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.
Click to Enlarge Image
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.
Click to Enlarge Image
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.
