[Boatanchors] ALERT: AM Under Attack - WA3VJB

[John Kolb]

On Fri, 20 Jun 2003, Colburn wrote:

> > >         Now 2.8KHz for SSB and 5.6 KHz for AM are now equivalent audio
> > > bandwidths. 6KHz is more appropriate.
> > With a proper BFO setting a 2.8 kHzx BW for SSB would be
> > transmitted audio freqs of 300 to 3100 Hz. Thus the proper
> > AM BW would be 6.2 kHz to get up to 3100 Hz in the audio
> > response.  John  KK6IL
>
> I agree that 2.8 is lots easier to listen to than 2.4! (Even though QRM
> frequently makes 2.4 necessary.) A max of 3.0 would allow for a little
> breathing room.
>
> Please explain why an AM signal (DSB with carrier) would require so much
> more than double the same bandwidth to duplicate SSB.  I really do not
> know.
>

Let me try this with ASCII art - it won't display properly
unless your brouser is set for a mono-spaced font.

                                  USB
                    Supressed
                     carrier
                         |      _________
                               /         \
                         |    /           \
                             /             \
                         |  /               \
                         0   +300        +3.1

A 2.8 kHz wide SSB signal would start about 300 Hz above the
supressed carrier, and extend up to 3100 Hz.


                        AM

                      carrier
                         |
                         |
          _________      |      _________
         /         \     |     /         \
        /           \    |    /           \
       /    LSB      \   |   /    USB      \
      /               \  |  /               \
      -3.1K       -300   0   +300        +3.1

To get the same 2.8 kHz of audio would require a lower
sideband starting 3100 Hz below the carrier and extend
to 3100 Hz above the carrier for a total of 6.2 kHz.

The audio wouldn't have to roll off below 300 Hz with AM
- in theory, the audio chain could be flat down to DC, but
there is very little signal in most human voices below
300 Hz (although I have a record with Ivan Rebroff singing
a 92.5 Hz note). The voice range considered necessary
for clear communication is 300 Hz to 3000 Hz, which is
why 2.7 kHz is a popular width for mechanical or crystal
filters.

John   KK6IL

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