Hi.
I’m Steve Albini, and today I’m going to talk about time-aligning multiple signals
from a single audio source.
[clunk, clunk.]
A common situation in the studio is that you have a microphone on an amplifier, but also
are recording a direct signal from an instrument: A bass guitar or a keyboard or a synthesizer,
for example.
This causes an arrival time difference between the direct signal and the signal on the microphone
to the recording medium.
One of the reasons for that is there’s an acoustic delay inherent in the microphone
recording the signal from the speaker.
I’ve got a little diagram here that I will show you…
Sound travels very approximately 1 millisecond-per-foot.
That’s a very — an extremely rough approximation, and it varies with humidity, temperature,
altitude, local air pressure, a lot of things —
but, as a “rule of thumb”, approximately 1 millisecond-per-foot.
So, if you have a sound source coming out of a loudspeaker, and there’s any physical
distance between the loudspeaker and the microphone - even a matter of inches - then there will
be an acoustic delay in the arrival of that sound, at the microphone.
If that signal is also being recorded as a direct signal,
then there is an imperceptible - or essentially zero delay for the signal that
is being recorded off of the direct signal.
So, the time difference between those two creates a phase difference in the two signals
if you were to try to combine them.
And, the phase difference can cause cancellation of some frequencies, a thinning out of the
bass sound, and a comb filtering effect which can be unpleasant.
[mumbling] “tell me when I can move…”
OKAY [production beep]
I wanna show you a technique you can use to compensate for that arrival time difference
by delaying the direct signal.
The sound source for this example is going to be this synthesizer.
This synthesizer is going through a direct box, directly to the desk.
It’s also going through an amplifier, and that amplifier has a microphone on it, which
is also plugged into the desk.
The direct signal is getting to the desk from the synthesizer almost immediately.
Not quite “speed of light” but very close to the speed of light. A significant fraction
of the speed of light.
But, that signal is getting to this amplifier, and it has to make its way through a pretty
elaborate system before it finally gets to this microphone.
It has to go through the input circuitry of this amplifier, which has a time constant,
[VOICE CHANGES AS VIDEO RUNS FASTER] …and then the preamplifier has to get the
signal to the power amplifier section, which then has to modulate the power supply,
[VIDEO AND SPEECH SPEED UP MORE] …through the output transformer, which then
needs to couple the primary and secondary windings,
[VIDEO AND SPEECH SPEED UP MORE] …to the output cable which goes to the loudspeaker,
which then gets its voice coil energized, and then that voice magnet drives that speaker
cone inside its magnetic gap,
[VIDEO AND PITCH ARE SO SPED UP THAT SPEECH BECOMES UNINTELIGIBLE]
… and the components of the cone needs to move the air that's in front of it...
and that air pressurizes and the pressure wave…
[UNINTELIGIBLE FAST-FORWARD RAMBLING] [OMINOUS DRONE SWELLS UP]
It’s a wonder it works at all.
Every system is different. Every amplifier is different, and every physical installation is different.
So this cumulative delay of all of these time constants can be anything from
less than a millisecond to several milliseconds.
And if there are effects pedals in-line, those effects can also affect the latency.
The arrival time difference is audible, but I can also visually show you the arrival time
difference on the oscilloscope by tracking the two waveforms against each other.
When I play notes on the synthesizer, you can see them displayed on the oscilloscope.
The bottom trace on the oscilloscope is the direct signal from the synthesizer, and the
top trace on the oscilloscope is the microphone signal on the amplifier.
When I play these notes, you’ll see that the waveforms don’t line up. That is, the
peaks and valleys will be in different locations on the timeline.
[SYNTHESIZER AUDIO PLAYS]
…and in extreme examples, they can be almost completely out of phase with each other.
[SYNTHESIZER AUDIO PLAYS]
Well, that sucked.
So, I’d like to correct the discrepancy between those two signals by inserting a delay
into the direct signal path.
Now, the delays that you’re gonna use for this kind of correction are going to be very
short - sometimes less than 1 millisecond,
and it’s important to have adjustability in the microsecond range.
I recommend using an analog delay, that’s a studio quality analog delay,
or, a digital device that allows for adjustment
in the microsecond range.
Here at Electrical Audio, we have some delays, that were made for us by the Eventide company,
that are adjustable in the single-microsecond range.
So, you’ll need to use a delay of this type, or an analog delay, or what’s called an
“all-pass” delay — “all-pass” filter.
So this is the signal with the delay bypassed.
[SYNTHESIZER AUDIO PLAYS]
I’m gonna engage the delay, and I’m gonna scroll it until the waveforms come into very good alignment.
[SYNTHESIZER AUDIO PLAYS]
You should be able to see the waveforms moving one against the other here.
[SYNTHESIZER AUDIO PLAYS]
I’m now satisfied with that. I feel like the waveforms are all in very good alignment
and the sound quality is very good as well.
So, the total alignment delay that we needed to use to compensate for the acoustic delay
between the microphone and and the loudspeaker and the mechanical system of the loudspeaker
in its compliance ended up being 774 microseconds.
That’s a significant delay, but the precision of the delay allowed me to get the waveforms
into very good alignment.
The audible effect of this is that the low frequencies are better supported because there’s
less phase cancellation, but you should also hear a cleaner, less comb-filtered effect
on the high frequencies as well.
[clunk, clunk.]
Another common scenario is to have two microphones on a single sound source - say, a guitar cabinet.
You might have two different microphones with different sound qualities, or you might wanna
create a stereo image from having — using two microphones that respond slightly differently.
Those two microphones, no matter how carefully you position them, are going to have some
slight difference in arrival time between the loudspeaker and the microphone.
And that arrival time difference can cause phase cancellation,
which can muddy the sound quality.
I have a oscilloscope set up here, in what’s called the “Lissajous” pattern.
[SMACK!]
And the Lissajous pattern is a display that shows you the phase relationship of two signals.
In this case, I have one microphone on the vertical axis, and another one on the horizontal axis.
And this dot will trace a squiggle that shows the phase relationship of the two microphones.
So, let’s go downstairs, and I will set these microphones up on the cabinet.
[WOOSH]
So these are the microphones that I’ve got up on the amplifier.
This is an old RCA ribbon microphone, which was — had its cable replaced by our technician,
Greg Norman, and when he did that, he ensured that it followed the
“Pin 2 Hot” convention of modern American microphones.
That is, a positive air movement in front of that ribbon will create a positive voltage
on Pin 2 of the output connector.
That’s standard for almost all microphones, but there are a few oddball microphones, and
a few very old microphones where that’s not true.
So, it’s always worthwhile to check to see if the microphones you’re using observe
the “Pin 2, Pin 3” convention.
This microphone is a Russian condenser microphone, and I suspect that it’s “Pin 3 Hot”
and not “Pin 2 Hot”, but we’ll confirm that when we listen to it upstairs.
Playing guitar for us is Jon San Paolo, America’s Top-Liked Guy.
He has a fucked-up knee, so he can’t leave. Let’s go see if he can play guitar.
[WOOSH]
Alright, so let’s take a listen to those microphones and see if we have any problems.
“Jon, can you play for me for a moment?”
[GUITAR STRUMMING]
OK.
I can hear that there’s a significant phase difference between the two microphones.
And it’s so extreme, I’m pretty sure one of the microphones is of opposite polarity than
the other.
You can see that this ellipse is quite wide, and it’s actually working in an anti-phase
direction. That is, in the direction opposite of what it would be if it were a mono signal.
So, I’m gonna try flipping the polarity of the microphone that is the culprit and
see if it brings it more into alignment.
Well, that sounds better to me.
And I can see, verified on the oscilloscope, that I have brought the two signals more into
alignment, so my hunch about the polarity of that microphone is correct.
So, from looking at the display, I can’t tell which of the microphones is too close
and which is too far. But by listening to the two microphones in stereo, I can hear
that the image is pulling very slightly toward the side of the condenser microphone.
That means my ears can detect an earlier arrival of that signal.
And that means that the condenser microphone, relative to the ribbon microphone, is too close.
Thanks, Jon.
[STRUMMING STOPS]
Let’s go down and see if we can correct that.
[WOOSH]
I could tell from listening that this microphone is a little bit farther away from the cabinet
than this microphone.
So, I’m gonna move this one in a little bit.
It wasn’t a drastic difference, but it was probably enough that I could measure the difference.
So, I’m gonna use my hands to see if I can estimate how much I need to move the microphone.
And it appears that this microphone needs to move about an inch, inch-and-a-half closer
to the speaker.
Alright, let’s go see what it sounds like.
[WOOSH]
Let’s see how that sounds with that adjustment.
Jon, can you play the guitar for me for a moment?
[GUITAR STRUMMING]
So, that sounds better to me and I can see from the Lissajous display that the ellipse
has narrowed considerably, which means the microphones are more closely in alignment.
There are, however, still some “curly-q’s” that indicate that there is some phase cancellation
going on at certain frequencies.
And when I listen in stereo, it sounds to my ear like the ribbon microphone is now slightly
closer than the condenser microphone.
So I wanna make another small adjustment. I think we can do better than that.
[WOOSH]
I could tell from listening that this microphone needs to be a tiny bit closer…
but not very much.
So, rather than try to move the stand, I’m just gonna slide it in the mic clip
a fraction of an inch.
That should be enough.
Let’s go see.
[WOOSH]
So let’s see what that adjustment sounds like.
Jon, can you play the guitar for me, please.
[GUITAR STRUMMING]
So, you can see here, that the two signals are in very close alignment:
making a very narrow ellipse, but there’s still some difference between the two microphones
that will allow us to hear the stereo image I was describing.
[STRUMMING STOPS]
You should never be afraid to experiment with things like trying multiple microphones.
Diagnostic tools like the oscilloscope and your own ears should guide you in moving and
correcting placement errors.
[VAUDEVILLIAN TRUMPET MUSIC PLAYS]
[HONK]
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