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Gang,

Please understand, my intention to transmit what has been refereed to as wideband audio is not to be HiFi, but to be something better than typical "telephone quality" audio.? I do not feel that I'm a part of the "government" where I can simply repeal the Laws of Physics that don't suit my needs - although I've tried.

For occupied bandwidth measurements I use an Anritsu MS2721B spectrum analyzer.? I set the span to 100 kHz, the RBW (Resolution Bandwidth) to 100 Hz, and VBW (Video Bandwidth) to 30 kHz, and also use peak store.? With the repeater in use that has a mix of fully quieting and noisy signals I let the analyzer sweep for 10 minutes or longer.? It's best if I make these measurements at the sample out of the transmitter, but I can get a fairly good representation of reality from home.

W6MEP runs 4 to 4.2 kHz peak deviation.? In the next few days I will post some spectrographs of the W6MEP repeater's occupied bandwidth.? In the meantime, I invite anyone in the range of the repeater to make occupied bandwidth measurements and show me where we are exceeding the limits imposed by good engineering, TASMA or any other coordinating group.? We do have a system that operates with its output 15 kHz below us and due to a deviation misadjustment on my part they were justifiably complaining about some interference.? Since I corrected the deviation they have not complained.

So, what am I using for processing?? I'm using an ORBAN 8100A, an FM broadcast processor.? A few people have gotten up in arms over this, so yes, this is a stereo processor with a stereo generator in it.? I have the stereo generator disabled which kills the stereo pilot and L-R sub channel.? It provides the following: Audio AGC, peak limiting, a protection peak limiter,(not clipping) and pre-emphasis.? Granted, the pre-emphasis is 75 usec, not the typical communications whatever pre-emphasis.? It's essentially flat from 50 Hz to 1 kHz and then begins a 6dB/Octave pre-emphasis boost above 1 kHz.? Between the processor output and the Direct FM Varactor modulator of the transmitter I have a 5 kHz low pass filter that's pretty steep.? Here's an important factor to be aware of: the limiter and protection limiter are "pre-emphasis aware."? This means that it anticipates pre-emphasized modulating frequency verses deviation and, for lack of a better way to put it, it keeps the deviation from climbing at higher frequencies.? On noisy signals, squelch tails, or just plain blowing squelch, it holds the deviation to set parameters.

The audio AGC has a fairly fast attack, which is adjustable, and a fairly slow release, which is also adjustable. What the AGC doesn't catch on attack, the limiter portion will catch. It's also gated, which means that when a quieting station stops talking, the AGC release stops and remains where it stopped until audio returns. A lot of us Hams speak right up in one sentence and then mumble in the next. Set up this way you don't hear it pumping up audio between words, sentences or other pauses in speech.? On noisy signals it's barely noticeable.

One of the things about having the 75 usec preempahsis is that it allows the low end to be there.? It's not pushed or boosted, but it's not lost. The W6MEP repeater sounds clean and what comes in is what comes out, with the exception that the audio level is pretty consistent, even when screaming or mumbling in the mic.

There are some that disagree, telling me I'm bringing up the noise when the audio expands.? Well, yes, but to hear someone that's only deviation a half kHz into a repeater, you have to turn up the audio if you want to hear them, and in doing so you're turning up not only any noise coming in to the repeater, but any noise between you and the repeater - Is that any better?? I get a lot of compliments on the fact that the W6MEP is easy to listen to; users don't have to turn the audio up and down, and it's clean sounding.

I hope the above epistle gives a little insight into how the W6MEP repeater audio is controlled.

There will be more.

Burt, K6OQK


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