Philips System4 Seriver Installation Instructions Manual Download

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The

Number of Frames Between Reference

setting determines how often the reference frame is updated. A

setting of 100, for example, means that every 100th frame becomes the new reference. The 99th frame would be
compared to the first frame, whereas the 101st frame would be compared to the 100th frame.

Block Difference

Each smaller image captured along with the compressed frame is made up of 368 blocks arranged 23

16. Each

time a new image is captured, its smaller image is compared block by block with the reference frame for that
camera. If the brightness of a block differs from the reference frame by more than the

Sensitivity

value, the block

is considered changed.

The

Sensitivity

setting is a percentage times 100. For example, a value of 100 indicates 1 percent, 5000 indicates

50 percent, and so on. A brightness change from black to white would be a change of 10,000, or 100%.

This comparison is performed for all 368 blocks in the frames. The total number of blocks that have changed is
counted and used with the

Percentage Area

setting, as described in the following section.

Percentage Area

The final step in motion detection is to compare the number of blocks that have changed (as determined in the
previous step) to the threshold computed from the

Percentage Area

setting. If this threshold is reached, motion is

detected.

Percentage Area

is the percentage of the 368 blocks in the image that need to change for motion to be detected,

multiplied by 100. For example, a

Percentage Area

setting of 250 (2.5 percent) means that nine of the blocks

(0.025

× 3

68) would have to change for motion to be detected. The lowest setting you can use is about 30 (or 0.3

percent), which requires a single block in the image to change from the reference frame for motion to be detected.
Setting the Percentage Area smaller than 30 would require zero blocks to change for motion to be detected, which
means motion would be detected all the time.

Masking Blocks

Any of the 368 blocks in a frame can be masked. Whenever a block is

masked

, it is completely ignored during the

block-by-block comparison. A masked block is never counted as one of the blocks that have changed, even if it
changes completely from black to white. If you mask everything in the picture, motion will never be detected no
matter what happens.

Even if you mask one or more blocks, the Percentage Area setting uses all 368 blocks for its computations. For
example, suppose you mask 268 blocks and set Percentage Area to 2000. In this case, 20 percent of all 368 blocks,
or 73 blocks, must change for motion to be detected. However, this requires 73 of the 100 unmasked blocks to
change for motion to be detected.

PAL Differences

The motion detection process is exactly the same for PAL, except that the sizes are slightly different. In PAL, the
original image is 768

576 and the smaller black-and-white image is 23

18 (compared to 23

16 for NTSC).

This means there are 414 blocks in the smaller frame (compared to 368 for NTSC).