Why should the ALC system be inactive when transmitting AFSK data signals?

Automatic Level Control (ALC) is a transceiver circuit designed to prevent sudden, loud voice peaks from overdriving the transmitter. It achieves this by aggressively compressing the audio or turning down the gain when the input gets too loud. While ALC is highly beneficial for Single Sideband (SSB) voice operation, it is detrimental to Audio Frequency Shift Keying (AFSK) digital modes like PSK31 or FT8. These digital modes rely on pure, linear audio tones to accurately transmit data. If the ALC activates during an AFSK transmission, its compression distorts the complex audio waveforms. This distortion creates 'splatter' (intermodulation distortion) across the band, causing interference to others and often making your signal impossible for the receiving station to decode. You should always adjust your audio drive level so that the ALC meter registers zero activity.