Hi Harold:
The WWII era AN/FGC-1 teletypewriter terminal had mark and space filters in it
that were set at 2125 and 2975 Hz.  The terminal was designed and built by
Western Electric.  It seems likely that Bell Labs provided some design help. 
I don't see anything in the literature about why those frequencies were selected
but there must have been a good reason.  I still have a pair of those filters.
    Jerry Murphy
-----Original Message-----
From:
Sent: Oct 7, 2021 8:14 PM
To: ad7i
Cc: greenkeys
Subject: Re: [GreenKeys] Tone frequency history?

 
 
On Thu, October 7, 2021 8:01 pm, ad7i wrote:
> Excellent question. I've wondered about this as well.
>
> I don't know that this is relevant to the reason for choosing 2125 & 2975
> Hz, but I note that
>
> 2125 = 17 x 5 x 5 x5
>
> 2975 = 17 x 5 x5 x 7
>
> Paul, ad7i
 
 
Thanks! I'm seeing that before Bell 103, there was Bell 101. But I'm
finding very little information about that. Some of the information gave
me the idea it used amplitude shift keying instead of frequency, but I may
have misread that.
 
Harold
 
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