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AFSK vs. FSK

Info from Andy G3SVD....


Re the frequency offset we talked about when running RTTY/PSK - FSK V AFSK .
Hmm "Center of Intelligence"  - John
15 Feb 90


The frequency of operation for data modes has long been a source of  confusion if not a point of contention for both  Amateur  and Professional radio operators. All Mark/Space data mode modulation can be generated using FSK or AFSK.   What  is  the difference?  FSK is  true  Frequency  Shift Keying  of the transmitter's carrier.  This shift can be  applied to  any  of the transmitter oscillators.  Audio  Frequency  Shift Keying  is  generated  by  shifting the  frequency  of  an  audio oscillator  that  is fed into the transmitter's  normal  transmit audio input. Unlike FSK, AFSK can be used for FM modulation. The  frequency of an FSK system is easily determined  by  reading the transmitter's dial or crystal frequency.  Frequency is not so easily  determined on a SSB transmitter modulated by AFSK  tones. In this case audio tone frequency and sideband must be known. For  discussion  purposes, let's assume that the AFSK  tones  are 1600 and 1800.  The following figure illustrates how to determine the resultant operating frequency.

 



Figure 1:
                      |           |          Mode: LSB
                      |           |
                      |           |
                      |           |
                      |           |
                      |           |
                      |           |                  |
         -------------+-----+-----+------------------+-------
                      ^     ^     ^                  ^
        1800 Hz ______|     |     |______ 1600 Hz    |
                            |                        |
Virtual Carrier or Center __|     Actual Suppressed _|
   of Intelligence                    Carrier


Note  that in the diagram, three new terms are  mentioned.   The Virtual  Carrier refers to an equivalent frequency to that  which would  be used if the system was FSK.  This can also be  referred to as the Center of Intelligence, or simply half-way between  the highest and lowest modulating tone.  The Suppressed Carrier is  a function of using a SSB transmitter and is its' dial frequency. The difference between the actual and virtual carrier frequencies is the potential source of confusion.  If Station A is using  FSK and  tells  Station  B who is using AFSK he  is  on  28.113  MHz, unless  they both "know the rules", is 28113 or (28113  +  1.700) KHz the correct frequency? .pa To  make matters worse, what if there are two AFSK  stations  one with  1600/1800 tones and the other with 2110/2310 tones. How  do they  tell  each  other the frequency?  If they  use  their  dial frequency,  their  virtual carrier frequency will not  match  and they can not communicate. The  FCC,  MARS, Military and  knowledgeable  commercial  stations solved  this by always referring to the Center of Intelligence  or Virtual  Carrier.   That put the burden of tone  offset  on  each station but made it unnecessary for each station to know or guess about the other station's equipment. I  recently became active on HF Packet radio and was  shocked  to find  references  to  frequency being made  by  using  AFSK  dial frequency.  One BBS even went as far to set his UNPROTO string to NET113,   the  dial  frequency,  when  actually  the  Center   of intelligence  was (28.113 - .001700) or 28.111300 MHz.   Since  I own a PK-232 which uses a tone pair of 2110/2310, I have to  tune my radio to 28.113510 MHz, still no relevance to 28.113! I  suppose it too late or futile to try to get the  Amateur  Data mode  community  to start talking Center of Intelligence  but  at least  they should be aware of it.  If they add notes  in  their BBS's  online tech info etc., they should mention the  center  of intelligence. Another  useful bit of related information is the simple rule  of thumb that the Bandwidth required is:                      BW Hz = Shift Hz + Baud For most HF packet this works out to be 500 Hz. Knowing  the  BW  required and actual  frequency  is  helpful  is picking an operating frequency.  In researching what Packet TNC's use what tone pairs, I  compiled the following data you might find helpful:

         TNC Type                          Tone Pair
         ========                          =========
Kantronic's KAM (default settings)         1600/1800
AEA PK-232 (USA normal ham)                2110/2310
AEA PK-232 (Sitor)                         1615/1785
AEA PK-232 (European)                      1250/1450

73's
Jim Lill
WA2ZKD


P.S.  I think MTTY uses the same tone pair as Kantronics - there is a Kantronics emulation setting in the setup.- john

 

 

 

 


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