectangular box shaped cover drops down over the
complete assembly and is held in place by four spring clips. This is
great for assembly in the workshop because everything is nicely
accessible. The problem comes when a service engineer at the top of a
ladder needs to work on it. Many engineers know that it needs four
hands to hold the clips clear and two to remove the cover. Especially
if it has been on for some time and the cover is welded to the rubber
seal. Once the cover is off everything is exposed to the elements and
it is no quick job to replace it.
Another type that was in vogue a few years ago and still
around is the extruded aluminium design. The housing is a complete box
and the camera plate slides out of the back in guides. The cable glands
are usually in the rear sealing plate therefore sufficient slack must
be allowed for the length of the plate to be slid all the way out. At
this point the engineer is faced with the whole assembly and cable in
his hands. It now needs two hands to hold it and two more to work on
it. With the camera and lens on a loose platform and the cables hanging
down it is really fun to focus the camera and set up the lens
adjustments.
There is a variation on the first type mentioned but instead
of using clips there is a pivot at the front. There are clips at the
rear and when released the cover swings right up and forward. This
exposes the complete interior and a stay rod holds up the cover. It's
just like opening a rear-pivoting bonnet of a car. In another design
there is a simple gas strut to hold the cover open. This needs one hand
to open the cover and leaves two hands to work on the inside.
Another design is a box like housing with two latches on each
side. When the latches on one side are released, the cover pivots open
on the opposite side latches. The cover may be opened in either
direction. If all four latches are released, the cover can be
completely removed.
There are other designs around that may be just as engineer
friendly. It is worth spending a little time on this often overlooked
item to make future servicing easier and cheaper.
Finally there is the ubiquitous dome. There has been a
proliferation of dome variants introduced recently probably more than
any other development. In addition they are becoming ever smaller and
faster. But they are not necessarily the panacea for all PTZs. The main
fallibility of dome housings is in the material and manufacturer of the
actual dome itself. Poorly moulded domes can lead to disastrous loss of
focus, particularly at long focal lengths. It is always advisable to
arrange a test and see the results for the longest focal lengths and
distances. If you intend to purchase a dome and fit your own
camera/lens make sure that there is sufficient clearance between the
lens and the inside of the dome to allow focussing at long focal
lengths.
Most domes now allow for quick release fixing of the pan tilt
mechanism and plug/socket for the telemetry and video. This can be
especially important when fitted at the top of a pole in a high street.
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