Audio Processing for AM

Discussion in 'Amplitude Modulation' started by WA3VJB, Aug 30, 2016.

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  1. WA3VJB

    WA3VJB Ham Member QRZ Page

    The Summer static crashes remind me of the importance of putting across a fully modulated AM signal so people have the least amount of trouble hearing my station.

    I'd like to initiate a discussion on what sort of measures you use in your AM station to improve readability.

    In my opinion, Hammy-Hambone type speech processors intended for SSB make a mess out of the potentially high-quality audio available on AM, where a full carrier delivers significant information so a listener can grasp the pace and punctuation of the person making a transmission.

    Typical SSB type processing involves levels of distortion and unflattering time constants that make them unpleasant to listen to. In some cases, it sounds almost as bad as the hysterical, screaming contestors on SSB. I heard two such AM stations on the AWA's gathering Sunday night on 3837Kc.

    There's a transition underway in the AM broadcast industry, toward "digital" audio processing. That means a lot of nice, older, analog processors are coming available, and are ideal for the hobbyist's spoken word.

    Late Monday I heard from Jack Williams, the retired CEO and Founder of a company called Pacific Recorders & Engineering. He was responding to my quest for information about a pair of PR&E audio processors that have been quite effective with my Collins 300-G on 40 meters AM.

    He confirmed that the AM MultiMax and AM MultiLimiter were developed in the early 1970s as an improvement over the dominant processors for AM stations back then, the CBS "Volumax" and "Audimax." The pair I have were almost buried underground where they sat in a rack when a station lost its lease to a former telephone company switching vault. An office complex now stands on the onetime transmitter site.

    Williams told me a lot more history surrounding his products that I'll share later. This is an article that I hope will motivate others to look around for such equipment and consider including in your "air chain."

    Over the years, I've experienced quite a range of audio processors intended for AM, and I highly recommend nearly any of them as an improvement over uncompressed audio many AM stations on our bands seem to transmit.

    My first experience was a box known as a Modulimiter, the UREI BL-40. This was an early 1970s box that featured a circuit that automatically switched input phase to maximize an asymmetrical waveform and improve apparent loudness. In practice, on a complex waveform like music played on a kilowatt daytime AM station where I worked, the switching included a "pop" in each direction that was more distracting than enhancing. Fortunately the feature could be disabled, and the rest of the processor did well making our station's signal audible out to the fringe of its coverage area on 900Kc AM.

    With the help of a number of guys in the AM Community, back in 1976-77 I completed a homebrew transmitter that someone had started in 1953-54, as best as I could tell. He had passed away and friends were settling his estate, which included an RF chassis and a Modulator deck, with components and a need only for a B+ supply and the point to point wiring to make it all work.

    That transmitter became my first beneficiary of a broadcast AM processor known as a Harris AM Limiter. It's a solid-state device, apparently it didn't sell very well, because I've never seen nor heard of another one, but it sure did a nice job on voice "programming" that I was delivering from a pair of 810s modulating a pair of 4-250s.

    The distinctive advantage of using the Harris was my ability to hit the input harder without hitting baseline and overmodulating. Simple compression of the dynamic range. It was fairly slow-acting, making it possible for a peak to get through and overshoot, but it was a big improvement in loudness over not running any processing at all.

    Williams, the retired executive with PR&E, said his company sold several thousand examples of the AM MultiMax and MultiLimiter to AM stations across the country. The "multi" in MultiMax refers to a multi-band compression scheme that takes bass, mid-range, and treble and processes them separately to great effect. The ability to handle the differing energy levels in several segments of audio spectrum make the processed, complete signal better than monoband processors like my earlier Harris. For example, low frequency audio can be so heavy that something as simple as a breath pop can cause the processor to drag down the midrange and treble response. The companion box is a fast-acting peak limiter, that takes the more gently handled output of the MultiMax, and further refines it against overmodulation.

    Circuit Research Laboratories, or CRL, also made audio processors for AM broadcast stations and used a similar multi-band compression scheme. They made a wide variety of boxes that were intended to be used as a system. Until recently, they were my "loudest" set of processing, used for what has sarcastically been called "full wartime conditions" on 75 meters AM. The CRLs offer very tight control against peak overshoots, and can be configured for very dense, very compressed full spectrum audio.

    Mike Dorrough, KO6NM, made two models of audio processors popular with AM broadcast stations. The Dorrough Discriminate Audio Processor, DAP-310 and later, the DAP-610, used a refined approach based on perceived musical loudness as to how the audio spectrum was parceled out, processed, and recombined. I have a DAP-610 that provides a gentle, but effective means to improve loudness on voice programming. Recommended if you don't feel the need for maximum audio, or wish for your listeners to avoid "earbleed" from the fatigue of audio that is constantly loud. Good control of peaks to avoid overmodulation.

    Bob Orban pioneered some audio processing designs that many broadcast engineers consider the pinnacle of the category, as analog equipment transitioned toward digital audio control and processing. There were several models of the "Optimod AM" starting in the 1980s, including the very popular 9100. Orban purchased CRL, and enjoyed a shared good impression among AM broadcast stations seeking loud, clean audio. I recently acquired a variation of the 9100 known as the Optimod-HF, designed for the shortwave broadcaster with processing intended to minimize problems from selective fading and receivers that failed to include adequate bandwidth for more traditional styles of audio.

    There are other audio processors out there that I don't know much about. The Gates Sta-Level is rather famous, as is the Gates Levil-Devil, the RCA BA-6A, and the Collins 26W.

    In sum, the variety of broadcast-quality audio processors can help the intelligibility of voice transmissions on AM. Your listeners should appreciate that you've taken the time to give them an easier time hearing you through conditions that aren't always the best.

    When you see such boxes on the market, I recommend consideration.
    Last edited: Aug 30, 2016
    KA4KOE, W2VW and KD2ACO like this.
  2. KA4KOE

    KA4KOE Ham Member QRZ Page

    Some of these boxes are hard to find and expensive when found. I settled on a used Inovonics 222 and Symetrix 528E combo. Stay away from Ebay if at all possible. I found reasonably priced used examples via

  3. WA3VJB

    WA3VJB Ham Member QRZ Page

    Attached Files:

  4. N2DTS

    N2DTS Ham Member QRZ Page

    Paul, its kind of crazy for you to ask about input, I think you set the standard on the air.
    There is no place t go but down from your audio.

    That said, I use cheap crap and have a poor radio voice so I have an excuse for sounding poor, and nothing is going to help anyway.
    If I can be understood its a big win.

    As of now, its a very old cheap radio shack dynamic microphone, a vx-2000 (crap box), some cheap ebay EQ, and a crl thing I found cheap on the web.
    The CRL does seem to increase the density.
    WA3VJB likes this.
  5. N2DTS

    N2DTS Ham Member QRZ Page

  6. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Coming from the broadcast universe (I was chief engineer of a 50KW directional AM station for 25 years) the most important thing you can do for voice is to set the audio phasing so that the largest peaks are POSITIVE. This allows you a large increase in peak power with NO compression. The typical male voice has between a 4:1 and 6:1 asymmetry. Of course, your modulation and RF chain needs enough headroom to handle the positive peaks with no distortion.

    The Harris Solid Statesman limiter, and a lot of copycats, did the phase flipping automatically, and if properly adjusted, inaudibly.
    WA3VJB likes this.
  7. KA4KOE

    KA4KOE Ham Member QRZ Page

    I've been told more than once that my Valiant (MJ2V) is the "best I have heard" on air. Of course, I've done most of Timmy's recommended mods short of changing out iron or different muddulator tubes. I use an EV320 with the above and am very happy with it.
  8. K5UJ

    K5UJ Ham Member QRZ Page

    Great thread; thanks Paul. The three legs on the AM stool: Carrier power, Audio processing, and antenna. A strapping signal needs all three. The hardest to achieve for most of us is the antenna. Few are blessed with the resources to have a dipole up at 100 feet or more on 75 m. But that doesn't mean everyone can slack off. You make the most of what you have. That might be a dipole at 40 or 50 feet but you make everything else as efficient as possible in the antenna system. But audio--clear clean compressed audio with a high average and strong carrier is better for shortwave (in my opinion) than less carrier power and super high positive peaks. No time to get into the reasons.

    Less is more (sort of what Bob Orban advises but I'm paraphrasing). With vintage gear, a crystal mic with a voice frequency response and an AM peak limiter driven into 6 to 12 dB gain reduction is great, again my opinion (maybe because that's what I do on 75 m. hi). If you can't get the wire sky high then process and put more fire in the wire.

    I run a lot of processing with the plastic radio and leenyar on 160 m. mic preamp to eq to an Orban 422 set up for pre-processing, to a CRL SEP 400B and a PMC-450. It verges on sounding nasty but that is a tough band to be heard on.

    yeah the boxes are not cheap unless you luck out. I once snagged a PMC400 at a hamfest for 20 bucks because the seller didn't know what it was. It had been sitting out in the wx at an Iowa bc station and needed cleaning up but then it worked FB and I sent it off to Derb, RIP. Lou W9LRS threw in a CRL pre-processor too. But often you have to pay more. But keep a look out. The old CRLs are pretty good. Don't forget Pixar or is it Texar? Some may need some work. Would love to try a 9000 or 9100 but they're still out of my price league. Inovonics has some interesting old boxes like the MAP. There's also Gregg Labs but they are seldom seen.

    WA3VJB likes this.
  9. W2VW

    W2VW XML Subscriber QRZ Page

    There are a few people who sound like Mr. Meek in person yet have plenty of voice quality on the air.

    Ditch that VX-2000 is you use the compressor. It sucks. The rest of the thing is OK.
  10. N2DTS

    N2DTS Ham Member QRZ Page

    I use the vx 2000 as a mic preamp and noise gate (downward expander) and a touch of EQ.
    The compressor is too slow, but I would say its better then nothing in most cases.

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