Ocean Optics SAD500 Communications and Control - Manual (Page 15)

15
Technical Note 1: SAD500 Data Compression
Transmission of spectral data over the serial port is a relatively slow process. Even at 115,200 baud, the
transmission of a complete 2048 point spectrum takes around 400 msec. The SAD500 implements a data
compression routine that minimizes the amount of data that needs to be transferred over the RS-232
connection. Using the “G” command (Compressed Mode) and passing it a parameter of 1 enables the data
compression. Every scan transmitted by the SAD500 will then be compressed. The compression algorithm
is as follows:
1. The first pixel (a 16-bit unsigned integer) is always transmitted uncompressed.
2. The next byte is compared to 0x80.
•
If the byte is equal to 0x80, the next two bytes are taken as the pixel value (16-bit unsigned integer).
•
If the byte is not equal to 0x80, the value of this byte is taken as the difference in intensity from
the previous pixel. This difference is interpreted as an 8-bit signed integer.
3. Repeat step 2 until all pixels have been read.
Using this data compression algorithm greatly increases the data transfer speed of the SAD500. The table
below shows the data transfer speed, in milliseconds, for various light sources and baud rates. Keep in mind
that these rates are for demonstration purposes only, and the speed of your computer may impact the data
transfer rates.
Comp 115 kb % faster 57.6 kb % faster 38.4 kb % faster 19.2 kb % faster 9.6 kb % faster
on 290 32.9% 426 45.2% 624 46.7% 1148 47.5% 2247 48.8%
dark
off 432 778 1170 2188 4391
on 290 32.9% 429 44.9% 624 46.6% 1141 49.6% 2192 50.1%
LS-1
off 432 779 1169 2266 4390
on 303 29.9% 465 40.2% 679 41.9% 1238 43.5% 2424 44.8%
HG-1
off 432 777 1169 2193 4391
The following shows a section of a spectral line source spectrum and the results of the data compression
algorithm.
Pixel Value
Value
Difference
Transmitted Bytes
185 0 0x80 0x00 0xB9
2151 1966 0x80 0x08 0x67
836 -1315 0x80 0x03 0x44
453 -383 0x80 0x01 0xC5
210 -243 0x80 0x00 0xD2
118 -92 0xA4
90 -28 0xE4
89 -1 0xFF
87 -2 0xFE
89 2 0x02
86 -3 0xFD
88 2 0x02
98 10 0x0A
121 23 0x17
383 262 0x80 0x01 0x7F
1162 779 0x80 0x04 0x8A
634 -528 0x80 0x02 0x7A
356 -278 0x80 0x01 0x64
211 -145 0x80 0x00 0xD3
132 -79 0xB1