PHY chips
Adaptive equalizer chip
Since security is a growth industry, it makes sense for an analog chip company to develop unique, new products that target this market. Intersil broke new ground in surveillance camera design with a pair of complementary product lines. One helps IP cameras communicate over legacy coax. The other helps conventional analog CVBS cameras communicate over Ethernet wiring. (“CVBS” is composite video, blanking, and sync. It’s a term that genericizes PAL and RS-170/NTSC.)
The problem that Intersil was addressing in developing the product families was the conflict between IP and CVBS cameras in new installations and upgrades. Intersil solved the problem by declaring that it didn’t exist and creating complementary product families. One, Security Link Over Coax (SLOC), makes it possible to simultaneously transmit analog CVBS, digital IP video, and RS-485 control signals (primarily to implement camera pan, tilt, and zoom motion) over a single run of coax. The other, MegaQ, makes it possible to send CVBS over a single CAT5 Ethernet twisted pair over a mile in length.
At first, that sounds like the definition of cognitive dissonance: digital video over coax, analog video over twisted pair. You have to admire the project engineer who sold that up the Intersil management chain. To understand why it makes sense, you have to understand the relative advantages and disadvantages of analog and digital video in surveillance applications.
IP: Great Pictures, Late Arrivals
IP cameras are appealing. Who hasn’t made at least one Skype video call from a laptop? Yet it’s a long way from having your 2-year-old talk to grandma, or even outfitting the vacation home with button-cams hooked up to a PC and the cable box, to real enterprise surveillance.
For one thing, there’s latency, the lag between the time something actually happens and the time its image is available for viewing. Latency doesn’t matter if you’re only recording events to identify suspects in a crime that might be committed under the eye of the camera, meaning that IP cameras are a fine solution in that scenario.
Indeed, the superior resolution possible with digital, along with the playback and image enhancement facilitated by a digital video recorder (DVR), makes the combination of an IP camera and a DVR ideal for post-crime investigations. On the other hand, the absence of delay means a great deal if you’re managing security at a casino and you want to catch a crooked blackjack dealer in the act, for instance, if you want to use that pan/zoom/tilt joystick in real time. In that kind of security situation, there’s nothing like analog CVBS.
Lots of Legacy Coax
According to analysts, even where latency is not an issue, there are factors that limit IP camera adoption—at least for the present. First is the enormous installed base of coaxial cable. Analysts estimate that there are more than 400 million coaxial cables installed worldwide in support of CVBS cameras, a disincentive to pulling new CAT5 through the cableways.
Second, even in new construction, CAT5 is not particularly well shielded compared to coax, so it is more susceptible to electromagnetic interference (EMI) and environmental noise that can jeopardize transmission signal integrity. Finally, digital IP video generally cannot (without network switches or repeaters) be transmitted beyond 100-yard distances per run of Ethernet CAT5.
That’s a good part of the case for Intersil’s SLOC (Fig. 1). The technology supports new and retrofit installations of IP surveillance cameras connected to DVRs by short runs of coaxial cable in situations where searching recorded video files after an event to identify participants is more important than intercepting a perpetrator in the act.
What’s the parallel case for MegaQ? There, you might be dealing with a shopping mall where a private security force is primarily charged with “keeping honest people honest” by presenting a physical presence of a few uniformed personnel, augmented by a system of real-time cameras for tracking lost children, old ladies who have fallen down the escalator, rampaging punks, and the guy who just walked out of Banana Republic wearing five shirts with the tags still attached.
That’s a good application for analog CVBS video. But why did the Intersil engineers stand the SLOC paradigm on its head and transmit the analog video over twisted pair instead of coax? The development team has two answers. One is that coax has its own range limitation (approximately a quarter-mile) when used for 5-MHz bandwidth video. One of the benefits of MegaQ is that it pushes that limit out to more than a mile using CAT5, which is much cheaper than coax. For no-latency surveillance video, it offers the best of all possible worlds.
The Hardware
Intersil’s SLOC comprises a pair of physical-layer (PHY) chips, the TW3801 and TW3811, that make it possible to simultaneously transmit digital IP video, analog CVBS video, and RS-485 control signals over a single coaxial cable.
Supporting both analog and IP video simultaneously enables surveillance system manufacturers to take advantage of legacy DVR, spot (multi-camera) monitors, and pan/zoom/tilt (PZT) controllers for viewing and controlling cameras.
System designers can also use the embedded analog CVBS video, which is typically available via the standard RF connector that manufacturers typically provide on the camera to help with setup, to ensure system redundancy.
At the receiver, real-time analog CVBS video can be extracted by filtering out the digital data. For real-time PZT, system designers can count on using the analog video for precise visual feedback, while using plain-old asynchronous RS-485 for commands.
Adaptive EQ
Meanwhile, MegaQ is an adaptive equalization solution embodied in a single chip that resides at the receiving end of one of the twisted pairs in a CAT5 or CAT6 Ethernet cable (or RG-9 coax, for that matter). There are five family members: the ISL59601, -02, -03, -04, and -05. (The terminal digits relate to the maximum cable length the chip can equalize.)
The -01 equalizes CAT5/6 up to a distance of 1000 feet, while the -05 equalizes up to 5300 feet. In other words, the ISL59605 compensates for high-frequency cable losses of up to 60 dB at 5 MHz as well as source amplitude variations up to ±3 dB. Equalization is achieved in a mixed-signal domain with digital control loops that tune the responses of analog active filters to recover the information from degraded signals (Fig. 2).
Because the camera’s analog video output is unbalanced (i.e., intended to feed coax), a passive balun must be used to convert it to a balanced differential signal. This could have presented a connection problem because composite video and sync are sensitive to polarity, but the MegaQ chips automatically invert the signal at their output if necessary.
Besides the balun at the camera, each video circuit requires one MegaQ chip, six resistors, and four capacitors at the monitoring end.
MegaQ technology also achieves a remarkable amount of signal recovery. Figure 2 shows one line of a multiburst test signal, but one demo that I saw included the multiburst in a screen-grab of a shot from a movie. (The actor in the scene was well known, so I cut him out and left the multiburst.)
While the multiburst signal is somewhat quantifiable, the restoration of the actor’s face provides a more vivid demonstration of how much information was being recovered. For example, if the actor had actually been walking out of a Hollywood set of a Banana Republic store while “overdressed,” it would have been a cinch to nail him for shoplifting.
On the other hand, the impact of that could have been lost in the transition to this article, so the information recovery is better illustrated by the multiburst pattern. If you aren’t familiar with analog TV test patterns, the illustration shows two versions of a single line of a multiburst video test signal (a white pulse, followed by four cycles of sinewave at 0.5 MHz, seven cycles at 1.0 MHz, 10 cycles at 2.0 MHz, 12 cycles at 3.0 MHz, 14 cycles at 3.58 MHz, and 16 cycles at 4.2 MHz).
The scope traces show what happens to the test signal after it has traversed 5300 feet of CAT5 cable and what it looks like after it has been reconstituted by the MegaQ chip.
Without adaptive equalization, the horizontal sync pulse and the white bar are severely attenuated, and their slew rates show the effects of the twisted pair’s capacitance. At the vertical scale of the scope’s screen capture, the color reference burst has entirely disappeared, and all that is visible of the burst packets is a little ripple from the 500-kHz packet.
After recovery, the burst has been restored, pulse rise and fall times are acceptable, and burst gain and setup are very close to where they ought to be.