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Phase Cancellation to Prevent Feedback Answered

The Grateful Dead's wall of sound prevented feed back by having two mics for each vocalist with one of the mics 180° out of phase of the other. Then mixing both mics on the same channel. So the only information that wasn't canceled was the voice. 

This gave me an idea. I took a handset and added a mic facing backward from the existing mic. Then wired the second mic out of phase. Apparently there's more to it electrically then just wiring the other mic backwards because the handset went dead.

Is this possible with just parts salvaged from old phone equipment?



5 years ago

Can you explain your goals a little?

Turning one mic 180 degrees in space won't create a 180 degree signal phase shift. But that's simple to do electronically--just invert the signal. Any typical common-emitter transistor stage, or an inverting Op amp will do the trick. Then a mixer to combine the signals.

But if the two microphones are close together, there isn't likely to be much difference in the signals, so the negative feedback from the inverted signal would more-or-less just attenuate everything, voice included.

Highly-directional mics might change that somewhat, but it would then be very dependent on the movement of the person wearing the mics...

I'm trying to prevent feed back through the shop's PA. One mic would be in it's normal position and the 2nd is facing backward from the other. I drilled a hole in the back of the handset to let the 2nd get signal. Thought since the person is speaking directly into the 1st mic their voice would be the only unique signal and not get cancelled. But hey, it's obvious that I'm working w/o a clue. I get a lot of ideas that are not incumbered by the thought process:)
Thx for the input.

Is there only one mic, and do you have lots of other competing audio (loud sounds)?

In the Dead's system, the background signal source (amplified instruments) was cancelled out. The fact that both mics were used for voices isn't crucial. You can still cancel background noise with a second mic. It doesn't need to be close to the voice mic, in fact it's better if it's not.

If it's just garden variety microphone feedback, then move the mic around the shop, or try adding an EQ...

The key to the method is that one mic is screened from the signal you want, and the other isn't. The idea's been around since at least WWII for cockpit mics, many modern mobiles also do similarly clever things.

Thanks, Steve. A form of the directional mics I was speculating about?  I'm familiar with the concepts behind noise-cancelling headphones and the like. I can see where two mics that are separated by at least 10 or 15 CM (or more) could be used effectively. But I'm not as clear on two mics being so close.

I didn't know about this technology was in use on mobile phones. Pretty cool. Seems like the closer the mics, the more engineering (and software) challenges...

Yeah, its an acousto-mechanical problem. You can imagine by the right design of mic capsule that you might feed ambient to the BACK of the diaphragm, as well as the front.

quoted from RFIC TECH:

Phase modulation Phase modulation (PM) is a form of modulation which represents information as variations in the instantaneous phase of a carrier wave. Unlike its more popular counterpart, frequency modulation (FM), PM is not very widely used (except perhaps for in the inappropriately named FM-synthesis for musical instruments, introduced by Yamaha around 1982.) This is because it tends to require more complex receiving hardware and there can be ambiguity problems with determining whether, for example, the signal has 0° phase or 180° phase.

Phase-shift keying Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of PSK, a finite number of phases are used. Each of these phases is assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent. Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used.

To say it is not as easy as it may sound at first is an understatement:   http://pdfserv.maximintegrated.com/en/an/AN686.pdf

You MAY find something in here (forum) of use....

And lastly,  another article that may be of some assistance....

In handsets, mikes are active things. they receive power and so are polarized. If you reverse it the output amplifier burns. As steveastrouk says you should amplify both mikes before substracting the signals or connect the 2 mikes to a difference amplifier.

The mics need independent amplification before you can try the trick