Where Acoustic & Analog Still Rule the Roost

Having sat in on more than two dozen recording sessions during the mid-60s at the old Capitol Records' studios at 151 West 46th Street in New York City, a frequent destination back then for Manhattan's jazz musicians after their late night club gigs, I am probably among the few still around to have seen and recall, albeit vaguely, their echo chamber. I don't have a photographic memory of it, probably because the setup was shown to me in passing by recording engineer John Chiuchiolo (credited with the famous “doo-lang” opening chorus in “He's So Fine” from The Chiffons' early 60s rock 'n roll hit). But I remember its lasting impact on me when it came to respecting acoustical/mechanical versus all-electronics designs and, in line with that, my reverence for simple analog versus digital designs in general.

The echo chamber appeared to be a simple rectangular closet space and not much more. And perhaps not even half the distance from home plate to the pitcher's mound. Nor do I remember seeing any kind of special sound-reflecting materials in it, or tunnels leading anywhere. So it didn't appear near as long or large or complicated as the subterranean concrete bunkers built 30 feet underground that I envisioned at Capitol's Los Angeles, Calif., studios. The echo chamber I saw in NYC butted up against the head of the audio-mixer console in the control room (in which studio I just can't recall) and looked as if it was fitted with several pickups to capture the reverberation effect.

The discussion that day wasn't so much about echo chamber designs. It was more generally about the quality of sound in acoustic-based versus electronics-based sound processing, notwithstanding musicians who used various fuzzbox systems to intentionally create distortion. John's opinion was that the echo effect was cleaner and truer coming from a system that had its roots in the instruments' own space-acoustics.

I respected John as a first-class audio and RF technician and as a musician. John demonstrated the acoustics-versus-electronics difference. For the particular example he chose that day (a playback of one of his own guitar instrumentals), he proved his point to me. I'm sure it wasn't the first time I recognized the link between analog and mechanical, and how such systems could do the job as well as pure electrical networks. But it might have been the first time that I could also see acoustic and analog solutions might be less messy. And this was before digital techniques transformed into much more than “a special case of analog.” It's rather taught the other way around today.

Not only could the analog solutions be less messy, sometimes they could be better. I inherently recognized the link between digital and unwanted switching transients, and what you had to do to get rid of those. Sure, we all saw the future of digital computing and super-speed operations in a world of future shock. I just didn't see, and still don't see, a need for high technology everywhere.

A few early examples based in amateur radio set that tone for me. At or near the top of the heap in their day, Collins Radio Company established one early standard for tight IF-stage filtering at 455 kHz using a mechanical-electrical transducer design in their communications receivers. These so-called mechanical filters could provide a bandpass from a few hundred hertz to a few kilohertz.

Somewhat related were the crystal filters. They were considered more of an electrical solution, but if you examine the filtering mechanism of one you'll see that they have a mechanical basis with similar filter characteristics. Either filter is superior to traditional transformer-coupled electric filters.

The transformer-based filters have only so much capability (at both large bandwidths and small) and, pound-for-pound, probably wouldn't be able to provide anywhere near the same bandwidth characteristics as either the mechanical-electrical or crystal filters mentioned above — unless you're considering an IF stage at an impracticably low frequency. In any case, DSP-based systems are in vogue today and touted for providing much tighter bandwidths with much less in the way of side effects when properly designed.

But there's the rub. I haven't seen that today's digital-based designs, be they RF or audio, have helped new hams develop into better radio operators. (It's actually ruined some old-timers who once were competent.) Quite the opposite: I've noticed they rarely can tackle a basic task any more — discriminating between three or four CW (Morse code) signals in a 2 kHz bandwidth.

That's because many came into the hobby leaning heavily on 200 Hz filters from the get-go and, without the mentoring system that widely existed in decades past, never learned to use their natural ear-brain filter to separate a desired signal from the rest. It's another case where high technology hasn't benefited them — because hearing and lining up three or four stations at a time for a quick contact is often what you need to do (especially in ham radio contests), much as a chess master needs to plan four or five moves ahead.

When it comes to the classic analog versus digital battle, though, one solution stands out above all. It came to the rescue for James Lamb's noise silencer, which was one of the great ideas for virtually eliminating impulse-type noise accompanying incoming radio signals. It was developed in 1936 using tube technology. Lamb's circuit uses offending noise pulses to quickly switch off the amplifiers in the receive chain, thus blanking the noise while letting the signal pass through.

This worked much better than peak-limiting the noise to about the level of the signal, as previous circuits did. One existing drawback in the noise silencer, certainly apparent in my 1970s era equipment (and even some modern gear) is that the switching nature of the circuit can create spurious products in the presence of strong RF signals 10 or 20 kHz away from the frequency you're tuned to. That can result in receiver blocking.

Maybe improved RF front-ends and DSP methods help address that issue today, although I've seen several $3k rigs in overload, too. But how about more of an analog solution for the problem back about 1970? A few at Bell Labs had some success, starting with a circuit I showed them in the Bellboy pager that had an indirect connection to the blanker issue.

The Bellboy receiver had a network somewhere in its control circuitry that performed waveform differentiation — versus traditional switching. I wasn't particularly a standout in analog design then (I'm still not). But through extending the idea of RC/LC differentiation, one of the more astute hams in the department made good progress on what was, for lack of a better description, the equivalent of a softly-switched yet sufficiently fast blanker circuit.

It incorporated a few not-so-clear leaps in logic that I had problems understanding. He wasn't short on guts, either — he ripped out much of the existing noise-blanker circuitry in his brand new Drake receiver, and showed how his modified design worked better on most types of noise.

It was the first piece of great analog design I'd seen. A few years later, I alerted Doug DeMaw, the editor of QST magazine, to what had been done. Doug was an excellent analog designer in his own right and I could see his mind formulate a design as I explained it to him. In the end, though, I never saw anything about it in any of the literature. Maybe it's now my time to reinvent a wheel no one knows already exists.

Have you worked on any of these noise-blanker type circuits? Have you worked with any “old” design techniques (tube or solid-state) that are ripe for reinvention and reuse?

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5 comments on “Where Acoustic & Analog Still Rule the Roost

  1. eafpres
    September 20, 2013

    @Vincent–did you ever encounter any waveguide filters in any of these applications?  They have been around for a long time, but aren't seen much in all the miniaturized electronics these days.  I think the idea of waveguide filters is really cool.  I visualize EM waves, traveling in multiple modes, interacting in resonant cavities and elegantly filtering out noise with no passives or actives at all.  You can't get much more mechanical/analog than that.

  2. vbiancomano
    September 20, 2013

    @eafpres—Curiously enough, I believe there was a connection between the waveguide filter and the mechanical filters that were developed at Collins Radio. Although I have no knowledge of the interplay, beyond a term common to both subjects, the “resonator.”   

  3. Netcrawl
    September 20, 2013

    It's really a cool stuff, waveguide filter is not hard to build ( in fact we can build it in our own garage) and results are predictable, I believe there's a strong to design a really high performance filter, with multiple sections.

    A feature of waveguide filter design concerns the mode of transmission, waveguide filters have much more similar with transmission line filters than lumped element filters; they do not contain any discrete capacitors or inductors.  


  4. samicksha
    September 21, 2013

    You are right @netcrawl: Waveguide filters seems more in common with transmission line filters, they do not contain any discrete capacitors or inductors. The design of waveguide filters frequently starts from a lumped element design and then converts the elements of that design into waveguide components.

  5. Navelpluis
    September 26, 2013

    I remember myself as a 14 year old. My neighbour played guitar and he had some old strings left. I decided to drilli a couple of holes in my (new) worktable to be able to get the strings straight with some pieces of metal to support them. An old relay coil was my pickup coil. The Stones “Tatoo You” was my source of 'sliding guitar' inspiration.

    I guess the combination of the mechanical, home build amplifier and the enjoyment of trying to play some tunes was the source of getting 'the nack', just like Dilbert 😉

    He came by and asked me how I created my “Damn Fat Fuzzy Sound”, well, that was just bad design from my first amplifier trials. At that time I could not put a finger on it why it was so nice. But he found the sound was great!

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