[CW] CW Speed Limit ???

[N2EY at aol.com]

In a message dated 3/6/05 10:38:17 PM Eastern Standard Time, 
w0oow at piperscreek.com writes:


> The text below was sent to bandwidth at arrl.org.  There were a few 
> individuals
> that tried to answer my questions, but all in all the ARRL did not nor has
> it yet addressed this issue as far as I have seen, nor has my question been
> answered.
> ... read on - my 2 cents
> Steve W0OOW
> CW - It's still magic !

I found the answers in my 1988 Handbook.


> 
> ===========================================================
> What values were used to calculate the data behind this ARRL statement?

5 ms rise and fall times, which equate to 150 Hz bandwidth. Since the 
proposal allows as much as 200 Hz, rise/fall as short as 3.75 ms would be OK under 
the proposed rules.

> 
> "200 Hz is intended to be the narrowest bandwidth to permit Morse telegraphy
> at all speeds that human operators can decode. The necessary bandwidth
> depends on speed and whether the circuit is fading or non-fading. An
> analysis by ARRL in the 1980s showed that 150 Hz is adequate and is based on
> rise and fall times of 5 ms. A bandwidth of 200 Hz will permit data modes
> such as PSK31 as well."

That's their proposal. I'd go with 250 or 300 Hz, myself.


> 
> The reason I ask is that according to the ARRL Handbook of 1987 p9-9, a
> value of K = 3 is used to non-fading circuits, and a value of K = 5 is to be
> used to fading circuits.
> 

Yep - applied to the speed in bauds. With Morse, WPM =  1.2 x bauds


> >From what I get, there is a speed limit imposed on an operator that is far
> below a speed that many hams currently use. That of course is allowing for
> fading/QSB which we all know rather too well.
> 
> "... Morse telegraphy at all speeds that human operators can decode."  Huh?
> 
> What value of K was used to make the above statement?
> 
> 

Let's work it out...

K is simply a conversion constant between the occupied bandwidth and the 
length of a dit. In theory, K can be as low as 1, but to most human operators 
Morse sent with such a low K sounds too mushy, or soft. 

Mathematically, K = bandwidth/baud 

Or, turning it around, baud = bandwidth/K

5 ms rise and fall time is 150 Hz bandwidth, as defined under the 23-db-down 
standard universally applied. If K = 3, we get 50 baud, and if K= 5 we get 30 
baud.

50 baud x 1.2 = 60 wpm (nonfading) and 30 baud x 1.2 = 36 wpm.

So with 5 ms rise and fall times, 150 Hz bandwidth is good for 60 wpm on 
nonfading and 36 wpm on fading circuits.

How many hams can do 36 wpm on fading circuits?

Now let's see how fast we could go with 3.75 ms rise/fall and 200 Hz 
bandwidth:

3.75 ms rise and fall time is 200 Hz bandwidth, as defined under the 
23-db-down standard universally applied. If K = 3, we get 66.6 baud, and if K= 5 we 
get 40 baud.

66.6 baud x 1.2 = 79.2 wpm (nonfading) and 40 baud x 1.2 = 48 wpm.

So with 3.75 ms rise and fall times, 200 Hz bandwidth is good for 79.2 wpm on 
nonfading and 48 wpm on fading circuits.

How many hams can do 48 wpm on fading circuits or 79.2 wpm on nonfading 
circuits?

There's a fundamental speed restriction on most rigs today in the form of the 
rise and fall times of the keying circuits.

Does that answer the question?


The best argument I can see against this part of the bandwidth proposal is 
that the 200 Hz limit is a bit too narrow. Make it 300 Hz, and a rig with 2.5 ms 
rise/fall can be used for speeds up to 120 wpm on nonfading and 72 wpm on 
fading circuits.

73 de Jim, N2EY