Stereo-Mono

[ignatz]

Steve:

I just noticed this thread and have some related questions.

I process a lot of mp3 files with Audacity--fading noisy tails, adding silence, noise removal, occasional tick and pop removal, etc.

At least half of my mp3s are in fact mono recordings (pre-1958), but are nonetheless two channel and presented in Audacity as two channel recordings.

At times, I will use tracks/stereo to mono to save disk space and in the possibly mistaken belief that it may help to reduce groove noise and distortion. I do this because long-term experience with cassette tape recording from vinyl and FM stereo reception has told me that a mono recording often has less distortion than a stereo recording of the same song.

Questions:

Should I do this stereo to mono as the first step in processing, the last step, or does it matter? My possibly tin ear cannot detect any difference in when I convert to a single track.

Does stereo to mono conversion in Audacity by itself tend to reduce unwanted noise or distortion on a mono recording? On a stereo recording? On any two channel recording, even if mono?

What is "split the tracks" as I see noted earlier in this thread? I have never used it and wonder why would it be used. I see a "split" command under the edit menu, but brief fooling with it doesn't seem to do anything, at least on the visible waveforms.

I just realized that stereo to mono is not available in 1.2.x, so this question may be more appropriate in the 1.3.x forum, but this is where I found the thread.

I will likely have some follow up questions, but that is enough for now.

[steve]

Hi ignatz, I have moved your post to its own thread so as to avoid confusion.

At times, I will use tracks/stereo to mono to save disk space and in the possibly mistaken belief that it may help to reduce groove noise and distortion. I do this because long-term experience with cassette tape recording from vinyl and FM stereo reception has told me that a mono recording often has less distortion than a stereo recording of the same song.

The reason that this works with tape or vinyl is that on mono recordings, the music signal is (almost) identical on both Left and Right channels, but the noise on one channel is independent from the noise on the other channel.

When you mix down to mono, you will need to reduce the volume level to half, because the music signals from each channel are being added to each other.
If you have a small "click" on one channel, then a small click on the other channel, when you mix them down to mono you will hear 2 clicks, but because you have turned the volume down to half, they will only be half as loud.

Now this raises an interesting question with "mono" files in digital audio.

If you have a "stereo" recording of a tape or record that is playing some "mono" music, then you have the same situation as the analogue case above and can mix the channels to achieve improved Signal to Noise Ratio (SNR).

If you record a "mono" source (for example Left or Right, or a mix of Left and Right) into a "stereo" (2 channel) track in Audacity, then you have 2 identical channels. The noise and the music on both left and right will be identical and there is no benefit in mixing them.

A simple experiment that you can try (I think you will need Audacity 1.3 to do this) is to try the "vocal remover" effect on your "stereo recordings of mono music".
The "vocal remover" removes sounds that are the same in both left and right channels, thus removing anything that is panned dead centre in the stereo field, which is where vocals are often positioned.
If the channels are identical, then the vocal remover will remover everything and you will get silence.
If the music is identical on Left and Right, but you have stereo noise, then the music will be removed and you will just hear the noise.
This is not a very useful thing to do, but an interesting experiment.

What is "split the tracks"

In Audacity 1.3.8 and later, you can click on the track name and there is a drop down menu. In that menu you will see several options including "Split Stereo Track" and "Split to Mono". These are only available on stereo tracks.

"Split Stereo Track" will turn a stereo track into two single channel tracks - one will be the left channel and the other will be the right channel. The upper of the new tracks will be set to play through the left speaker only (it is set to output to the left audio channel), and the lower track will be set to output to the right channel.

Mono tracks may be set to output to the Left channel, the right channel, or to "mono" (output to both left and right channels).

"Split to Mono" is very similar to "Split Stereo Track" except that the two single channel tracks that are produced are automatically set to output to mono. (This was my little contribution to Audacity )

"Split Stereo track" is useful if you need to apply different processing to the left and right channels of a stereo track.
"Split to Mono" is useful if you record 2 instruments at the same time with one panned to the left and one panned to the right. By splitting to two mono tracks you will then have one instrument on one track and the other instrument on the other and so are free to process and mix them as you like. (This is a neat trick for recording say keyboard and vocal at the same time).

[ignatz]

Steve:

test

Thanks for the details on “split the tracks”. I will soon be upgrading to 1.3.8 and will experiment with it. It may have limited use for me. The only outright “recording” I do with Audacity is occasional “what you hear”, directly to mp3 on sounds found on the web. Most of my Audacity use is processing of ripped mp3s (all two channel; some mono and some stereo) and occasional conversion of mp3s to WAV so I can burn them on a CD.

I appreciate your details on the stereo to mono thing, but I am still unclear on a few points.
I posed a few questions in my original post and hope you can answer them specifically. Here they are, with my best guess as to the answer. These questions assume we are dealing with an mp3 ripped from a CD in its original form (two channel stereo or two channel mono, but shown in Audacity as two channel in all cases).

Please correct any errors in my guesses:

Should I do this stereo to mono as the first step in processing, the last step, or does it matter?

My guess: I have no idea, but my ear can’t distinguish any difference.

Does stereo to mono conversion in Audacity by itself tend to reduce unwanted noise or distortion on a mono recording?

My guess is that it does not, if mono means “ two channels, but (near) identical content in each channel."

On a stereo recording?

My guess is that it does in all cases to the extent that each channel has different content and that this effect is more pronounced if the stereo recording is taken from vinyl with slight wear. You give up the stereo effect in exchange for less distortion (primarily groove noise), which is frequently a desirable trade-off.

On any two channel recording, even if mono?

My guess is no. If the content in the two channels is nearly identical, stereo to mono has no advantage in reducing distortion, but will reduce file size.

Incidentally, I have had a lot of luck with the “noise removal” filter on noisy 45s and 78s with the default slider settings. As little as ½ second of noise before the music begins is enough of a sample to make the filter work well. I have even run the filter twice on certain 78s that were pressed on poor shellac and noticed that the second pass provides even further noise reduction with no audible deterioration in the music.

Thanks for your time and comments.

[steve]

Should I do this stereo to mono as the first step in processing, the last step, or does it matter?

Unless you are applying any stereo effects, which you are probably not, then I don't think that it matters. At least not as far as sound quality is concerned. Working in mono will be a little bit quicker and use less disk space.

Does stereo to mono conversion in Audacity by itself tend to reduce unwanted noise or distortion on a mono recording?

If the left and right channels are the same, then there is no point merging them - they are the same, so whether you use one, or the other, or both, it will always sound the same.

On a stereo recording?

You mean "Does stereo to mono conversion in Audacity by itself tend to reduce unwanted noise or distortion on a stereo recording?"

If the music is a stereo recording then it is likely to sound better in stereo. It all depends on how much you want to sacrifice in listening pleasure for the sake of reducing file size. If you decide that you do want to reduce the file size, then you need to decide whether you want a mix of left and right, just the left channel, or just the right channel. In most cases a mix of left and right will sound better than just left or right on its own.

You give up the stereo effect in exchange for less distortion (primarily groove noise), which is frequently a desirable trade-off.

The noise level will not be much lower than keeping it stereo. In fact in some cases it could be worse.

Our first assumption is that if the original (pre-CD) recording was mono, then the music, when it finally gets into Audacity, is still mono. That is to say, it is exactly the same in left and right. This means that if we add (mix) them together, then the music component of our sound gets doubled.

Our second assumption is that noise in the recording is stereo noise. That is to say that the noise on the left channel is different to the noise in the right channel. This means that if we mix/add them together, the noise components do not reinforce each other, so the peak level is less than double.

With a stereo recording, the music in the left and right channels are different - that's what makes it stereo. So the two music signals will not reinforce each other to the same extent. Also, in the case of vinyl clicks, there is a good chance that any major scratches will affect both left and right channels, in which case they may reinforce each other and come out twice as loud.

On any two channel recording, even if mono?

What we are looking for is mono music and stereo noise. In this case the recording is likely to benefit from mixing the two channels. Some recordings will benefit more than others, but if it is mono music, then it is likely to do more good than harm.

I have had a lot of luck with the “noise removal” filter on noisy 45s and 78s with the default slider settings....

Take care not to overdo the noise removal. Too much noise removal will start to produce a bubbly metallic kind of sound to the music. It is usually most noticeable on high frequency peaks, such as cymbal crashes. It's quite hard to describe, but if you try using extreme amounts of noise removal then you will hear what I mean. The thing is that you can always do more noise removal later, but once you have Exported the audio out of Audacity you can not go back and undo noise removal, so any bubbly metallic sounds you are stuck with.