Mono Compatibility and Phase Problems
One of the biggest compromises audio engineers have to live with is the fact that not everything that sounds good in stereo will sound good in mono. That's why Ozone has tools specifically designed to troubleshoot the interaction between both sides of the stereo image.
Checking your mix for Mono compatibility as you go is a good preventative measure. Sometimes just because a mix sounds good in stereo doesn't mean it will sound good when the left and right channels are combined into a mono signal. In some cases, you may hear what is known as comb-filtering, which will color the sound of your mix and cause peaks and dips in its frequency response. In some cases, instruments may lose their integrity or even seem to completely disappear from the mix!
Why should you care if your mix sounds good in mono? Who listens to music in mono!? Well, as it turns out, a lot of people. Live music venues and dance clubs are a good example. Running a PA or sound system in mono is common practice to ensure music sounds good everywhere in the room. Because mono systems don't create a sweet spot or introduce the complex phase issues of stereo, they're often more desirable in that type of setting.
In some cases, part of your mix, specifically the low-frequency part, may be summed to mono. Many home stereos, home theater systems, monitoring systems and PAs use subwoofers these days. Did you ever notice that that subwoofer under your desk has just one speaker, but there are two speakers sitting on your your desk? It's likely that somewhere in that system your signal is being put through a crossover, the low frequencies summed to mono before being sent to the sub.
You get the idea ... always check your mixes for mono!
Troubleshooting with Ozone:
In Ozone's Stereo Imaging module, there are several tools that can help troubleshoot mono compatibility.
Enable "Channel Ops" to see the Mono switch. By engaging this switch, you will hear the result of your project summed to mono. Often simply hearing your mix in mono can bring problems to the surface that you can then address.
Correlation Meter and Vectorscope
The Correlation meter is extremely useful for seeing how your left and right channels interact with each other. When both left and right channels have almost identical output, the meter will be peaking all the way to the right at a reading of +1 (mono). The meter approaches 0 when the left and right channels are very different. When the meter starts hitting between 0 and -1, this is when you will see mono compatibility problems in your mix. A reading of -1 means signals from the left and right channel completely cancel each other out. Generally for a normal stereo mix, this meter will read between 0 and +1 most of the time.
The Vectorscope provides a view of the stereo image of the signal. Typically, stereo recordings produce a random pattern that is taller than it is wide. Vertical patterns mean left and right channels are similar (approaching mono, which is a vertical line). Horizontal patterns mean the two channels are very different, which could result in mono compatibility problems.
Checking Individual Tracks and Busses
If you need to further narrow down what might be the cause of your mono compatibility problem, just patch Ozone to your mains bus, and solo individual tracks or busses of your project while watching Ozone's meters and listening with the Mono switch engaged. This can help locate issues within your mix.
Mono Compatibility Causes:
Any situation where more that one microphone is used to record a source. The phase problems introduced might not be evident when the mics are spread across the stereo field, but when you check the mix in mono, you may hear comb filtering and a loss of quality. In some circles like classical recording where having many microphones is necessary, sometimes mono compatibility has to be sacrificed to make a better stereo mix. But in cases like recording solo acoustic instruments, pay attention to mic placement to avoid this problem.
In some settings, you may be able to go back and manually time align or invert the phase of individual tracks in your mix to fix a mono compatibility problem. For example if two mics were placed at different distances from an instrument, you can manually fix their time alignment. Zoom in on two related tracks with your DAW of choice, and then nudge one track slightly to bring it in alignment with the other. For mics that are facing opposite directions, for example two mics set up to record the top and bottom of a snare drum, inverting the phase of one track can help solve a lot of mixing problems.
Simultaneous direct + mic recording
If you've ever recorded a guitar simultaneously through a direct box and a microphone, you may have noticed the time alignment problems this causes. This type of situation can often be fixed by manually aligning waveforms in your DAW. Try bringing the microphone's waveform backwards to match the DI box recording.
Working with synthesizers and digital effects
This one might not be as obvious, but if you use a lot of synthesized sounds, you should still take a look at your source material for mono compatibility issues. For example, a very wide pad sound from a synthesizer might sound amazing in stereo, but how does it sound in mono? Also, exciters and stereo wideners can make tracks really jump out of your mix, but always pay attention to how this affects the stereo image.