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Gyyr DVMS 400 User’s Guide
than reversible algorithms. Non-reversible algorithms don’t give you back
exactly the same data when they are de-compressed. Obviously this is not
suitable for some types of data, such as numerical data or text, but nonreversible compression algorithms that have been especially designed for
video images create very small changes in the image that do not significantly
alter the overall look of the picture. These small changes are called digital
artifacts.
How noticeable digital artifacts are depends on the amount and type of
compression . By carefully tailoring compression the digital artifacts can be
reduced to a virtually unnoticeable level. The DVMS 400 uses Wavelet
compression, which produces images that do not show any obvious digital
artifacts. Other compression schemes, for example JPEG compression, will
show digital artifacts such as block artifacts, or “stair stepping”, where the
edge of a diagonal line has a slightly jagged edge.
The DVMS 400 uses compression rates that vary from 10:1 to 35:1 in
increments of 5. A 10:1 compression ratio reduces the throughput for NTSC
resolution to approximately 2 MB/s. Most hard drives will have no problem
operating at this throughput rate. However, at this rate a 9 GB drive would
be full in approximately one hour. You must balance the picture quality
desired against the hard drive storage space available when you are selecting
a compression ratio.
NTSC
Compression Data Capture Approximate time that could be captured on a 9 GB drive
Ratio
Rate
60 fields/sec
10 fields/sec
1 field/sec
10:1
2.0 MB/s
1.3 hours
7.6 hours
76.7 hours
20:1
1.0 MB/s
2.6 hours
15.3 hours
153.4 hours
30:1
0.67 MB/s
3.8 hours
23.0 hours
230.1 hours
Calculation Method:
Width in pixels x height in pixels x 2 bytes per pixel x number of fields per second x
3600 seconds per hour = bytes per hour (for a single frame, uncompressed)
720 x 243 x 2 x 3600 = 1,259,712,000 bytes/hour (for a single frame, uncompressed)
9 GB = 1024 B/KB x 1024 KB/MB x 1024 MB/GB x 9 = 1,077,741,824 x 9
= 9,663,676,416 B
For 30:1, 60 fields/second: 9,663,676,416 / (1,259,712,000 x 60 / 30) = 7.671 x (30 / 60)
= 3.8 hours
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