Lee Middleton, Technical Sales Engineer at Dedicated Micros, answers your queries on CCTV
Sense and sensitivity
Q: As an installer with little time to spare, I have recently started using CCTV control equipment with automatic camera sensing facilities. The problem I am having is that the equipment does not always sense correctly. Can you explain why?

A: I suspect that the main problem is your 'video level'. Most equipment with auto-sensing capabilities requires a good clean composite signal in order to sense correctly. It should be useful if I explain a few facts about the subject:

  • The signal level or strength can normally be adjusted via the camera settings, or through some telemetry boards
  • The term composite video refers to a 1 volt peak-to-peak signal voltage used as a standard in the CCTV industry
  • The voltage is divided into sections for measurement of the colour, grey scale, synchronisation pulse and telemetry information
  • It refers to one line of video signal (64 microseconds).

    Below is a breakdown and description of the 1v p-p signal
    0 volts to 0.3 volts
    This is used for the line (horizontal) synchronisation pulses. It ensures that scanning of the image is in precise step with the viewed image.

    Colour (Chroma burst)
    The colour sub-carrier sits on the 0.3v rail at 4.433MHz and is mixed with the luminance signal to produce a colour composite video signal. Changes in the phase of this signal enable the monitor or multiplexer to correctly determine the hue (colour), and changes in the amplitude determine the saturation (amount) of colour.

    Back porch
    Here is where the signals for FSK 'up the coax' telemetry sit.

    0.3 volts to 1 volt
    Known as the grey scale, this uses a staircase waveform where each step has a voltage value:

  • 0.3v is black, 1.0v is white, therefore any change in contrast will represent a change in the voltage
  • Mid grey is 0.65 volts and so on.

    As you can probably imagine having read the above components of a composite signal, it is critical to get as near to 1v p-p as possible for all of this information to be read correctly. Hence, any deviations in the signal voltage can cause all sorts of trouble.

    In order to test the signal for accuracy, oscilloscopes are by far one of the best ways. However, we all know that they can be cumbersome and expensive. Fortunately you can now purchase small hand-held video signal test equipment that will give a quick and easy read-out of p-p voltages and synchronisation pulses, enabling correct set-up of your composite video equipment.

    • Source

    Security Installer

    Postscript

    Send your questions to: Can we help?, Security Installer, The Builder Group, Exchange Tower, 2 Harbour Exchange Square, London E14 9GE. Fax your queries to 020 7560 4416 or e-mail alan_hyder@buildergroup.co.uk