Field Guide · protocol

Also known as: SSTV, Slow-Scan TV

SSTV (slow-scan television) is an amateur-radio mode that sends still images across an ordinary voice channel by encoding each pixel as an audio tone whose frequency represents brightness. That audio subcarrier then modulates the RF carrier — via SSB on HF or FM on VHF — so a single frame trickles through in seconds to a couple of minutes rather than the 30 frames per second of broadcast television.1 The “slow scan” name captures exactly that trade: image bandwidth is squeezed down to the ~2.5 kHz a voice radio already passes.

2300 Hz 1200 Hz time along one scan line tone = pixel brightness sync
Each scan line begins with a 1200 Hz sync pulse; brightness then rides as a tone swept between roughly 1500 and 2300 Hz.

Overview

An SSTV transmission is a sequence of horizontal scan lines. Before the image, a short VIS (Vertical Interval Signaling) code — a digital header of FSK tones — announces which mode follows, so the receiver knows the line count, timing, and color order. Each line then starts with a 1200 Hz sync pulse, after which the luminance (and, in color modes, the separate red/green/blue or Y/R-Y/B-Y components) is sent as a tone gliding between about 1500 Hz (black) and 2300 Hz (white). The receiver measures instantaneous frequency, paints one pixel per time step, and assembles the picture line by line.

Technical characteristics

Property Value
Signal Audio tone 1500–2300 Hz + 1200 Hz sync
Header VIS code (FSK) identifies the mode
Common modes Robot 36, Scottie 1/2/DX, Martin 1/2, PD90/120/180
Frame time ~8 s to ~4 min depending on mode
RF carrier SSB on HF, FM on VHF/UHF
Popular frequency 14.230 MHz (20 m), 144.500 MHz FM

History

SSTV grew out of 1950s experiments by Copthorne Macdonald, who demonstrated that a narrow-band, long-persistence image could fit a voice channel. Early systems used monochrome and slow electromechanical or long-persistence CRT displays; the arrival of frame stores and, later, PC sound-card software turned it into an accessible color mode. The International Space Station periodically runs SSTV image events on VHF, drawing large numbers of casual receivers.1

Deployment

SSTV is purely an amateur and experimental mode. It appears on HF phone segments (notably 14.230 and 14.233 MHz), on VHF FM simplex, and from the ISS. Because it is analog and tolerant, even a weak or noisy copy still yields a recognizable, if speckled, picture.

Decoding it with GopherTrunk

GopherTrunk targets digital land-mobile trunking and a handful of data modes; it does not decode SSTV. SSTV is an analog image mode outside GopherTrunk’s decode chain, and it is well served by dedicated tools (MMSSTV, QSSTV, RX-SSTV). The relevant GopherTrunk building blocks — FM demodulation and subcarrier handling — are the same primitives such decoders rely on, but no SSTV frame decoder ships in GopherTrunk.

Sources

  1. Slow-scan television — Wikipedia, for SSTV’s tone-to-brightness encoding, VIS headers, mode families, and history. ↩ ↩2

See also