APRIL TECH's FILES
(advance release)
©2005 by Eddie Ciletti
This past winter I taught two classes a week, Analog Recording Techniques and Studio Maintenance. Classes started the week after the Quantegy's "restructuring" announcement, so it was a bit odd to say the least. As a result, we've been hoarding the good "rust" and using recycled stock for class work. The maintenance class started with soldering techniques, basic DC circuits series and parallel resistance, Ohm's Law and the Power formula plus an overview of test equipment.
From these two extremes comes this month's material, including a rant about patch bay wiring conventions and optimizing small spaces with off-the-shelf acoustic panels.
The Kick of Zen and Now
As you may know, I am particularly interested in acoustics and bass perception issues. Optimizing and tweaking the acoustical environment is the fifth critical "ingredient" in that most important list that includes talent, mic choice, placement and mic preamp. The emphasis on low frequency perception is the reason I keep returning to kick drum as the primary example. The goal is for the kick to translate on all speakers large and small and when that doesn't happen, I believe it's due to excessive processing too much low-midrange being sucked out and / or too much 80Hz is being added.
Bass-ically, if you want to hear more bottom from the kick, either use a subwoofer or take advantage of a spectrum analyzer plug-in, especially if you need proof. More than likely, the LF energy you want is already there and needs much less tweaking than you might think.
Click here for for mic and placement options and sonic samples
Click here for 5.1 drum overview -Includes Spectral Response
THEN and NOW
When I started recording, Auratones were the popular near-field monitors. Even then, the geek in me brought unconventional tweaks into the mix, like having a passive subwoofer to augment my customized mini-monitors or using an LA-2A as a mic preamp. What I love about teaching is the ability to integrate the stuff that worked for me then with stuff I know now; it's been a blast.
THEN, recording kick drum to multi-track @ 30IPS was a bit of a crap shoot, so while getting drum sounds, I'd always listen in REPRO to see how hard the tape could be pushed before saturation turned punch into mush. NOW, the Studer A827 and Quantegy 499 "combo" is closer to what "input" sounded like. That machine's dual metering (VU and LEDs) plus the tape's extra dynamic range yields less worry and more predictable results, especially when recording drums in the control room a process that precludes any possibility of nuance monitoring.
I've always preferred drums in an open space where they can breathe rather than confining them to a booth, in a corner or near a wall where low-end "reflections" can muck things up. Most drum booths should have a good deal of hidden space filled with bass traps, but when that is not the case, an easy fix is detailed below.
One of my favorite spaces to record drums was at Skyline Studios in NYC. They had such a great sounding lounge just outside the studio that I placed the drums at the sound lock all doors open to capture the natural ambience. Studios are smaller these days, so the modified approach has been to locate the drum kit just in front of the ISO-booth. This seems to have many benefits.
The kick drum has a figure-of-eight radiation pattern in free space, but when placed in front of the booth the radiation is now more cardioid. One or two mics in the booth delivers a dry, well-balanced kit mix with plenty of low-end kick support. With three different spaces tested so far, each has useable results.
Acoustic Triangulation
If your booth is acoustically treated, but not all you'd want it to be, perhaps the problem is that the RT60 the "decay" time is uneven across the frequency spectrum. The average booth is similar in size to a "full" bathroom and you know how that sounds "untreated." Yes, there are some things we like about the natural reverb of a tiled room, but think about that one low-mid frequency, in the 200Hz to 250Hz range, that the male voice will typically trigger.
The most common mistake is to over-treat the walls for reflections, but ignore the requirements for effectively treating below 500 Hz. When there is a build-up in the low-mid to low frequency region, the higher density trapping that is required takes up quite a bit more space than "just" placing absorbers on the walls. One of our "classrooms" is a control room with an ISO booth (that needed such treatment), but no studio, the perfect experiment.
Unlike upper midrange and treble, which "reflect" like light, low-mids and bass tend to build up at a room's boundaries especially corners, and not just the obvious, but where floors meet walls and walls meet ceilings. My experience with MiniTraps taught me enough to know what was easily possible, without major modifications. (Thanks Ethan!) See Figure-1.
Since the school already had some "floating" absorber panels, I had students place them as mentioned. The booth is approximately 12-foot by 8-foot space with 8-foot ceilings. Even after two panels, everyone noticed the difference, but we kept going figuring the drums would produce much more energy than the human voice and require the additional absorption. Drum samples are already online as proof both a mix as well as isolated mics.
FIGURE-1: Triangle placement of simple absorbers is effective at trapping bass. Original image courtesy RealTraps. Image modified by ec.
PATCH BAY LOGIC
Questions about Patch Bay wiring, via phone and e-mail, set me off this month, big time. Two issues were raised the semantics of "out-over-in" versus "in-over-out" and then grounding, sheesh! Like the expression "too many cooks," experimenting with grounding can really mess up da pasta sauce.
For years, I thought the way I'd been wiring patch bays was the way it was done the gravity method, where console sources (outputs) are on the top row while destinations (inputs) are on the bottom row. The exception is Outboard Gear! When I first encountered an Outboard Gear bay wired out-over-in, I simply made the assumption that some wire monkeys decided to maintain continuity with the rest of the bay, NEVER bothering to contemplate chicken and egg er, uhm microphone and preamp, source and destination. You're welcome to write in, but can you see why it would make me nuts?
If you start at the very beginning, microphones are the "first" sound source and while not all bays have mic lines IF in the bay, they would be located at the top-most row with preamp inputs directly below, simple enough. Equally logical, the next patch bay group is preamp out (insert #1 send) followed by the insert return. After that, things are literally upside down. I believe that outboard gear should be wired IN over OUT, because in terms of signal flow it is both logical and gravitational. Imagine a horizontal slice across the insert send and return bay, in between which the outboard gear bay is inserted. Does it make sense now? I had to ask my Record Plant mentor, Paul Prestopino, to confirm this for me for fear I was losing my mind. Even though I'm closing in on the big half century this summer, at least those brain cells are still functioning.
While on the subject of Patch Bays, I also want to point out that while there have been many schemes to resolve ground current issues such as flying shields at one end the "problem" has never been with the wiring, it's the gear that's at fault. Eighties era gear often used "plastic-insulated" jacks mounted directly to a Printed Circuit Board (PCB) making the shield connection the Trojan Horse of audio. In this example, the shield dumps noise on the Printed Circuit Board (PCB) ground, to which many high gain amplifiers are referenced. Other noises get in the way too, like Radio and Television interference (RFI / TVI). Not matter what type of connectors are used, this is known as the "pin-1" issue.
While I've mentioned this over and over, the most familiar example of good "pin-1" implementation is Mackie's use of ALL metal quarter-inch jacks directly secured to a metal chassis an electrical "firewall" to noise. Minimizing noise susceptibility is almost that simple and it certainly starts here. My point is not to accommodate bad gear, but to identify, fix, modify, wrap transformers around or chuck equipment that will not cooperate.
Keep in mind that unbalanced audio signals are supposed to be the domain of consumer electronics simple systems where very few components are close together with perhaps three feet of cable between them, max! What goes on behind an audio rack is no place for unbalanced cabling. With just a shield for noise protection and one wire for signal, the gear and its destination have no way to defend against being too close to a wall-wart, inside of which is a hum radiating transformer.
While a "firewall" stops noise from sneaking into the box, it alone doesn't stop noise induced into the signal cables, but the addition of a balanced signal path does (a twisted pair of wires for signal plus a noise shield). There are three types of "balanced" signal configuration transformer balanced (passive), signal balanced (active) and Impedance Balanced.
Two identical signals travelling down a shielded twisted-pair cable, one a mirror image of the other (180 degrees out-of-phase) is easy to visualize. If brought up on a mixer as two independent channels, the signals would cancel. If subtracted by a differential amplifier (that's what balanced inputs are) the signals add up while canceling the noise common to both wires. The relationship of amplifying the audio while rejecting noise is known as the Common Mode Rejection Ratio or CMRR.
Impedance Balanced is a little more squirrelly to grasp because both signal wires have the same source impedance (stay with me now), but only one of the wires carries a modulated signal. For example, let's say the signal-modulated wire is being driven by an output amplifier with a 47-ohm impedance. (It's a "source," get it?) By tying the un-modulated wire to ground through a 47-ohm resistor, hum will be equally induced into both "signal" wires so that the destination a balanced input / differential amplifier can do its job of canceling the noise common to both wires.
I've always wired unbalanced gear with balanced cabling and this does reduce induced noise by an obvious amount, albeit not as much with a true impedance balanced circuit. If you knew the output impedance of an unbalanced device, the trick in the paragraph above could improve things considerably. OR, using a transformer at the destination would be the easier fix.
Eddie has always been thankful that his technical background has always kept his bag of tricks full and a bit ahead of the curve, but he sometimes regrets not getting Psych and MBA degrees