Analogue SSTV vs digital picture modes
Digital sounds like the obvious upgrade: error correction, perfect pictures, real files. It is an upgrade in some conditions and a downgrade in others, and the difference is not marginal — it is the difference between a noisy picture and no picture at all. Here is the honest comparison, including where digital loses.
The fundamental difference
Analogue SSTV encodes brightness directly as pitch. There is no compression, no checksum and nothing to verify — the receiver draws whatever it hears, faithfully, including the noise.
A digital picture mode does something completely different. The image is compressed into data, that data is wrapped in forward error correction, and the result is modulated onto the air. The receiver demodulates, uses the error correction to repair whatever the path damaged, and reconstructs the file.
Everything else follows from that one distinction.
How each behaves as conditions worsen
| Conditions | Analogue SSTV | Digital |
|---|---|---|
| Good | Clean picture | Perfect picture |
| Fair | Light speckle | Perfect picture |
| Marginal | Noisy, readable | Perfect — or nothing |
| Poor | Recognisable fragments | Nothing |
| Bad | Something arrived | Nothing |
That column of “nothing” is the cliff edge. Error correction repairs damage right up to its design limit and then stops abruptly; one decibel either side of the threshold is the difference between a flawless image and a blank screen.
Analogue has no threshold at all. It just gets worse, gradually, and never stops producing something.
What digital genuinely wins
- The real file. Analogue sends a picture of your picture — resampled to 320×256 or similar, transmitted as tones, re-digitised at the far end. Digital can send the original file byte for byte. What arrives is the same file.
- Resolution beyond the mode. Analogue resolution is fixed by the mode: PD290 tops out at 800×616 and costs 289 seconds. A digital mode is limited by data rate and compression, not by a scan-line standard from 1975.
- Perfect or honest. A digital picture is either exactly right or absent. It never quietly shows you a slightly wrong colour.
What analogue genuinely wins
- Everyone can decode it. Any SSTV software, on any platform, from any era. There is no negotiation, no version, nothing to be incompatible with.
- It degrades instead of failing. On a path that will not support digital at all, analogue still gets a callsign banner through — which is often the entire point of the contact.
- It is trivially simple. Implementable from a published spec by one person, which is precisely why it is still universal sixty years on.
A third option: hybrid
There is a way of dodging the trade entirely when both stations have an internet connection. Send the full-quality photo over the internet, and put only a short reference to it over the air.
Open Signal calls this hybrid mode: about five seconds of airtime rather than two minutes, at genuinely full quality, and stations without an internet connection still receive a readable link rather than nothing. It is not a radio contact in the purist’s sense, and it is not pretending to be — it is a different tool for the times when what you want is the picture.
Choose per contact, not per station
These modes share the same bands and the same software. A sensible operator picks analogue for a marginal path or an unknown correspondent, digital when the path is solid and the file matters, and hybrid when the airtime is the constraint.
Open Signal implements all three, plus byte-exact file transfer over radio — the waveform design behind the digital mode is described in under the hood.
Analogue vs digital — FAQ
Is digital always better than analogue SSTV?
No. Digital gives you a perfect picture or no picture; analogue always gives you something. On a marginal path, analogue delivers a noisy but recognisable image where a digital mode delivers a blank screen. Which behaviour is better depends entirely on the day.
What is the cliff edge?
The point where a digital mode's error correction runs out. Above it every picture is pixel-perfect; a decibel below it, nothing decodes at all. Analogue SSTV has no cliff — it degrades smoothly all the way down to unrecognisable.
Can digital modes send the actual photo file?
Yes, and that is their real advantage. Analogue SSTV sends an image of a picture — resampled to the mode's resolution and re-digitised at the far end. A digital mode can send the original file byte for byte, so what arrives is genuinely the same file that left.
Which should I use?
Analogue SSTV when you want to be decoded by everyone, on any software, including equipment from the 1980s. Digital when the path is good and you want the real file at full quality. There is no reason to pick one permanently — the modes coexist on the same bands.
More about Open Signal
Four ways to send a picture
Classic SSTV, Open Signal's error-corrected digital mode, byte-exact photo files, or hybrid. Pick a mode chip and transmit — the same program does all four.