This contribution is courtesy of my colleague Renato Bordin.
Video signals from cameras are transmitted to the receiving end via coax cables, normally RG59, RG11 and so on. The camera’s composite video output buffer has a series output resistance of 75Ω and requires an end of line load impedance of 75Ω, hence all video equipment such as matrices, DVR’s, monitors etc have a cable termination of 75Ω to match the cable and camera specifications. This transmission method is referred to as an unbalanced signal path and requires a return path which the coax cable provides in the form of the screen or braiding. The concept is the same as a light bulb connected to a battery with two wires, remove one wire and the bulb stops burning since there’s no electrical connection to complete the current flow. This method of video transmission has one major disadvantage; the video is referenced to ground making it susceptible to common mode interference and provides a path for potential differences between two ground points. This often results in poor quality pictures received on the monitor and when a lightning ground surge moves between the camera and monitor the potential difference is now at a dangerous level resulting in camera failure. Other disadvantages are the attenuation properties of coax cables, RG59 for example cannot reproduce a high resolution image at 300m, the high frequency attenuation, and hence the upper end of camera resolution is reduced. The net result is a high resolution camera installed for a particularly demanding application but the received picture is in fact more a medium resolution image. Bear in mind that most applications involve a digital recording device that further reduces picture content or resolution. Digital recording devices require the highest possible camera resolution or rather video bandwidth to reproduce an acceptable image after digitization and then compression.
The alternative to using coax cable as a video transmission medium is the twisted pair cable but we have some challenges to overcome. Let’s take a look at the most popular of twisted pair cables – CAT5e, widely available and well known in the IT industry. For starters the cable impedance is 100Ω and not 75Ω as required by a camera and has no screen, so induced common mode interference becomes a problem. More importantly the cable is designed for a balanced or differential transmission method such as Ethernet and RS485/422. In the active domain differential input and output operational amplifiers (Opamp’s) are used to convert an unbalanced signal to a balanced or differential signal appropriate for twisted pair transmission and visa versa at the receiving end. In the passive world a Balun transformer is used to match impedances. In simple terms this device receives video from a camera and converts it to balanced video matched for a cable impedance of 100Ω and the receiving end converts this balanced video signal back to its original format suitable for a coax connection to the DVR, matrix etc. There are only advantages in using CAT5e cable for video transmission. 1. Since we now have balanced video, common mode interference is rejected far more than coax, resulting in less noise present on the picture. 2. CAT5e cable frequency response specifications are ideal for video so longer cable runs are possible. RG59 cable runs must not be longer than 220m – 250m using high quality cable but CAT5e cable runs can be at the 300m mark and up. 3. Since CAT5e has 4 twisted pairs, one cable can carry 4 video signals. 4. Some twisted pair video solutions have full ground loop isolation. 5. Installation ease and costs are less than a coax based installation. I’m sure several installers reading this can see several other installation related advantages.
The VIDEO CAT range of products available from BFR Digital currently includes the VC-01, a direct camera mount twisted pair converter and the VC-016R. This is a 16 channel rack mount version with full ground loop isolation. Both products feature BNC’s for camera or head end video connections and terminals for twisted pair connections.
Renato Bordin
http://www.bfrdigital.co.za/videocat.html
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