[RTTY] One more thought on emergency comm

[Kok Chen]

On Dec 12, 2013, at 2:56 PM, Joe Subich, W4TV wrote:

> All things being equal - similar levels of convolutional coding,
> similar symbol rates, etc. - as the number of carriers increases
> and the bandwidth gets wider, for a constant PEP output, the peak
> power available to each carrier drops and while the noise in each
> decoder channel remains the same and the number of potential hits
> due to selective fading and interference increases.  

Joe,

Many of the modern modes either do not use sub-carriers (check out the Harris mil spec modems), or use very few of them.

The crest factor of Pactor 4, for example is only 4 dB.  That is only 1 dB worse than the crest factor of PSK31.

So, basically, the next step up from Pactor 3, in terms of getting more bits/secsond, is *not* more sub carriers (my comment to RM-11708 states as much, that just increasing bandwidth by using the same modulation mode as Pactor 3 will not get you what you want).  Although, if you start using 256 or more subcarriers, the mean crest factor again drops since the probability of all subcarriers to peak at the same time approaches zero -  the same reason why SDRs do not saturate from the peaks of the 60 MHz bandwidth that the ADC sees, as some people superhet people expect the SDRs to do.

Vy 73
Chen