[AMRadio] Signal Bandwidth


rbethman rbethman at comcast.net
Wed Mar 10 13:24:26 EST 2010


We've been beating this DEAD horse.

Don Chester made some very good points regarding the way human process 
audio presented to the ear.

Some may disagree.  The reality is just what Don stated.  The consonants 
do indeed appear up around 3500cps.

If one were to have an audiogram, hearing test, performed, you would be 
amazed what your actual hearing span is.  I am pushing right at 60 yrs 
old.  I STILL get the 15Kc sound in my ears whenever I pass by a TV.

The rules and regulations, Part 97, list the Emission as A3E.  Regarding 
that, there is NO question!

Then there is the VERY real reality!  I look at the audio chain in MY 
transmitters.  I suspect many would find "very similar" specifications.  
The audio transformer that takes the output of the first audio amplifier 
stage, lists clearly on its label: 400 to 3500 cycles.

This already demonstrates that the bandwidth is 100cps over the 
3,000cps.  Now when you lay it out in a graph or on a scope, the carrier 
sits in the middle.  Each sideband is offset from the carrier by the 
lowest audio frequency passed.  In my case the carrier stands alone with 
300cps open on each side of it.  That is where the sidebands start.  
Then take the width of audio chain response, and you have each sideband 
3100cps in width.

Now, add the entire width.  3100cps, a 600cps gap with the carrier in 
the middle, and another 3100cps.  This makes the entire signal 
3100+600+3100.  IOW, 6800cps in total width.

Unless the top 400cps is trimmed off or filtered down, you absolutely 
cannot stuff it into a 6000cps width signal!

I am posting this as a little more comprehensive view of AM DSB.

The entire discussion has concentrated on a 3000cps sideband audio.  Yet 
it hasn't been presented as the ENTIRE signal as it looks like and IS.

This is MY $0.02 worth, but I feel it needs to be laid out as it is.  AM 
is indeed structured in this manner.

Bob - N0DGN



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