[an error occurred while processing this directive]

CATV/Video, (Signal Balancing)

This article outlines an overview for video signal balancing for CATV/Video.

By Doug Moses, Jr. - January 22, 1997

What makes video look and sound good or bad?

The Decibel Notation System is used to determine video signal strength. The human perception of video quality is directly associated with this system. This unit of mesurement is commonly noted as dB (Decibel). The human eye and ear can generally detect a change in video signal quality around plus or minus 3dB. When designing a video distribution system or adding a new component to an existing system, one must try to take into consideration all factors that influence dB. This can, however, be harder than you might expect. In a perfect world, the video quality you perceve on your TV is the same quality as the video signal source. In the real world, this is not usually the case. The video signal is usually degraded by a factor of dB from the source to the TV. The type and length of cable used, the number of splitters, the type of video amplifiers, the quality of the incomming video signal, all influence picture quality and dB differently. The key to good picture quality is to evenly amplify the video signal to all video devices without exeeding the 15.5dB maximum allowed by the FCC.

Using a Tilt Compensator/Low Frequency Attenuator

A Tilt Compensator attenuates lower frequency video signals. Simply put, a Tilt Compensator provides leanear tilt or a weakening of video signals at lower frequencies. One reason you may need to use a Tilt Compensator in your system design is to overcome the problems a video signal amplifier creates in combination with long cable runs. Generally, video signal amplifiers amplify lower frequency signals much more than higher frequency signals. When this happens, in combination with long cable runs, video signal strength in low and high channels is unbalanced. The Tilt Compensator is used to equalize the video signals and bring the two frequencies back into alignment. A Tilt Compensator is usually needed for every 250ft run of RG6 coax cable.

Using an Attenuator pad

An Attenuator pad reduces dB. This component is usually necessary to equalize video signals with different values of dB. For example, you may want to combine the video signal sources from a short cable run with a long cable run. The video signal from the short cable run may be 15dB and the video signal from the long cable run only 5dB. In this case, you should insert a -10dB attenuator pad before combining the two signals on the short cable run. This equalizes both video signals to 5dB.

Using an Amplifier

The video amplifier is one of the most important components for producing a good quality picture. This device increased dB. Video amplifiers usaully increase the dB more at lower channels than higher channels.

Using Terminators

A Terminator is a small cap that screws on the end of an open coax connector. A Terminator is used to prevent video signal bounceback and ghosting. All connections in your system should be terminated either by a video device or a Terminator. Make sure you use only 75 ohm Terminators in your system. This is the proper resistance in a CATV system.

Understanding each components influence on dB

The following is a generalized chart for calculating dB per component. You should check with the literature accompanying your video components for specific details.

******************************************************************
*          Componet          *          General Ranges           *
******************************************************************
*  Attenuator pads           * -3dB, -6dB, -10dB, or -20dB       *
*  Amplifiers                * +5dB to +35dB adjustable or fixed *
*  1x2 Way Splitter/Combiner * -4dB to -6dB                      *
*  1x4 Way Spitter/Combiner  * -7dB to -9dB                      *
*  100ft RG6 Coax Cable      * -4dB to -6dB                      *
*  Tilt Compensator          * -12dB low frequencies             *
******************************************************************

The Final word

This information is only a general explanation of how to balance CATV/Video signals. Other factors such as noise play an important part in the quality of a video signal. The more components you intruduced into your system, the greater your chance of producing a poor quality picture. Each device and component produces noise. This noise is usually hard to remove once intruduced. Sometimes even after you have carefully calculated each device and component, you may still finish with a poor quality picture. This is usually due to the fact that the video signal is passing through many diferent devices and components and the combined noise is great. The best bet for a good quality picture to make the video source and destination distance short and the number of componets involved few.

You can also contact me (Doug Moses) via E-Mail at: kdmoses@ix.netcom.com or visit my home page: at Doug Moses' Home Page