A portion of the camcorder market is making a slow transition to hard-disk-drive (HDD) storage over magnetic tape, and with the change come some challenges old and new. To sum up the old, every camcorder (and digital still camera) with a long zoom lens suffers from hand shake in the sense that the long telephoto optically amplifies any vibration and unsteadiness. Regarding the new, the use of a hard drive for storage ropes in concerns that major bumps or drops can induce a disk head crash, wiping out your video masterpiece and seriously maiming the camcorder in the process.
The GZ-MG20 HDD-based camcorder from JVC, Figure 1 , is an example of next-generation camcorders, employing a 1.8-inch, 20-Gbyte miniature drive from Hitachi for a claimed four to 24 hours of video storage, depending on recording-quality level.
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Figure 1: JVC GZ-MG20 HDD-based camcorder packs a lot into a small package.
The JVC design also uses a whopping 25x optical zoom to allow for tight shots from long range. All in all, JVC had the challenge of keeping the hard drive humming and the video steady.
To tackle HDD safety, a MEMS device was brought to bear on the problem of preventing head crash, Figure 2 .
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Figure 2: A combination of gyros and MEMS sensors from different vendors senses both motion (hand shake) and the camera falling.
The principle is pretty simple: Detect an imminent collision with the concrete and park the drive quickly, getting the damage-inducing read-write head arm safely off the platter and over to a neutral location. Using an H48C two-chip, three-axis MEMS accelerometer from Hitachi Metals, JVC was able to buy its customers some insurance. By monitoring the MG20 for sudden changes in acceleration (such as when a camcorder leaves the hand and heads for the pavement), the system can quickly hunker down before the potentially dangerous jolt actually unfolds, a technique that's analogous to an automobile's airbags.
For taking care of the shakes, the MG20 design employs a pair of special sensors to detect rotation around the X and Y axes comprising the plane of the image being recorded. The EMC-03MA and EMC-03B piezoelectric gyroscopes contain ceramic elements whose output voltage changes when rotational forces are applied to the structure. Monitoring angular velocity by way of the proportional output of the sensors, the system can gauge the degree of vibration and movement in the image plane and, correspondingly, the image itself.
At this point, the acquired picture can be electronically subframed to extract a consistent, shake-free portion of the image from frame to frame. While some imaging systems use similar gyroscopes to steady the optics, JVC takes the digital approach, sacrificing some image quality-discarded pixels, really-but saving significant cost in implementation.
Thus, by using some slick little analog sensors, JVC hopes to keep accidents and the shakes at bay.
About the author
David Carey is president of Portelligent, www.teardown.com. The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.