The ARS Sojourner

OOK vs 21st Century

Page history last edited by Russ Carpenter 8 mos ago

OOK Versus the 21st Century

By Russ Carpenter, AA7QU

Amateur radio is often a curious blend of the old and the brand new. For example, many of us cheerfully engage in OOK, invented in the 19th century, and a few minutes later fire up a one of this generation’s digital modes.

A confession. Before tackling this article, I didn’t know the meaning of OOK either. It's the fancy way of saying on-off keying, also known as Morse Code, or CW. It is, of course, older than radio itself.

Why on earth are we still messing with OOK, especially when all the other rats have deserted the ship? My answers may not hold much longer, but many of us appreciate the simple equipment that can create and process OOK. It works well at low power, its musicality is rewarding, and its link to past centuries may be satisfying in its own right.

In this article, we’re going to do something that, to the best of my knowledge, hasn’t been published before. We will use a sophisticated HF path simulation program to compare OOK with two other popular modes of communication: PSK-31 and single sideband. Our comparison will not be quantitative, because I’m not aware of a way to compare these three modes by the numbers. It will be subjective, but it will also be illuminating and interesting.

Our software is called cocoaPath, an elegant and powerful program written by Kok Chen, W7AY (in fact, everything Chen writes has these attributes). cocoaPath uses the Mac OS. It, and the rest of Chen’s programs, are powerful reasons to own a Mac (especially in light of the Mac’s ability to run Windows software just as well as traditional Windows machines).

The input section of cocoaPath creates an audio signal, either by using one of its internal generators or using an external sound file. For this article, we generate CW and PSK-31 messages within cocoaPath. The SSB audio comes from a file that yours truly recorded using the Mac’s built-in microphone.

Our audio signal is then sent through a “propagation model” section of the program, where the computer simulates the distortion arising from multipath delay, Doppler spread and noise. We decide to use four distortion environments for this article:

Mid latitude good conditions

Mid latitude disturbed conditions

High latitude moderate conditions

Flutter

We add noise to the signal path to simulate relative signal strength. Since most of the readers of this article will be interested in low power, we select a low signal to noise ratio (0 dB) that produces almost error-free copy of PSK-31 under mid latitude good conditions. That is our baseline.

We set cocoaPath’s bandwidth at 3 kHz for all three modes. To create a fair test, we then deal with that bandwidth in the following ways:

The bandwidth for SSB stays at 3 kHz.

Ultimately, when the PSK-31 signal is decoded in cocoaModem (another of Chen’s marvelous programs), the software will create a much narrower bandwidth (which will improve the signal to noise ratio). We don’t need to change any of the cocoaPath settings to reflect this improvement.

Ultimately, when the CW signal is processed by the HF receiver, the bandwidth will be set to a relatively narrow value by the receiver’s CW fitlers. To roughly simulate the effect of the CW filters, we increase the signal to noise ratio in cocoaPath by 6 dB.

We handle the distorted audio that cocoaPath produces for us in two different ways.

The CW and SSB signals are recorded to WAV files (which are then uploaded to the ARS wiki).

The PSK-31 audio is sent to cocoaModem, where it is decoded. We make a screen capture of cocoaModem’s interface and upload that file to the ARS wiki as well.

We'll offer a few observatons before you plunge into the graphics. PSK is fine as long as you stay in simple distortion environments (like mid latitude, single hop, quiet conditions). But it collapses when the going gets tough, and increasing power doesn't help. Samuel Morse's invention is amazingly robust, even in the face of stout multipath delays and Doppler shifts. At the low powers we have simulated, the only word for SSB is wimpy.

There are many other experiments you can run for yourself. If you are a Windows user, try PathSim. www.moetronix.com/ae4jy/pathsim.htm And don't miss the excellent article by Daniel Crausaz in the Nov/Dec, 2007 issue of QEX.

And here is the multimedia presentation you’ve been eagerly awaiting.