You don't have to understand the technical details of FAX480 to enjoy the mode and its many advantages. The information on this page is included to encourage hardware and software implementation of FAX480 in new SSTV products.
Image aspect ratio: 1:1
Horizontal line rate: 3.7416 Hz (Note 1)
Direction of horizontal scan: left to right
Clock/Pixel/Sample rate: 1953.125 Hz (Note 1)
Grayscale values: 16/32/64 per pixel
Frame scanning lines: 480
Direction of vertical scan: top to bottom
Video modulation: FM subcarrier
Image transmission time: 2 min 18.3 seconds (10 seconds for start tone and phasing plus 480 image lines)
~ 5 seconds of 244 Hz square-wave modulation between black and white limits. For transmit, the subcarrier is set to 2300 Hz for 4 clock cycles (2.05 mS), followed by 4 clock cycles of 1500 Hz. This sequence is repeated 1220 times to generate the start tone.
For reception, my technique was to take blocks of 488 samples (1/4 second) at the clock rate, keeping track of the black to white or white to black transitions. A total of 61 transitions would indicate a valid start tone. In both the ViewPort and Pasokon versions of the software, any value between 59 and 62 is accepted to allow for sampling error.
This consists of 20 lines, each line consisting of 10 clock cycles (5.12 mS) of 1200 Hz sync, followed by 512 pixels of white (2300 Hz), each representing one clock cycle. Each line thus represents a total duration of 522 clock cycles. On receive, standard sync detection routines are used to detect the time for the line start.
Each of the following 480 lines of video starts with 10 clock cycles (5.12 mS) of 1200 Hz, followed by 512 pixels (each one clock cycle) of 16, 32, or 64 grayscale video. 16 grayscale steps were specified in the original standard for compatibility with the basic 640 x 480 x 16 VGA standard, but there is no reason not to code for 64 grayscale steps on transmit.
Clocked receive mode:
The preferred display method is to reference image display to the system clock. Assuming the start time for the image lines was determined during the phase interval, each line is loaded as follows: Delay 10 clock cycles at the start of each line to bypass the 1200 Hz sync pulse Proceed to sample and display pixels once each clock cycle for a total of 512 pixels. Repeat until all 480 image lines have been displayed. This technique provides optimum noise immunity and pixel registration and will yield images superior to normal SSTV with very poor video SNR. With a clean signal, the resulting images are superb.
Absent a precise crystal clock, the images can be received using line-sync triggering as in conventional SSTV. In that case, noise immunity is no better than SSTV, but it is a simple approach and also works with recorded pictures.
Note 1: In the original standard, the clock frequency of 1953.125 was the result of using a 4 MHz reference crystal and a 2048 divider chain.
Ralph E. Taggart WB8DQT (email@example.com)