Basics

Slow Scan Television (SSTV) is a system designed for sending still television pictures over an audio channel. This is achieved by slowing down the scan rates and the number of frames sent so that an unmodified amateur transmitter and receiver can be used.

Originally, in the 1960s, the SSTV monitor used a P7 phosphur surplus RADAR picture tube which could hold an image about 8 seconds before it completely decayed. 120 horizontal lines (instead of 525 lines on commercial TV) gave a reasonable resolution. 8 seconds/image divided by 120 lines meant each horizontal line scanned across in 1/15 of a second. Video frequency transitions were limited to under 3 Khz to fit the audio passband of the usual amateur SSB or FM transmitter and receiver.

A typical transmission sent 3 frames of the same picture, taking 24 seconds total. The receiving station sat in a darkened room, watching the images slowly paint down the P7 RADAR screen. Then the receiving station sent 3 frames. Because the signal was a low frequency audio signal, a cassette tape recorder wwas often used to record the incoming SSTV for future playback.

Image capture took 3 basic forms. A flying spot scanner using photomultiplier tube(s) was used to scan a picture. Special vidicon tubes that worked at the SSTV rates were sometimes used to capture a picture. Another camera system used a standard video camera slightly modified, rotated 90 degrees, and captured samples of the image at the SSTV rate. But these all required a completely still subject for 8 seconds, or you would get a forehead in one spot, a nose in another and a mouth somewhere else. (sometimes this was done on purpose, just for fun!)

In the 1970's digital logic ICs & memory ICs started to become available at low cost which permitted making the monitor and the camera out of solid state ICs and eliminated the dim P7 monitor system. Toward the end of the 1970s the PC based SSTV systems started to appear, and the hardware/software approach to SSTV became popular.

Color SSTV systems became very popular in the early 1980s, and by the 1990s the standard SSTV images had moved away from the original 8 second P7 mode and now resulted in a much higher resolution full color picture. Since the audio bandwidth was kept the same, the basic frame time was extended from 8 seconds to somewhere between 36 seconds and about 5 minutes. Several SSTV firmware designers came up with "standards" so that most of the current pictures heard on SSTV these days take about 1 to 2 minutes transmission time. Since everything is now stored digitally, only a single copy of each image needs to be sent.

In the 1990s, most of the SSTV systems were utilizing an IBM PC clone. In the beginning of the 90s, dedicated supplemental PC Boards were plugged into the PC to provide SSTV demodulating and capture capability, but by the end of the 90s, most stations were using the soundcard in the the PC to receive and transmit the SSTV audio tones. The much more powerfull computers did all the timing conversions in software. Software designers became king at this point, and a complete SSTV novice could now download a software SSTV program from the Internet, and be receiving SSTV in less than an hour, instead of being a hardware technician, hand building the SSTV station in about a year back in the mid 1960s.

(I need pictures of:)

P7 monitors, Macdonald camera, FlySpot Scanners, Sampling Cameras, SSTV Keyboard, Scan Converters, Color Systems, Computer SSTV systems, SSTV boards (Pasocon, etc)               E-Mail to W0LMD

Click "History of SSTV" for more information.