[RTTY] Baud racing

[Kok Chen]

On Jun 7, 2011, at 11:30 AM, Anthony (N2KI) wrote:

> After that, gibberish. I was using an I7 900 processor with 12 gig ram and a video card with 1 gig on Win 7 64 bit so no lack of processing. It must have to do with the bit rate settings. 

I suspect there could be some filters inside MMTTY.  Someone who knows how to, can check on that.  As I mentioned earlier, a 300 baud RTTY signal with a 170 Hz shift requires a bandwidth that is in the vicinity of 1 kHz.  If you insist on receiving 300 baud RTTY, it might just be a matter of changing that filter.

For Baudot RTTY, cocoaModem for example includes a roofing filter that passes only up to 75 baud RTTY signals, irrespective of the baud rate the matched filter is set for.  cocoaModem widens that particular filter when it is switched to using 110 baud ASCII RTTY, or for SITOR-B reception.

Ahh, I just check fldigi, and it defaults to a 1070 Hz input filter when RTTY is set to 300 baud with a 170 Hz shift.  This happens to be the same as the "170 + 300x3"  formula which I had posted a day or two ago.  At least the fldigi author and I are in agreement that the bandpass needs to let through the third harmonic of the keying fundamental.

73
Chen, W7AY