Sensoray 615 - Manual (Page 4)

4
Introduction
Model 615 allows simultaneous capture of full resolution JPEG compressed images and scaled down
uncompressed bitmaps from up to 16 asynchronous standard video sources at a combined frame rate
of 30 frames per second for NTSC, or 25 frames per second for PAL/SECAM. The bitmaps may be used
for visual monitoring, or motion detection. An optional text caption (64 characters), modified on a
frame-to-frame basis, may be overlayed on the compressed image.
As could be seen from the block diagram, model 615 implements a 16x4 crosspoint video switch and 4
separate video capture channels (decoders) with individual frame buffers. This allows cycling through
all 16 input channels at a full capture rate.
Operation
The JPEG compression engine works at a frame rate of 30 frames per second (NTSC), or 25 frames per
second (PAL, SECAM). At the start of every frame the board generates an interrupt. There is a very
narrow time window (less than 2 milliseconds) during which the driver has to prepare the board for the
next compression pass: the crosspoint switch has to be switched, an input channel has to be selected,
and the caption text and settings have to be uploaded. As long as under Windows operating systems
the communication between the driver and the application cannot be ensured within this time
constraint, the approach of the command queue is implemented in the software. The commands are
placed into the queue by the application, and retrieved from it by the driver, as necessary. The
command specifies the decoder channel to capture from, caption text and parameters, the input
channel to switch the selected decoder to after the capture is complete, and some other parameters. If
a command is not present in the queue by the time the interrupt occurs, the next frame is skipped.