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Option Bandwidth for Mark and Space Filter |
Choosing the filter bandwidth for the Mark and Space tones is a compromise between noise reduction (selectivity) and signal integrity (time response).
Here are the recommended values for the most common scenarios:
Standard Setting: 100 Hz
When: Normal RTTY operation (45.45 baud, 170 Hz shift).
Reason: This width allows the main spectrum of the signal to pass through and offers good balance. It is more forgiving of minor tuning errors and provides clean time response (low ringing).
Narrow / Contest Mode: 50 Hz to 75 Hz
When: With very weak signals (close to the noise floor) or when strong neighboring stations (QRM) are directly adjacent to the signal.
Reason: Significantly improves the signal-to-noise ratio (SNR).
Caution: The filters will start to ring. This can blur the bit boundaries at 45 baud. You have to be very precise with the frequency, as even a 20 Hz deviation will attenuate the signal.
Wide / Search Mode: 150 Hz to 200 Hz
When: For quickly searching across the band or when the other end drifts significantly (old tube equipment).
Reason: Facilitates tuning, as the signal is still decoded even if you are not exactly in the middle of the filter slope.
Disadvantage: Allows a lot of noise through, which increases the error rate with weak signals.
High-Speed RTTY (75 / 100 baud)
When: Commercial services or special modes.
Bandwidth: 150 Hz to 250 Hz.
Reason: A bit is much shorter here. A filter that is too narrow would round off the edges of the bit so much that the decoder would no longer recognize clean level changes.