Noisy, torn or half-missing pictures
A picture arrived, but not the one that was sent. What it looks like tells you what went wrong — and a good deal of the time the answer is that nothing did, because analogue SSTV is supposed to degrade rather than fail.
What it looks like, and what it means
Find the row that matches what you are looking at.
| What you see | What it means | What to do |
|---|---|---|
| Speckled or grainy all over, but complete | A weak or noisy signal. The decoder heard every line, just not cleanly. | Nothing is broken. This is what a marginal path looks like — the picture degrades gracefully rather than failing. |
| Fine at the top, degrading toward the bottom | The signal faded during the transmission. An SSTV frame runs for minutes, and HF does not stay still that long. | Normal on HF. A shorter mode finishes before the fade does. |
| Stops dead partway down, rest is blank or grey | The signal was lost, the transmitting station stopped, or something on your end took the audio away mid-frame. | If it happens on every picture at the same point, suspect your end — a power-saving setting or another program grabbing the audio device. |
| Horizontal streaks or blocks displaced sideways | Tearing — individual lines arriving misaligned, usually from impulse noise or a dropout. | Local electrical noise is the usual culprit. Not the same thing as a uniform lean. |
| The whole picture leans consistently | Clock error, not signal quality. Every line is slightly the wrong length. | This one has its own page — it is a calibration issue and entirely fixable. |
| Colours wrong or separated into bands | The mode was misidentified, or sync was lost early enough to shift the colour components. | Check the decoded mode matches what was sent. A missed VIS header sends everything downstream wrong. |
| Nothing decodes at all, though you can hear it | The decoder never triggered — no VIS header detected, or audio is not reaching it. | Confirm audio is arriving first. A waterfall showing the tones proves the path; a flat one means the problem is before the decoder. |
Noise is not a fault
Analogue SSTV has no error correction. That sounds like a weakness and is in fact the reason it still works after sixty years: there is no threshold below which it stops. A signal that would fail a digital mode outright still produces a recognisable, speckled picture.
So a grainy picture from a distant station is the system working. The question worth asking is not “how do I remove the noise” but “is this as good as the path allows” — and that is a question about antennas and band choice rather than software.
When it is your end
- Every picture breaks at the same point. A signal that fades is random; a fault that repeats is yours. Suspect power saving, or another program taking the audio device mid-frame.
- Tearing on a strong signal. Impulse noise from something electrical nearby — a charger, a thermostat, LED lighting. Switch things off one at a time.
- Nothing decodes though the tones are obvious. The audio is not reaching the decoder, or the level is so low it cannot find the header.
Faults with their own answer
Two things look like picture faults and are not signal problems at all, so they have pages of their own rather than a line in the table:
- A consistent lean is sound-card clock error — every line fractionally the wrong length. Slanted pictures and how to correct them.
- Your own transmissions reported as poor while your power looks fine is usually drive, not propagation. Audio levels and ALC covers what the level should be.
Reading the report you were sent
A reception report tells you more than a screenshot does. Signal-to-noise in dB says how far above the noise you actually were, which is the number that predicts whether the picture would have survived — what those reports mean.
And if a mode keeps failing on a path that is genuinely marginal, the fix is usually a shorter mode rather than more power. The mode reference lists airtimes side by side.
Common questions
Why is my received SSTV picture noisy?
Usually because the signal is weak relative to the noise floor, and that is normal rather than a fault. Analogue SSTV degrades gracefully — a marginal path gives you a speckled but complete picture rather than nothing at all. If the picture is complete and merely grainy, nothing is broken.
Why does my picture fade out toward the bottom?
Because an SSTV frame takes minutes to send and HF conditions change within that time. The top arrived while the path was good and the bottom arrived after it faded. A shorter mode finishes before the fade does, which is one practical reason to pick Robot 36 over Martin M1 on a marginal path.
What is the difference between noise and tearing?
Noise is speckle spread evenly over the picture — a poor signal-to-noise ratio. Tearing is whole lines displaced sideways, caused by impulse noise or a brief dropout rather than a generally weak signal. Noise means turn up the signal; tearing means find the interference.
My picture leans to one side. Is that the same problem?
No, and it is worth knowing the difference. A consistent lean across the whole picture is a sound-card clock error, not a signal problem — every line is fractionally the wrong length. It is a calibration issue with its own fix, and no amount of signal improves it.
Why did the colours come out wrong?
Almost always a mode mismatch. Each SSTV mode carries colour differently, so if the VIS header was missed or misread the decoder applies the wrong timing to the colour components and you get banding or shifted colours. Check what mode the decoder thinks it received against what was actually sent.
I can hear the signal but nothing decodes.
The decoder never triggered. Confirm the audio is genuinely reaching the software — a waterfall showing the tones proves it, a flat one means the problem is upstream in the device selection or cabling. If audio is arriving and it still does not trigger, the VIS header may have been lost to noise, and starting a manual decode in the right mode will often recover the picture anyway.
More about Open Signal
Or send pictures that survive the path
Open Signal decodes every SSTV family automatically, and its own digital mode is error-corrected — a picture either arrives whole or tells you what was missing. Receiving is free forever.