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VEXA is a tool to show the beginning and the end of optical exposure within every single video field of a PAL or NTSC video camera or video modul.
Tested camera was my CCIR monochrome Video camera WAT-120N with cable remote controller box. This camera can run in normal video mode and in frame integration mode. In normal (OFF) mode 50 fields per second get exposured and transmited as CCIR signal. In frame integration mode one can choose the integration of two fields to one frame image or the integration of 2, 4, 8, 16, 32, 64 , 128, 256 frames to one image. The integrated image will be transmited as normal CCIR signal in real time. This means that in integration mode multiple identical images are transmited as normal CCIR signal in 50 fields per second.
Example:
If you choose 32x frame integration with cable remote controller box you will get the first image out after 32 x 0.04 = 1.28 seconds. This image will be transmited from now on 32 times as normal CCIR signal in 50 fields per second until a new integrated image is ready to be transmited.
Additionally Gain, Gamma, Iris and image freeze can be controlled with the external box. But this does not affect our measurements.
All measurements were made inside a dark room using a 50mm f=1.7 Objektive only lying free in front of the camera to get a focus on VEXA-LEDs.
The measurement sequence with images
Mode 00: range = 0 - 20ms, LED step = 2ms |
| Mode 00 is the first step to begin the measurements with an overview. This type of camera always use nearly the full time of a field for exposure. A test with a artifical light source confirmed that there is no automatic or other possibility to shorten this exposure window when recording brighter objects. Only optical measures like aperture stop or filters can be used to decrease recorded brightness. |
Mode 03: range = 0 - 10ms, LED step = 1ms |
| Mode 03 now shows that LED 1 and 2 in the range of 0 - 2ms are not full lit. More measurements with higher accuracy should bring clearness to this. |
Mode 08: range = 0 - 5ms, LED step = 0.5ms |
| Mode 08 shows in the range from 1.0ms to 1.5ms a dark LED3. But is this LED3 really complete dark? |
Mode 14: range = 0 - 2ms, LED step = 0.2ms |
| Mode 14 now shows that the LEDs 5 - 8 in the range of 0.8ms to 1.6ms are really dark. The same behaviour could be seen also in measurements with separated odd and even fields. |
Mode 14: range = 0 - 2ms, LED step = 0.2ms - Odd field only |
| Mode 14 with LED row lit only in odd fields shows the same window. |
Mode 14: range = 0 - 2ms, LED step = 0.2ms - Even field only |
| Mode 14 with LED row lit only in even fields shows the same window. |
| Mode 14: range = 0 - 2ms, LED step = 0.2ms Both fields integrated electronically in the camera to one frame  |
| Again mode 14 but with both fields integrated electronically in the camera - a surprise. At the moment I can not explain why the camera now exposures without this dark window shown in the images before. In integration mode the last or the delay of the several exposure windows seems to change. This behaviour does exist on all nine integration modes of this camera. |
Mode 27: range = 0.72 - 0.92ms, LED step = 0.02ms |
| Back now in non integration mode of the camera VEXA mode 27 shows the begining of the dark window in higher resolution. LEDs are lit in both fields. |
Mode 27: range = 0.72 - 0.92ms, LED step = 0.02ms - Odd field only |
| Mode 27 shows the begining of the odd dark window at around 0.79ms. LED4 is not lit full. |
Mode 27: range = 0.72 - 0.92ms, LED step = 0.02ms - Even field only |
| Mode 27 shows the begining of the even dark window at 0.76ms. There is a differnce of around 0.03ms between odd and even field. This is the half time of a 64 microseconds lasting PAL line. |
Conclusion:
In integration mode "OFF" this camera has an internal delay of 760 microseconds in Even fields and 790 microseconds in Odd fields between Vsync output signal and the end of the optical sampling field.
This is exactly the same value as measured by Geoff Hitchcox from Christchurch in New Zealand on some (not on all) of his PAL video modules and also the same value as measured on my video modul SK-1004XC/SO.
At the moment the behaviour of the WAT-120N in integration mode could not be explained by me. If there is no delay between Vsync output signal and the end of the optical sampling field or always a full exposure window in integration mode. I don't know at the moment. Any suggestions are welcome.
05. August 2006 |
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