[CW] Data transmission rate

[Ken Brown]

>>
>> Please explain how lower morse code speeds are more noise immune than higher speeds?
>>     
Think of it in terms of energy transmission. First, you need to know 
that energy is power integrated over time.

( When you pay your electric bill, you pay for watt hours, not watts. 
You are paying for energy, not for power. And you're paying a lot of 
taxes too, but that's a different subject.)

If you can accept that it takes a certain amount of energy to get a 
signal detection, at a specific degree of certainty (that is, you know 
the signal is there or not there) then the degree of certainty can be 
increased by increasing the energy transmitted. If you can integrate the 
received power over time, then integrating for twice as much time, at 
the same transmission power level, will have the same effect on the 
certainty as transmitting at double the power for the same amount of 
time. Either way the energy is doubled.

Similarly if you want to communicate more information, you can transmit 
for a longer period of time, or you can transmit with a wider bandwidth 
for the same period of time. If you use a wider bandwidth, spreading the 
power over more bandwidth you will also need to increase the power level 
to keep the signal above the noise.  If you try to communicate twice as 
much information, it will either take twice as much time, or twice as 
much bandwidth (and twice as much power to maintain the same 
power/bandwidth density), so twice as much energy.

Since noise power is not necessarily uniform over time and frequency, 
there are cases in which the math does not work out quite so simply. 
Therefore as others have pointed out, sending a quick burst may be more 
effective than sending slowly.

If you are thinking about this, you may realize that in twice as much 
time there is also twice as much noise energy. So then how can there be 
any advantage to transmitting twice as long? It is because the noise is 
random and the signal is not. Integrating over time the signal builds 
and the noise doesn't.

In radio astronomy we use the principal of integrating received power 
over time to detect spectra of common molecules, such as carbon monoxide 
and carbon dioxide, from billions and billions of light years away, by 
integrating as long as it takes to see the spectral lines rise out of 
the noise. With really strong sources the spectral lines may be detected 
in a second or two. With really weak (and probably distant) sources, the 
integration time may have to be several minutes in order to detect the 
spectral lines.

DE N6KB