VV5801B001 Ver la hoja de datos (PDF) - Vision
Número de pieza
componentes Descripción
Fabricante
VV5801B001 Datasheet PDF : 23 Pages
| |||
VISION VV6801/5801 PRELIMINARY CUSTOMER DATASHEET Rev 1.1
5. Operating Modes
There are six main operating modes for the sensor:
1. Still Image Capture with a Frame Buffer
2. Correlated Double Sampling (line by line FPN cancellation)
3. Live-Video Mode
4. Subsampled Mode (Horizontal and Vertical ‘Cine’ modes)
5. Parallel Integration
6. Accumulate
These are explained in the following sections, together with detailed timing requirements for the various
control signals necessary to operate the sensor. An additional suggested mode of operation (‘Multiple Dark
Current Periods’) is explained in Section 5.7.
5.1 Still Image Capture with a Frame Buffer
y This is the recommended operational mode for high quality still image capture in camera systems where
r there is an electro-mechanical shutter in front of the sensor and a Frame Buffer for temporary image storage.
FPN cancellation is central to this mode of operation, and is described in detail. Other operational schemes
a that may be devised can include all or some of the techniques employed in this example, but the elements
are essentially the same. (See Section 5.7 for a discussion of variations to this FPN cancellation scheme.)
in Note: For the simplest possible image capture mode, with no FPN cancellation, see the description of the
Vertical Shift Registers above.
The basic still image capture cycle starts with the shutter closed. The array is released from reset by taking
the input to the reset vertical shift registers, FI, high. The system controlling the camera must then wait for
1306 lines to allow this “integrate wavefront” to propagate through the shift register, before opening the
lim shutter. When FI goes high FR should also be pulsed high for 2 lines to initiate the Read sequence. Reading
each pixel as soon as it is released from reset yields a reset image which contains both the fixed pattern noise
component for each pixel and the random reset noise due to that particular reset operation. This image
should be stored in a frame buffer.
e When the shutter has closed after exposure FR must be pulsed high again for 2 lines to re-read the array and
obtain the exposed image data. Again, it will take 1306 lines to read all of the array pixels. FI should fall when
r FR falls, to return the active pixel array into reset. As the image frame is read out the appropriate pixel reset
value, as stored in the frame buffer, is subtracted from the current pixel value and the result written to the
P frame store. This removes both pixel reset noise and pixel to pixel DC offsets from the image.
LCK
EVEN
FI
Shutter
FR
AVO
1306 Lines T1=Exposure 1306 Lines
1306 Lines Integrate=T1 1306 Lines
Image Frame
Dark Current Frame
Valid Video Line
Black Ref Line
AVO Not Valid
Figure 5.1 : Relative timing of still image capture with frame buffer
Note: See Section 7. for exact relationships.
Due to the length of time taken to read out an image (200 ms, assuming a 5 MHz clock rate), the dark current
in each pixel is a significant part of the image data. To remove the fixed pattern noise injected by the dark
current a ‘dark image’ must be captured with the same integration time as the exposed image but with the
shutter closed. Subtracting the dark image from the exposed image removes the dark current fixed pattern
cd24082b.fm
09/09/98: PRELIMINARY
17
VISION VV6801/5801 PRELIMINARY CUSTOMER DATASHEET Rev 1.1
noise, leaving a ‘clean’ image. This process can be summarised as follows:
1. With the shutter closed, release the sensor from reset and immediately read a frame into the buffer
memory; this captures the array threshold FPN and reset noise (‘VReset’)
2. After 1306 line periods, open the shutter and expose the sensor to the required scene (the exposure
time can be determined by ‘Parallel Integration’ or ‘Accumulate’ — see below)
3. Close the shutter and immediately read the array; as each pixel is read, subtract the value for that posi-
tion stored in the frame buffer, and overwrite that pixel location with the difference — the memory now
contains the image plus dark current FPN (Vim + VDark)
4. After the 1306 line periods of the second read, repeat the image capture cycle, but do not open the
shutter; this time, load a second frame buffer with first the VReset value and then the VDark value (after
subtraction)
5. After the second integration period, subtract the VDark value for each pixel that is stored in the second
frame buffer from the (Vim + VDark) value for that position stored in the first frame buffer and overwrite
y that pixel location with the result.
The frame buffer now contains the corrected image values, which can be processed for colour and so on,
r then transferred to permanent image storage memory.
The pixel voltages for this method are illustrated schematically below:
ina Reset[n]
Read[n]
Shutter
Vpix
VBlack
Prelim VRT
VDark1
VReset
VD+ark1
VD+ark2
Not to scale
Vim
VDark = VDark1 + VDark2
Image
Exposure
VDark2
VWhite
Read 1
Read 2
Read 3
Read 4
Figure 5.2 : Pixel voltages during still image capture with frame buffer
Note: Since the ‘integrate wavefront’ must propagate through the VSR, the point at which the open shutter
exposure occurs will vary progressively from line to line of the array — from close to ‘Read2’ on the bottom
line to close to ‘Read1’ at the top.
5.2 Correlated Double Sampling (line by line)
This is an alternative FPN cancellation mode for camera systems where there is only a Line Buffer available
for temporary image capture, and not necessarily a mechanical shutter in front of the sensor. The method
outlined below, using the CDSR signal, relates to a still image capture in a shuttered camera system, but the
same principle could also be applied to exposure control with the FI pulse duration in Still Frame and Live
cd24082b.fm
09/09/98: PRELIMINARY
18
Share Link: 