QRSS (Part 3)


In the previous two articles under this heading, we discussed “extremely slow” Morse Code, such that it could only be displayed on a crawling computer screen display, sometimes known as a “waterfall display”. (It is still called a waterfall display even when the movement of the display is horizontal rather than vertical). Such a Morse Code mode is known as QRSS, frequently with a trailing number such as 3, 10, 60 or 120 to indicate the dot length in seconds. The dot length is the fundamental unit of timing in Morse Code. The time between the dots or dashes of each letter is one dot length, the time length of a dash is 3 dot lengths, the time between the end of each letter and the beginning of the next is also 3 dot lengths, and the time between each word is 5 dot lengths. The whole message signal can be thought of as made up of “dot length” impulses, (or the absence of them), one impulse for a dot and 3 contiguous impulses for a dash etc. The shorter the dot length, the shorter the impulse. (Incidentally, and to put all this into context, the dot length for Morse at 12 words per minute is 0.1 seconds). On the other hand, a voice signal with a bandwidth of 3kHz would need to be a succession of impulses of varying amplitude, (or absent impulses), of 1/3000 second. Hence, if t is the duration of an impulse, a bandwidth of 1/t is needed to convey and reproduce the voice. This argument is not mathematically exact but it establishes the principle that the more “impulses” you send per second, the wider the bandwidth.

The actual bandwidth of a Morse transmission thus depends on the fundamental dot length, and importantly, how the dot is formed, for actually it is the rate of change of amplitude which ultimately determines bandwidth. After all, a carrier that was turned on a long time ago and continues to infinity has a bandwidth of zero Hz. From the above discussion, it would seem that a Morse transmission at 12 words per minute would require a bandwidth of only 10Hz. In practice, depending on how “sharply” the dots and dashes are formed at their leading and trailing edges, (sharp edges are necessary for clarity and easy reading), 30 to 50Hz is usually required, but the principle is the same; The higher the speed of the Morse the higher the bandwidth needed to convey and reproduce it at the receiver. Hence very slow Morse needs a very low bandwidth, and as explained in the second article, a very low bandwidth receiver can then be used. This in turn means a very low noise level is present at the display leading to good readability of very weak signals.

An interesting extension of this principle of “narrow bandwidth leading to good signal to noise ratio” is the reception of very weak signals from outer space. Whereas a domestic television receiver outputs several tens of millions of pixels per second to produce a good picture, the “Rovers” on Mars only transmit, (and we receive), only a few pixels per second, so it takes a long time to build up a high quality picture from Mars.