FSK vs AFSK RTTY: Why FSK Has the Edge

In the world of RTTY, two primary methods of generating the characteristic mark and space tones exist: Frequency Shift Keying (FSK) and Audio Frequency Shift Keying (AFSK). While both achieve the same end result, FSK offers several advantages that make it the preferred choice for many seasoned RTTY operators.

FSK directly shifts the radio's carrier frequency between two distinct frequencies to represent the mark and space tones. This is typically done by applying a keying voltage directly to the radio's oscillator circuit. 

AFSK, on the other hand, generates audio tones that are then applied to the radio's microphone input, much like phone operation.

The primary advantages of FSK include:

• Spectral Purity: FSK produces a cleaner signal with less unwanted sidebands. This results in a narrower transmission, reducing the chances of causing interference to nearby stations.
• Linearity: FSK is less affected by non-linearities in the transmitter's audio chain or power amplifier, as it bypasses most of the audio circuitry.
• Simplicity: Once set up, FSK requires no adjustment of audio levels, reducing the risk of overdriving and distortion.
• Better Signal-to-Noise Ratio: FSK often achieves a better signal-to-noise ratio at the receiving end, especially in weak signal conditions.
• Less Susceptible to Frequency Drift: FSK is typically more stable and less prone to frequency drift compared to AFSK.
• The Ability to Use Narrow Filters: Many radios with a dedicated FSK RTTY mode have associated narrow filters and audio peaks for Mark & Space frequencies. 

While AFSK has its place, particularly in situations where direct FSK keying is not available, FSK remains the gold standard for serious RTTY operation. Its cleaner signal, improved weak signal performance, and resistance to overmodulation make it my go-to choice for contesting and DXing.

As someone who's operated RTTY for decades, I've found that the extra effort to set up FSK is well worth it. The resulting improvement in signal quality and reception can make all the difference when trying to snag that rare DX or maximize your score in a contest.