I want to know more about Polarity (books?)

[Franco]

Hi, there are some concepts of polarity inversion that I'm a little familiar with, but I'd like to really dig deep into the subject and am wondering what resources are available for me to pick up.

In Adobe Audition (2), there is a "Channel Mixer" that allows you to work with a +/- Left/Right value for each channel (L/R), but there is also a "Center Channel Extractor" function that has the ability to extract audio from the Center, Left, Right, Surround and there's even a Custom alternative where one can choose a Phase Degree, Pan (in percentage), Delay (in ms.) and you can also customize the start/ending frequency ranges. There's also a "Discrimination" setting that allows you to alter the Phase Discrimination, Amplitude Discrimination, Amplitude Bandwith, Spectral Decay Rate and finally, a "Granularity & Resolution" that allows you to change the FFT size and change the interval size (in ms.) with a "Window Width" in percentage.

(btw, I know of the many free tools out there like Stereo Tools and the Voxengo plugs, to separate the mid/sides, but I really would like to know what all of the above is about, so I'd like to know what sorts of books I can pick up that would help me understand all of it better). Thanks.

[Kiwiburger]

Polarity is simple. Plus or minus - choose one. It's up or down. In or out. Black or white. Negative or Positive. A polarity inverted signal will completely null the original signal.

Polarity has nothing to do with phase, although it's often called a 'phase' switch. I believe this is because in a balanced signal path the positive and negative wires are called 'phases', and the switch swaps the two wires (phases) around to achieve the polarity reversal.

Calling a polarity reversal or inversion a 180 degrees phase shift is a mistake. Why?

Phase is all about time within a cycle. Think 'phases of the moon'. There are 360 degrees in a cycle - and infinite divisions of those. Continuously variable - not 2 states like polarity.

True - if you clone and timeshift a sinewave 180 or 360 degrees of the cycle, you will achieve a null, similar to a polarity reversal. But if you change the frequency of the sinewave, that particular time offset no longer represents 180 or 360 degrees of the phase, and the nulling stops. Or if the signal is non-repeating it can't null.

With a polarity reversal, all frequencies, all signals null.

Preaching to the choir, but I think many people don't really know the difference.

[Kiwiburger]

The whole Mid/Side or Sum/Difference thing is another issue.
We are used to stereo Left and Right. But interestingly, it's possible mathematically (or electrically) sum & invert this data to create M/S or S/D data.

Taking a simple maths example - imagine the Left & Right data consists of:

Left: 1,2,3,4,5
Right: 5,4,3,2,1

We could transform this into:
Sum: 6,6,6,6,6
Diff: -4,-2,0,2,4

And it's also obvious that we could transform this back again, with no loss:
Left: 1,2,3,4,5
Right: 5,4,3,2,1

If these numbers happen to be audio, an interesting effect occurs.

Summing left and right is the same as panning towards the centre.
The common, centred, dry portions of the signal combine to achieve a 6dB boost.
The different, wide, ambiant wet portions of the signal partially cancel or at least don't get the same amount of boost. This becomes a mono "Mid" or "Summed" signal - signicantly dryer.

The Difference between Left and Right can be created by summing a polarity inverted Left (or Right - it doesn't matter) to leave the Difference. The centred dry stuff cancels out (because of the polarity reversal), leaving the wide, ambiant, "Difference" signal. Remember - this is mono'd, but it's very wet. This effect is basically your karaoke vocal removal trick. It sometimes happens by accident if you use a mono plug on stereo headphones, and the sound goes all mono and reverby.

And by magic, just like the numbers, we can transform Mid and Side signal back into Left and Right with simple summing (on one side) and summing with one signal polarity reversed again on the other side.

The reason you might want to do this is it allows you to alter the relative mix of dry and wet sound in a stereo signal. If you haven't played with this before, it's like magic.

Think of it as being a pan control for Front to Back. There is no reason why stereo equipment couldn't have two pan controls - Left/Right and Front/Back.

[Kiwiburger]

OK - the FFT thing. This is digital stuff. Fast Fourier Transform.

Basically, it can be shown mathematically that any complex signal (audio or otherwise) can be created by a mixture of sinewaves. A square wave can be created by an infinite number of sinewaves - so that's going to be the hardest to process.

Think about Hammond organs or DX7 synths or additive synthesis. You can make a whole range of noises using just sine waves.

FFT analyses a signal and breaks it into as many sinewaves as it can handle, and then allows you to put them back again. Unscramble that egg.

FFT is brilliant for digital eq. If you can break any signal into it's sinewave components, you can vary the levels of them very precisely.

I believe the software you are describing goes beyond simple Mid/Side processing by using FFT as well.

If you've played with the karoke voice removal effect, it doesn't work if the vocal is not dry, centre panned. But using FFT (busting it into sinewaves) we can tell the software to look for common signals at a particular pan position. Or with a particular phase offset. Cool stuff like that.

Hope that helps - and hope I haven't muddied the pool of knowledge with crap information. The real gurus will tell you exactly how wrong I am, but this is how this dumb muso understands this stuff.

[skygod]

Originally Posted by Kiwiburger

The whole Mid/Side or Sum/Difference thing is another issue.
We are used to stereo Left and Right. But interestingly, it's possible mathematically (or electrically) sum & invert this data to create M/S or S/D data.

Taking a simple maths example - imagine the Left & Right data consists of:

Left: 1,2,3,4,5
Right: 5,4,3,2,1

We could transform this into:
Sum: 6,6,6,6,6
Diff: -4,-2,0,2,4

And it's also obvious that we could transform this back again, with no loss:
Left: 1,2,3,4,5
Right: 5,4,3,2,1

If these numbers happen to be audio, an interesting effect occurs.

Summing left and right is the same as panning towards the centre. The common, centred, dry portions of the signal combine to achieve a 6dB boost. The different, wide, ambiant wet portions of the signal partially cancel or at least don't get the same amount of boost. This becomes a mono "Mid" or "Summed" signal - signicantly dryer.

The Difference between Left and Right can be created by summing a polarity inverted Left (or Right - it doesn't matter) to leave the Difference. The centred dry stuff cancels out (because of the polarity reversal), leaving the wide, ambiant, "Difference" signal. Remember - this is mono'd, but it's very wet. This effect is basically your karaoke vocal removal trick. It sometimes happens by accident if you use a mono plug on stereo headphones, and the sound goes all mono and reverby.

And by magic, just like the numbers, we can transform Mid and Side signal back into Left and Right with simple summing (on one side) and summing with one signal polarity reversed again on the other side.

The reason you might want to do this is it allows you to alter the relative mix of dry and wet sound in a stereo signal. If you haven't played with this before, it's like magic.

Think of it as being a pan control for Front to Back. There is no reason why stereo equipment couldn't have two pan controls - Left/Right and Front/Back.

I'd like to think of your ‘Front/Back’ brilliant suggestion it as pan controlling a time delay effect. Same can be done with reverb densities and tails. So how does one engineer a time delay pan effect lol? Interesting concept. At least somebody's head is NOT meshed all the way up their arse and is thinking outside of the box in this forum!!! Good job mon!

Also be advised that polarities are a little complex when ANY type of wave propagation is concerned. I discovered this by accident in a severe electrical storm one night as all my mobile units communicate intra- via CB AM/SSB and Nextel Chrip. I was talking over the radio to another truck and suddenly a crack above head and I lost my radio output finals that blew from a static electrical discharge burst in the atmosphere. All radios are gounded to vehicle chassis plugs, SWRs set at 1.2or completely flat, and radios swinging to at least 70 watts each w/o kickers. All three mobile radios fried that night. Lesson learned: The negative charge of the dc volt static burst overhead was close enough that it blew all three of my Connex 3300HP radio finals. Was it a +ve or -ve charge? I suspect the latter. Cost a pretty penny to ship all three radios back to TX and have the circuit boards and finals redone and retuned. Faak! Never turn on a radio during an electrical storm.

So how does this apply to pro audio? Well .....
1) static discharge from fingers when fiddlefvvcking around the inside of any piece of progear. Make sure you are grounded or you'll fry RAM, and other critical ICs esp if your standing on carpet,
2) static discharges when recording live performances indoors and outdoors will destroy an expensive mixer, FET & tube mics, etc. It will also pick up that noise floor and ceiling into the performance. Try to de-ess that out or limit that out or multiband PEQ it out. Its a fvvking nightmare to fix. Too many artifacts and peaks, hum, wow, flutter, hiss, screech, sheesh and sheeat!

Most small to large clubs to arenas have the worst fvvking ground and electrical circuits available to the human race. Know why? Because they love us out there don't you know. If you cannot dig a hole in time and plant a ground pole, or if a real earth is NOT available, look for major piping in a basement and run ground to that. Worst case scenario run a low number gauge ground wire to a thick tree like a 10 or 12 gauge wire, otherwise make sure you have a floating ground system (NOT recommended) that you can ground your outdoor rigs to into a load resistor box or something similar, 3) All recording devices build up static electricity with usage, particularly tape decks that wind, rewind, by reel forward and backward over magnets.

Your -ve terminal must be grounded to 6 feet below ground surface to establish a 'real earth' if you will. Make sure your personal and pro studios have a separate ‘star ground’ system from the rest of the building that all your gear is grounded to. Connecting to batteries: Positive first then negative. Unhooking batteries: do the reverse negative first then positive.

Electricity is analogous to gravity folks. Its not the positive G forces that kill you when your parachute doesn't open WHEN YOU SLAM INTO THE GROUND AT TERMINAL VELOCITY, its the 'negative' ones coming up against you from the earth's center. You know like a volleyball game where you thought you had it and missed and the ball is on the ground again? Yup. Gravity is a muthfukka! Same thing with electricity -- its not the positive voltage pressure and amperes that will kill you and your precious gear, rather the negative electrons ....

Also be aware of positive and negative connections when series-ing and paralell-ing mults between tw0 or thee or four pieces of gear esp when sidechaining via using patch boxes or rack splitters or patchbays, or when combining two electrical devices when spliting the same signal thru two similar or different devices, e.g., an SSL G solid state compressor and an other compressor like a tube compressor then rejoining them and blending them by bringing one up under the other. Watch for phase anomalies. Watch for voltage anomalies esp in the –ve voltages. Watch for additonal anomalies when sidechaining pultecs and other eqs into those same paralelled compressors multiple or single. It gets complex and you can ruin a lot of really spiffy and very expensive gear notwithstanding the precious time you spent and lost on the recording that you just royally fvvcked up on the 8 bus or 2 bus.

there many ways i connect tandem compressors from a basic two channel output signal (L/R): series, parallel, series parallel, , dual parallel, dual-parallel-sidechain, series-parallel-sidechain etc. get out there and experiment and find out alternate ways of 'coloring you sound' other than preamps, and how to create 3d depth to your mono, and stereo, dual mono, and surround mixes and masters.

passing thoughts about -ve voltages and other tawdry and sundry pornographia


~skygod~

[Alécio Costa]

very good!

So.. can we say that comb filtering only happens when we have a phase shift, right?

Example: It would be similar to when Voltage is at Zero degree reference (1 +j0) and current, thru an inductive or capacitive circuit is (1+j10) or (1-j10). It will provide less than ideal Real Power "P" , but S=P+-jQ.