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Stereo to mono - "you know it ain't easy"

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Stereo to mono - "you know it ain't easy"

Postby Guest » Mon Dec 03, 2018 1:03 pm

I have an old Sony HR-MP5 processor which has mono and stereo inputs but only stereo outputs, whereas I would like a mono output. Apparently if you just pan both channels dead centre, hard right or left, or use a mono utility in a DAW, true mono is not produced because of the phase information - although it may not be a problem on a lot of stereo material.
May question is, what is the correct way to convert stereo to mono?

Re: Stereo to mono - "you know it ain't easy"

Postby Sam Inglis » Mon Dec 03, 2018 1:38 pm

If you are taking a stereo output from the Sony unit, then panning both sides to the same place, or using a mono plug-in in your DAW will both create a true mono signal. So too would just using one of the outputs and discarding the other. However, if the unit is processing in true stereo then it's inevitable that some of what it does will be lost in this process, and depending on the effect, the results of collapsing to mono can indeed be pretty disappointing. If you find that it alters the sound too much, the best option would probably be to see whether the unit can be made to operate in a mono mode internally.
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Re: Stereo to mono - "you know it ain't easy"

Postby Hugh Robjohns » Mon Dec 03, 2018 1:44 pm

Still Vibrations wrote:May question is, what is the correct way to convert stereo to mono?

Stereo has two audio channels of related-but-different information, whereas mono is a single channel of audio.

The standard way of reducing stereo to mono is to sum the left and right channels, so all of the original information in both source channels is carried into the mono signal.

If the two source channels are identical (coherent) -- eg, a solo vocal panned centrally -- the mono sum will be 6dB louder than either source channel on its own, and some stereo-to-mono converters always attenuate the mono output by 6dB to prevent this level rise causing overloads.

If the two source channels are similar but different (incoherent) -- eg. a normal stereo mix with panned instruments and reverb, etc -- the mono sum will typically be around 3dB louder than either source channel on its own... and again, some stereo-to-mono converters attenuate the output by 3dB to prevent overloads...

The potential problem, though, as you mentioned above, is that if there is information in one source channel which happens to have the opposite polarity of that in the other channel, they will completely cancel when summed to mono, and so that information -- which would be audible when listening in stereo -- will disappear from the mono summation.

While total polarity inversions are unlikely (although it can and does happen), this problem does rear its head often when there are large phase shifts (close to 180 degrees) between information carried in the two channels at some specific frequencies, and that is very common with some stereo reverbs and some stereo chorus type effects, as well as some stereo widening effects.

What tends to happen in these situations is that the reverb will appear to be much drier in the mono sum compared to the stereo version, or it may sound coloured or 'nasal'. The latter may also apply to stereo chorus type effects too. The same can also happen with some stereo keyboard instruments -- especially sampled pianos -- when mono-ing the output of a stereo keyboard. (That's why most stereo keyboards have separate mono piano sound options).

If you don't like what happens when you sum a stereo source into mono there are two alternative ways of trying to improve the situation.

The first and easiest is simply to only use one channel of the stereo source. This usually works fine stereo reverbs and chorus effects processors where both channels carry the same information but with complex and varying phase relationships. It won't work with things like stereo piano outputs where the two channels replicate the effect of separate spaced mics so that one channel has more low end and the other more high end, etc. Taking just one channel here will give a radically different tonal balance...

The other option is to use an all-pass phase shifter in one channel before the mono sum to introduce a fixed phase shift at all frequencies -- typically 45 or 90 degrees. This usually has the effect of reducing (but not completely removing) any phase cancellations that render a normal stereo-to-mono sum unacceptable.

In your case where you just want a mono output from your effects box, I'd just hook up one output channel!

Hope that helps.

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Re: Stereo to mono - "you know it ain't easy"

Postby Wonks » Mon Dec 03, 2018 1:50 pm

I'd not worry about 'true mono', as the unit was designed to provide stereo outputs with slightly different effects applied to the L and R channels, so there is no 'true mono' signal.

But if using just one channel sounds OK, or summing in the DAW sounds OK, then just use it as it is.

Think about the effects being used. There's no point using any ping-pong delays because that will require stereo to work, so just use a mono delay. If the unit tends to put a dry signal on one channel and an effected signal on another, then it should still sound OK in a mono mix, just not as expansive. Stereo reverbs will have different information on the two channels, but again, either the right or left outputs should both have useful sounds on them, though you may be better just using one channel in this instance if the mixed sound doesn't sound as good as a single channel does.

If it sounds OK, just don't worry too much about it.

And how you set up the unit will determine how you want to edit the FX settings. If it's an insert effect, then you'll generally want the unaffected signal at a high level, but if it's a send effect, then you'll want wet signal only.
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Re: Stereo to mono - "you know it ain't easy"

Postby Martin Walker » Mon Dec 03, 2018 2:42 pm

Wonks wrote:If it sounds OK, just don't worry too much about it.

At the risk of sounding too simplistic, Wonks has hit the nail on the head in my opinion.

A multi-effects processor like this is designed to add your choice of effects to your audio signal, so if you only need mono effects combine the two channels in any way that sounds good in your mixes, or simply use one or other of the two in isolation.

Whatever sounds good IS good in this scenario!

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Re: Stereo to mono - "you know it ain't easy"

Postby Guest » Mon Dec 03, 2018 5:23 pm

Many thanks for all of your replies and Hugh, it certainly did help, that explanation means I am more confident in converting stereo to mono and understand what the potential pitfalls can be.

After reading all these comments I did an experiment, taking a recording I made in a lift that was used in a work performed last Friday and compared the following mono files:

stereo saved as dual mono files and panned centre
dual mono files left and right played separately
stereo file with both sides identical using the Izotope RX7 mixing module
stereo file with the DAW utility to convert stereo in to mono out

They all sound the same except for the volume increase that Hugh described. This has demystified stereo for me (like Brian Wilson and Phil Spector I don't really use stereo, particularly as the music I produce is invariably played through a PA system).

Here's an initial set up I'm going to try:
Sony L and R outs to separate (physical, not virtual) mono mixer channels for mono, avoiding any ping pong or phasing effects in the processing. Should I occasionally need stereo I can pan them left and right.
The Sony can be used as two parallel channels but I usually use the two processors together.

Thanks again - this has cleared my brain. Now I have to learn how to use the compressors and other software I bought in the Black Friday sales.

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