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Equaliser improves signal quality for 600m cable

Posted: 17 Jun 2008     Print Version  Bookmark and Share

Keywords:adaptive equlaliser  twisted-pair cables  lossless transmission  video signal 

Maxim Integrated Products introduces the MAX7474, the industry's first adaptive equaliser for composite video over twisted-pair cables. Maxim solves the challenge of lossless transmission of video over long, unshielded twisted-pair (UTP) cables in security video systems. The MAX7474 automatically compensates for cable losses that occur in the transmission of a composite video signal with blanking and sync (CVBS) over UTP cable lengths of up to 300m. The MAX7474 also greatly improves signal quality for cable lengths up to 600m. The device intelligently adapts to cable length and adjusts the video signal gain appropriately, and it automatically switches over to fixed equalisation for monochrome CVBS. The MAX7474 is compatible with NTSC and PAL standards.

Any video application that needs to run CVBS over long UTP cables will benefit from this product. Key applications include security video systems, video switching systems and automotive rear-view cameras.

Compensating for transmission losses
Traditional CCTV security systems typically consist of many CVBS cameras, routed through a video switch matrix, viewed and then recorded on a small number of monitors and recorders. These cameras are often routed using coaxial cable or CAT2-7 UTP cable.

Differential UTP cables are becoming common now because their cost is lower than coax cables. Differential UTP cables do, however, experience transmission losses. As most cameras and switchers use single-channel coax connectors, these systems thus need some type of single-ended-to-differential video conversion. Designers traditionally choose among multiple passive solutions, such as baluns or twisted-pair drivers/receivers, to provide active compensation and drive the video signals through the UTP cables. While effective, these solutions are limited to fixed-length cable equalisation and require manual gain adjustments. Those additional passive components also demand extra design effort and consume valuable board space.

The MAX7474 overcomes the problem of transmission losses over long UTP cables without adding extra design time, components or cost. "The MAX7474 adaptive equaliser intelligently compensates the CVBS signals," said Jim Fox, Maxim's director of business management for video products. "It brings the video signal back to its original amplitude after long-distance transmission has degraded the signal quality." For monochrome signals, the MAX7474 detects the absence of burst on the input signal and applies a user-selectable fixed equalisation to the signal. "The MAX7474 also eliminates the need for fixed-length cable equalisers," Fox added.

Key advantages
The MAX7474 is the only part that automatically adapts to cable length. It monitors the sync and colour burst of the colour video signal at the output, and adjusts the gains accordingly. When it detects monochrome signals, it applies an AGC function together with user-selectable fixed equalisation to the signal. It thus continuously adjusts the low-frequency luminance band gain and the higher frequency chrominance band gain to correct for cable losses anywhere between 0m and 300m of CAT2-7 cable. The MAX7474 provides full equalisation up to 300m of cable, and greatly improves performance up to 600m.

The MAX7474 accepts differential video input signals in NTSC or PAL standards and automatically sets the optimal level of equalisation for any cable length between 0m and 300m. The device includes an output unity-gain video driver and adjustable back-porch DC level for additional flexibility. It also features loss-of-sync and loss-of-burst detection logic outputs for system monitoring.

The MAX7474 saves significant board space and design time, while providing automatic cable compensation for the colour CVBS signals over differential cables such as CAT2-7. It operates over the -40°C to 85°C industrial temperature range, and is offered in a 16-pin SSOP package. Prices start at Rs.240.13 ($6) (1,000-up). Samples and evaluation kits are available.

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