AM receive band width

[Mike Scott-7]

>So my questions are: (1) Is audio distortion on AM a known problem with the
>K3? (2) What does the LCD BW actually read? Is it an IF BW, an audio BW, or
>what, and does the displayed BW actually correspond to reality?

On my K3 I see and measure the following:

6 KHz AM filter AM mode
Band width displayed as 3.00 KHz,
Spectrum lab -10 dB audio band 100 Hz to 3100 Hz

6 KHz AM filter LSB mode
Band width displayed as 4.00 KHz
Spectrum lab -10 dB audio band 120 Hz to 3500 Hz

--------------------------------------------------------

13 KHz FM filter AM mode
Bandwidth displayed as 5.00 KHz,
Spectrum lab -10 dB audio band 100 Hz to 4200 Hz

13 KHz FM filter LSB mode
Bandwidth displayed as 5.00 KHz
Spectrum lab -10 dB audio band 120 Hz to 3600 Hz

If I had a second 13 KHz filter in the sub receiver I would try linking VFOs
and place one on upper sideband and the other on lower sideband (otherwise
know as ISB mode).
----------------------------------------------------------

AM audio much more understandable (pleasant) on the FM filter, the extra 1
KHz helps.

AM on the 6 KHz filter on LSB is okay and using LSB/USB is best possible
reception of AM if you only have a 2.7 KHz filter.

I will leave it up to you to decide what the LCD BW corresponds to.

Mike Scott - AE6WA
Tarzana, CA (DM04 / near LA)
K3-100 #508/ KX1  #1311



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[K7TV]

> AM audio much more understandable (pleasant) on the FM filter, the extra 1
> KHz helps.
>

To those who feel that AM reception with the 6 kHz filter is not very
understandable:

I had that same impression the first day or so after assembling my K3. Then
I discovered that, with my small hi-fi speakers, excessive bass was the
culprit. It appears that broadcast AM transmissions are balanced to sound
pleasant with a wider receiver passband. The 6 kHz filter removes some
highs, so to restore balance some of the lows need to be cut. This was
easily achieved with the K3's receive equalizer. Of course, if you are using
a speaker designed for communications, this is not likely to be needed.
Anyway with the equalizer cutting off frequencies below 200 Hz or so, and
some attenuation at 400 Hz, I found AM receive quality on voice to leave
little room for improvement, and better than good SSB (assuming no ESSB).
For music the FM filter would be a significant improvement of course, but
when I listen to SW broadcasts, music is not generally what I am looking
for, so I haven't been able to justify a 13 kHz filter.

Incidentally, I find a similar situation with the surround sound system in
my living room. When the tuner is set to AM, I need to adjust controls to
cut some bass, probably because the manufacturer takes pride in a deep bass
response. The car radio and small portable radios don't seem to have that
problem, perhaps because the manufacturers expect the products to be
actually used for AM, and of course in many cases less bass means lower cost
of manufacturing.

73,
Erik K7TV


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[AC7AC]

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[K7TV]

Very interesting Ron!

This suggests that the radios where AM broadcast sounds good (portable
radios and car radios) have built-in
bass 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 emphasize
the 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 the
manufacturer from counteracting the broadcasters' roll-off, lest the
expensive box look non-flat on paper.

73,
Erik K7TV

----- Original Message -----
From: "Ron D'Eau Claire" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, February 01, 2009 2:55 PM
Subject: Re: [Elecraft] AM receive band width

> Perhaps many US listeners would be surprised to learn that the AM
> broadcast
> stations they listen to limit their bandwidth to 5 or 6 kHz for several
> reasons.
>
> In many tests listeners prefer the reduced bandwidth because it improves
> the
> AM signal/noise ratio when signals become marginal. They never miss the
> highs because the most common AM radios today - car radios - have filters
> that limit their audio bandwidth to less than 5 kHz!
>
> Broadcasters have discovered they can "sell" the un-needed bandwidth they
> are entitled to use for non-listener functions, so they have a double
> incentive to limit the broadcast bandwidth.
>
> Here's a couple of on-line resources for the curious:
>
> http://www.rwonline.com/article/1672
>
> This report notes: "These objective measurements established that the
> majority of current analog AM receivers have audio bandwidths of less than
> 5
> kHz. In fact, with only a few exceptions, the frequency response of
> individual receivers falls off above 1 or 2 kHz.
>
> As shown in Fig. 1, the combined frequency response of all receivers
> through
> the test bed (the middle curve, in blue) was -3 dB at 2450 Hz and -10 dB
> at
> 4100 Hz.
>
> Another interesting report is here:
>
> http://findarticles.com/p/articles/mi_m0EIN/is_2007_Feb_2/ai_n17166612
>
> I suppose the moral is that bandwidth is like money: It's not how much you
> have but what you do with it that counts in the end.
>
> Ron AC7AC
>
>
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[Julian, G4ILO]

K7TV wrote

Since the K3 is primarily for amateur communications, and many amateurs have
microphones that emphasize
the 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.)

Yes, that would really be nice (including disabling equalization for data modes.)