[Elecraft] Poor AM audio

[Guy Olinger K2AV]

Before MLB.com came up with their odd method of streaming HD (really very
good), I would listen to WCBS am out of New York for the nightYankees
games.  What actually worked the best, even though I had the 6 kHz filter,
was 1.8 kHz filter SSB reception with the shift adjusted for best clarity,
and usually with some noise blanking.  This had no fading distortion, just
missing content at the cancelling frequency.

The K3 was good enough and stable enough to set the frequency and then hear
music without shift artifacts in the tonality.

When the signal is good enough on the various HF BC frequencies, I use
synchronous AM detection and pick the best sounding sideband.

Of course I'm just listening.  Transmitting is another matter.

On Sun, Dec 30, 2012 at 5:21 PM, KU4AF <johnmars at mindspring.com> wrote:

> 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.
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