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6 Baud, 60 WPM, Say What?

I’ve seen several reports from hams who have tested the FSQ “digital chat mode” described in the September 2015 edition of QST. That illustrates just how easy it is to implement a new digital mode with a modern Amateur Radio station. The article also provides an example of just how hams can experiment with new ways to communicate in various conditions. What really started a gestalt with me, though, was the claim of 60 words per minute with only six baud. I was still stuck in a classic radioteletype (RTTY) 45 baud for 60 wpm. What gives?

Amateur RTTY is the granddaddy of digital modes. WW II surplus equipment made it possible for Amateurs. Transmitter modulation was by keying a shift between two tones. That ‘one of two’ is a binary system. Five bits was needed to look up a character in a code table called the Baudot code.

Baud is a measure of stress on a communications channel. It is essentially how many changes in signal are made per second. Bits per second (bps) is a measure of how much information is transferred. With RTTY, baud and bps are pretty much the same as one change in signal from one tone to the other is needed to change from a 0 or mark to a 1 or space or vice versa. The ‘pretty much’ is because sometimes you have several 0’s or 1’s in a row where the tone won’t change. Another complication is being able to tell you’ve got several 1’s or 0’s in a row and where to start and stop and other management issues. Management is information, too, so the effective information transfer rate isn’t the total information transferred.

What changed is all about those two tones. Back towards WW II making a filter to detect one or the other of the two RTTY tones was a difficult task. That resulted in fixed frequencies that needed careful tuning. This only started to change in the 70’s.

In the early days of micros it was really hard to read analog signals. There were no monolithic A/D converters; you had to buy an epoxy-potted module to sample signals, and 12 bits was about the max resolution that was possible. These things cost hundreds of dollars back when inflation hadn’t hollowed out the greenback. Long defunct companies (like Analogic and Burr-Brown) made these, as well as some still with us (e.g., Analog Devices). Conversion speeds were typically in the tens of microseconds.

The following picture shows a typical 12-bit unit dating from the mid-70s. The MAS-1202 retailed for $270, or about $1100 in today’s dollars, and was an enormous 4 by 2 inches: [Jack Ganssle on Microchip’s new PIC16F18877 family]

The advances in electronics with IC’s for signal handling and the availability of personal computers in the 80’s opened things up a bit for Amateurs but a custom circuit in a box – a modem – was still needed for digital modes. The computer served as a terminal and modem control unit. This was pretty much the case until Intel dropped the Audio Codec ’97 specification with hardware. AC ’97 was when a lot of the advances in digital audio became a commodity item in personal computers. Now these computers could process audio signals digitally without a custom modem. Filtering could be done with mathematical operations on a digitized signal rather than a special circuit.

That was the gestalt with the FSQ article. Instead of two tones like in RTTY, the new digital mode uses 32 tones. Instead of needing as many as five frequency shifts for one character, now only one was needed. That’s a 5:1 reduction in baud for the bps which was the basis for the 6 baud yielding 60 wpm in the new system.

With modern personal computers, the audio system turns the analog signal into digital measurements. The digital measures could be mathematically translated from a time domain to a frequency domain. The frequency domain could then be inspected to see what frequencies were in the signal. Old style filtering can only see specific frequencies and required re-wiring or tuning a circuit to see other frequencies. This new technique could see all the frequencies in the selected band and simple changes in software could look for different ways to represent information. The experimentation with those simple software changes is described in the FSQ article. 

This is one reason why the Fldigi program has so many digital modes to choose from. Most digital modes use FSK or frequency shift keying like RTTY. Once a band of radio frequencies is translated down to the audio spectrum, the PC audio system can convert it to a digital signal for processing. Once you have a frequency domain representation, it is a matter of timing, tones, encoding methods, and management techniques to decode the signal. All of that is easily handled with software. The result is a lot of experimentation and a plethora of digital modes to play with.