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I received this bit of 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
PS I think MTTY uses the same tone pair as Kantronics - there is a
Kantronics emulation setting in the setup.- john