[Lowfer] Audio limiting using diodes, and capacitor

[Mike.WE0H]

Wonder how that would work with WSPR decoding? I tried using DSP while 
receiving WSPR signals and it caused the software to stop decoding even 
though the signals were easier to hear. Don't know...I think Dave /17 is 
transmitting WSPR on 600 meters. See if you can get decodes with the 
noise reduction.

Mike
WE0H




Andy - KU4XR wrote:
> Hi all:
> 
> As many of you probably have done; I have been reading the thread
> on the RSGB LF reflector where Jim - M0BMU has been using diodes,
> back to back, across the audio line to the sound card. The purpose
> being; to reduce the noise impulse action from lightening.
> OK; curiousity got to me, so I had to try it out myself and see 
> if there is any benefit. The finding here is; YES. And surprisingly
> so. An entire evening in a nutshell; I found that Germanium diodes
> worked better than Silicon diodes. I found that there is a " sweet -
> spot " on the volume adjustment where the distortion to the audio
> is minimal, but the reduction in impulse noise in quite notable.
> Too much audio will reintroduce the impulse noise but in a wideband
> clipped form, and will create all kinds of crud on the audio, and
> the waterfall display. I found the best place to insert the diodes
> was before the active audio filter. I obtained additional reduction
> in noise by placing a .33 µF capacitor, and a variable resistor,
> going to ground from the output side of the active audio filter.
> The resistor is used like a tone control to reduce high frequency
> response on the audio line. This works to offset what the diodes do
> to the audio. The noise becomes wideband, and shifts a great deal
> of audio impulse noise energy to higher frequencys. The " Tone "
> control will roll off the high frequency noise. With the right 
> settings of volume, tone resistor, and tone capacitor value, 
> I was able to almost eliminate 20 dB over S-9 lightening crashes.
> I could see them on the S-meter, but just barely hear them on the
> audio.
> I forgot to mention that I was using the carrier of DCF-39 to see
> any changes on the waterfall of Spectran, plus it is audible here,
> and I could " hear " the reduction in lightening noise, and any
> changes to the desired audio. There was one setting that would give
> maximum noise reduction, and best Signal to Noise ratio on the
> signal I wanted to hear thru the noise.
> I have a 60 second audio clip of the results. The first 20 seconds
> is un-filtered audio. There is a 1500 cycle tone in the background,
> that is DCF-39 set for a 1500 cycle offset. The next 20 seconds,
> ( 20 to 40 ); is with the diode clipper, and capacitor tone control
> optimized for 1500 cycles. Then the final 20 seconds, ( 40 to 60 ),
> is the filtered audio, and the radios Noise Blanker turned ON.
> 
> See if you can hear any difference in the static crashes, and the
> received signal. And imagine this to be a WSPR signal:::
> 
> Hear the audio clip at :    http://webpages.charter.net/ku4xr/
> 
> Click on the Diode & Capacitor noise reduction MP3 file.
> The recording was made with a 2.4 KHz Bandwidth for plenty of noise.
> 
> 
> 73; and have a great weekend everyone.
> 
> 
> Andy - KU4XR - EM75xr - Friendsville, TN. USA
> LOWfer Beacon " XR " @ 185.29875 KHz ( QRSS-60 )
> Coordinates:  N:  35º 43' 54" - W:  84º 3' 16"
> http://www.myspace.com/beaconxr
> http://webpages.charter.net/ku4xr/