CCTV Interview Questions & Answers

Question 1. What's The Difference Between Hardware Compression And Software Compression?

Answer :

Hardware Compression: Both Capturing Video Signal and Compressing Video Signal are done by DSP chipset integrated on DVR Board. It doesn't need the computer's CPU to do this work. It's low cost of CPU and RAM resources. In this system the computer CPU's task focus on answering network request, streaming the video/audio to network and saving recorded data to local hard disk.

Software Compression: The DVR Board only capture video signal but doesn't compress it, it is the computer CPU and RAM to do this compression work. It's high cost of CPU and RAM resources. In this system the computer CPU and RAM are often overloaded. It is easier to crash than hardware compression system.

Question 2. How Do I Choose The Correct Camera For My Application?

Answer :

• 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. Color 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. Color 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 eclipse function. This has the effect of bringing detail to areas and making an object clear, that would otherwise be shadowed.

Question 3. How Do I Set Up A Camera And Lens For Use In Low Light Conditions And Or With Infra Red Lighting?

Answer :

When setting the back focus of a Color 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.

Question 4. What Are Osd Cameras?

Answer :

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.

Question 5. Why Do I Have A Clear Sharp Picture During The Day And It Is Out Of Focus At Night?

Answer :

This is due to the depth of field changing as the light conditions change and can be easily overcome by following set procedures.

Question 6. When Can I Use A Manual Iris Lens?

Answer :

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.

Question 7. How Do I Back Focus A Camera Fitted With A Fixed Focal Length Lens?

Answer :

This is achieved by following five simple steps:

1. Set the physical focus of the lens to infinity (clockwise from the front).
2. Aim the camera at the subject to be viewed.
3. Release the camera back focus mechanism.
4. Adjust the back focus to obtain the best possible picture.
5. Secure the cameras back focus mechanism.

Question 8. How Do I Connect An Auto Iris Lens To A Camera?

Answer :

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.

Question 9. What Is The Difference Between Auto Iris And Direct Drive Lenses?

Answer :

1. 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.
2. A Direct Drive 'DD' lens does not have this Amplifier and can only operate with a camera fitted with one.
3. A camera specification will indicate the available output options.

Question 10. Does The 'f' Stop Matter When Choosing A Lens?

Answer :

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’.

Question 11. How Do I Back Focus A Camera Fitted With A Zoom Lens?

Answer :

This can be achieved by following these steps:

• Set the lens to full wide angle view.
• Set the physical focus of the lens to infinity (clockwise viewed from the front).
• Aim the camera at an object at least 30 Meters away.
• Release the camera back focus mechanism.
• Adjust the back focus to obtain optimum clarity.
• Zoom the lens in to full telephoto and focus on a nearby object.
• Keep this object in view as you slowly zoom out and if all is set correctly it should remain in focus (track).
• Secure the back focus mechanism.

Question 12. How Do I Set Up An Auto Iris Lens?

Answer :

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.

Question 13. What Size Monitor Should I Be Using?

Answer :

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.

Question 14. What Is Video Termination?

Answer :

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.

Question 15. What Camera Housing Should I Use And At What Ip Rating?

Answer :

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.

IP655:

• Protection against low pressure jets of water from all directions - limited ingress Permitted.

Question 16. What Is The Difference Between Simplex And Duplex Multiplexers?

Answer :

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.

Question 17. Which Pan And Tilt Unit Should I Use?

Answer :

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.

Question 18. What Type Of Illumination Can I Use With Color Cameras?

Answer :

Only lighting within the visible wavelength should be used with color cameras. Tungsten Halogen is often the recommended source of lighting.

Question 19. Can I Use I/r Lamps With Color Cameras?

Answer :

The answer to this is a definitive NO. Color 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.

Question 20. Do I Have To Use A Regulated Power Supply?

Answer :

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 manufacturer’s specifications prior to the connection of any power supply.

Question 21. What Is The Maximum Distance I Can Run 12vdc When Powering A Camera?

Answer :

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.

Question 22. What Is A Ground Loop?

Answer :

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.

Question 23. How Can I Eliminate Ground Loop Faults?

Answer :

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.

Question 24. What Is The Correct Level For A Video Picture?

Answer :

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.

Question 25. Manual Or Auto Iris Lens?

Answer :

• 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 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.