星空传媒

DVR-based security video systems are comprised of numerous CCTV cameras that are positioned several hundred to several thousand feet away from the head end, as the readers of Security Installer would undoubtedly be well aware.

As distance increases, the video image begins to degrade until it is undecipherable by the DVR equipment.

In this case, in order to provide a reliable connection over UTP, an active twisted pair hub is needed at the head end in order to bring the video signal up to an acceptable level. (See diagram 1).

However, due to the transmission properties of copper twisted pair - Cat5 - the amount of signal amplification that must be applied is determined by the cable length.

Furthermore, in order to provide the optimum image quality in terms of brightness, colour and sharpness, the correct amount of amplification must be applied to the low and high frequency components of the video signal respectively.

Therefore, in order to ensure that the DVR receives the right amount of colour, sharpness and brightness, active twisted pair hubs require gain compensation for each camera port on the hub.

Given the recent technological developments, there are now two ways in which gain compensation may be achieved on an active hub: manual gain control and auto-gain control.

Traditional approach

The traditional approach to applying gain compensation is via manual DIP switches on each hub port.

On some equipment there is just one set of switches for overall gain.

The position of the switches determines the amount of low and high frequency gain to be applied based on cable length.

On other hubs, there are two switches, one for low frequency and one for high frequency.

By tweaking the switches, one is able to optimise the picture quality for a particular camera.(See diagram 2).

Auto Gain Control

Recently, active CCTV cabling solutions with auto-gain control (AGC) have started to appear on the market.

Although auto-gain control is a standard feature in security video cameras and monitors, it is a new phenomenon in security video UTP cabling equipment.

Basically, cabling hubs with auto-gain control perform the same function as manual gain control, but automatically.

There are no manual adjustments. The hub "reads" the video signal and automatically applies the correct amount of amplification (gain) to each portion of the video signal based on cable length.

As cable length increases, AGC compensation increases. (See diagram 3)

Due to the complexity of processing both the low and high frequency components of the video signal, there can be a few seconds' delay before the AGC stabilises. This is normal. (See diagram 4)

Advantages of manual

Manual gain control has a number of advantages.

Manual fine tuning

One advantage is the ability to fine-tune the image according to the individual eye.

By allowing the users to adjust the gain themselves, it allows for personal preferences in regard to brightness, colour and sharpness levels.

Material cost

Hubs with auto-gain tend to have a higher cost per port.

This is due to the greater number and complexity of components inside.

Typically hubs with manual gain adjustment sell for as low as US$160 (拢90) per port.

Auto-gain hubs, on the other hand, regularly list at around US$250 (拢140) or more.

Running costs

Manual gain hubs generally consume less power and generate less heat, therefore electricity and air-conditioning requirements are lower than more power-consuming auto-gain hubs. In larger installations where dozens of hubs are installed, this cost can be significant.

Advantages of AGC

There are also a number of advantages to automatic gain control.

First time installation

It is estimated that it takes between five and ten minutes per port to manually adjust one port on a manual gain active hub.

In medium to large installations, where tens, if not hundreds of cameras are installed, the 'tweaking' time can add up to several hours, or days.

For example, if 100 cameras require manual gain adjustment, this translates into 16 hours.

At US$65 (拢36) per hour labour rate, this translates to over $1,000 (拢560) just for 'tweaking'.

Furthermore, if distances are beyond 3,000ft, sometimes an active transmitter balun is required at the camera, and the gain must be adjusted at the camera balun as well.

On-site maintenance

Changes in cabling conditions such as temperature, cable routing, or changes in CCTV equipment can necessitate a readjustment of gain on one or more hub ports.

The advantage of auto-gain is that it re-adjusts automatically as cabling conditions change.

For example, if different camera and cable length is connected to a hub port, the hub automatically re-calibrates the gain, thus eliminating an on-site service call.

According to one security and surveillance manager, one of his technicians spends a good portion of his time going around and re-calibrating manual gain hub ports. (See Diagram 5)

Downtime

Possibly the most important advantage to auto-gain control is the elimination of downtime due to poorly calibrated hub ports.

If a camera port is out of adjustment, it can adversely affect the ability to see details, and for a certain period of time, the camera is effectively out of commission.

This is a major issue where sensitive or valuable assets are under surveillance. Auto-gain control ensures that from the cabling standpoint, there is zero downtime.

Space management

With the proliferation of electronic hardware in a building, space utilisation is a key issue.

Since there are no adjustments required, auto-gain hubs may be installed in a remote telecom room where access is controlled. Sited here, the advantage is that they do not consume valuable real estate near digital video recording equipment.

Practically speaking

A number of major installations have successfully deployed auto-gain hubs and are reaping the benefits.

One site is a major casino in North America. One of the largest casinos on the west coast, it houses over 1600 cameras.

The system features Ultrak PTZ cameras feeding into twisted pair cabling hubs. Some of the hubs are active and feature auto-gain control.

From the hubs, the video feeds into Honeywell cross-point matrix switchers.

From the matrix switchers, video is routed to real-time monitors and video archival servers located in different rooms. the difference between the cameras and cabling hubs is around 700 to 1,500ft. (See diagram 6).

Advanced Audio Concepts, the systems integrator for the project, installed MuxLab auto-gain hubs to support copper twisted pair cabling between the cameras and the head end.

According to Doug Gilroy, the company's president: "The client is very satisfied with the image results. Despite the normal glitches in getting the new system up and running, the hubs performed as expected and have saved an enormous amount of installation and maintenance time."