RUMlogNG Version 6.3

[DL2RUM]

RUMlogNG 6.3 is available in the App Store: https://itunes.apple.com/app/rumlogng/id964454561?mt=12

English manual: https://dl2rum.de/RUMlogNG/docs/en/
German manual: https://dl2rum.de/RUMlogNG/docs/de/

[DL2RUM]

Version 6.3.1 with an improved RTTY Decoder can be downloaded from here: https://dl2rum.de/RUMlogNG/RUMlogNG.zip

A stand alone RTTY application with a dual receiver can be found here: https://dl2rum.de/files/RTTYlite.zip

[DL2RUM]

There are four option available to affect the RTTY decoding quality:

• AGC
• Limiter
• Synchronisation
• Bandwidth for separate mark and space filters

AGC

The AGC (Automatic Gain Control) in the RTTY decoder is a double-edged sword. It tries to keep the input signal at a constant level so that the subsequent filters and the decoding logic always work with an optimal amplitude.

Here are the scenarios when you should turn them on or off:

1. Switching on AGC (ON) makes sense for:

• Fluctuating signal strengths (fading/QSB): This is the main reason for an AGC. If the signal is constantly louder due to the propagation conditions and becomes quieter, the AGC automatically equalises these differences. You don't have to constantly turn the volume control or the gain slider.
• Search on the conveyor belt (tuning): If you crank over the belt and meet stations with completely different transmission powers, the AGC ensures that each station is immediately processed at the correct level.
• Convenience: For normal operation, the AGC is a "Set and Forget" feature. It ensures that the decoder does not "overdrive" (clipping) when a signal suddenly becomes very strong.

2. Switching off AGC (OFF) is useful for:

• A lot of noise during the breaks (noise pumping): In broadcast breaks, the AGC only sees the soft noise and tries to increase it to the target level. That leads to the fact that the noise during the pauses becomes extremely loud ("inflate"), which in turn can lead to many missed start-up bits and character salad.
• Strong neighbour signals (QRM): If a very strong station appears right next to your desired signal, the AGC reacts to this high overall energy and regulates the level down. This will push your actual (weaker) signal below the decoding threshold. Here it is better to turn off the AGC and manually adjust the level so that your desired signal arrives optimally.
• Strong atmospheric disturbances (cracking/static): Short, extremely loud crackling noises (e.g. By thunderstorms or electrical devices) the AGC can briefly "Stuffing." The reinforcement collapses and takes a moment to recover, during which time valuable signals are lost.
• Use of the radio-internal AGC: If your radio already has a very good, fast-responsing AGC, it can cause unwanted vibrations or control effects occur when two AGC systems are connected one after the other. In this case, it is often more clean to turn off the software AGC.

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Summary & Recommendation:

* Standard: ON. Most users prefer the convenience of an automatic control.

* Manual (OFF): If you are a professional user who wants to maximise his signal-to-noise ratio (SNR), turn off the AGC, set your radio to a

Pro tip: When you turn off the AGC, make sure that you still have enough "Headroom" at the input of the decoder so that loud impulses do not distort.


Limiter

Switching on the limiter is basically a decision between SNR (signal-noise distance) and dynamic handling. Above all, it makes in the following situations:

1. In case of strong fading (QSB)
   If the signal constantly fluctuates between "extremely strong" and "barely audible" (typically on shortwave), the AGC (Automatic Gain Control) can sometimes be too slow react. The hard limiter radically cuts off all amplitude fluctuations above a minimum and provides the filters with a signal with a constant level. The helps the decoder to find the zeros of the signal more clean.
2. With very strong interference signals (QRM) in the neighbourhood
   If a very strong signal appears right next to your RTTY signal, it could "push down" the AGC so that your actual signal becomes too quiet. A Limiter ensures that the frequency information (the zero passes) takes precedence over the amplitude.
3. When the CPU load has to be reduced extremely (theoretically)
   A limiter simplifies the signal so much that subsequent calculations must be less precise. In our implementation, however, this plays a subordinate role, since the dual-tone logic is running anyway.

When should it stay OFF (Limiterless)?

In most modern scenarios, Limitless (i.e. NO) is better:

• With a lot of noise: A limiter raises the noise during the breaks to the same level as the signal. This leads to more "error start bits". Without limiter remains the noise quietly, and the matched filter integrator can better pull the signal out of the dirt.
• With selective fading: Since we now have the dual-tone logic that levells mark and space independently of each other, we already compensate for the fading on a more intelligent level. A hard limiter in front of it would destroy the subtle differences in level that our new noise reduction logic uses.

Recommendation:

Leave the limiter out by default. Only turn it on if you have a very strong but fluttering signal (fast flat fading) where the decoder despite good level "out of step" falls.


Synchronisation (Flywheel)

1. Switching on the flywheel (ON) makes sense for:

• High noise (QRN) and weak signalling: This is the main purpose of use. If the background noise is so high that the decoder constantly recognises "false starts" (false start bits), the flywheel ensures that only flanks that are in the rhythm of the RTTY signal are accepted.
• Diddles (idle signals): When the transmitter transmits between the words "Diddles" (usually the LTRS sign), the flywheel remains perfectly synchronised. It prevents short rattling during the breaks from getting the decoder out of the way.
• Longer texts: With a fluid text, the flywheel offers enormous stability. Once synchronised, the decoder "knows" when the next character is coming, and almost completely ignores interference between them.

2. Switching off the flywheel (OFF) makes sense for:

• Very strong and clean signals: If the signal is far above the noise, a flywheel is not necessary. A purely asynchronous decoder reacts a tick faster to the very first letter of a broadcast, as it does not require a synchronisation phase (the first character).
• Stations with unstable timing: Some very old mechanical teleprinters or poorly programmed controllers have a slightly "jittering" timing (the bits are not exactly the same length). The flywheel could mistakenly block valid start bits here if they are slightly outside the expected window.
• Short "Bursts": If only very short information (e.g. B. a short "R R" or "73"), the flywheel could swallow the beginning while it is still waiting for synchronisation.
• Tuning process: If you are still looking for the right frequency, it is often better to leave out the flywheel so that the decoder reacts immediately to each flank and you see if anything decodable arrives at all.

Summary (pro tip):

In practice, the flywheel is usually left out with modern software decoders, as long as the signal is easy to read. As soon as the signal sinks in the noise or "trash signs" appear in the pauses, it is switched on to stabilise the decoding.

Bandwidth for Mark and Space Filters

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.