The OOPS Effect

Theory

What is OOPS?

OOPS means Out Of Phase Stereo. It is an effect obtained by combining the sound of one stereo channel with an inverted image of the other channel. It is also called "Left Minus Right" or simply "L-R"
What is the result? First, we must remember that, if a wave is combined with its inverted image, they will cancel themselves and the result will be no oscillation - if we are dealing with sound waves, the result will be silence.
This way, it's easy to understand that, by combining the sound of one channel with the other channel inverted, everything that is EQUAL on both channels will DISAPPEAR, and the remaining sound will be a mono waveform combining everything that is "different" between the channels - usually sounding very different than the original recording.

OOPSing stereo recordings

The "position" of each sound on a stereo recording depends on the amount of it that is recorded each channel. If a sound is recorded only on the left channel, it will be heard completely to the left. If it is recorded on both channels, more on the left than on the right, it will appear between the left and the center. Finally, if it is recorded equally on both channels, it will come from the center. (Of course it is the same for the right channel!)
OOPS does the following:

If a sound is heard only on one channel, it will remain unchanged.
If a sound is heard more on one channel than on the other (between the side and the center), its volume will be reduced. The more "centered" the sound, the more reduced it will be.
Finally, if a sound is centered, it will disappear.

This way, we can notice sounds that are barely heard in stereo (for instance: a sound heard at low volume on one side, on a recording where most of the sound comes from center). Also, in most songs we get only the rhythm track, in karaoke-style! (when the voice comes from center and the rhythm track from the sides). And many other special effects!

OOPSing mono recordings

Nowadays, mono recordings (as Beatles' ones) are actually stereo recordings with the same signal recorded on both channels.
OOPSing such a recording will result in silence. However, in some recordings some sound can be heard after OOPS, as a result of the following anomalies:

The mono sound is not perfectly centered between the stereo channels: this may be caused by incorrect setup of the mixing/mastering equipment during, while splitting the mono signal between stereo channels, or a mono tape played on a stereo machine (see below). The OOPS result is the same original sound, at low volume. The more unbalanced the track, the higher is the volume. Also, the OOPS sound seems to be trebly, for unknown reasons. (maybe the bass sound tends to be more centered?)
The recording is a mono tape played on a stereo machine:In this case, the original mono sound, coming from 1 track, is splitted in 2 tracks in the player's head itself, so it's very easy to obtain an unbalanced sound. Also, the original tape (usually old) may have some volume drops and noise caused by surface defects on the tape, and this noise will probably appear off-centered while playing the tape on a stereo machine.
OOPS will emphasize these noises, as they appear "in stereo". The main sound, which is closer to the center, will be at lower volume.
(You can simulate such a recording at home easily: pick an old tape with some surface defects, record something on it using a portable mono recorder, then play it on a stereo system.)
Processed recordings:This happens several times on Anthology: Mono recordings had some reverb added in stereo, to give them a better ambience. In stereo, you hear the sound coming from the front, with some "echos" around. The OOPSed sound is mainly reverb and echo.

OOPSing a mono recording can reveal these (d)efects, sometimes barely noticed when hearing the recording even on headphones. It will point out what is true mono (centered) and what is processed or incorrectly mastered.

Anatomy of the OOPS signal

In general terms, a stereo recording can be divided in three "signals": the "left" (L), the "right" (R), and the "center" (C). This is a simplification, but is almost true for many early stereo recordings, such as Beatles ones. During mixing, these signals are divided between the channels to obtain the desired positioning:

Left channel: L + C

Right channel: R + C
The "center" signal is mixed equally between both channels, while the "side" signals are mixed only on one side each.
What happens when we OOPS a signal? If we invert the right channel, it will be (-R-C). Combining with the left channel will result: (L+C) + (-R-C) = (L-R). The center signal will disappear and the result will be (L-R). That's why OOPS is also called "Left Minus Right".

If the OOPS sound is the "side" signal, one may think that we can do OOPS again, this time between the OOPS sound and the mono combination of channels (L+R+C) and then obtain the "center" signal alone. This is NOT true, because on OOPS, the "sides" are reversed. Let's see:

(L+R+C) + (L-R) = (2L+C)
Only one side is canceled, and the other is emphasized.
To get the "center" alone, we should obtain a (L+R) signal. I've made many calculations but could not obtain it, and I don't think it's possible. But if someone knows how to get this signal (or the "center" alone), only by combinations/inversions of the stereo channels and its results, please let me know.

Practice

Analog OOPS

This is the method described by Dave Haber on his Internet Beatles Album . Basically, all you have to do is connect your speaker this way:

               
               + -------------\
             L                 \
               -                Speaker
Amplifier                      /
               + -------------/
             R 
               -

(Check Dave's page, he has a GIF of this!)

In short words: connect only one speaker, to the (+) terminals of both channels. I have asked an electronic engineer about this, and he assured me that will be no damage to your amplifier and speaker.

I have tried this technique, but I was not completely satisfied. I think the output of any equipment - amplifier, CD player, etc - is many times unbalanced (that is, the gain of each channel's amplifier is not exactly the same), which will affect the quality of OOPS - center sounds won't be canceled completely. So I thought a bit and invented...

Digital OOPS

This is OOPS made by computer! You will need a soundcard and a program that allows inverting a waveform and combining it with another. A CD-ROM drive is highly recommended.
All you have to do is record a sample of the sound you want to OOPS, in stereo and then process it this way:

Invert the waveform of one channel, leaving the other channel untouched.

Combine both waveforms.
The result will be: Pure OOPS! I have tried this with Wave Studio 2.0 and Cubic Player 1.5: here are instructions:

Wave Studio: Record the sample, then go to the Special menu, select Invert Waveform and choose one channel. Then go to Convert Format and convert it into mono (you don't have to change the bits and frequency).
Cubic Player: This program already features OOPS! Select the entire wave, go to the Transform menu, then select Channel Mixer. Choose the preset "Voice Cut" and that's all. Note that the result is not mono, is a stereo sound with the same signal on both channels.

For best results, you have to record the sound from CD, playing on the CD-ROM drive.
The disadvantages of this method are: you cannot OOPS the sound in "real time" (while playing), and it takes some time to process it. Also, unless you have plenty of free space on your HD (and a very fast PC), you cannot OOPS an entire song at full quality.
I usually record my samples at 16-bit, 22KHz. 8-bit also produces good results, but in this case there is a lot of noise that may cause lower sounds difficult to hear.

Quality of OOPS

The only way I could get perfect OOPS was using Digital OOPS, and recording from CD, playing on the CD-ROM drive. I tried with Live at the BBC, a mono recording, and this was the only case I could get perfect silence. I also tried with an external CD player, connected to "Line" input of the sound card, and the result was not so good.
As you may notice, it's very hard (I'd say impossible) to get perfect OOPS from tape or LP. It's very difficult to obtain the perfect alignment stylus/groove (LP) and head/tracks (tape), not to mention surface noise and recording defects, that may be inaudible while listening to the sound "normally", but are brought up by OOPS.
The quality of OOPS is specially important if you are researching mono tracks, to see if they are unbalanced or processed. If the OOPS is not perfect, you will always get some sound after OOPS, even if the sound is centered.

Examples

Here are the OOPS results for some Beatles records:

Dave Haber's page has also a list of some Beatles songs that are worth OOPSing and the results.

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