Audio levels and ALC — the thing everyone gets wrong
If you listen on 14.230 for an afternoon you will hear pictures arrive torn, streaked and buried in splatter from stations running plenty of power. Almost none of that is propagation. It is drive: too much audio into the radio, ALC clamping down on it, and a signal that was distorted before it ever left the shack.
What ALC is doing to your picture
Automatic Level Control protects the transmitter by pulling gain back when you feed it more audio than it can handle cleanly. That is a sensible thing to do to a voice. It is a destructive thing to do to a picture.
SSTV and digital picture modes carry information in the precise amplitude and frequency of a continuous tone. ALC acting on that tone is, by definition, changing the amplitude of the thing carrying your data. The receiving station does not see “a bit compressed” — they see banding, tearing and a decode that fails on the noisy lines first.
Worse, the distortion products land outside your signal, several kHz either side. That is the splatter, and it is why an overdriven station is unpopular long before anyone tells them.
Setting drive from scratch
- Put the rig on a dummy load, or find a genuinely clear frequency and identify.
- Set the software transmit level low — around 20% — and start a test transmission.
- Raise the level slowly and watch power output. It will climb, then stop climbing.
- The point where power stops rising is where ALC took over. Back off until the ALC meter barely moves, then back off a little more.
- Note the number. Set it once and stop touching it — this is not a control that needs adjusting per contact.
Open Signal makes the last step easier than most: after every transmission the station log reports TX audio peak was N% of full scale, so you can see what actually left the sound card rather than inferring it from a meter.
The duty cycle problem nobody mentions
This matters more than the levels, and it is almost never said out loud: an SSTV transmission is continuous full-power output for one to five minutes. Scottie DX runs 269 seconds. PD290 runs 289. There are no gaps between syllables the way there are in speech.
- Most HF radios are rated for a 100% duty cycle at a fraction of their peak power — frequently 50%, sometimes less. Check your manual.
- Running 100 W of SSTV on a rig rated for 50 W continuous will get hot, and repeated over an afternoon it shortens the life of the finals.
- Turn the fan on if you have the option, and consider running the long modes at reduced power. It costs you very little: a clean signal at half power reaches further than a hot, distorted one.
FM is a different measurement
On 2 m FM there is no ALC to watch. The equivalent adjustment is deviation, and the target is around 3 kHz.
- Too little deviation: your picture arrives noisy even at full quieting.
- Too much: the receiving discriminator clips, and the damage looks like harsh horizontal streaking.
- FM is far less forgiving of the wrong level than SSB, because there is no graceful degradation — see SSTV on 2 m FM.
How to tell it is you
Overdrive is easy to diagnose from the far end and almost impossible from your own shack. Ask for a report, or read the ones you get: if stations copy your picture with the same banding regardless of distance and conditions, the problem travelled with the signal. If it varies with the path, it is propagation.
Reading reception reports covers how to tell those two apart from the numbers.
Audio levels — FAQ
How much ALC should I run on SSTV?
As close to none as you can manage. On SSB, ALC works by reducing gain when it is driven too hard, and that gain reduction distorts the waveform. For voice it is tolerable; for a picture it widens your signal and corrupts the decode at the far end. Set drive so the ALC meter barely flickers.
Why do I get poor reports when my power output looks fine?
Because power output is the wrong meter to watch. Overdriving produces plenty of indicated power — much of it in distortion products either side of your signal, which do not carry your picture and do bother your neighbours. A clean 25 watts decodes better than a distorted 100.
Does this apply to FM on 2 metres?
The principle does, the meter does not. FM has no ALC; the equivalent measure is deviation, and you are aiming for roughly 3 kHz. Too little and you are quiet and noisy, too much and the receiving station's discriminator clips the picture.
Why did my levels change on their own?
Almost always the operating system. Windows in particular ducks, mutes or auto-adjusts audio device levels — for a call, for an app that requested exclusive access, or after an update. Open Signal reports the stream and the device volume at startup and restores its own mixer volume if it finds it altered.
More about Open Signal
See what you actually transmitted
Open Signal reports the peak audio level of every transmission in the station log, and shows you the reception reports that came back — so setting drive stops being guesswork.