[Elecraft] K3 2.7kHz filter DSP FC

[Jay Bromley]

Hi Guys,

I know I am a couple days late on this, but I have been wanting to comment 
on this audio thing.

Billions of folks may be content with restricted audio, but I wouldn't be. 
Let me first state I do a ton of listening or monitoring on all modes. 
Right now I am on 7.020 using a wide filter set for 3.6 KHz listening to CW 
stations coming and going as I work in the shack.  A also have a few AM 
programs on the weekend I like to listen to.  With my outboard audio amp and 
bookshelf speaker system it reminds me of the old days when REAL 
communication receivers had room filling volume.

Let me move away from phone operation and talk digital, I do a little of 
that as well.  Using programs such as HRD's DM780 and MixW if you have a 
receiver rolling off the low or high end response you will loose the ability 
to monitor more effectively on modes such as PSK31.  Now for you guys that 
can only think narrow as well as listen that way, let me explain what I do 
in my shack.  When I am monitoring the bands on PSK31, I use the widest 
filter possible.  At the extremes of the rig's bandpass their maybe a 
problem on transmitting a very low or high tone.  However both programs 
mentioned above have a centering function that grabs the signal and puts it 
at 1500 hz.  If you are using CAT with  your rig, this is all automatic, it 
centers the signal right on 1500 Hz and adjust the VFO accordingly.  Say I 
find a brand new country and find him at 100 Hz using DM780, I hit "Center" 
and it moves the station of interests to 1500 Hz.  Then I can point and 
click for a narrower receive filter.  I am now ready to transmit.

If you have ever gone fishing, you catch more fish with a large net than you 
do with a small one.  You can always throw back in the small ones or 
unwanted ones.  If your receiver is restricted, you will and can miss 
stations that might interest you!  Operators that use DM780 with 
SuperBrowser know what I am talking about.  Just for fun I decide to see how 
many stations I would missed with a rolled off response on the low end of 
300 hz, typical "communication grade" receiver of yester year.  I fired up 
non communication grade receiver and set the cursor at 70.1 hz, I still got 
good print on PSK31!  So using a channel width of 30 to 50 hz on PSK31 means 
I could  miss up to 7 to 4 station respectively.   This doesn't even count 
for the higher end response!  For those that would like to see, I have jpg's 
of stations received at 70.1 Hz and 3604.3 Hz that printed cleanly I can 
send to those that are interested.  I have other receivers that could do 
better on the high end, but that is not the point I am making here.

Over the years I have a ton of receivers, to be honest I would not know how 
to classify a "communications grade" receiver.  When I hear "communications 
grade" I think heavy, military like ruggedness, and versatility!   I never 
think 300-300 KHz, but that is just me.

So end the end, this cool aide drinker would like a choice of favors, the 
more versatility, the better for me.  I hope in the end the K3 has the 
ability to go wide on IF/audio frequency response and then let me choose how 
narrow I need to go on all modes.  There a lot of reasons to have a nice 
flat, wide, low distortion bandpass, not just for good sound, but that would 
be great also!

I would be curious to what the K3 ultimate bandpass would be and would that 
also be completely adjustable?  Yes I did glance at the various pdf.s, but 
didn't see a spec for this.

Thanks for letting me add my 2 cents worth.

73 de w5jay/jay..

> Paul Christensen wrote:
>> Bruce, as Barry points out, the K3's audio response really needs the 
>> ability to extend down significantly lower than the current ~ 250 Hz
>
>> we're not concerned with the broadcast of music in our service, then the 
>> male voice (lowest relevant fundamental of 70 Hz) and certain digital
>
> Billions of people are quite happy with the 300kHz cutoff used by the 
> public telephone system (a total of 3.1kHz between 300 and 3.4kHz).  The 
> critical frequency for speech communications is the lowest formant 
> frequency; it is the resonances in the vocal cavity that convey speech, 
> not the fundamental.
>
> The lowest formant in normal speech (formant 1 for "u") spans about 200 to 
> 400Hz, so 250Hz probably is a reasonable compromise; it will include the 
> peak and only slightly distort the lower edge.
>
>
>> modes in SSB mode (e.g., PSK31) may be used to establish the lower 
>> response threshold.
>
> When you use digital modes with the receiver configured for SSB, you are 
> really using the audio path as a final IF of about 1kHz.  As long as the 
> audio passband is flat across the actual digital signal, it doesn't matter 
> what its centre frequency (final final IF frequency) is.
>
> That's basically the strategy used by telephone modems (except 56kbs ones, 
> which play tricks with the digitisation of the signal).  The faster ones 
> use a carrier of about 1800Hz (nominal channel centre is 1850 Hz).
>
>
> -- 
> David Woolley
.elecraft.com