[RTTY] Low tones

[Kok Chen]

On Mar 24, 2016, at 9:21 AM, John Merrill wrote:

> Low tones?? Why?

The reason I have often heard for preference of low tones is that the op actually listens to, and tunes a superhet radio using a knob.  

As one gets older, the higher tones are too shrill;  in mild cases, it is just painful to listen to, and in more severe cases, the tones are also so attenuated that you cannot hear them.

With modern demodulators, Mark and Space tones are decimated to a baseband I/Q signal before any filtering takes place.  The actual demodulator mechanism always see a baseband signal (i.e., tone is at "DC").  So, low or high tones both appear the same to the demodulator.  

There is no performance penalty, either way.

Arguably, RITTY is the first amateur modem that can be considered "modern," but I don't know if it uses baseband I/Q processing.  cocoaModem (ca 2004) used baseband processing because "textbook" signal processing are done in terms of analytic signals.  In various discussions with the authors of fldigi and 2Tone, the processing in those modems are also baseband.

The main disadvantage of using low tones is really not on the receiving end, but because superhets tie the transmit tone pair to the receive tone pair.

If you overdrive an SSB transmitter, you will put out lots of garbage since the third harmonic of the low tones will go through the transmit filter.   The third harmonic (even second harmonic) of the high tines are usually outside the transmit filter.

That being said, cocoaModem included a feature that sends the received tone to the computer's speakers.  However, it also included a frequency offset before sending the tones out to the speakers.  So, the modem could be tuned to a high tone while the human ears hear a low tone on the computer's speakers.   You can have your cake and eat it, as long as your software can do some extra work for you.

73
Chen, W7AY