I know that encoding from an MP3 to an MP3 loses quality, but is it possible to convert somehow stereo MP3s to mono MP3s?

I listen to my iPod a lot and when I am with people I have one earbud in and the other out so I can hear what they are saying over my music. The problem is some bands record one guitar on one side, the other on the other side, some put vocals on one track, etc. So is there a way to smush the two together or do I have to re-rip?

With 100% M/S joint stereo encoding, in theory it's possible, but in practice no one has coded it.

With pure stereo encoding, or content-adaptive joint stereo (i.e. some frames M/S, some frames L/R - in other words, normal "joint stereo" behaviour) it isn't.

Cheers,
David.

Looking further 'upstream', downmixing stereo to mono also depends on the original recording. If 'pure intensity' stereo, all sounds arrive at both speakers at the same time and in phase. If the original recording used a classical technique of two mikes in front of an orchestra this is not the case. If a binaural recording there would also be differences in time and phase. In these cases any downmixing would introduce distortions.

Also, if pure intensity stereo any sound dead center would come out 3db louder when the two channels were mixed.

Woody

If you don't mind the larger file size of stereo MP3s, why not downmix in the analogue domain instead?

Either get a mono ear piece or an adapter like this (available in most "Radio Shack" type electronics stores). Quick, cheap, easy, and lets you listen in stereo when you want to.

Shorting out one of the channels...not a good idea.

How much money does it cost to PWNZ0RZ your iPod?... $3.98

Yeah, there's adaptors which will combine stereo to mono, but I haven't seen one that has a stereo connector on both ends. You could make one though, for sure. Just connect the L & R conductors.

There is of course better ways to convert from stereo to mono for lots of program material, other than simply summing... but nothing you can do passively with just a cable or adaptor.

Shorting the left and right channels together is not a good idea either for two reasons.

1/ You may damage the amplifiers.

2/ Any sounds that appear on both channels but phase-shifted will lead to partial cancellation and affect the apparent volume of certain sounds. It won't be true mono.

Cheers, Slipstreem.

re: 1... it's a headphone amp, so the load is relatively small, with a decent impedance. nothing is being "shorted" by combining L & R.

re: 2... define "true mono". I know what you're saying (and i mentioned already that simple summing is the crudest method to "create" mono), but how is summing any different to just recording a sound with one mic? Won't the phase-shifts are still going to hit the one mic and be recorded just the same as if you recorded with two mics (that are relatively close together) and summed to "tape"?

Well that's how everyone has generated mono since people stopped making dedicated mono mixes!

You can do some clever phase shifting to preserve out-of-phase elements of stereo mixes even in mono, but it's really not worth it - the onus is on the record producer to check mono playback. Broadcasters still do!

Cheers,
David.

Re:1 Your load argument is incorrect. We're talking about the output impedance of the amplifiers here which will be very low (milliohms) if they're designed properly. Dumping current into the output of one amplifier from the output of another is very bad practice and is likely to lead to distortion at the very least as the protection circuits in the amplifier (which most have) attempt to stop you from frying it. Mixing them with resistors that have a value equal to the minimum design load should be OK though.

Re:2 Maybe I was being a little pedantic as is revealed by Dave's post above, but the example you give of a mono microphone doesn't stand up in the real-world either. Naturally generated sound comes from, potentially, an infinite number of directions due to reflections from physical objects. It doesn't come from two discrete spot sources. A single microphone would capture the sound in a similar way to one ear, not the mathematical sum of two ears in different positions and pointing in opposite directions.

As Dave rightly points out, summing the channels is the commonly accepted method of converting stereo to mono and will provide acceptable results in the majority of cases. It's still not "true" mono though.

Cheers, Slipstreem.

use :

lame -a input.mp3 output.mp3

you can also add more settings

For a Blumlein crossed pair, an M/S array (matrixed to L/R), and a soundfield mic (decoded appropriately), the sum of the two (or more) channels really is mono.

(One of the raw outputs from an M/S array and from a sound field mic is mono).


For spaced microphones, there really is no way to create "true" mono - i.e. the signal that would have been picked up by a single microphone placed somewhere between the two. I bet someone has tried to get close.

This is all academic, as the kind of recordings the original poster describes (i.e. instruments panned 100% left so that they are not audible on the right channel) derive their stereo effect from pan pots, not microphone placement. As such, a simple sum of the channels won't cancel anything unless the phase is intentionally inverted (though there is an issue with perceived levels).

It's great that such a simple (and impossible) question has lead to such an interesting discussion!

Cheers,
David.


That will transcode everything to 64kbps CBR mono.

That could be quite a quality hit if the original is 320kbps stereo! Or pretty much anything.


lame -V2 -a input.mp3 output.mp3

...might be kinder to the audio, but is still transcoding. Given the intended use, it's probably not worth worrying about transcoding artefacts.

Cheers,
David.

thats why I add "you can also add more settings", you can use any encoding settings

http://www.rockbox.org/ has a mono option, which i use a lot!

When Joint stereo to Mono , many people do not notice that "Cancellation" occur and hurting the sound quality..

Cancellation has nothing to do with mp3 coding. But yeah, I don't want to split hairs.