I get why it's a good idea to do this. Here's my question: How do people do this? Do you just listen to two (or more) sources independently and together? Is there a phase checking tool that you keep by the console? Is it a matter of trusting your ears, or are there more quantitative ways of doing this?

I like to give a feel of the room that I record in, and I'd like to have a better handle on the practical side of checking prase.

Any help is, of course, appreciated.

= Justin

Justin from Queens wrote:I get why it's a good idea to do this. Here's my question: How do people do this? Do you just listen to two (or more) sources independently and together? Is there a phase checking tool that you keep by the console? Is it a matter of trusting your ears, or are there more quantitative ways of doing this?

Justin,

Most people rely on their ears for testing phase. You don’t need any equipment other than phase reverse switches on your console/preamp (or, if that's not available, a bunch of cables with pins 2 and 3 flipped at one end). Phase cancellation isn’t an issue until you have at least two input sources active at once-so as you check each microphone, pop the phase rev. switch in and out while still listening to the mics you’ve already checked. Usually, the correct decision is obvious. When the mics are far away, it’s not always so easy. At such distances, you’re unlikely to get drastic cancellation; rather, the tone appears to shift brighter or darker as you flip the phase. Use your ears to determine which setting sounds best (usually the setting with more low end is more acoustically in-phase), or if you’re not satisfied, consider moving the microphone.

There is at least one device I’ve seen which tests “absolute phase.” Someone has to hold a probe in front of the mic in question while someone else holds a receiving probe in front of the speaker. The first probe emits an audible pulse, which travels through the electrical path and out of the control room monitors, where the receiving probe tells you if the signal is in-phase with itself. This assures you that the path between the mic and the speaker is a-ok. When you have a bunch of microphones open, it supposedly helps you determine whether the mic you’re checking is in phase with everything else, which seems like a stupid waste of time to me because we’re not recording pulses, we’re recording instruments with varying dynamics and frequencies. Nevertheless, the hi-fi jazz people swear by it and will spend inordinate amounts of time fiddling with such gizmos. I think it’s a pretty retarded device myself, and using it will guarantee you to look like a tool in front of the studio personnel.

An oscilloscope can also tell you much about phase coherence, in addition to making some neat little squiggly patterns that “dance” to the music.

Take care,

-greasygoose

justin

phase problems happen when a signal is in stereo

you just sum the stereo channels to mono to check phase (pan 'em both to the middle if you need to do it that way)

if the stereo channels are perfectly out of phase, you get no sound in mono

if they are perfectly in phase, you get a perfect composite of left and right

every other situation is someplace between these two

sometimes phase mismatch is complementary in some way--drum overheads are sometimes this way, if they whack out unwanted room boominess or tom overtones or something

but if it sounds 'funny,' you have some phase problems that you might should want to fix

to fix it, you can move one of the mics, or you can try flipping the polarity of one of the channels

'funny' is usually kind of a nasal sound--phase problems kill low frequencies more readily than high ones:

high freqs have short wavelengths, low freqs have long wavelengths. an offset in mic position will more readily result in a 360-degree-or-so phase shift at high freqs than low freqs. and a 360-degree phase shift in two channels at a given frequency means the channels are back in phase at that frequency.

tm

Tim and Goosey Grease.

Thanks both for the input. Summed up well what I'd gathered from different sources and made me feel a little better about how I've muddled through in the past.

It leads me to wonder a bit more - how does the oscilloscope help?

Any additional thoughts from anyone will take at least 25 minutes off time in purgatory. I promise.

= Justin

It's interesting that this came up, because I found a similar error in statements on the TapeOp board yesterday. These are great comments, but I have to take exception with the statement that phase issues only occur with two mics (or audio sources). Phase cancellation is an acoustic phenomenon and can occur with one microphone if, for example, the mic in question is close to a reflective surface in additon to the direct source. A good example of this occurs frequently when using a music stand. A single microphone placed near a music stand can receive the source from the player (ie-a vocalist or horn player) as well as reflections from the stand that can cause phase cancelations. Not to be too nit-picky, but it's something to be aware of.

Chris Garges
Charlotte, NC

cgarges wrote:It's interesting that this came up, because I found a similar error in statements on the TapeOp board yesterday. These are great comments, but I have to take exception with the statement that phase issues only occur with two mics (or audio sources). Phase cancellation is an acoustic phenomenon and can occur with one microphone if, for example, the mic in question is close to a reflective surface in additon to the direct source. A good example of this occurs frequently when using a music stand. A single microphone placed near a music stand can receive the source from the player (ie-a vocalist or horn player) as well as reflections from the stand that can cause phase cancelations. Not to be too nit-picky, but it's something to be aware of.

I don’t think I was in error at all. I was talking about electrical phase cancellation in order to answer Justin’s question about “checking phase,” as that is usually what people are talking about when they use such terminology. You’re describing acoustic phase cancellation, which also fits with what I said in that it requires, in your words, “a reflective surface in additon to the direct source.” This is the acoustic equivalent of my statement that electrical phase cancellation requires “at least two input sources.” I suppose you could “check” for acoustic phase cancellation by considering the reflective surfaces around the microphone, including music stands (draping a towel over them often helps), the floor, walls and ceiling, microphone stands, other microphones, instrument parts (piano lids), gobos, chairs, musicians, etc. I prefer to call that basic microphone technique.

Justin from Queens wrote:It leads me to wonder a bit more - how does the oscilloscope help?

Any additional thoughts from anyone will take at least 25 minutes off time in purgatory. I promise.

Here’s a good starting point:

http://www.egr.msu.edu/classes/ece482/T ... sajous.htm

All in all, though, I'd have to say the neat squiggley lines are still the most useful feature in studio oscilloscope use. It’s also worth mentioning that some consoles and metering devices provide a simplified version of this information (no squiggley lines) in the form of a “phase meter” (a VU or digital meter with a null point in the center and phase + and – at either end).

Try this next time you are doing a vocal overdub with a singer who’s uncomfortable using headphones. Route your mix through a pair of speakers 180 degrees out of phase with one another. Find the point of maximum cancellation, put your microphone there, and let them sing away. You’ll get little bleed from the monitors, and with a little luck, the singer will get a headache and you get to go home early.

-greasygoose

I would certainly agree that this is basic mic technique, but so many people are unaware of even the simplest of acoustic phase issues that I thought it relevant to point out. I was responding primarily to greasygoose's comment that "phase cancellation isn't an issue until you have at least two input sources active at once" and tm's statement that "phase problems happen when a signal is in stereo." Sorry for the confusion, I just didn't want Justin to forget that phase cancellation happens in circumstances other than those mentioned as well. Thanks for the great comments!

Chris Garges
Charlotte, NC

chris makes a good point

i too was concentrating on electrical phase issues

i was simplifying by saying electrical phase problems occur when a signal is 'in stereo'--the real issue (electrically) is: how many different mics are picking up a signal? if it's more than one, phase might be an issue.

similarly, regarding acoustic phase, you can ask: how many paths does a signal have to a given mic?

this whole thing also applies to speakers in reverse: how many speakers are transmitting a given signal? how many paths does the transmitted signal have to the listener's ears?