RTTY Demodulator Development

Kok Chen, W7AY
w7ay (at) arrl (dot) net

September 1, 2011

This document is an overview of the formative years (1960s) of RTTY demodulator development.

It is hoped that the document can provide both a historical perspective for anyone curious about early RTTY development, and a technical background for anyone wishing to implement new hardware or software modems. I have attempted to assemble together the important ideas that "worked," while also including ideas that were impractical to implement back in earlier times.

Many of the early circuits end up in the demodulators that we use today (in algorithmic form in the case of software modems) and which we take for granted. Some of the older ideas, a number of which were too complex to implement back then, became "lost art" and are not present in many of today's demodulators even though it is now practical to implement them.

Arguably, two aspects of an RTTY demodulator contribute most to its performance. One is how well a discriminator handles different propagation conditions. The second is how well the RTTY filters provide the best signal-to-noise ratio to the discriminator while providing the need interference rejection.

We will discuss these two aspects separately in the
Limiterless Demodulator page and the RTTY Filters page.

When it comes to designing high performance demodulators, dynamic range is always a concern. Because of that, certain circuits could not be realized in hardware. Today, we have affordable sound cards with dynamic ranges that exceed 110 dB, and some of the "impractical" ideas in the past may now be practical. Using floating point arithmetic on today's fast desktop computers, software modems have practically no limitations than the ingenuity of their authors. The dynamic range of an FSK decoder today is largely limited by the receivers which feed the modems.

The other area where RTTY modems can be improved upon is the design of the filters. Some of the early RTTY experimenters eventually realized that optimal decoding of FSK cannot be done with just any arbitrary narrow band filters. However, it was not easy to design these optimal filters back then; using DSP techniques, fewer compromises have to be made today.

Some of the early demodulators had to compromise on performance to keep the cost reasonable. We often find phrases in the old articles such as "
not implemented because it requires 8 additional op-amps." Things that were impractical in hardware can be implemented with literally just a few extra lines of software code today.

What got us here?

Radioteletype's roots, and thus the challenges of demodulating RTTY, can be traced all the way back to
Howard Krum's 1918 patent.

Trained as an electrical engineer, Mr. Krum was the son of the founder of the Morkrum Company. The elder Krum had partnered with the head of Morton Salt to form Morkrum. Morkrum merged with Kleinschmidt in 1925 and changed their name to Teletype Corporation in 1928.

The challenge back then was to economically transmit characters from a teletypewriter over a landline. The teletypewriter's keys were encoded into what we would today call a
5-bit code.

The principal claim of the U.S. 1,286,351 patent consists of
"initiating the operation of [a receiver] in response to transmitted impulses at the beginning of each signal" and "restoring [the receiver] to a condition of rest at the completion of each signal."

In modern technical nomenclature, the claim can be described as "serializing the 5-bit parallel data by adding a start bit before the data, and adding a stop bit at the end of the 5 bit data."

Krum's patent permits one to transmit 32 states from a teletypewriter through a single wire. The start-stop mechanism synchronizes and maintains the bit order of the five data bits between the originating teletypewriter and the receiving teletypewriter.

This patent gave us the way to eventually transmit a
radioteletype signal by modulating a transmitter between just two states.

We take the operations of
UARTs and serial ports for granted today. Back in the beginning of the 20th century, the start-stop method had to be invented. The '351 patent was filed in 1910 and issued in 1918. Today, we would call it a "killer app."

(Technology has come full circle
many of the advanced digital modulation modes will transmit 16, 64 or even more states with a single modulation symbol. Modern codes such as Varicode used in PSK31 are also "self clocking," and no longer use start-stop bits for synchronization.)

Selective Fading

Very early radioteletype (RTTY) had used on-off keying (OOK) of a carrier (as done for Morse transmissions) to encode the two states (on and off) that Howard Krum's system generated. After dealing with the various problems of OOK, amateurs quickly moved to sending one state with a carrier at one frequency, and the second state as a carrier at a different frequency (in other words, Frequency Shift Keying or FSK). One of the two FSK frequecies is called the Mark frequency and the other is called the Space frequency.

However, FSK has its own set of problems. One of which is caused by selective fading.

Selective fading causes unequal signal strengths in the Mark and Space channels. Selective fading in a Rayleigh channel can cause two carriers to randomly and independently fade to different depths. A panoramic view of a HF spectrum often shows a notch that pans across a wide band signal.

FSK demodulators started out being of the limiter-discriminator variety. The audio FSK signal from a receiver is first passed through a limiter to remove any amplitude variation. The limited signal then passes through two resonators that are tuned to each of the two FSK tones.

In his
June 1963 article in the RTTY bulletin, Frank Gaudé K6IBE shows graphically what happens when a signal with equal Mark and Space durations but with different amplitudes is passed through a limiter; the demodulator output shows unequal durations!


On the left (Figure 1 in Mr. Gaudé's article), an FSK signal with a Mark tone that is 24 dB stronger than the Space tone is first limited and then detected. Figure 2 shows an FSK signal with the same amount of selective fading that first is put through a comb filter.

The comb filter consists of two narrow bandpass filters that are centered around the Mark and the Space tones to reject interference elsewhere. Today, the more popular name for the "comb filter" is a "dual peak filter."

Figure 1 shows unequal (25 ms/19 ms) output durations even though the input has equal Mark and Space durations. After passing through a dual peak filter, the output discrepancy is even worse, becoming 37 ms and 7 ms.

Gaude's article pretty much drove modem designers to implement
"AM" detectors.

Optimal Filtering

In a 1964 article "
Filters For RTTY," Victor Poor K3NIO (of Frederick Electronics Corp.) wrote:

"No other factor in the design of RTTY equipment bears as much on performance as the filters used to separate the received signal from the surrounding noise.
At the same time, however, there is probably less understood [sic] by the average amateur about the design of filters than any other aspects of RTTY." [emphasis is mine - kc]

Mr. Poor goes on to explain that an optimal data filter needs to satisfy the slicing and sampling points that he showed in the following figure:


The slicing and sampling points are defined by the "Nyquist Criterion" to avoid Inter-symbol Interference (ISI) between adjacent data bits. (H. Nyquist, "Certain Topics In Telegraph Transmission Theory," AIEE Transactions, Vol 47, April 1928 pp 617-644.)

This topic is discussed in more detail in a following section on