AM receive band width on K3

[Don Rasmussen]

I have the FM filter.

I learned last week after experiencing less than optimal
FM audio quality on receive that the Rx EQ is enabled
on all modes. I've never seen that on FM.

The complicating factor is that the 8 equalizer sound
band values vary from the K3 documentation whenever the
filter width changes (this always happens with
mode changes).

I had Rx Eq frequency band 8 set to -16 (who needs
that much high end response), only to find that on SSB
the expected attenuation was at 3.3khz, but on FM the
attenuation was at 2.7 khz in the audio.

I also had band 7 reduced from level.

AM is also different with respect to the 8 Rx Eq
bands than is documented.

The spectragram displays are enligtening.

So, widen it up then make sure the Eq is correct.



[Elecraft] AM receive band width
Erik N Basilier ebasilier at cox.net
Sun Feb 1 17:21:39 EST 2009

Previous message: [Elecraft] AM receive band width


Very interesting Ron!This suggests that the radios where AM broadcast sounds good (portable radios and car radios) have built-inbass suppression and/or treble emphasis for AM, which supports the need to use the equalizer on the K3.Since the K3 is primarily for amateur communications, and many amateurs have microphones that emphasizethe highs, it makes sense for the K3 to rely on the equalizer rather than impose a non-flat response on all.(However, it helps make the case for eventually getting mode-specific equalization on the K3.)When it comes to the living-room stereo that sounds lousy on AM I guess spec-manship prevents themanufacturer from counteracting the broadcasters' roll-off, lest the expensive box look non-flat on paper.73,Erik K7TV

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[KK7P]

> I learned last week after experiencing less than optimal
> FM audio quality on receive that the Rx EQ is enabled
> on all modes. I've never seen that on FM.

The Rx EQ is for the receiver. It currently applies to all modes.

> The complicating factor is that the 8 equalizer sound
> band values vary from the K3 documentation whenever the
> filter width changes (this always happens with
> mode changes).

There is but one Rx EQ routine in the DSP and it has but one set of
parameters. It does not vary by mode; it is completely independent.

Note that the way the EQ works is that it creates, or attempts to
create, a smooth slope from the indicated frequency (say, 1.60 kHz, EQ
band 6) to halfway to the next indicated frequencies (in this case,
halfway to the 0.80 kHz band (thus 1200 Hz), band 5) and halfway to the
2.40 kHz band (thus 2.00 kHz), band 7).

The Tx EQ works the same way.  This is similar to the way the W2IHY
equalizer works,or the formerly-common "graphic equalizer" sliders on
stereo equipment.  The indicated frequency is the "center" of the band
of frequencies over which it operates.  (I put "center" in quotes as you
may not consider 1600 Hz to be the center frequency between 1200 Hz and
2000 Hz as explained above.)

Finally, the upper frequency correction value (3200 Hz, band 8) extends
to the top of the audio passband.  If you set it for - 4 dB, for
example, the -4 dB correction will be applied from 3.20 kHz to 4 kHz, or
4.2 kHz, or whatever the audio frequency cutoff happens to be for the
mode you have selected.

The net result is that you can create a smooth frequency roll off
characteristic.

While on the topic of the ERQ, some people have asked me if the DSP
delay can be reduced if the EQ is set to OFF (flat).

The answer is no.

The Rx EQ is *not* a separate process added to the audio path.  The EQ
parameters are used when building the AF filter response for the radio.
  There is *one* DSP AF filter, which includes the effects of
SHIFT/WIDTH/MANUAL NOTCH *and* Rx EQ.

The Auto notch, and noise reduction, are additional processes in the
path.  So is AFX in the right channel.

73,

Lyle KK7P
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[Jack Smith-6]

Speaking of narrow band FM, does the K3 implement pre-emphasis and
de-emphasis? I haven't had a chance to measure the FM transmitter and
receiver characteristics yet, but hope to later in the week.

The EIA/TIA land mobile spec is 6 dB/Octave receiver de-emphasis
300–3000 Hz and corresponding transmitter pre-emphasis.

Jack K8ZOA

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[KK7P]

> Speaking of narrow band FM, does the K3 implement pre-emphasis and
> de-emphasis?

Yes :-)

73,

Lyle KK7P
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