DIGITAL VIDEO RECORDER PROBLEM SOLVING
DIGITAL VIDEO RECORDER PROBLEM SOLVING
NEW TECHNOLOGY:
The new digital revolution has brought us many new and exciting products for the CCTV industry. The DIGITAL VIDEO RECORDER or (DVR) may be one of the most important upgrades that can be made to any CCTV system. The DVR takes a camera video signal and converts it to a digital bit stream and saves it on a computer hard drive. It can then be played back with remarkable fidelity. This new technology brings with it a new set of problems and solutions. This article will discuss some of the problems and solutions for DVR installations. To understand the problem we must look at the standards.
STANDARDS:
A video standard was developed by the Institute of Radio Engineers so that all manufactured video equipment would be compatible. The unit of measure for this standard is the I.R.E. Unit. One I.R.E. unit is equal to .007142 Volts peak to peak. A Black and White (B/W) video signal is 140 I.R.E. units equal to 1 Volt peak to peak. A color video signal has three vital standard measurements the SYNC = 40 I.R.E. units, the WHITE = 100 I.R.E. units, and the COLORBURST = 40 I.R.E. units. Both the B/W and Color signal measure a total of 140 I.R.E. units, but unlike the B/W signal the Color signal measures 1.142857 Volts peak to peak. The additional 0.142857 Volts peak to peak is caused by the color information called “CHROMA” in the video picture. It is a common misunderstanding that all video is 1 Volt peak to peak. Only a B/W video signal is 1 Volt peak to peak, a Color signal is 1.142857 Volts peak to peak. Now how does all this relate to the DVR problem?
THE PROBLEM:
When properly installed the DVR does not output a video picture, displays a “NO VIDEO” image or has a blank blue video screen. Yet if the video input signal is connected to a monitor it displays a video picture. At first you might think that the DVR is defective, that is very unlikely. It is more likely to be a video level standards problem.
HEADROOM IN THE DVR:
Most DVR’s are designed to accept a video signal of 1 Volt peak to peak with some extra range known as “headroom”. This headroom allows the video signal to exceed the 1 Volt peak to peak by some percentage. Usually 20% over the 1 Volt peak to peak video level. That makes it possible for the equipment to accept a video signal of up to 1.2 Volts peak to peak. The standard Color signal measures 1.142857 Volts peak to peak so this amount of headroom should be adequate. The amount of headroom varies from one manufacturer to another, so you may discover one brand works better than another under certain circumstances, but the problem does not necessarily lie with the DVR.
THE REAL PROBLEM:
One of the most over looked problems in CCTV installation is the output level adjustment of the camera. The standard for camera output is SYNC = 40 I.R.E. units, WHITE = 100 I.R.E. units, and the COLORBURST = 40 I.R.E. units. However some camera manufacturers have “fudged” the standards some what. We find that WHITE levels in auto iris and auto shutter control systems to be padded up to 120 I.R.E. units (1.285714 Volts peak to peak). That is 20% above the standard maximum level established by the Institute of Radio Engineers.
When a camera’s WHITE level is set anywhere above 100 I.R.E. units the video signal can exceed the maximum headroom allowed by the DVR and the system will go into digital overload resulting in the failure of the DVR to record a video signal. When a monitor is attached to the video signal the video can be viewed because a video monitor is not sensitive to overload. This would lead you to believe that the DVR is non-functional, when actually the camera level is not set correctly.
SOLUTION:
To avoid DVR problems at installation and subsequent service calls the camera’s in the system must be adjusted to the proper standard levels so that the video will not overload the DVR. It is recommended that a Camera Master or waveform monitor be obtained to set the camera’s level accurately. If the type of camera you are using will not allow the WHITE level to be set to 100 I.R.E. and you have already purchased the camera’s, then you can insert a video correction amplifier like the GB-60 and adjust the gain below unity ( a loss of signal) to solve the problem. Any amplifier that works with video and will adjust below unity gain will do the job. Video attenuator pads can also be used if you measure the amount of over level and correct it with a fixed value in line attenuator.
Proper setting of the video levels will correct the problem and an understanding of what causes the problem will help you to solve other level related problems with digital equipment in the future. If you would like more information regarding this or other CCTV related problems call FM SYSTEMS, INC. CCTV HELP LINE at: 800-235-6960.
Don McClatchie
FM SYSTEMS,INC.
3877 SOUTH MAIN STREET
SANTA ANA, CA. 92707
TEL: 800-235-6960
