[Elecraft] K3 Dual Receive Surprise

[Stephen W. Kercel]

Matt:

At an unconscious level of cognitive processing if you were listening to 
two replicas of the same signal, with one slightly delayed, if the delay 
were on the order of 1/100 second and if your ear were very highly 
trained, you might be able to tell that something is different about the 
two signals, but you would not be able to describe what. The delay would 
need to be on the order of 1/10 second or more before you would 
consciously recognize that one signal is delayed compared to the other.

Recall that radio waves move at 3 x 10^8 meters per second. If you look 
at multi path propagation at HF radio waves over intercontinental 
distances, and suppose that your two receivers were responding to two 
different signals taking two different paths, the differences in time of 
arrival of the different signals would be 1/10000 second or less.

Also supposing that you were listening to the same wave on two antennas 
3 meters apart, the delay would be 10^-8 second.

Thus, by either (or both combined) delay mechanism, the delay is many 
orders of magnitude too short for your internal cognitive processes to 
detect the difference in arrival time of RF wave fronts.

On the other hand, DSP delays can be very long by comparison, and if you 
are using two receivers with non identical DSP parameters, you might 
very well be able directly to hear the delay (a difference in latency in 
the jargon of the trade) audibly.

One curious phenomenon I've noticed is comparing digital versus analog 
TV signals. My cable service uses the same channel on cable as the "over 
the air" VHF channel for the local broadcast TV stations. At the present 
time the local stations are still simulcasting an analog signal on their 
VHF channels that I can pick up with an antenna and receive on the TV, 
and a digital signal on UHF. The cable company captures the digital 
signal, converts it to analog and retransmits it over the cable circuit 
on the analog VHF channel. I have an RF switch that lets me switch 
between the antenna and the cable service. If I'm listening to the VHF 
analog "over the air" signal and quickly switch to cable, I can hear the 
last syllable or so of the transmitted dialog repeated quite distinctly. 
This suggests a delay for the digital modulation and conversion back to 
analog on the order of 1/10 to 1/2 second.

Thus, I'm more inclined to believe that what you're hearing is an 
artifact of the DSP in your receiver rather than an effect occurring Out 
There.

73,

Steve Kercel
AA4AK




Matt Zilmer wrote:
> I was working on 160m CW during the contest and noticed an interesting
> phenom when hearing static crashes.  The subRX (vertical polarized
> antenna) received the noise later than the receiver on the long wire
> (horz pol).  The K3 was set in diversity receive mode.
>
> Being new to receiver diversity, I'd never thought through all of the
> implications.  Is it possible that two wavefronts arrive at times
> different enough that this can actually be detected audibly?
>
> Anyone else ever noticed this or have any kind of explanation?  I
> don't know if it's a DSP audio delay thing, or something physical Out
> There.
>
> 73 and :)
> matt zilmer
> K3 #24
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