This page contains some of the important references. The works of the major contributors (Gaudé, Poor, Hoff, etc) are listed in their own pages. A following page shows where to obtain these documents.
Relevant Inventions
U.S. Patent 1,286,351 Howard L. Krum, Morkrum Company, "Electric Selective System." (1918)
This is one of
the earliest patents that pertains to RTTY. It describes
a start-stop mechanism that allows multi-bit data to be
encoded into a single bit data stream and synchronized,
thus making it practical to transmit 5-bit teleprinter
data.
U.S. Patent 2,443,434 Robert M. Sprague, Press Wireless, Inc., "Automatic Signal Bias Control Means and Apparatus." (1948)
This is the basis
of the Automatic Threshold Correction (ATC) circuit in
RTTY that allows copy of unequal Mark and Space
amplitudes (selective fading). The ATC also provides
Mark-only and Space-only copy.
Frank Gaudé K6IBE was one of the first amateurs to point out (June 1963 article in the RTTY bulletin) that the use of a limiterless demodulator in conjunction with an ATC circuit fixes one of the major source of error from a limiter-discriminator detector that until then was the preferred FSK demodulator method in amateur circles. Mr. Gaudé calls his implementation of the ATC a slideback detector.
Frank Gaudé K6IBE was one of the first amateurs to point out (June 1963 article in the RTTY bulletin) that the use of a limiterless demodulator in conjunction with an ATC circuit fixes one of the major source of error from a limiter-discriminator detector that until then was the preferred FSK demodulator method in amateur circles. Mr. Gaudé calls his implementation of the ATC a slideback detector.
U.S. Patent 2,999,925 Elmer Thomas, Page Communications Engineers, Inc., "Variable Decision Threshold Computer." (1961)
In the days
before diddles are used with RTTY, the ATC's main problem
occurs when nothing is being typed. A prolonged Mark
signal moves the slicing threshold incorrectly towards a
value that is half the Mark amplitude. The DTC works by
bleeding off the Mark bias when the Mark signal is held
down over a long period.
The existence of this patent had kept some manufacturers at the time from correctly implementing DTC, and those modems had to fall back to using a manual switch to remove the ATC circuit when slow manually type radioteletype data is received..
Victor Poor K3NIO was one of the first in amateur circles to adopt the DTC circuit. In a May 1964 article ("Comments on DTC circuit"), Mr. Poor said "If you have a sudden fade while in steady mark, the slideback detector will produce an error. The DTC will not produce an error and this is true regardless of [the time constants]."
In his Mainline TT/L Demodulator, Irv Hoff K8DKC had adopted a DTC circuit to allow limiterless copy. Because a DTC will not properly copy Mark-only and Space-only signals, the TT/L Demodulator included a switch to defeat the DTC action and make the circuit behave like a plain ATC circuit.
The existence of this patent had kept some manufacturers at the time from correctly implementing DTC, and those modems had to fall back to using a manual switch to remove the ATC circuit when slow manually type radioteletype data is received..
Victor Poor K3NIO was one of the first in amateur circles to adopt the DTC circuit. In a May 1964 article ("Comments on DTC circuit"), Mr. Poor said "If you have a sudden fade while in steady mark, the slideback detector will produce an error. The DTC will not produce an error and this is true regardless of [the time constants]."
In his Mainline TT/L Demodulator, Irv Hoff K8DKC had adopted a DTC circuit to allow limiterless copy. Because a DTC will not properly copy Mark-only and Space-only signals, the TT/L Demodulator included a switch to defeat the DTC action and make the circuit behave like a plain ATC circuit.
Relevant Professional Papers
J. W. Allnatt, E. D. J. Jones, H. B. Law, "Frequency Diversity in the Reception of Selectively Fading Binary Frequency-Modulated Signals," Proceedings of the IEE - Part B: Radio and Electronic Engineering, March 1957. Vol 104 Issue 14 pp 98-110.
This paper's
theoretical analysis and experimental measurements show
that "limiterless" demodulators are superior to
limiter/discriminator FSK demodulators for a selectively
fading channel.
The paper's Summary suggests that by avoiding the use of a limiter, one can take advantage of frequency diversity.
An "assessor" circuit is applied after the mark and space signals are demodulated, to provide what we today know as a threshold correction circuit. In fact, the "assessor" circuit is practically identical to the ATC circuit shown in the earlier (1948) Press Wireless patent (mentioned above). The paper thus appears to provide a theoretical proof of the effectiveness of the Press Wireless circuit.
The analysis assumes that fading is slow compared to the symbol rate [thus may not yield good results for the a fluttered signal - kc]
The paper's Summary suggests that by avoiding the use of a limiter, one can take advantage of frequency diversity.
An "assessor" circuit is applied after the mark and space signals are demodulated, to provide what we today know as a threshold correction circuit. In fact, the "assessor" circuit is practically identical to the ATC circuit shown in the earlier (1948) Press Wireless patent (mentioned above). The paper thus appears to provide a theoretical proof of the effectiveness of the Press Wireless circuit.
The analysis assumes that fading is slow compared to the symbol rate [thus may not yield good results for the a fluttered signal - kc]
H. Nyquist, "Certain Topics In Telegraph Transmission Theory," AIEE Transactions, Vol 47, April 1928 pp 617-644.
This is the paper
that forms the basis of Victor Poor's "Filters For RTTY"
article in the May 1964 issue of the RTTY bulletin.
Textbooks
Marvin E. Frerking, "Digital Signal Processing in Communication Systems," Chapman & Hall, 1994, ISBN 0-442-01616-6.
Chapter 8 ("Data
Transmission") of the Frerking textbook includes a
section on Frequency Shift Keying, where the
implementation of Matched Filter detector ("AM"
detection) and of limiter-discriminator detection ("FM"
detection) are presented. Other topics in the section
includes FSK power spectrum and bit timing. More unusual
and seldom addressed by other textbooks, is his inclusion
of a variable threshold ATC algorithm to counter Rayleigh
(selective) fading.
Frerking's ATC equations basically describe the ideal version of the circuit in the Page Wireless patent.
Frerking's ATC equations basically describe the ideal version of the circuit in the Page Wireless patent.