[RTTY] trying to understand afsk

[Kok Chen]

On Feb 6, 2009, at 2/6    6:59 AM, FireBrick wrote:

> I was trying to explain afsk and offset.
> The fellow said he operates USB with his RigBlaster as per it's  
> instructions
> on all bands.
>
> Does that sound right?
>
> I thought the norm was LSB for asfk on all bands.

Yes, using USB for RTTY is perfectly normal.

The only "standard" for RTTY, as far as tone pairs are concerned, is  
that on the RF spectrum, the Mark tone appears as the higher of the  
two RTTY tones. The other standard is that we use a 170 Hz separation  
between the two tones.

If you are using AFSK (SSB transmitter), this means that if your  
audio modem is putting out a lower audio frequency for the Mark tone,  
you will have to use LSB for transmission.  But you can use USB, by  
exchanging the audio Mark and Space tones.

For example, if your audio modem is using the 2125/2295 Hz tone pair  
and you have chosen LSB, then the 2125 Hz tone needs to be the Mark  
tone and 2295 be the space tone.

If you want to use USB, just use 2295 for Mark and 2125 for Space.

I.e., continuing using the 2125/2295 tone pair... if you are using  
LSB, the Mark tone will be 2125 Hz below the suppressed carrier and  
the Space tone will be 2295 Hz below the suppressed carrier.  This  
places the Mark tone at the hiher RF frequency on the air.

If you are using USB and have swapped the 2125/2295 pair, the Mark  
tone will now be 2295 Hz higher than the suppressed carrier, and the  
Space tone is now 2125 Hz above the suppressed carrier, so the Mark  
tone is still the higher of the two tones in the RF spectrum.

Both works just as well.

This is why a lot of packetcluster spots are 2125+2295 Hz off (about  
4.4 kHz).   But then, some people spot the suppressed carrier  
frequency of their SSB transmitter -- and those spots are completely  
useless unless you know both what tone pair they use (I use a tone  
pair centered at 1615 Hz myself) and whether they operate in LSB or  
USB.  Most of us cannot read someone else's mind, otherwise we  
wouldn't be using radios to communicate :-).

73
Chen, W7AY