What is the difference between direct FSK and audio FSK?

Audio FSK (AFSK) works by generating audio tones, typically a mark and a space tone, using a computer soundcard or a modem. These audio tones are then fed into the microphone or audio input of an SSB transmitter to modulate the signal. Direct FSK, on the other hand, bypasses the audio stages entirely. It operates by applying a digital control signal directly to the transmitter's Variable Frequency Oscillator (VFO), shifting the RF carrier frequency back and forth between the mark and space frequencies. The other options are incorrect because the method of modulation (Direct vs. Audio) does not inherently dictate the bandwidth or the maximum baud rate. Those factors are determined by the amount of frequency shift and the symbol rate being transmitted, not by how the signal is injected into the transmitter.