[Ham-Mac] Slightly off topic, history of 850 Hz shift?
Steve Hellyer
shellyer at sympatico.ca
Sat Sep 30 01:01:50 EDT 2006
Hi Chris,
Found this opinion on the Internet on this subject from a fellow who
was in Military...
http://groups.google.com/group/rec.radio.amateur.misc/browse_thread/
thread/39d4dab85c50dcf2/b2a7547952cc8f0c%23b2a7547952cc8f0c
Reads...
-----------------------
The problem of HF RTTY frequency shift standards and usage is
probably a combination of engineering criteria, practical
application and some arbitrary selection. When I was in the Army
(1969-72) assigned to the US Army Transmitter site in the Canal
Zone, we used 10 and 40 KW PEP transmitters on three curtain
rhombics. When propagation was good, we probably had a signal to
noise ratio of better than 30 dB to the distant end receiver site.
The military did use multi channel telegraph as it was called with
a 42.5 +/_ Hz shift with a range of 85 Hz shift. These
transmissions were made with ISB (independent sideband) and
inserted carrier -20 dB down. The distant end locked on to the
reduced carrier signal. Besides, we might have a couple HZ drift
in a 24 hr. period (probably less than 2 Hz). There was no problem
decoding 42.5 Hz shift at 74.5 baud. Because of the fact that the
receive side locked on to the pilot carrier, very narrow shifts
were practical. This enabled the military to transmit multiple rtty
channels on the same signal ( I think there were 24 channels of
teletype traffic).
The problem with using narrow shift is that due to unpredictable
S/N ( when your not using a 40 KW transmitter) and taking into
consideration Shannon's Law regarding bandwidth vs. signal rate
(baud rate) there is a practical limit to the fsk speed. If the S/N
is poor, then reliability even at 170 Hz can be a problem. In the
amateur radio world we don't send a 20 dB down pilot tone and we
don't run 10 KW or more for better S/N. So in the old days of
single channel radio teletype, to ensure reliable communications
in a noisy environment when frequency stability was also not as
good as today, 850 Hz became the standard. A analysis of an 850 Hz
shift signal at 45.5 or 50 baud, results in a bandwidth approx.
occupied by a single nominal HF voice channel. As receiver and
transmitter stability improved over the years, lower shifts were
selected to reduce occupied bandwidth.
The value of 170 Hz was standardized by the Navy as a shift (a neat
multiple of the old military 42.5 Hz shift system). 170 Hz was
selected as it met the criteria for a combination of the S/N needs
on single channel HF RTTY transmissions, state of the art in
transmitter/receiver stability and the need to transmit higher baud
rates. If they tried to send higher baud rates which are common
today, at 42.5 Hz shift the transmission would be garbled due to
intersymbol interference within the FSK envelope. Also, when
selecting a new single channel radio teletype frequency shift
standard the military did not want to always count on having
enormous amounts of RF power and antenna gain to have very high
S/N ratios.
Seth T. KC2WE: The opinions are mine and not those of any other
party.
----------------------------------------------------------
73 Steve
On 28-Sep-06, at 9:45 AM, Chris Smolinski wrote:
> Someone asked me why much of the military RTTY on HF commonly uses
> 850 Hz shift. I am curious if anyone knows the history behind the
> selection of this shift rate, or if it was just arbitrary.
>
> --
>
> ---
> Chris Smolinski
> Black Cat Systems
> http://www.blackcatsystems.com
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