[GreenKeys] RE: [MMTTY] ve2lx reason for 2125 rtty freq

[Veenstra, Lester]

Here is an expansion of the answer I gave offline to Gilles.

 

As has been stated by others, there was a tendency to  follow the Bell
System standard practices, hence the use of multiples of 85 Hz. But
there is another twist to this story. In the beginning, there were
available 425 Hz tuning fork audio frequency standards from commercial
terminal equipment , (this is part of my folklore although I have not
recently been able to find found documentation, as a consequence I will
post this reply on the RTTY (Greenkeys) reflector where the real experts
in this area lurk). Keep in mind that this was in the day when a
frequency counter was the size of three microwave ovens, and very
unlikely in the hands of a RTTY ham basement.

 

Thus precise audio frequencies could be determined by lissagous patterns
against the 425 Hz standard using a simple oscilloscope. 

 

Also keep in mind that the audio terminal units started with a hard
limiter,  you did not want the second harmonic of the audio mark
frequency to fall inside the pass band of the receive filter. 

 

So with constraints of using multiples of 425 Hz and the 850 Hz shift
"standard" her's how the possibilities worked out:

 

425   Hz Mark  (1:1) 1275   Hz Space  (3:1) 2x Mark =  850  3x mark =
space,  not a good choice

850   Hz Mark  (2:1) 1700   Hz Space  (4:1) 2x Mark = 1700 = Space Freq,
not a good choice

1275 Hz Mark  (3:1)  2125  Hz Space  (5:1) 2x Mark = 2550  Still
possibly in band on some filters but first real possibility

1700 Hz Mark  (4:1)  2550  Hz Space  (6:1) 2x Mark = 3400  Good
candidate

2125 Hz Mark  (5:1)  2975  Hz Space  (7:1) 2x Mark = 4250  The winner,
second of mark outside most receiver passbands

 

In addition the selection of the 5:1 and 7:1 combination allowed
reasonable filter designs without the need to use excessive inductance
values. And of course these frequencies worked well with the 88 mHy
"loading coils" that were also available from telco sources.

 

When, later the improvement in equipment frequency allowed narrow
shifts, starting from the standard for wide shift,  850 Hz shift with
2125 mark and 2975 space, the narrow standard of 170 Hz shift was
implemented with the same mark frequency retained, 2125, and the space
frequency, 170 Hz higher, became 2295.  For some of us cheeep types, it
allowed us to reuse the existing mark filter networks (discreet
components) we had already.

 

 73

  Les

     K1YCM/3

 

 

Lester Veenstra
Senior Engineering Program Manager
Intelsat General
6550 Rock Springs Drive, Suite 450
Bethesda Maryland, 20817
+1-301-571-1212
e-mail: lester.veenstra at intelsatgeneral.com
  

-----Original Message-----
From: MMTTY at yahoogroups.com [mailto:MMTTY at yahoogroups.com] On Behalf Of
Brian Heywood
Sent: Wednesday, July 27, 2005 4:34 PM
To: MMTTY at yahoogroups.com
Subject: Re: [MMTTY] ve2lx reason for 2125 rtty freq

 

Good morning Gilles,
This mode started to be used by amateurs in the 1940's.
There are some hams still using the old methods of generating the tones.
Some even use mechanical RTTY units; old line teletype systems 
manufactured by Creed, etc
The use of the frequency is so that any harmonic of the tones fall 
outside of the audio passband of the radio.

73
Brian, ZL1IE
Tauranga, New Zealand

gilles wrote:

>
> hello
>
> is there a member that know the answer
>
> I am looking for that information
> Could you help me to find a reference where to search
>
> in RTTY
>
> the default frequencies chosen is 2125 and 170 hz of shift
> there is also the lower tone
> why these ( eg: 2125 ) frequencies were  chosen instead of any other 
> at the begining of RTTY...?
>
> It could of been anything lower or higher then that but keeping the 
> same shift
> was that done to eliminate the risk of ( harmonic ) problem in 
> reference of our audio passband or other harmonic reason...?
>
> wishing to ear from you
>
> Gilles Lecompte   VE2LX
>
>             
>       
>       
>




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