> What we do today (and what Hoff did) is precisely the same as
> modulating an AM transmitter and then removing the carrier and one
> of the AM sidebands. The result is the two audio tones are shifted
> into the RF spectrum by the modulation process (i.e., convolution
> of the AFSK waveform with an RF carrier).
In fact, Amateur 45.45 baud, 170 Hz shift FSK would be known as 350HF1B
using the ITU system of modulation designation - 350 Hz wide, FM, one
channel of digital information without a subcarrier, intended to be
decoded by machine. Amateur 45.45 baud, 170 Hz shift AFSK would be
known as 350HF2D (or 350HF2B) - 250 Hz wide, FM one channel of digital
information *using a subcarrier*, data (D) intended to be decoded by
machine (B).
That the two forms of modulation appear identical when viewed in the
frequency domain is not material - in fact they differ in "frequency"
by the frequency of the subcarrier (Mark) but one can not determine
that by simply looking at the RF spectrum *IF* the 205F2D transmitter
is properly adjusted and the modulating subcarrier is clean.
That a given signal can be classified two ways is not unique to
RTTY ... the same situation arises with *any* FM transmission.
Although we generally refer to VHF FM signals as 15KF3F (15 KHz wide,
FM, single channel analog voice) it could equally be described as
15KG3F (15 KHz wide, Phase Modulation, single channel analog voice)
since FM and PM differ only in the level of sidebands vs. modulating
frequency. In fact, the two are equivalent depending on preemphasis
of the modulating signal.
73,
... Joe, W4TV
On 8/24/2011 12:16 AM, Kok Chen wrote:
>
> On Aug 23, 2011, at 8:29 PM, Bill, W6WRT wrote:
>
>> Somewhere along the line, the term AFSK came to be used to describe audio
>> tones
>> being used to modulate an SSB transmitter, which does in fact produce a
>> transmitted signal which is identical to a frequency-shifted carrier.
>
> Irv Hoff (K8DKC back then, before he became W6FFC) wrote the article "Audio
> Frequency-Shift Keying for RTTY" in the June 1965 issue of QST.
>
> In the introduction, he wrote:
>
> "Audio FSK originally applied in the VHF bands where tone-modulated keying is
> permissible, has come into use on lower frequencies because it appears to be
> an "easy" way to get FSK with an SSB transmitter. There are inherent
> dangers, in terms of meeting the purity-of-emissions requirements of the FCC
> regulations."
>
>> Way back, the original meaning of AFSK was a carrier, either AM or FM, which
>> was modulated with two audio tones. If you recall your modulation theory,
>> this is very different from shifting the carrier frequency back and forth by
>> 170 Hz, as we do it on HF today.
>
> What we do today (and what Hoff did) is precisely the same as modulating an
> AM transmitter and then removing the carrier and one of the AM sidebands.
> The result is the two audio tones are shifted into the RF spectrum by the
> modulation process (i.e., convolution of the AFSK waveform with an RF
> carrier).
>
> 73
> Chen, W7AY
>
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>
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