Slow-scan television (SSTV) is a mode of video communications in which a sequence of fixed images is sent and received at intervals of several seconds. SSTV is practical for transmission of images over plain old telephone service ( POTS ) lines and in other applications where the available bandwidth is severely limited. The bandwidth of an SSTV signal is comparable to that of a voice signal in analog telephone or wireless communications (3 KHz or less).
SSTV was used to send the first video images back to earth from the Apollo 11 moon mission. SSTV is sometimes used in medical imaging, security systems, and in the remote monitoring of hazardous equipment or natural phenomena. Amateur radio operators occasionally use SSTV in their communications. Most SSTV communication is done in grayscale . However, color images can be sent and received in SSTV mode with some sacrifice in resolution , or a longer interval between images, or both.
An SSTV signal consists of a rapid sequence of audio tones having variable pitch. When heard directly, it has a characteristic warbling sound. An SSTV communications station consists of a telephone connection or radio transceiver, a personal computer, a program called a scan converter , and a video camera. The scan converter employs two data converters, one for receiving (by means of the microphone or line input port) and the other for transmitting (by means of the speaker or line output port). The scan converter changes incoming SSTV tones into images suitable for viewing on the computer. These images can be viewed in real time , stored individually, or stored as a sequence of video files. The scan converter also changes video data from the camera to audio tones that are fed to the telephone set or the microphone input of the radio transceiver.