total and partial phase cancelation
over the last few months there has been a battle raging on the Nuendo and other forums over how important zero output is when waveforms are in phase reversal - phase reversal should result in NO OUTPUT AT ALL on the mixdown or playback of a song - so what is phase reversal and how can 1 track cancel out nother ? -
phase reversal and its effect is easy to explain, less easy to understand and even harder to display in graphic form -
in screenshot 1, i have inserted some wave files, to explain how phase reversal works i have added a MECHANICAL element to the equasion -
attached to tracks 1 and 2 is a ‘mechanical’ version of a playback/mixdown engine -
OK, tthe ‘mechanical’ components are, a rod, attached to this rod are two fixed rollers and a pen (pen is the red dot) -
as the rollers move across the timeline they follow the peaks and dwells of the waveform - as both the waveforms are the same amplitude and IN PHASE, on playback or mixdown you would hear is what the red pen has drawn - this being one wave file which is the result of the way audio is summed inside the DAW -
of the next two tracks to make things clearer i have moved the pen (red dot) to a new position, on track 4 the waveform is 180 degrees out of phase with track 3, so here you can see that the rollers (being fixed) are in confrontation with the waveform and as the rollers cannot go up and down at the same time they are locked in one position, in this situation the pen is unable to draw any output so (in theory) there is no audio output -
http://i235.photobucket.com/albums/ee177/eurodog6/myscreenshot-29.jpg
Although this is somewhat important in forensic audio work, in many cases it is just
academic - but PARTIAL PHASE CANCELLATION can be a real nightmare in recording -
in screenshot 2, i have inserted three tracks - tracks 1, 2, and 3 are pure tone tracks all are PARTIALLY IN PHASE, are of the same signal level but are at different frequencies,
track 4 the actual mixdown of tracks 1, 2, and 3, and show how the results of partial phase cancellation can have serious and suprising results in the final mixdown -
http://i235.photobucket.com/albums/ee177/eurodog6/myscreenshot-30.jpg
Dr J
The problem is, of course, knowing when you have files with phase cancelation. do you know of a way to spot the problem?
Bax
its down to your ears, - it is a case of comparison -
where there is a difference between how you know a track sounds in isolation and how it sounds in the mix, say for example a guitar is great in isolation but is either lost or sounds ‘lumpy’ in the mix, there is a good chance that a ‘like’ instrument is too close in its frequency and amplitude thus partial cancelation takes place -
one way to check this out is to mute tracks either in turn or in multiples, when the guitar stands out again as it did on its own, the muted track(s) need attention -
you can then very slightly reposition the track(s) that had been muted left or right a fraction, very slight adjustment should not affect the mix and can resolve the phase cancelation -
if the problem can be nailed down to one track that is the spoiler but re-positioning it does not cure the problem it may have to be recorded using a different instrument -
Dr J
Dr J…
I just purchased a new receiver for my studio and I was listening to some finished projects.
On some, the sound (volume) kept “fading” in and out.
Is this a phasing problem?
Could it be my speakers being wired out of phase or do you think it is out of phase tracks?
I never noticed the problem with my old receiver but it was a little flakey - thus the new one.
cliff
if your speakers are out of phase then when 1 speaker is ‘pushing air’ the other is ‘sucking air’ - our ears will detect the one that is pushiig air as louder than the other one -
have you checked the new amp by running a few CDs through it ? -
if it passes the above ‘speakers in phase’ and plays CDs OK - then new amp is oK -
so now where ? - you will have to have the original song to open in N not on a CD - play the song through noteing where the fade occurs, then MUTE all but the top track and BYPASS all plugins, play te top track and see if it is ok - then unmute tracks in turn - if you cant find the track that causes the fade, switch the plugins on one at a time to see if it is one of them that is at fault -
if it is a track fault and as it sounds like the fade goes on for quite a time, you will either havs to not use it, or change the instrument on that track for a different type - if is a plugin try not to use it if you can -
Dr J