|[AMRadio] Signal Bandwidth|
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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