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audio/videoZONE Products for the week of July 14, 2008
Maxim Integrated Products Says…
MAX7474: Automatic Equalizer for Twisted-Pair Cables in Security Video Systems
Adaptive Equalizer Compensates for CVBS Cable Loss Characteristics Up to 300m
Maxim Integrated Products solves the challenge of lossless transmission of video over long, unshielded twisted-pair (UTP) cables in security video systems. Maxim introduced the MAX7474, the industry's first adaptive equalizer for composite video over twisted-pair cables. 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 equalization 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 in traditional video system designs
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 equalization 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 equalizer 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 equalization to the signal. "The MAX7474 also eliminates the need for fixed-length cable equalizers," Fox added.
EN-Genius Says…
If they had been keeping an eye on the ball this should have been a product from Intersil. The EL9110 won a 2003 Product of the Year Award from us for the innovative correction of a video signal at the end of 300 m of Cat-5 cable, making use of the repeatable characteristics of such cable.
Making that an automatic process was not a difficult jump to believe in and I mentioned such a possibility in my review.
The MAX7474 is a little different and, in some ways, only does half the job, with no correction that I can see for cable delay differences that occur with twisted pairs.
The device looks to line sync to determine the gain correction for the low frequencies and to the color burst to determine the gain needs for the high-frequency end of the video signal spectrum. Fixed gain equalization settings are applied in the absence of a color burst – ie a monochrome signal.
The part accepts an ac capacitively-coupled (0.22 µF in each leg) differential video input with a maximum 2.4 V pp swing when sync tip is dc restored (Maxim misuses the word clamp) to a typical 2.0 V on the positive input and 3.2 V on the negative input. An AGC circuit looks at the sync pulse amplitude and applies appropriate gain to the low frequency spectrum of the signal while the automatic equalization control circuits set the color burst at the output to 293 mV (for both NTSC and PAL).
A loss of signal output is triggered when the input sync amplitude falls below a preset threshold for 32 lines of video. For the same 32 line duration a loss of burst detection switches the equalization to a fixed mode. The three positions of fixed gain at the NTSC subcarrier of 3.58 MHz are a nominal 0 dB, 4.5 dB and 10.5 dB.
After the adaptive equalizer the part sets the sync output dc level (another dc restoration circuit mislabled as a clamp) to get the back porch to between 1.0 V and 1.6 V. A single-ended output stage, with 0 dB gain, provides a typical output voltage swing of 1.2 V pp capable of driving either an ac-coupled load (220 µF needed), or a dc-coupled load, that has been double-terminated in 75 Ω.
This is a nominal 5 V part with a normal mode supply current of about 50 mA. The PSRR is a typical 65 dB. Differential phase and gain are an immaterial (for security applications) typical 0.3º and 0.3%.
It is difficult to see phase delays on the data sheet characteristic curves but they look to be of the order of 100 ns to 150 ns at 300 m at low frequencies and 2T pulse frequencies. There will therefore be visible – but not dramatically annoying – secondary imaging reflections on the output signal.
The noise performance of the gain equalization is particularly impressive. A full signal amplitude recovery at 300 m reach is extremely useful and with acceptable picture recovery out to 600 m most security systems will benefit. Those with longer reaches are, hopefully, using intermediate line amplification techniques powered perhaps by PoE systems using the same twisted pairs.
The lack of phase correction is far from a deal breaker although the perfect solution for video over twisted pairs would be for an automatic pre-distortion system triggered by the output signal with a feedback to the input. The MAX7474 will do extremely well and the pricing is set to challenge the EL9110 head on with its initial release price of $6.75 (1000-piece) eased back to just over $6.
The MAX7474 is in production in SSOP-16 priced at $6.00 in 1000-piece lots.
Data Sheet
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