The AM Forum

http://www.idiompress.com/scaf-1-am-xmit.html

Saw the above today and though others would like to read
about this filter.

Chuck

Thats a good idea. Way to many AM stations out there that run 20+ KC wide. 


C

To me it just looks like a tone control.

With the lows down at 50 cycles he probably sounds like the rest of the stations that do that and cant be copied in QRM/QRN unless they are loud.

"Repeat your name again OM, my speaker cone was still reverberating and dust is falling from the basement rafters"

Sometimes you need a "like" button!

--Shane
KD6VXI

You obviously never heard Stu.

Yeah, Pete, your comment reminded me of my first experience with  tone controls.  It's funny how we learn stuff.

Way back when, I played with various capacitors in the standard .001uf to .005 /500k ohm tone control added to just a smidgen higher end AM radios to the 50C5's plate line.  Other than hearing the difference, impedance demands of the capacitor and variable resistor combination just didn't dawn on me until I saw the very same circuit in transistor amplifiers but with bigger caps and smaller resistors.  "So, that's how that works !" -Another W3RSW/JN "Eureka"  moment.

So now I know much more about multi-pole filters, exponential roll-off, simple lead and lag networks and all kinds of fancy explanations for what I heard as a kid with a simple capacitor change. 

Unfortunately, also now I can turn the treble control all the way and still sounds the same....   ;D

The SCAF is a multi-pole lowpass filter. No tone control will give this sort of response nor will any tone control provide variable corner frequencies.

5th or 8th order butterworth or bessel switched capacitor filters. The MAX74xx series of IC. They also offer evaluation boards, so you may not have to do much soldering.

http://www.maximintegrated.com/datasheet/index.mvp/id/1894/t/al

I believe this circuit or one like it, as well as the SCAF-1, were discussed here before, maybe in more detail.

http://amfone.net/Amforum/index.php?topic=30547.0
http://amfone.net/Amforum/index.php?topic=7904.0
http://amfone.net/Amforum/index.php?topic=29015.0

And could the ICs be seriesed for a sharper cutoff?

That would be a wrong assumption, OTOH he was strong and not limiting highs as suggested in that article.

He used the filter and I heard him experimenting with it. No highs were lost and there was no booming bass. The reality is that most of this "lost highs" stuff is pure mythology. Myself and others have used various lowpass filters on the air for years (well over a decade in my case) without saying anything about it. Few (1), if any, ever noticed when the filter was in or out.

1. Few on channel. Those off channel noticed much less interference.  ;)

So you are saying that studies showing that audio out to 6-7KHz or so does improve intelligibility under weak signal conditions and that the lows below 200Hz carry nothing of interest is mythology?

No, I'm saying I've been running a lowpass filter for years and no one noticed. Weak signal conditions are irrelevant. My signal is never weak.  ;)

-certainly not weak but severly distorted on the majority of my receivers due to supermodulation in the audio you do let through your low pass.   ;D

Very strong mid range and driving my poor Collins 75A series distortion generating diode to saturation.  Could you dial it back to say, 180%? heh, heh.

(ok, ...so in what few QSO's I do have with you, but I listen a lot.)

Now on the QS1R your signal is tight.   8)

All I know is that whenever I spend much time in Steve's station, I come home with a sunburn on the side of my face that was closest to the transmitter.  ;D

Last time I checked, Stu was still a member here. I bet he'd be more than happy to elaborate or answer any questions about his article & filter. Anytime I heard or worked him he never had anything like excessive lows. In fact, I'd say his lows were lacking a bit, but I've never met him in person. If he doesn't have a lot of lows in his voice, he probably sounded just as he is supposed to. To me, nothing is worse than the guys who add excessive low end to make up for the broadcast voice they'll never have.

Chuck:

Based on Stu's work and the Idiom Press unit, I homebrewed an elliptical LPF and set the corner at about 6kc using two MAX294's in series (one would be enough). I used a negative diode clipper to shave off the infrequent HF overshoots that occur due to the extreme phase shift as you reach the corner frequency. Some other more gentle filters tend to slightly muddy the audio as their slope starts much lower in the audio passband than when using an elliptical filter.

http://www.maximintegrated.com/datasheet/index.mvp/id/1443?utm_expid=50713806-2&utm_referrer=http%3A%2F%2Famfone.net%2FAmforum%2Findex.php%3Ftopic%3D29015.0

You could easily buy a SCAF, then change out the chip to the MAX294.

73,
Dan
W1DAN

Hi Dan,

Thank you for your post. 

Would you approach provide a steeper and variable cutoff frequency approach like the original design?

73,
Chuck

I believe I have found the right thread I wanted, to post this article on audio bandwidth versus intelligibility.  When I read this topic many days ago I didn't have time to dig out the article I was reminded of.

The attached pdf is by Jeff Rodman CTO of Polycom.

Chuck:

Yes. The details are in the data sheet for the chips, so all you need to do is compare the graphs in them. The flatter response up to the cutoff frequency will cause massive phase shift as you near the cutoff frequency. So either phase correction (not easy) or careful clipping afterwards will retain your HF peak amplitude to a pre-defined level, at the expense of a very small amount of out of band splatter. This is also true with the smoother LPF of the SCAF, but to a lesser degree. I feel I retain a little more of the highs ("esses") in my voice with the elliptical filter. I have never had a complaint of splatter and spectrum tests with an SDR showed a well defined occupied bandwidth.

It is REAL easy to breadboard this. A single IC is fine (mine as SMT), as we really do not need such a deep stop-band. Tuning is done with a single capacitor, so you can have a multi-position switch for different cutoff frequencies.

Tom: This paper is good, and shows the obvious fact that a wider bandwidth is easier to listen to. It seems to be a little of a sell job. Intelligibility tests were conclusively done with Bell labs in the 1920s and 30s.

73,
Dan
W1DAN