Phase reverse (reversal)

[Chris Swanberg]

Can some explain the technical aspects of this? The how's where's why's and give some practical advice on this function and its use?

Thanks in advance.

Chris

[Steve House]

Originally Posted by Chris Swanberg

Can some explain the technical aspects of this? The how's where's why's and give some practical advice on this function and its use?

Thanks in advance.

Chris

What context are you referring to? Microphones setup, speakers, an option on a track in your editing software, ????

[Alexandru Petrescu]

Originally Posted by Chris Swanberg

Can some explain the technical aspects of this? The how's where's why's and give some practical advice on this function and its use?

Thanks in advance.

Chris

Imagine a signal whose graph looks like this for a while: /'\
and then see the same signal whose phase has been inverted: \,/

One practical aspect of this is Surrond Sound: that one can extract some frequency ranges out of a signal, invert the phase of these ranges, or just rotate at some 32 degrees, join to the original signal. 'Virtual' or 'Surrond' stereo is obtained out of a Mono signal also by employing phase inverting or changing techniques. I suppose Dolby and THX and the others all employ phase rotation somehow.

One should use phase inverting to decode a typical Mid-Side signal for stereo speakers Left-Right.

[Steve House]

There are negative consequences of phase reversal as well sometimes and the switch can be used to fix them. For example, some XLR to TRS adapters that people use (improperly) to send a pro mic into a camcorder stereo mic input are wired so that the single mic signal appears on both the left and right tracks with one track phase inverted with respect to the other. (A properly wired adapter won't do that.) When they're mixed to mono for broadcast, the dialog vanishes because the two tracks cancel each other out. Inverting the phase on ONE of the two channels will fix such problems.

[Chris Swanberg]

Thanks for the replies. What specifically generated my question is a phase reversal switch on a tube preamp I have... Do I just set it to produce the most pleasing sound to my ear from the mic attached or is there something more I need to know?

[Colin McDonald]

Basic stuff first:

if you have a conventional hi-fi system with two separate (left and right speakers) you can hear what happens to the quality if you invert the phase of one channel (by wiring one of the speakers the wrong way round) - it sounds duff, because much of the sound is cancelled out. That's why speaker cable has one connector marked with a stripe or ridge. In this context phase inversion is a fault. But it is used deliberately as the principle behind noise cancelling headphones where the ambient sound is cancelled out by adding a phase reversed version of the ambient noise to the signal you want to hear.

"Phase inversion" is normally reserved for 180 degree phase shifting - like wiring the other way round.

Phase shifting by other values is another more complex matter and is used for various effects and purposes like guitar pedals, stereo imaging and surround sound.

EDIT: Sorry, you posted your reply while I was writing this so I didn't see it!

[Alexandru Petrescu]

Originally Posted by Chris Swanberg

Thanks for the replies. What specifically generated my question is a phase reversal switch on a tube preamp I have... Do I just set it to produce the most pleasing sound to my ear from the mic attached or is there something more I need to know?

No expert here, but I believe reversing the phase of a single sound source will give no audible result - sounds precisely the same. Differences appear when we talk mixing two signals (one of them being supposedly a phase-rotated instance of the first, or some frequency ranges from the first).

A single way preamp phase inversion could probably produce audible effects if the sound reflected from the walls is phase-changed compared to the phase of the straight sound hitting one's eardrum. But that's more like one would turn a potentiometer, not a on-off switch.

Is your tube preamp stereo (dual) or single? Is your phase switch a on-off or a potentiometer? Do you listen on hifi or on dual headphones?

Alex

[Steve House]

Originally Posted by Chris Swanberg

Thanks for the replies. What specifically generated my question is a phase reversal switch on a tube preamp I have... Do I just set it to produce the most pleasing sound to my ear from the mic attached or is there something more I need to know?

All the mics in a multi mic setup need to be in the same phase, ie, an increase in pressure on the diaphram causes a positive voltage to appear on the "hot" output terminal. Almost all pro mics are wired to be consistent regardless of manufacturer but there are a few odd-balls out there. On an XLR connector, pin 2 is "hot" and pin 3 is "cold" and on an extension cable pin 2 on one end should connect to pin 2 on the other and the same for pin 3. That switch is to let you reverse the phase of the mic going to that preamp so you can correct for the occasional reverse wired mics or to compensate for a miswired cable.

[Chris Swanberg]

Thanks all. The tube preamp is a mono model, and I recently picked it up to add some tube sound to a voice over setup. I was a little perplexed at the switch, and using (at the time for test purposes) an Octava 219 large diaphragm mic, though I could detect a difference in the sound (through my mixer and coming through my headphones) when switching back and forth on the phase reversing switch...

[Steve House]

Originally Posted by Chris Swanberg

Thanks all. The tube preamp is a mono model, and I recently picked it up to add some tube sound to a voice over setup. I was a little perplexed at the switch, and using (at the time for test purposes) an Octava 219 large diaphragm mic, though I could detect a difference in the sound (through my mixer and coming through my headphones) when switching back and forth on the phase reversing switch...

If you're listening to a mono mic all by itself it's most likely anything you hear is psychological rather than real. "Positive phase" doesn't sound any different from "negative phase" when by themselves because they're the same waveform and frequency, etc, just mirror images of each other in terms of polarity. It is possible that there is some slight electrical imbalance existing in the signal paths for the different switch settings that could make it sound different but it's not very likely. If you're speaking into the mic yourself or have phones that don't completely isolate you from the sound source, you might be hearing an interaction between the sound heard through the headphones and the sound heard conducted through your head to your ears or leaking around the earpieces. In a truly blind test where you can't hear the source feeding the mic at all except through the preamp and your cans there should be no audible difference,

[Dave Blackhurst]

To add a bit of complexity (but perhaps more understanding) there are TWO elements to "phase" reversal...

#1 there's a mechanical element introduced by proximity of the input source to the recording device (or on playback the distance of the listener from the sound sources/speakers).

IN THEORY, if the distances to/from two identical sources are out of phase with each other the signals will cancel each other out.

IN PRACTICE, since it's almost impossible to achieve perfect distancing (aside from applications like noise cancelling headphones or generating a cancelling wave at the same source as the original "noise"), the more likely result will be a thin, nasal sort of result on the semi-cancelled signal - thus reducing the effective overall output at least at some frequencies.

#2 is the ELECTRICAL portion of the equation - as already noted, speakers, mics, and other sound recording devices typically have a "+" and a "-" - they have a polarity if you will.

IDEALLY all sound recording devices are correctly polarized and so phase aligned...

IN THE REAL WORLD, you may sometimes encounter devices (much like some people!) that have their "poles" reversed, and thus will when put with other devices will cancel out the signal. By reversing the phase of such a device ELECTRICALLY, you will restore the sound to full output as you no longer have the cancellation effects of the out of phase signal.

OK, so there's more than you ever wanted to know about what that little switch does <wink>! Most of the time it won't matter, but if you were for instance using a multiple mic/preamp setup, it could be important! IOW a single signal source can't technically be "out of phase" even if it's polarity were wrong, but take two sources, put one 180 out of phase... let the fun begin!!

FWIW, I come from a guitar/music industry background, phase effects are BIG FUN and have made for some great music when creatively abused! <all engineers in the room may now feel free to cringe>

[Chris Swanberg]

I kinda wondered if it couldnt just be in my head, more or less for the reasons you mentioned....Sound and human perception of it is a very interesting subject.

Maybe I should start another thread, but I do want to know more about the famous (infamous) 12AX7 tube and its use in tube amps and the effects. I notice old 12AX7 tubes selling for large sums on E-Bay.... Anyone familiar and knowledgeable?

[Dave Blackhurst]

Well, in a lo-fi (guitar amps) way, I've got a fair amount of knowledge/experience. What's the question?

I'm a bit rusty, as I try to stay away from high voltage circuits with small children running about, but my memory is mostly intact, despite aformentioned small children... so ask away, I'll see if I can shed some light (at least a pleasing soft glow... ahhh tubes) on the subject.

[Chris Swanberg]

Well, it seems from things I have read that the newer chinese made tubes are not as good at producing the "colored" tube sound as the older tubes, certain ones by certain mfgr's and certain designs are allegedly better- much better. A look on E-Bay will demonstrate the price variations on these tubes.

So, what's the real skinny on these? Which ones are really better (best) and why? Anything to stay away from? Anything to look for?

[Dave Blackhurst]

OK, I'll preface by saying I haven't tested any of the current crop of tubes, and so I'm about 10 years out of date...

BUT, when I was tuning amps, generally the Russian tubes were durable, but sounded like crap, the functionality was there, but the tone was not so good, not warm and smooth as you typically wanted. Really nto a big jump on just using solid state...

Chinese tubes were less relaible, but had more of the desired warmth, so that was what I used most of hte time since NOS 12AX7's were prohibitively expensive and the imports were cheap.

For a while (this was probably about 15 years ago as I think about it) Magic Parts was importing a custom brewed Chinese tube they dubbed the "silver special" - the original ones of these were amazing, higher output, warm and smooth. Then I understand the Chinese government shut the factory down and moved the equipment outside into the weather... about that time I got rather discouraged about the state of tube manufacturing and availability, modeling amps were beginning to get the tone and feel pretty close to the real deal, so I haven't done much with tubes since.

So that's some general info - a tube isn't a tube - they have differences in tone, as well as gain, and with careful selection you can make some fairly sizable changes in how a given piece of equipment sounds, not to mention some tweaking of the surrounding circuitry can alter things too...

One of these days I may sample current tubes as I've got a couple amps I still keep around, but I kept a personal stock of silver specials when I got wind of them becoming unavailable...

The best thing I can suggest is to buy a few different tubes and A/B them in your preamp to see if you get noticeable and pleasant tone changes - depending on the circuit design and the tube you might get a little, or a lot. Then it's a matter of personal taste.

That's a basic primer on how to start looking... if you have more specifics, ask away.