THE SLIDE RULES
The August Tech's Files for MIX Magazine
©2005 by Eddie Ciletti
This month Ill attempt to correlate the
technical with the artistic using two different show and tell examples
multi-mic'd drums and an ancient analog calculator. I must be crazy
The use of multiple-mics on drums requires
polarity awareness. Sure, polarity can be toggled in the analog domain,
but once in digital form, time can be manipulated from the
sample level (phase) all the way to delays that are larger than life (and
I dont mean Tom DeLay). So, if you've never looked for kick drum leakage
in each mic and attempted to re-time the kit, Ive got some pics and clips
for ya. Geek first, then the tweak reward. (ALT: Ruffage first, then the
Our thought processes vary depending on
the task at hand from the logically-linear time manipulation example
above to the non-linear (if not abstract) world of art. Our ability to
distinguish level changes varies with each person audio geeks being hyper-perceptive
compared to their average consumer counterparts, a.k.a., the sonically
unwashed but from a scientific perspective, the ability to perceive change
is considered to be logarithmic and that's what the Decibel is all about.
Similarly, the Inverse Square Law mathematically describes the way changes
in distance affect gravity, light, sound and electro-magnetic radiation
(EMR). Don't gimme dat look! If you're a guitar player (or know one) then
you've dealt with EMR.
Uh, you guessed it! It's a Slide
We all have a feel for the decibel
(dB) because it relates directly to our perception of changes in Sound
Pressure Level (SPL). At the circuit level, voltage and power changes created
a ratio that gets converted to decibels using logarithms. We take for granted
that the scientific calculator has simplified our pursuit of things mathematical,
but it was not so long ago that the Slide Rule a physically linear device
was an essential part of an engineer's tool kit. What an abacus is to
basic math like multiplication and division a slide rule is to higher
math, such as the aforementioned logarithmic and exponential equations.
You should all know what a "power" is
ten-to-the-second-power or "ten squared" is 10 x 10 = 100, ten
being the base and two being its exponent. Ten raised to
nice, round "natural number" powers (1, 2, 3, etc.), yield 10, 100 and
1000, respectively. An exponent can also be a decimal fraction (a rational
number). So, given only the exponent of ten that exponent being known
as a "common logarithm" it is possible to determine the number using
a calculator, slide rule or logarithmic table.
In our audio world, for example, a signal
raised from one volt to two volts or lowered from two volts to one volt
is, respectively, doubled or halved. Two divided by one equals two; one
divided by two equals 1/2 or .5 (point five). The log of that doubling
or halving yields the same number, but one of opposite numeric polarity
(the log of 2 is .3, the log of .5 is -.3), so that ten raised to the ".3
power" is 2 and ten raised to the "-.3 power" is .5 the log of those
numbers, when multiplied by 20, equals +6dB and -6dB, also respectively.
FIGURE-1: The decibel is a much easier
way to compare two voltages. When comparing two powers, in watts, substitute
a "10" for the "20" in the formula.
BACK TO THE ART
Like it or not, DAWs have heightened our
awareness of the technical. The ability to correlate what is heard to precise
amounts of amplitude, frequency and phase is sweet 2 me, but that bugs
lots of engineers who prefer to fly blind and just listen. The old school
put tape over the VU meters, but here in geek-ville, the SPCM* prefers
a switch to re-scale the meter. (* = Society For The Prevention Of Cruelty
Pre-workstation, there were analog computers
that did all sorts of things, a VU meter being one of the best examples.
Simply by changing its fascia, a meter (which is essentially a motor with
wristwatch-style pivot points) can be used to indicate dB, volts, current,
WRONG SIDE OF THE TRACKS
Aside from its obvious emulation of traditional
sonic tools (mixer, EQ, dynamics, etc.) your workstation is also an oscilloscope,
dB meter and time measurement / manipulation device. Figure-2 shows
three waveforms of kick, overhead and acoustic guitar mics. Relative
to the kick impulse, each waveform is delayed simply by the mic's position
and its relationship with the speed of sound. The overhead mic was positioned
eight to ten feet above and behind the kit, the acoustic guitar was ten
to fifteen feet further behind the kit and surrounded by gobos.
FIGURE-2: The speed of
sound reveals itself in these three waveforms. At top, the initial impact
of a kick drum and below, its leakage is delayed by the time it reaches
the kit overhead mic as well as an acoustic guitar mic.
You all should know by now that
one of my soapbox issues is Low
Frequency Management. The point of this exercise is to help the low
end without additive EQ. Note the polarity of the "kick leakage" into the
overhead and the acoustic guitar all of it conspiring to kill the kick's
low end punch. Of course, the tracks can be left in place, polarity reversal
being the only option in a purely analog world. But once
realigned in the digital domain, the polarity of the acoustic was flipped
so that all kick leakage was in phase. Think of it as time alignment.
Tweaking a fourth track, the acoustic guitar
DI, was much more difficult because its waveforms were so radically different
from the mic. There was no drum leakage in the DI, but once the acoustic
guitar mic was in place and locked in time to the kit, the DI track was
visually ball-parked. I then alternately monitored the acoustic tracks
in stereo and mono, sliding the DI track and flipping its polarity until
the "center" image came into focus and when combined, the pair had the
least amount of comb filtering. This was admittedly a little tedious, but
well worth the effort, especially for a simple guerilla recording using
a student guitar that was less than optimum.
To show how time alignment affects "the
mix," check out the .wav samples in the table below. They
clearly show the before-and-after difference, both flat as well as peak
limited and EQ'd versions (using mix buss processing only; the raw tracks
are always flat). The track was a cover version of For What It's Worth
by Buffalo Springfield. Click
Here for Student mixes.
Raw mix of four tracks
- kick, oh, acoustic guitar (mic and DI) - as originally captured and detailed
Time Aligned (TA) mix with
faders in identical postion to previous mix. Note the extra bottom!
RAW, but normalized to
compete with Track-2
RAW after peak limiting
TA after peak limiting
RAW after peak limiting
TA after peak limiting
versions of Four tracks - kit, overhead, acoustic (mic + DI) - mixed
in mono with identical fader settings and processing to show how Time Alignment
of individual tracks improves bottom end without EQ.
Did anyone notice the distortion on the
overhead track? It wasn't obvious during tracking I didn't notice until
taking the tracks home and zooming in for time alignment but there it
is, the negative excursion is clipped. Fortunately, the threshold of clipping
"worked," a type of overdrive that it is very similar to peak limiting.
zooms out to show the unnaturally even peaks, the result of using a very
powerful mic into an external preamp (at minimum gain) feeding the Digi
002's line input. If you ever have a problem like this, Audio Technica
has an in-line pad, model AT8202 ($49 list, $38 street) that is phantom
Figure-3: This Drum Overhead
track has unnaturally even peaks, the result of the inability to attenuate
an exceptionally hot mic signal before the preamp.
So there you have it, the math
that parallels our hearing perception. I hope the correlation of slide
rule and drum timing was not too much of a stretch. Once the image was
in my head, it would not shake loose. If you do get a chance to time align
the kit keep in mind that you should be looking for phase issues in all
the kit mics anyway let me know if it works for you (or not).
Eddie would like to thank the musicians
in his spring AE282 class and Logan Erickson for the guerilla PT rig.
All of Eddie's mixing and processing was done using Adobe Audition, V1.5.