The AM Forum

The discussion on peak limiting (parallel thread) revived some ideas ...

The Western Electric research done in the 1930's on telephone toll line audio intelligbility (sp?) demonstrated that a flat transmit audio response is not optimum under noisy conditions.  Amateur radio and military practice has demonstrated the veracity of this statement.  It seems that quickly tailorable audio would have a good application here.

I had a discussion with Jack, WB8BFS ( yup - radio big brother ) about doing a two knob quick setup and he supported the notion.  so here goes ...

a coupled multiple pole low pass - high pass switchable corner frequency ganged filter....this forms a band pass filter with easily changed roll off frequencies... suggest 3, 6, 9 and 15 kHz corner frequencies for the low pass function ... and 20, 50, 100, and 200 Hz corner frequencies for the high pass function ... in addition a 6 db/ oct high pass section with corner freq at 1 kHz could be switched in separately giving a 'rising high end' response similar to crystal mics commonly used ... the trick to all this is minding phase shifts

there are many ways to do this and I have thought of a few ... I am not familiar with new technology of the last 10 years or so .... are switched capacitor filters similar to National MF 10 still available?
what else is out there ... maybe this yet another job for the computer sound card ?

Wouldn't an EQ be easier?

good question ... has someone already done this ? ... if you are speaking of graphic eq, these are too imprecise, too hard to change quickly, can have excessive phase shift, IMHO ....73 ...John

I'll agree with John. A few slider changes to roll off the bass and the normal rise 2kc to about 7kc will get the audio where you need it for the noisy or crowded conditions.
And the SDR stuff is so much fun to play with. Variable bandwidth, EQ adjustments, etc for transmit audio or receive. It's like having 2 or 3 premium radios at your finger tips.
It never hurts, though, to roll your own. We probably have gotten out of the habit of designing things and building. It's all done by processors or software. There's an art-form in either approach. The ability to engineer an effective signal on the air and to be able to hear stations when MaNature is crapping on us.

Fred

A 1/3 octave eq is more than precise enough for voice work, especially for amateur radio applications.

You should never run your audio flat on the radio. A rising response starting at 800 Hz or so with at least 6 dB per octave is needed, even under good conditions. If you have that in the mix, the only thing needed under poor conditions is to roll off the low end (different for each voice but probably starting at around 200 Hz).

Don't let anyone talk you into making audio far more complicated than needed. It's not required.

Hi:

Simple op-amp filters will work fine here and are easy to do. For a 6db/octave presence boost (highly desireable in any event) a simple resistor and capacitor in parallel will do this.

Maxim makes a set of switched capacitor filters that are updates to the MF10. I used the MAX 294 for an elyptical LPF.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1443

This series of IC's has a slightly limited SNR. I use mine after audio processing and it is not an issue.

As stated above, a consumer equalizer will allow for easy experimenting. Set one channel up for good condition/fi-fi and the other channel for communications quality, then switch between the two.

Good luck!

73,
Dan
W1DAN

You can pull this off with a simple, 1 knob "Tone" control, like a slightly modified tone circuit from a "Big Muff" guitar fuzz pedal from the 60's/70's era..

(http://riley-music.com/BowsStuff/BroadcastAudio/AudioTone.JPG)

Although it is passive, you might need a little more gain to get your audio level back

I used an Altec tube type multi-channel mixer board while my main station was in transit between the upstairs room in the house and the shack.  It is a high quality unit with top-hole audio transformers throughout.  In addition to separate step attenuators for each channel and a master output control, it has treble and bass controls.  I could achieve the  desired effect by turning up the treble control, but I had to keep the bass control flat because any attempt to boost the bass resulted in low frequency phase shifts.  I could set the phase so that with normal voice the asymmetrical peaks with the greater amplitude went in the positive modulation direction, but at bass frequencies the peaks would go in the opposite direction.