CCTV Technical
FAQ's
How do I choose the correct camera for my
application?
This in general is a comparatively difficult decision. Many aspects of
the installation must be taken into consideration in order to obtain the
correct performance that meets your requirements.
A high resolution camera should be considered where greater detail of
scene is required. E.g. Colour 460 TVL, Monochrome 570 TVL. Choosing a
more sensitive camera will improve reproduction in poorly lit areas. The
sensitivity of a camera is indicated by the minimum amount of light in
order for the camera to produce a usable picture. e.g. Colour 1.0 Lux at
F1.2.
A conventional camera produces a pale backdrop when an object is shot
against a bright background. BLC (Back Light Compensation) will counter
strong light sources retaining picture quality.
Concentrated light sources directed towards the camera (e.g. car head
lamps) can be inverted by an optional peak white inverter or an eclipser
function. This has the effect of bringing detail to areas and making an
object clear, that would otherwise be shadowed.
How do I set up a camera and lens for use in
Low Light conditions and or with Infra Red Lighting?
The answer to this is the same as that for questions relating to Back
Focusing on Page 31 and 34, with the addition of the following:
When setting the back focus of a Colour camera for low light conditions
you should place an ND1 (Neutral Density) filter in front of the lens.
When setting the back focus of a Mono camera for low light conditions you
should place an ND3 (Neutral Density) filter in front of the lens. When
setting the back focus of a Mono camera fitted with I/R lighting for low
light conditions you should place an IRP (Infra- Red Pass) filter in
front of the lens.
Should you not have any of the above filters you may have to attend
site during the hours of darkness.
I have installed a new camera and lens why am
I unable to obtain a sharp image?
The most common resolve to this is to ensure that both camera and lens
are the same mount i.e. CS mount lens on a CS mount camera and a C
mount lens on a C camera.
What are OSD cameras?
OSD (On Screen Display) cameras have a menu system within the camera
assembly that can be accessed in order to set functions such as Iris
levels, AGC on/off and most features of standard and advanced cameras.
Why do I have a clear sharp picture during the
day and it is out of focus at night?
This is due to the depth of field changing as the light conditions
change and can be easily overcome by following set procedures.
When can I use a manual iris lens?
A general rule of thumb is only to use a MI lens in an internal
application. This is because you are reliant on the electronic circuitry
of the camera compensating for light changes in the scene and this is not
able to compensate to the same degree as that of an Auto Iris lens.
How do I back focus a camera fitted with a
fixed focal length lens?
This is achieved by following five simple steps.
- Set the physical focus of the lens to infinity (clockwise from the
front).
- Aim the camera at the subject to be viewed.
- Release the camera back focus mechanism.
- Adjust the back focus to obtain the best possible picture.
- Secure the cameras back focus mechanism.
Can I fit a 1/3" Lens to a 1/2" camera?
The simple answer is NO.
How do I connect an Auto Iris lens to a
camera?
This is usually performed by a simple plug-in connection to the rear or
side of the camera. However you should always refer to the relevant
camera handbook.
What is the difference between Auto Iris and
Direct Drive Lenses?
An Auto Iris lens is one that automatically adjusts its iris for
changes in the scene lighting levels. The motor that opens and closes the
iris is driven by an Amplifier that processes a small electronic signal
changing with the light level.
A Direct Drive 'DD' lens does not have this Amplifier and can only
operate with a camera fitted with one.
A camera specification will indicate the available output options.
Does the f stop matter when choosing a lens?
Yes, lenses are usually specified as having a minimum and maximum f
stop rating; the f stop is a measure of how efficiently the lens allows
light from the scene, to pass through the lens and onto the camera CCD
sensor. The maximum aperture (when the lens is fully open), is the
minimum f stop number and the minimum aperture, (just before the lens
completely closes) is the maximum f stop number.
A low minimum f stop number means that the lens can pass more light
through during dark conditions, which will produce better pictures at
night.
A high maximum f stop number may be necessary where there is a high
level of light or reflection. This will prevent the camera whiting out.
How do I Back Focus a camera fitted with a
ZOOM Lens?
This can be achieved by following these steps.
- 1. Set the lens to full wide angle view.
- 2. Set the physical focus of the lens to infinity (clockwise viewed
from the front).
- 3. Aim the camera at an object at least 30 Metres away.
- 4. Release the camera back focus mechanism.
- 5. Adjust the back focus to obtain optimum clarity.
- 6. Zoom the lens in to full telephoto and focus on a nearby object.
- 7. Keep this object in view as you slowly zoom out and if all is set
correctly it should remain in focus (track).
- 8. Secure the back focus mechanism.
How do I set up an Auto Iris lens?
An Auto Iris lens has two pots on the side commonly marked ALC
(Automatic level control) and LEVEL.
The ALC control has settings of PEAK and AVERAGE (P+A).
The LEVEL control has HIGH and LOW settings H+L.
ALC
The adjustment allows control over any bright areas in the scene e.g.
sun reflection through windows, street lighting etc. There are two
settings PEAK and AVERAGE.
If set to PEAK, bright areas in the scene are taken into account more,
reducing the contrast in the surrounding area. This allows more detail to
be seen in the bright areas.
If set to AVERAGE the lens takes the bright areas less into account
which usually causes over brightness or flare in these areas, but raising
the contrast of the surrounding area.
LEVEL
The only correct way to set the VIDEO LEVEL is by the use of an
oscilloscope, for most Engineers this is not an option.
A more practical method is to use a service test monitor and a camera
that you know has been set up correctly to 1 volt peak to peak.
Put the video output from this tested camera into the test monitor and
adjust the contrast and brightness until you are satisfied with the
picture. Mark the contrast and brightness controls so that you can set
them to this position again.
Set up each camera adjusting the ALC (as above) then adjusting the
LEVEL to obtain a picture similar to that achieved with the test camera.
(Making sure that your test monitor is set to your marked positions)
NB: On most zoom lenses the ALC adjustment is a speed control for the
Iris motor and is best left in the mid position. The Amplifiers on Auto
Iris lenses are sensitive; so adjust the LEVEL and ALC with a proper
trimming tool instead of an ordinary screwdriver, which can induce small
voltages.
What size monitor should I be using?
The correct size monitor is dependent on its use e.g. the number of
images to be displayed at any given time, the viewing distance and the
available space.
What is Video Termination?
This is the end of line resistance of any CCTV system and this should
be set to 75 ohm. Should you encounter any double image or ghosting this
is more often than not caused by two pieces of equipment in series both
having the 75-ohm switch set on. Only the last piece of equipment should
be set at 75-ohm.
What camera housing should I use and at what
IP Rating?
Camera housings come in various shapes and sizes. With regard to the
correct IP rating protection, this will range from dust and water
ingress. This system is governed by a number of European and British
standards.
IP55
- Protected against dust - limited ingress.
- Protection against low pressure jets of water from all directions
limited ingress permitted.
IP65
- Protection against dust - no ingress.
- Protection against low pressure jets of water from all directions
limited ingress permitted
- Protection against high pressure water from all directions - limited
ingress permitted.
What is the difference between simplex and
duplex multiplexers?
Video multiplexers are designed to allow recording of several cameras
onto one recorder.
Simplex
A simplex machine is best suited to applications where recording only
is the priority. Simplex machines cannot display multi screen images
(i.e. quad, 9 way and 16 way split) while in the record mode.
Duplex
If an operator is monitoring the system (i.e. security guard) then a
duplex machine is more suitable. A duplex machine can provide screen
splits and user selectable images without affecting what is recorded onto
the recorder. Should you use two recorders, it becomes possible to record
and playback simultaneously.
What is the difference between Standard VHS
and SVHS recordings?
In general the main difference apart from cost is the increase in
overall resolution. A standard VHS machine may achieve in the region of
300 TVL (mono) whereas S-VHS machines may achieve in the region of 400
TVL. However this being achieved will depend on the correct ancillary
equipment being used.
Which pan and tilt unit should I use?
The choice is wide and varied dependent on the system requirements. You
may require Top mount, Side mount, 230V AC or 24V DC to name just a few.
TOP MOUNT
- Pro: Can fit two IR lamps on the side of the Pan/Tilt. These act as
a counter balance enabling you to use a lighter duty Pan/Tilt head.
- Pro: Compact size.
- Con: Restricted tilt often -45 to 0 dependent on the housing fitted.
- Con: Cannot be inverted.
SIDE MOUNT
- Pro: Can be inverted.
- Pro: Often cheaper.
- Pro: Large tilt often +or- 180
- Con: Difficult to mount IR lamps.
- Con: Generally large size.
What type of illumination can I use with
colour cameras?
Only lighting within the visible wavelength should be used with colour
cameras. Tungsten Halogen is often the recommended source of lighting.
Can I use I/R Lamps with colour cameras?
The answer to this is a definitive NO. Colour cameras are typically
fitted with an IR cut filter and will not allow IR light in excess of
700Nm to pass resulting in the camera performing poorly in these
circumstances.
What is an ISDN Telephone Line?
An ISDN (Integrated Services Digital Network) Line is a digital system
allowing a camera to be updated at a rate of 15 frames a second.
What is a PSTN Telephone Line?
A PSTN (Public Switched Telephone Network) Line is an analogue network
allowing a camera image to be updated every few seconds and is therefore
slower than that of an ISDN network.
What is an ADSL Line?
Also known as Broadband, an ADSL (Asymmetric Digital Subscriber Loop)
line is a form of Digital Subscriber Line in which the bandwidth
available for downstream connection is significantly larger than for
upstream. Although designed to minimise the effect of crosstalk between
the upstream and downstream channels this setup is well suited for web
browsing and client-server applications as well as for some emerging
applications such as video on demand.
What is a Leased Line?
Leased Line, sometimes known as Kilostream, Megastream or Private wire.
This is NOT a dial up line but is connected 24hrs a day and is usually
subject to an installation charge and a quarterly bill.
What is full picture update?
Full picture update is a technology that enables a transmission system
to update a high quality full picture refresh at very fast speeds.
Do I have to use a Regulated Power Supply?
In general the answer is yes. Most manufacturers will recommend the use
of such power supplies as standard with their equipment. You should
always consult the manufacturers specifications prior to the connection
of any power supply.
What is the maximum distance I can run 12vdc
when powering a camera?
This is a commonly asked question and there is no simple answer. Some
manufacturers may recommend that their cameras can be run over (X)
distance with (Y) cable. This however should still be considered as a
general guide. Cable conductor size and installation route must also be
taken into consideration. If you are unsure, we would recommend that you
contact Technical Support for guidance.
What is the difference between RG59 and URM70
Coax?
In general there is little difference. However RG59 is a hard drawn
conductor and is best suited to fixed camera installations. URM70 has a
multi stranded centre conductor and is more suited to installations such
as PTZ cameras.
What is a ground loop?
An AC current that can be produced in a cable. This is usually caused
by parts of the system being fed from different electrical sources
resulting in different earth potentials at each end. The result is
interference on the signal, usually in the form of dark bands across the
monitor and on occasion tearing in the top third of the image.
How can I eliminate ground loop faults?
This can be achieved in a number of ways, the easiest of which is the
installation of a Ground Loop Isolation Transformer. This is best
installed at the monitor or recording end of the system.
What is the correct level for a video picture?
The correct level is 1 volt peak to peak. This can only be accurately
set either with an oscilloscope or with a video level meter.
How often should I replace my video tapes?
In general it is recommended that all video tapes should be replaced
after a period, no greater than 20 passes of each tape. That being 10
record and 10 play back. This will also be dependent on the quality of
tape being used and that the recording equipment is in good operational
condition.
Manual or Auto Iris Lens?
The iris is the part of the lens that determines how much light falls
upon the camera CCD sensor.
The Manual Iris 'MI' lens has this fixed at the time of installation.
As the light levels change in the scene, the lens can do nothing to
prevent either too little or too much light entering the camera.
Virtually all cameras employ an Automatic Electronic Shutter 'AES' to
compensate for these variations when fitted with an MI lens. However they
are only able to cope with a relatively small change in light levels.
An MI lens should never be used in an external situation, as the camera
will be unable to cope with large changes in light levels. Use an Auto
Iris AI lens in this case, or indeed anywhere where large scene
illumination changes take place.
The advent of new sensor technology such as PIXIM may result in a
change to this situation in the future.
How often should I replace my IR Lamp Bulb?
Installers and end users are often disappointed by the life they get
from Halogen bulbs.
Manufactures quote life expectancy figures as 'Mean Time'. They are not
a guarantee of the bulb life. This simply means that on average after a
number of hours quoted by the manufacturer HALF OF THE BULBS WILL HAVE
FAILED. If the 'Mean Time' of 4,000 hours is quoted, half the bulbs will
fail within that 4,000 hour period (5.5months).
Matters appear worse during the period of October to March, obviously
because of the longer hours of darkness. During this period, lamps may be
on as much as 16 hours each day. A simple calculation may be used in
order to provide adequate maintenance on these systems.
If you have 5 external cameras each with 2 lamps that gives us a total
daily lamp usage of:
10x16hr = 160 hours per day.
If the lamp has a mean life expectancy of 4,000 hours, this means that
you can expect a bulb to fail every:
4,000/160 = 25 days.
You should make sufficient provision for this within a maintenance
contract.
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