[Elecraft] Poor AM audio

[KU4AF]

This limitation of audio bandwidth of AM signals to 1.35 kHz is only true
when the AM carrier is centered in the roofing filter passband. As Don says,
AM is a double sideband signal, but you only need to receive one sideband to
hear all the audio. You can use the big knob on the K3 to tune slightly
above or below center to recover some of the higher modulation frequencies.
The 'gotcha' is that the AM carrier must be in the passband for the audio to
demodulate properly so if you tune too far off center, the carrier falls
outside the filter skirt and the audio distorts. With a 2.7 kHz roofing
filter, there isn't much room for error and the audio bandwidth is still
pretty restricted. As Don points out, you can use AM Synchronous mode which
lets the K3 reinsert the carrier and puts that carrier right at the filter
skirt, so you get the maximum bandwidth possible with the filter. Even so,
AM will sound better with a 6 or 13 kHz filter.

To digress, this issue has been around ever since narrow filters tailored to
SSB appeared in ham receivers. Here's a snippet from the manual for my
Collins 75A-3 (circa 1951) with its 3 kc mechanical filter:

The tuning techniques used with a 75A-3 receiver differ somewhat from those
used in tuning a conventional receiver.  Because of the flat top and almost
vertical sides of the pass band, it is possible to tune either sideband of
an AM station and reject the opposite sideband.  To get the most out of your
75A-3 take advantage of this unusual feature.  Tuning "on the nose" results
in loss of the high frequency audio components with, in most cases, a loss
of intelligibility.  Select the sideband that contains the least
objectionable adjacent channel interference.  ...  Tune toward one side of
the carrier being received until "S" meter reading drops sharply.  Then
carefully tune into the signal again until the "S" meter reading increases
to its former value and the modulation is readable. The receiver is now
tuned to one sideband plus the carrier of the AM station.

So, what's old is new again!

John, KU4AF
Pittsboro, NC



Dave,

AM is a double sideband signal, so the audio you receive will be only 
about half that of your filter setting.  The 2.7 kHz roofing filter is 
limiting the demodulated audio to about half the width of that filter - 
1.35 kHz - so you will not hear any high audio frequencies no matter how 
wide you set the DSP bandwidth.  Even though the DSP bandwidth can be 
set higher, the internals of the DSP currently impose a 4.2 kHz limit on 
the audio response.

You will not be able to transmit AM until you install the 6 kHz filter 
(or with the latest firmware the 13 kHz filter can be used).
In the meantime, for reception only, you can use the 2.7 kHz filter to 
listen in synchronous AM mode.

73,
Don W3FPR




--
View this message in context: http://elecraft.365791.n2.nabble.com/Poor-AM-audio-tp7567914p7567946.html
Sent from the Elecraft mailing list archive at Nabble.com.