A digital signal that looks fine on your screen can still be messy on the air. The usual cause is transmit audio set too high, which pushes the transceiver into ALC action and can turn a tidy, narrow signal into a wide, distorted one. The fix takes about ten minutes and a little patience. This guide covers setting sound card and radio drive for FT8, PSK, sound-card RTTY and similar modes, so your signal stays clean for you and for everyone nearby.
What ALC does, and why you want it idle
ALC stands for automatic level control. It is a protective circuit that reduces gain when the signal arriving at the transmitter is too strong. On voice, a little ALC movement on peaks is common and usually tolerated. On digital modes you want it to do essentially nothing. If the ALC meter is moving, the radio is already fighting your audio, and you are past the point where the signal is clean.
What overdrive does to your signal
A transmitter is only linear up to a point. Push past it and the radio creates distortion products: unwanted energy on frequencies either side of your intended signal. That is splatter. On a busy digital segment, it can raise the noise floor for stations close to you and make weak signals harder to decode.
Extra audio also does not buy you a stronger signal. Beyond a certain level, the power output stops rising, but the distortion keeps growing. Well-set drive is not timid. It is the point where you get your full clean output and nothing more.
Test safely and legally
Transmitting requires the right amateur licence and following the rules where you operate. Those rules differ between Canada and the USA, so check your own regulator's requirements. If you are still studying, our exam practice is a good place to build confidence first.
For setup, a dummy load is ideal because it lets you send long test tones without radiating. If you must use an antenna, listen first to make sure the frequency is clear, keep tests short, and identify as your rules require. Continuous tones run the transmitter hard, so keep power modest and make sure nobody can touch the antenna while you transmit.
Switch off the extras first
Before adjusting any levels, remove things that alter your audio:
- Put the radio in its data mode (often labelled DATA, USB-D or PKT), which normally routes audio from the computer instead of the microphone.
- Turn the speech processor or compression off. Processing is designed for voice and will damage a digital signal.
- Turn off any transmit equaliser or noise reduction that applies to the transmit audio.
- In your computer's sound settings, disable audio enhancements, effects and any automatic level features on the interface used for the radio.
- Stop system sounds and notifications from playing through the radio interface. Better still, give the radio its own sound device so nothing else can sneak onto the air.
Set the computer side first
Match the sample rate your software expects, which is commonly 48 kHz, and choose the radio's audio device as the output in your digital program. Set the computer's device volume for that interface to a fixed mid-range level, then leave it alone. Use the program's own output slider as your everyday adjustment. Having one control that you change and one that you leave alone is much easier to reason about than trimming both at once.
If your radio has a setting for USB audio or data input level, set it near the middle to start with. Names and ranges vary by model, so check your manual.
Step by step: setting transmit drive
- Connect the dummy load and select your digital mode on a reasonable frequency.
- Set the radio's power control to the output you actually want. See the next section for how to choose.
- Start the software's Tune function, or another steady test tone, so the radio sends a constant signal.
- Watch the power and ALC meters. Slowly raise the audio level until the power output reaches your set level.
- If the ALC is showing at that point, lower the audio slightly and, if needed, raise the radio's power setting a touch, then repeat. You want your target power with ALC idle.
- Stop the tone. Note the settings so you can restore them later.
Some radios show ALC as a bar with a marked zone. Read your manual for what it recommends, since meters differ, but the aim is the same: little or no deflection.
Choose a sensible power level
Many digital modes transmit for long stretches. FT8 sends for most of each 15-second period, and when you call repeatedly you may transmit about half the time. That is a heavy duty cycle compared with a voice conversation. Many radios are designed with voice in mind, so running a good deal below full rated power is a widely recommended habit. The exact figure depends on your radio's cooling and specifications, so treat any percentage as a rough guide and check your manual.
Lower power is also often enough. Digital modes are built to work with weak signals, so a clean signal at modest power usually does the job better than a strained one at high power. If you use an external amplifier, keep the drive low enough that the amplifier stays in its linear range, and follow its manual.
Set your receive level too
Good receive audio matters as much as a clean transmit signal. In WSJT-X, the receive level meter is intended to sit at roughly 30 dB, in the green, with the exact target described in the program's guide. Other programs have their own meters. Adjust the level on the computer side or the radio's audio output setting so the meter is neither starved nor clipping. Accurate clock time matters for timed modes too, so see our guide to syncing your PC clock for FT8.
Check your work
Once levels are set, look for evidence that the signal is clean. A second receiver or SDR listening nearby can show your signal on a waterfall, where a well-behaved digital signal should look like a narrow, tidy trace with nothing spreading out either side. Some transceivers have a built-in monitor or scope that can help. Reception reports from other stations are useful, but a report that looks good does not prove the signal is clean, so treat them as one clue among several. When you are new to the process, FT8 for beginners walks through the first contact end to end.
Common mistakes
- Raising audio until the output "looks strong" without watching ALC.
- Leaving the speech processor or compression on.
- Adjusting the computer volume, the software slider and the radio's input level all at once, so you lose track of which one mattered.
- Letting notification sounds share the radio's audio device.
- Running near full power for long periods on a radio not built for it.
- Ignoring RF feedback. If levels or meters jump when you transmit, look at cable routing, ferrite chokes and grounding.
- Setting levels once and never revisiting them after changing modes, bands, antennas or software.
A short checklist
Data mode selected, processor off, enhancements off, dedicated audio device, power set to your target, tune tone sent, ALC idle, receive meter in range, settings written down. If you operate through a remote setup, the same principle applies at the station end, and Station Bridge connects the radio, audio interface and computer there. Keep a record of your settings with your contacts in the free online logbook, so a change of band or software never sends you back to square one.
Wrap-up
Clean digital audio comes down to one habit: set the power you want, bring the audio up only until the radio delivers it, and keep ALC quiet. Test on a dummy load when you can, check live HF propagation before you go on the air, and revisit your levels whenever your setup changes. A cleaner signal is kinder to your neighbours on the band, and it is simply better operating.
How HamRadioList can help
Once your levels are dialled in, keep a note of your settings alongside your contacts in the free HamRadioList logbook so you can return to them quickly. Check the live HF propagation page to pick a time when the bands are active, and if you run a remote setup, Station Bridge and Remote Rig can carry your radio, audio interface and computer together.
- Free online ham radio logbook - log every contact, import and export ADIF, sync with QRZ.com and keep your awards organised.
- Live HF propagation - see current band conditions before you call CQ.
- Station Bridge - the software that connects your radio, audio interface and computer to Remote Rig.
- Remote Rig - operate your own radio from anywhere on Android, iPhone, Windows or Linux, with live push-to-talk audio and CAT control.
- Ham radio FAQ - plain answers to the questions new operators ask most.
New here, or getting back on the air? Create your free HamRadioList account to log contacts, check band conditions and practise for your exam, or browse more guides from the blog. HamRadioList is built by amateur radio operators in Nova Scotia for operators everywhere.