Digital Mode Audio Levels: Getting Them Right the First Time

Bad audio levels are the #1 problem with digital mode stations. I see it daily on the FT8 waterfall: signals that are too wide, too distorted, splattering across adjacent frequencies. The operator thinks they're running 50W clean. They're actually running 50W of garbage that interferes with everyone nearby.

The rules are simple. Most people get them wrong because they skip the setup and go straight to making contacts. Take 10 minutes to do this right and you'll never cause problems.

The Core Principle

Digital modes generate audio tones on your computer. Those tones feed into your radio's audio input. The radio transmits them. The chain is:

WSJT-X (software) → Sound Card → Radio Audio Input → Radio PA → Antenna

At EVERY stage, the signal must be within the linear range of that component. Overdrive anywhere = distortion = splatter = you're the jerk on the waterfall.

The ALC Rule: Zero Movement

This is the single most important rule for digital modes:

Your ALC meter must NOT move during digital transmission. At all. Ever.

ALC (Automatic Level Control) is your radio's internal protection against overdrive. When ALC activates, it's compressing your signal. Compression on a voice signal is fine (it's actually desired). Compression on a digital signal is catastrophic — it creates intermodulation products that spread your "50 Hz wide" FT8 signal across 500+ Hz.

How to Achieve Zero ALC

1. Set your radio to the desired output power (e.g., 30W)
2. Start a digital transmission (WSJT-X: Tune button)
3. Watch the ALC meter
4. Reduce computer audio output until ALC reads ZERO
5. Your power output will be below the set level — that's correct
6. Now adjust radio power setting until you reach desired actual output

Alternatively:

1. Set radio power to maximum
2. Reduce computer audio until desired power output is reached with zero ALC
3. This is equivalent and gives you power adjustment via software

Setting Up: Step by Step

IC-7300 (USB Audio)

The IC-7300's USB connection carries both audio AND CAT control. Setup:

1. Radio settings:

   • Menu → Connectors → USB Audio: ON
   • Menu → Connectors → CI-V USB port: Unlink from [REMOTE]
   • Menu → Connectors → USB send: OFF (let software control)
   • Mode: USB-D (or DATA-USB depending on firmware)

2. Windows Sound:

   • Sound Settings → USB Audio CODEC (radio) should appear as both input and output device
   • Set these as default for WSJT-X (not system default)

3. WSJT-X Audio settings:

   • Input: USB Audio CODEC
   • Output: USB Audio CODEC

4. Level setting:

   • WSJT-X: click Tune
   • Watch IC-7300 power meter and ALC
   • Adjust WSJT-X "Pwr" slider until ALC shows zero AND power is at desired level
   • Typical: Pwr slider at 20-40% for 25-30W output with zero ALC

Other Radios with External Sound Card (SignaLink, Digirig)

1. SignaLink/Digirig output → Radio mic/data input
2. Radio set to correct mode (USB, DATA, DIG)
3. Start transmission in software
4. Adjust SignaLink TX knob or computer audio level until ALC = zero
5. Then set desired power with radio's power control

Key Difference: Mic Input vs. Data Input

Many radios have separate mic and data/accessory audio inputs:

• Mic input: Has preamp gain, designed for -60 dBm microphone signals. VERY easy to overdrive with line-level audio from a sound card.
• Data/ACC input: Designed for line-level signals. Much harder to overdrive. Proper for digital modes.

Always use the data/accessory input if available. Mic input + sound card = constant fight with ALC unless you attenuate severely.

Common Mistakes

Mistake 1: Running Too Much Power

FT8 decodes at -24 dB below noise. You do NOT need 100W. In most cases:

• 20-30W is plenty for worldwide contacts
• 50W is the practical maximum on a crowded band
• 100W makes you the equivalent of a floodlight pointed at everyone's eyes

More power doesn't get you more contacts if everyone can already decode you. It just makes your signal wider than necessary and reduces available spectrum for others.

Mistake 2: Sound Card Too Hot

Windows master volume at 100%, WSJT-X at 100%, radio gain up. Result: massive overdrive, ALC pinned, signal splattering.

Fix: Start with everything at minimum. Bring up SLOWLY until you reach desired power with zero ALC.

Mistake 3: Wrong Mode on Radio

Transmitting FT8 in "SSB" mode instead of "USB-D" or "DATA." In many radios, the "DATA" modes disable speech compression, disable TX EQ, and use the correct audio input. Using plain SSB may route through processing that distorts digital signals.

Mistake 4: Multiple Audio Paths

Windows sends audio to the radio via USB AND through a sound card analog output simultaneously. The two signals arrive out of phase and create distortion. Ensure only one audio path exists between computer and radio.

Mistake 5: Transmitting During Receive

VOX set too sensitive, computer notification sounds trigger transmission. Disable VOX when using software-controlled PTT. Use CAT PTT or dedicated PTT line.

Monitoring Your Signal

Power Meter and ALC: Your Primary Indicators

During digital transmission:

• Power output should be STEADY (not fluctuating)
• ALC should show ZERO
• No RF protection/fold-back lights

Second Receiver (If Available)

Tune a second receiver to your transmit frequency. Listen to yourself. A clean FT8 signal sounds like a steady tone. A dirty one sounds raspy or buzzy.

PSKReporter Bandwidth

After operating, check PSKReporter. Your FT8 signal should be decoded by others as a single trace on their waterfall. If people report seeing a wide, fuzzy signal — you're overdriving.

IMD (Intermodulation Distortion) Measurement

Some software (like WSJT-X in monitor mode) can show your transmitted signal's IMD if you loop audio back. IMD should be -30 dB or better. Above -25 dB, you're causing problems.

Mode-Specific Settings

FT8/FT4

• Power: 20-50W (higher only if needed for specific path)
• Audio level: minimal ALC
• Duty cycle: 50% (15s on, 15s off) — but radio must handle continuous 15s key-down without overheating

PSK31

• Power: 20-30W typical
• Audio level: IMD below -30 dB critical (PSK31 is very visible when distorted)
• NEVER use speech compression

RTTY (FSK or AFSK)

• Power: 50-100W acceptable (constant envelope mode)
• If using AFSK: same ALC rules as FT8
• If using true FSK: radio generates the signal, no audio level concern

VARA HF (Winlink)

• Power: 20-40W for reliable connections
• Audio levels: same ALC-zero rule
• Longer transmissions than FT8 — verify radio doesn't overheat

Duty Cycle and Overheating

Digital modes demand more from your radio than SSB voice:

• SSB: You talk, you pause, you listen. Average duty cycle: 20-30%.
• FT8: 15 seconds transmit, 15 seconds receive. 50% duty cycle.
• PSK31/Winlink: Potentially minutes of continuous transmission. 90%+ duty cycle.

Most radios rated for "100W" cannot sustain 100W continuously. At 50% duty cycle, derate to 50-75% of rated power. At continuous duty, derate to 25-50%.

If your radio's fan runs at maximum and the case is hot to touch — reduce power. Heat kills PA transistors.

Quick Reference Card

SETUP CHECKLIST:
1. Mode: USB-D / DATA (not plain SSB)
2. Compression: OFF
3. TX EQ: FLAT or OFF
4. Start TX test tone
5. ALC = ZERO? If not → reduce computer audio
6. Power = desired level? If not → adjust radio power control
7. Everything stable for 15 seconds? → Done
8. Note these settings. Don't touch them.

Once set correctly, these levels shouldn't need adjustment unless you change something in the audio chain (new cable, software update, Windows audio settings reset after update).

The Social Contract

Digital mode operating is shared-spectrum cooperation. FT8 packs 50+ stations into 3 kHz of bandwidth. This only works if everyone's signal occupies ONLY their 50 Hz slot. One overdriven station splattering across 500 Hz ruins the experience for 10 other operators.

Take the 10 minutes to set your levels correctly. Check them periodically. When you see complaints about wide signals on the WSJT-X forum — make sure it isn't you.