Talk:Automatic Picture Transmission (APT)
A complete line is 2080 pixels long[edit]
So at a "2400Hz subcarrier" there could be theoretically transmitted 2400 bytes per second by the amplitude-modulation.
Why is the image width 2080 and not 2400?
(well its still possible of course, just have no idea why they would do that)
Reading official documentations the width of 2080 never seems to appear (there is only the 2400hz reference)
Can sombody explain how that value of 2080 gets calculated?
(not to mention that the baudrate of 4160 would then be wrong too, for 2400 it would be 4800 baudBaud (unit symbol Bd) is the unit for symbol rate or modulation rate in symbols per second.)
Answering myself[edit]
All processing is accomplished in the digital domain before being converted to an analog signal for output on the APT transmitter. APT data format is shown in Appendix B. The averaging and geometric correction which produces 909 data samples per channel per line has an effective bandwdith of 2080 HzHertz (Hz), unit of frequency, defined as one cycle per second (1 Hz).. To minimize distortion in the data, the signal is low-pass filtered prior to amplitude modulation. The final output is AMAmplitude Modulation modulated on a 2.4-KHzKiloHertz (kHz) 10^3 Hz subcarrier and transferred directly to one of the two APT transmitters.
So 2080 pixels is correct.
Source: ftp://ftp.library.noaa.gov/docs.lib/htdocs/rescue/TIROS/QC8795U4no95.pdf
Not official source of a graphic explaining the total width of 2080 pixels: https://sourceforge.isae.fr/projects/weather-images-of-noaa-satellites/wiki/Apt_
