Re: KX3 to computer connection.

[Elecraft mailing list]

On 05/04/17 17:08, David Deitrick <[hidden email]> wrote:
> I have a KX3, the KX3-PCKT Cable set and a computer that does not have separate mic & headphone jacks, but a single combination jack for both mic & phones. Any hints as to how to connect the mic & phone cables from the KX3 to the computer?
>
> Thanks
>
> Dave
> KB8XG
> [hidden email]
Hi.

Take care with the levels...

The I/Q output from the KX3 is a stereo feed at "Line" level.  (But is
still useful as a mono feed at times, especially if you want the speaker
to be active too, else you'll need to make a "Y" cable from the phones
jack, to serve both the PC's input, and an external speaker or phones.)

The 'phones' output from the KX3 is even higher level, and is also a
stereo feed.

The "Mic" input of a PC is very low level.

So, you'll need at least a simple two resistor attenuator between rig
and PC, so as not to overload the PC, and keep the RX noise level down
(as seen by the PC)   I use a rough value of some 20 to 30dB, depending
what parts I have about.

The source impedance of the KX3 is fairly low (Ohms) and the load
impedance of the PC will be fairly high (kOhms) making the needed
calculations easier.


Similarly, the 'headphone' output from the PC will be a fairly high
level, while the Mic input to the KX3 again is a very low level, so
again you'll need a simple resistive attenuator between the two.
And again, the PC output is a LowZ and the input to the KX3 is a
High(ish)Z, again, simple calculations.
Also turn OFF the Mic Bias option in the KX3 menu.

Just make sure the resistor values used swamp the actual source and load
impedances, this is audio frequency signals, so no need to "match" the
source to the cable to the load etc.

Run the PC's output at a relatively high level, but not into distortion
(test with headphones, so as to swamp any spurious PC related noises.)  

Use the rig's mic gain control to get an ALC level of 4 bars, with the
5th flickering when transmitting, and use the RF Power control to set
the TX power level.  (Use "Data A" mode, so the rig turns off any TX EQ
you may have set for your voice.)

If you find you have to have a very low mic gain, and it's a very coarse
adjustment, then you need more attenuation between the PC and Radio.  
Turning down the PC's output level can help, but then any spurious
noises become significant elsewhere in the audio passband, hence it's
best to run the PC at a high(ish) level, and attenuate it all down to
suit the mic input, but in a way that gives you decent and fine control
of the mic gain.

Likewise, you should see a more or less flat frequency response in your
digimode (or whatever) software when just RX noise is present.

Similarly, if you find you have the PC's mic gain set very low, AND the
Radio's output (volume) set low, you need more attenuation between radio
and PC.

A request please.
Check your PC is not set to make bings and burp etc noises by default
from the internal sound card if that is what you use.    Most external
(USB type) sound cards are "quieter" (noise wise) than most PC internal
cards, and also allow you to keep whatever OS related noises you want on
the internal system, keeping them away from the radio.

It takes a little juggling of levels etc, but you can get a very good
results this way.

73.

Dave G0WBX.


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[Jim Brown-10]

Several thoughts. There's a detailed tutorial on setting audio levels
for digital modes at

http://k9yc.com/USB_Interfaces.pdf

It also includes advice about making suitable attenuators.

I've found adapters on amazon to split a TRRS jack into mic in and line
out connectors. I seem to recall the brand was Star. They're pretty
light weight, and I haven't used them (wanted them for the ready box),
so I have no idea how well they will hold up. But they're cheap enough
to buy two. :)

Based on my testing, that good, low cost Numark listed is a very good
unit for our purposes, and can significantly improve decoding as
compared to internal sound cards.

BTW -- the noise you describe is often caused by failure to bond the
computer to the radio.

73, Jim K9YC

On Wed,4/5/2017 10:02 AM, Dave B via Elecraft wrote:
> Run the PC's output at a relatively high level, but not into distortion
> (test with headphones, so as to swamp any spurious PC related noises.)


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