Sensoray 2255 - Manual (Page 6)

Operation basics
Each channel of the 2255 behaves as a separate video capture device (frame grabber). It could be
configured for a specific operating mode and capture rate independently of other channels.
The total capture rate from all 4 channels is limited to that of 2 full resolution color channels. It
means that one could capture at full frame rate (30 frames/sec each channel for NTSC) from 2
channels, or at half frame rate (15 frames/sec each channel for NTSC) from 4 channels. Capture
rate is limited by the amount of data transferred through the USB link, so reducing this amount
increases the capture rate. For example, switching to monochrome mode allows capture at full
frame rate from all 4 channels at full resolution.
In case of JPEG capture the frame rate is limited by the hardware capability to perform JPEG
compression, but the resulting numbers turn out to be the same as with uncompressed video
capture.
The 2255 could be configured for single frame or continuous capture. In single frame capture
mode a command has to be sent to the board each time the frame is required. In continuous
capture mode the frames are sent to the application at the selected capture rate. Choosing an
appropriate mode allows balancing capture speed and CPU utilization.
The 2255 allows capture of the interlaced frames (640x480 and 704x576 resolutions) or non-
interlaced fields (all other resolutions). Interlaced images of fast moving objects, when viewed on
the computer screen, display specific motion artifacts (“jaggies”, “feathering”) which result from
the fact that two fields of one interlaced frame are captured by the camera sequentially in time.
Alternatively, a capture mode is provided that recreates the second field using interpolation. That
eliminates the motion artifacts at the expense of somewhat reduced vertical resolution.
In dual field capture mode fields are sent to the host separately, and an application is notified after
each field is complete, 60 (NTSC) or 50 (PAL) times per second, as opposed to regular, frame-
based capture update at 30 (NTSC) or 25 (PAL) times per second. That may be beneficial in some
cases, i.e. moving objects tracking.
System Requirements
Transfers of large data amounts over the USB require more CPU attention than, for example, over
the PCI bus using bus mastering devices. In addition to that in case of the 2255 image format
conversions may be performed on the host side, depending on the output color format selected
(only YCrCb planar and Y8 formats do not require any conversion).
Due to the factors mentioned above the host system has to satisfy the following minimum
requirements: a Pentium III 700 MHz CPU with 512 MB of RAM. Sensoray recommends at least a
Pentium IV-class 1 GHz CPU with 1 GB of RAM. A high-speed USB 2.0 port is required.
Below is a screen capture of the CPU usage graph from the system running a 2255 demo
application capturing full size color NTSC video at 30 frames/sec from 2 channels in RGB format.
The system is Dell Precision 380, Pentium D 2.8 GHz, 1GB of RAM. (Other applications were
running concurrently, but were not using any significant CPU resources).
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