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| VC30P NTSC Prototype, docked to a JVC SVHS Recorder, for proof-of-concept testing. | First version of the VC25P: items were added on as the design evolved. | VC25P in its repackaged version. |
This is a one-of-a-kind "film-look" digital camcorder custom-built specifically to shoot the independent feature film, Cold Day in Hell. The camera module is based on a Hitachi HV-C20A/E, 1/2" format, 3 CCD industrial PAL camera with 700 lines of resolution.
Background
One of the primary factors that makes video look like video is the motion artifact caused by the interlacing of two fields to create a frame. Progressive-scan cameras do not have this artifact because either:
1) The entire frame is exposed in one time period (True Progressive), or;
2) Both fields represent the same time period (25P using 50i format, PAL).
There are a number of prosumer cameras available now, and more coming soon, that have progressive-scan capability. However, all of these cameras use the small 1/3" CCD format, and most still use tape.
The VanceCam25P Concept
Given the limitations of the prosumer cameras, one alternative is to go with a professional progressive-scan DV camcorder. However, these start at about $40,000 and go up, and their rental rates are disproportionately high compared to motion picture film cameras. Another alternative then, is to attempt to achieve progressive-scan operation using less-expensive interlaced equipment.
Researching progressive scan technology reveals that there are several very high quality industrial cameras available (Hitachi and Sony) that have a "frame-integration" mode. While this mode is intended to increase the low-light capability of the camera, it also enables progressive scan. Basically, in the frame-integration mode the CCDs are allowed to accumulate light (charge) for TWO field periods (1/25s). Because of this, both fields are seeing the same light (at the same time) for 1/2 the frame period (each field sees the same light for 1/50s). So then, if the light could be admitted ONLY during the period when the fields are overlapped, then both fields would be capturing the same image at the same point in time, thus producing a progressive scan image, captured as two separate fields: 25P masquerading as 50i. The necessary light control can be achieved using an external shutter.
(Note: the "frame-integration" mode is not to be confused with simple slow-shutter settings (e.g., 1/25s, 1/12.5s) that are available on some camcorders. Progressive scan imaging can only be achieved with interlaced cameras that have a frame-integration mode.)
SHUTTERS
The shutter is positioned between the lens and the camera. This necessitates use of a lens with a greater back-focus distance ("register") than the camera, to allow room for the shutter.
The progressive scan concept was first tested using a liquid crystal shutter. The LC shutter is very easy to synchronize to the camera pulses, and the drive circuit is relatively simple. The outcome of this test was dramatic and impressive: the motion artifact is eliminated!
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| LC (Liquid Crystal) shutter. | Proto-board and the mechanical shutter (black). |
The LC shutter has certain limitations, because it essentially uses stacked polarizing filters to "open" and "close." So, for example, shooting some LCD screens may cause problems, depending on the orientation of the shutter with respect to the orientation of the polarized film on the LCD screen. Because of the polarizers, the LC shutter also loses a lot of light, about 2-1/2 stops.
A motorized shutter is optically better because it leaves the light path totally unobstructed when it's open. However, a motorized shutter requires a somewhat more elaborate control and sync system, is heavier, and requires a bit more power. The motor not only has to be synched in speed but also in precise position. This is achieved by letting the motor start up normally, then sensing when it reaches the correct speed (1500 RPM), then capturing it with synchronized camera pulses to lock it in the correct position.
See the Circuit here.
The VanceCam25P
The VanceCam30P was constructed as a proof-of-concept prototype. The VC25P, a PAL version, has been completed and working. This system was used to shoot the independent sci-fi feature The Shivering Earth. PAL is the preferable format for film transfer because the frame rate or 25fps is close to the film rate of 24fps, and the shutter speed of 1/50s is close to the normal motion picture shutter speed (180° shutter) of 1/48s. Progressive scan PAL video is transferred frame for frame to film, without any pulldown schemes; the audio tracks are slowed and pitch-corrected to match the new frame rate. PAL also provides about 20% better resolution compared to an NTSC version of the same camera.
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| The shiny black box is the YUV to DV converter. |
In the VC25P, the component signals from the camera are fed through an analog component-to-firewire converter (ADS Pyro A/V link) which feeds the DV signal directly to a Laird CapDiv hard disk recorder, for a tapeless system.
Peripherals
An audio preamp was constructed to interface a microphone input to the A to D converter's line-level input.
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| All controls are on the rear panel. |
What's next?
Maybe a 1080+/24P direct to disk system...maybe a 3D system...maybe something completely different...
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© Copyright 2006 GDI/Dan Vance