external DSP audio filters (was Re: [RTTY] AA5AU Contest Notes for CQWW RTTY now on-line)

[Jerry Pixton]

Jeff, et al

Since the JPS NIR-12 filters were not available, last spring I opted for 
the MFJ 784B Audio DSP filters which I put in series from my radios, 
FT1000mp and IC756proII, to the sound cards. In spite of good filtering in 
both radios, the audio filters make a BIG difference. I have the audio 
filters set up with a 500 hz and a 250 hz position as well as a twin peaked 
position. With the hundreds of stations on 20 meters last week end I could 
search in the 250 hz position and generally seperate almost all stations. 
The occassional stations that were sitting almost on top of each other, I 
just put on the bandmap and moved on unless they were a multi. If they were 
50 hz or so apart then the twin peak filter was useful. I would say that 
the audio filters made a big improvement in my station.

Jerry, W6IHG


At 06:25 PM 10/1/2004 -0500, Don Hill AA5AU wrote:
>Jeff et al,
>
>The JPS NIR-12 audio filters give me lots of great advantages to no filters.
>The difference between the audio through the filter and with the filter turned
>off is tremendous.  Audio straight from the radio with no filter, no matter
>which radio I use, is full of "snap, crackle & pop" no matter how quiet the
>band.  I can't stand to listen to that at all anymore.  I refuse to.
>
>With the filter, the audio is much cleaner with no transient noise, not 
>only for
>my ears but also to the demodulators - both MMTTY and the HAL DXP-38.
>
>The NIR-12 (no longer made but I found one on E-Bay) has two outputs - one for
>the headphones which can be adjusted by the volume control on the front of the
>unit and an Audio Out jack with a constant output (independent of the volume
>control) that I split two ways to go to the sound card and the HAL unit.
>
>Furthermore, I take audio from the headphones jack on each NIR-12 and 
>bring them
>to my Dunestar 2 Radio headphone selector.  I can then adjust the audio 
>with the
>volume control on each unit for a comfortable level to each side of the
>headphone (left headphone for left radio, right headphone for right 
>radio).  It
>just seems perfect this way.  I can even switch either radio audio off 
>with the
>Dunestar switch, but rarely do that.
>
>I doubt very much if the DSP of the NIR-12 units has any degrading effect on
>MMTTY.  When callsigns highlight out of nowhere when I can't even hear them, I
>know something must be working.  That happened a lot of 15 meters this past
>weekend.  No noise, but couldn't hear a signal either, but those weak EU
>stations were still highlighting.
>
>I used to use the NIR-10, but the problem with that unit is that it has 
>only two
>outputs - speaker and headphones.  And both outputs are tied to the volume
>control on the filter which is not good.
>
>It's unfortunate JPS has gotten out of the Amateur business.  Although 
>expensive
>in their time, the NIR-12 is a so valuable I cannot think of RTTY contesting
>without them (time to look for a spare on E-Bay again).
>
>73, Don AA5AU
>
>-----Original Message-----
>From: Jeff Stai WK6I [mailto:wk6i at twistedoak.com]
>Sent: Friday, October 01, 2004 2:08 PM
>To: Don Hill AA5AU; rtty at contesting.com
>Subject: external DSP audio filters (was Re: [RTTY] AA5AU Contest Notes 
>for CQWW
>RTTY now on-line)
>
>At 11:30 AM 10/1/2004, Don Hill AA5AU wrote:
> >For anyone interested.
> >
> >http://www.rttycontesting.com/contestnotes/2004_cqww.htm
>
>hi Don - thanks for the writeup! And thanks again for the excellent info 
>on the
>rest of your site - it really helped me get my first SO2R attempt going 
>for this
>contest.
>
>Question: you have talked about using NIR-12 DSP filters in your contest 
>setup.
>What are you really achieving with those filters? And how?
>
>It seems to me that MMTTY is a DSP filter in concert with a RTTY demod, and if
>you put another DSP in front of it, you may end up losing some of MMTTY's
>effectiveness by removing information that MMTTY could use.
>
>(As far as cleaning things up for my headphones - my ears can suffer, as 
>long as
>I am getting the best print...:-)
>
>Or are you using it mainly with your external HAL modem?
>
>Or am I missing something? thanks! - jeff wk6i
>
>
>--
>Jeff Stai               jds at twistedoak.com
>Twisted Oak Winery      http://www.twistedoak.com/
>
>
>
>
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------------------------------------------
Dr. Jerry R. Pixton, PIXOS Designs LLC
http://www.pixos.com/designs/RadioTuner/
jpixton at shentel.net
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