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Understanding time and phase alignment, chapter 1.
12 years 2 months ago #9811 by Pasi
Understanding time and phase alignment, chapter 1. was created by Pasi
Hi guys and girls!
As promised i'll share my experience and knowledge about sound systems here to help all of you in your mission for good sound. We will start this alignment thing by checking with some pics what different situation we have and how they can be sorted. Sorry about picture quality, it is my limited softwares and knowledge to use them smiley23 . I may use some different terms or ways of doing these things, so be patient and remeber that this is just my way, someone else has different approach. I've managed to get good results with this way like i'm doing it so it gives you at least a good starting point.
Picture 1 under here shows us 4 different situations with soundwave we have everyday.
1. This is our reference wave, completely normal. Blue dot indicates the reference point to all other waves.
2. This wave is otherwise same than number 1, but it has phase shift of 180 degrees.
3. This is otherwise same as first wave, but this has reversed polarity.
4. And this last one has both reversed polarity and phase shift of 180 degrees.
Now it is time to understand difference between phase shift and polarity. Phase shift includes always time and is caused by time which equals to half of the wavelength. Polarity reverse on the other hand doesn't include time. Results may look similiar, but notice the location of blue dot. That is our time reference point. For example waves 1 and 4 are in phase but in different time caused by phase sift and polarity reverse.
In picture number 2 we have corrected all problems and all waves are in phase and in time.
To make it easier to understand what has been done, numbers are also moved and mirrored when needed.
1. This wave is now delayed by half of the wavelength involved. For example to 100Hz this would mean about 1.7 meters and about 5.5ms in time.
2. Wave two is unprocessed. It is allready in right phase and it was delayd in first picture. Since we can't take that delay off, we had to delay wave number 1 to match phase with this wave. Using polarity reverse between wave1 and this would have corrected phase issue but not time.
3. Here we had to do bit more processing and we needed to use delay which again equals to half of the wavelength and polarity reverse to match time and phase with wave number 2. (and also processed wave number 1)
4. This was quite easy to fix, just polarity reverse and we are done.
In my opinion it is important to understand difference between phase shift and polarity reverse. First one includes always time, latter one doesn't include time. Phase shift of 180 degrees looks like polarity reverse, but it isn't the same thing. It happens only when delay time equals to half of the wavelenght. More or less will give different phase angle, for example 145 dgrees.
Polarity reverse occurs for example in your Loudspeaker Management System when you select polarity reverse or when you make a mistake smiley4 (or deliberately) solder pins 2 and 3 crossed in XLR cable. These will swap the wave upside down in real time and they don't cause phase shift.
Questions and comments are wellcome. We will continue from this in next session and see, how we can recognize these phenomenas in real life with and without computer based measurement systems. We will also see what causes these issues and is there any way to avoid them?
Pasi
As promised i'll share my experience and knowledge about sound systems here to help all of you in your mission for good sound. We will start this alignment thing by checking with some pics what different situation we have and how they can be sorted. Sorry about picture quality, it is my limited softwares and knowledge to use them smiley23 . I may use some different terms or ways of doing these things, so be patient and remeber that this is just my way, someone else has different approach. I've managed to get good results with this way like i'm doing it so it gives you at least a good starting point.
Picture 1 under here shows us 4 different situations with soundwave we have everyday.
1. This is our reference wave, completely normal. Blue dot indicates the reference point to all other waves.
2. This wave is otherwise same than number 1, but it has phase shift of 180 degrees.
3. This is otherwise same as first wave, but this has reversed polarity.
4. And this last one has both reversed polarity and phase shift of 180 degrees.
Now it is time to understand difference between phase shift and polarity. Phase shift includes always time and is caused by time which equals to half of the wavelength. Polarity reverse on the other hand doesn't include time. Results may look similiar, but notice the location of blue dot. That is our time reference point. For example waves 1 and 4 are in phase but in different time caused by phase sift and polarity reverse.
In picture number 2 we have corrected all problems and all waves are in phase and in time.
To make it easier to understand what has been done, numbers are also moved and mirrored when needed.
1. This wave is now delayed by half of the wavelength involved. For example to 100Hz this would mean about 1.7 meters and about 5.5ms in time.
2. Wave two is unprocessed. It is allready in right phase and it was delayd in first picture. Since we can't take that delay off, we had to delay wave number 1 to match phase with this wave. Using polarity reverse between wave1 and this would have corrected phase issue but not time.
3. Here we had to do bit more processing and we needed to use delay which again equals to half of the wavelength and polarity reverse to match time and phase with wave number 2. (and also processed wave number 1)
4. This was quite easy to fix, just polarity reverse and we are done.
In my opinion it is important to understand difference between phase shift and polarity reverse. First one includes always time, latter one doesn't include time. Phase shift of 180 degrees looks like polarity reverse, but it isn't the same thing. It happens only when delay time equals to half of the wavelenght. More or less will give different phase angle, for example 145 dgrees.
Polarity reverse occurs for example in your Loudspeaker Management System when you select polarity reverse or when you make a mistake smiley4 (or deliberately) solder pins 2 and 3 crossed in XLR cable. These will swap the wave upside down in real time and they don't cause phase shift.
Questions and comments are wellcome. We will continue from this in next session and see, how we can recognize these phenomenas in real life with and without computer based measurement systems. We will also see what causes these issues and is there any way to avoid them?
Pasi
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