Joined: 14 Feb 2007
Posts: 1933
Location: Shah Alam
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i just want to share about "what is phase cancellation" as i always get a question about it once i use the term phase cancellation.
Im no expert and no physic knowledge in so many level... so i try to keep it simple by what i understand through experience and own studies. Other that are well than me should share with us and correct me if im wrong.
| Quote: | Its All About "Phase"
Phase is the time difference between to similar waveforms. When two signals are close in frequency and amplitude (level) but out of time with each other, there is a phase difference. One cycle of sound is considered to have 360 degrees. When a second signal starts a half-cycle later, it is called 180 degrees "out-of-phase" with the first signal. When two signals are 180 degrees out-of-phase, the peaks of one signal are in time with the dips of the second signal and the result is a cancellation of the signals’ energy. Its sort of like someone pushing on a door as another person pulls on the same door - it doesn’t move.
When two signals have their valleys start at the same time and their peaks start at the same time, the two signals are said to be "in-phase" and the energy of the two signals will double when the signals are combined. This is sort of like two people both pushing on the same door - it opens faster and/or further. |
The quote above are the best simple answer i found with a very proper explanation.
to get more clear picture , here are the video demo how phase cancellation happen.
http://www.youtube.com/watch?v=BBOxQZhjxsM
also see this video on how 2 mic on a snare is using (top/bottom) can make a phase cancellation
http://www.youtube.com/watch?v=9jtYkdh_NLw
as we know (or been told), upon recording, "phase cancellation" is highly recommended to be avoid as it will result a very thin sound and also comb filter because of multiple mic interference. Especially miccing a drum which need to consider about the mic type and it placement. Great drum tracks come from a combination of proper mic choices, good mic placement, decent preamps, and, of course, good playing.
| Quote: | MORE DETAIL
Every instrument creates sound waves for each frequency covered in its range, and each frequency has a specific wavelength. These waveforms look like a rising and falling wave, each having a peak and a dip in one complete cycle. A peak occurs at 90 degrees and the dips at 270 degrees, with a complete cycle lasting 360 degrees total. Now, when you are using multiple microphones to record the same source, these frequencies are usually going to arrive at the two microphones at different times, each being in a different part of the cycle. The whole waveform is still accurately represented, but the microphone further away from the source is recording a delayed waveform. With our example of micing a snare drum, the top mic picks up the direct waveform from the surface of the drumhead, but the bottom mic receives a delayed signal as a result of the sound waves having to go through two drumheads and forcing them to react to it. So you can see where the second mic may pick up the signal in a completely different point of the cycle than the first mic.
Obviously, a musical source is going to create a spectrum of frequencies when played, and because each frequency from a source has a specific wavelength, their waveforms will be at a certain positions when they reach both microphones and are converted to electrical energy. Microphone 1, which is closest to the source, will record a certain frequency having only completed half of it’s waveform, whereas microphone 2 (a few more inches away from the source) will pick it up as having completed 1 full cycle of its waveform. Thus, when the two certain frequencies are played back at the same time, that frequency is going to be canceled out completely. This is because one is in its dip and the other is in its peak, both at the exact same moment. Each individual cancellation would then be considered 180 degrees out of phase.
That example is just one frequency from the sound source, so imagine what is going on with all the other frequencies. It is this "phase cancellation" with multiple frequencies at the same time that causes a comb-filtering effect, where there are a series of missing frequencies that create a very unnatural and possibly unpleasant sound. Of course, there will be frequencies that are not cancelled out, but rather amplified because of two identical waveforms at the same point in their cycles being summed together. In other words, both would be at either a dip or a peak at the same time, thereby doubling the power of that frequency. Most frequencies, however, are not completely amplified or cancelled, but are in varying positions within its cycle in relation to each other. There are varying degrees of phase cancellation and amplification going on here, some more noticeable than others. Just remember that anytime the two identical waveforms are not lined up perfectly, there is a certain amount of phase shift going on.
Source : http://www.recordingeq.com/EQ/req1001/mmi.htm |
Phase cancellation also can happen from "Acoustic Reflections", and this is always happen to newbie bedroom recording when they ignore the distance between mic , sound source and wall.If a reflective surface is close to the sound source, the reflection can reach the microphone at almost the same strength as the direct sound. Since the reflection has a longer path, it will phase-cancel the direct sound giving the same type of thin, hollow Comb Filter frequency response of multiple microphone interference.
As matter of fact, this phase cancellation can be avoid by The 3 to 1 rule.
To get more detail about how The 3 to 1 rule works, please take your time go to this useful link http://www.recordingeq.com/articles/321eq.html
Also using nowdays DAW technology, phase cancellation can be fix in the mix using method of Phase Inversion/Phase Invert/Phase mirror by simply with 1 click. But chances to get a good result out of it is 50 - 50. Some may cause the signal are just getting louder, clipping or distortion. Or the other way around, may cause phase cancellation on other different frequencies.
(I dont know about Cubase, but Sonar and Ableton Live got this function, In sonar you can see the button on your channel/track slip, In Live you can check under utility, other DAW may need to refer to respective manual or website)
AWARE: Copy and paste a same wave track will not cause phase cancellation but it just make the signal louder or clipping as it just double up the signal because they are sharing the same frequencies, level and amplitude. If you phase invert one of the copy track, it may cause TOTAL PHASE CANCELLATION. Hard panning L/R double signal will only result in halving it amplitude,and pretty much the signal can be heard and considered as CENTER, not HARD L or R. This copy and paste method are highly not recommended unless you got different purpose using the advantage of this method. THIS IS A REALLY SLOPPY WAY OF DOING THINGS. Dont copy and paste, just double tracking or quad tracking if you want to; example make the guitar track sound heavier. Dont be lazy to experiment.
***2 copied track maybe can be giving cool effect when you apply new output bus channel then you expand the channel before you chain back to master output, but that is another story of another technics. What we try to learn is to achieve good input, good output
Something new i just cross to learn using the advantage of Phase Cancellation is make an acapella using 1 full mix track and 1 instrumental track from the same song. This is really cool, so i just share the video from youtube with you guys, how come i never think about this method before hahah its really cool.
http://www.youtube.com/watch?v=FeYZ5-d-TxE&feature=related
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