Monday, October 25, 2010

How to increase volume in one channel of stereo MP3 files

Due to poor recording and encoding, sometimes the audio volume on the two channels of a stereo audio file do not match. When such an audio file is played back on the computer or on a music player, one speaker plays louder than the other, and the stereo surround effect is lost. Depending on how bad the mismatch is, it may be impossible to hear the low playing channel over the high playing one. Trying to adjust the volume with the volume knob is futile since the volume control acts on both the channels at the same time.

A workaround to this problem is to adjust the left-right speaker balance on the music player.

winamp-balance

But this is not a permanent solution and you have to set the balance back once you are done playing with the faulty file. In this article we will see how we can fix, what I believe, is a common problem.

You will need a free, open source program called Audacity. You will also need the LAME MP3 encoder plugin for Audacity. Download both from the links provided below:

- Audacity
- LAME MP3 encoder

Install Audacity and then install LAME MP3 encoder to a location which you can easily access. We will need that later.

Open Audacity and load the faulty MP3 file from the File > Open menu.

Directly to the left of the track appears the Track Control Panel, as indicated by the red marker.

audacity-track-control

Click on the small black arrow on the right of the Track Control Panel to reveal a menu. Select the option “Split Stereo Track” from the menu.

audacity-split-track

Once this option is selected, a second Track Control Panel will appear adjacent to the second audio channel of the stereo file. The track on the top is for the left channel while the one at the bottom is for the right channel.

audacity-split-track2

By now you already know which channel has the low volume. If the volume on the left speaker is low we have to increase the volume of the left track (top). If the volume on the right speaker is low we have to increase the volume of the right track (bottom). BUT, what if your speakers are set up incorrectly? That is, the left speaker is placed on the right and the right speaker on the left. If so, then what you think is the left channel is actually the right and what you think is the right channel is actually the left.

To clear this confusion, click on the Solo button on the Track Control Panel of the top track and click on Play. This should play the music through the left speaker and right speaker should be silent.

audacity-solo

Once you have correctly identified the channels, move the Gain slider of the channel that has a lower volume towards the right to increase the gain or the volume of the track. Listen to the result by clicking the play button and adjust accordingly. (Make sure the Solo button is unselected).

audacity-gain

When you think the volume of both the channels are appreciably equal, click on the File menu and choose the option “Export to MP3”. Here you will get an alert box telling you that the LAME plugin is missing, and then ask you if you want to locate the lame_enc.dll file.

Click YES and locate the said file on the directory where you installed LAME MP3 encoder. Finally save the exported MP3 file.

A note of caution: Increasing the gain of one channel (or both) may lead to ‘clipping’ of the audio signal, producing distortion. Keep a close eye on the volume level indicator at the top as you adjust the gain. Try to keep both green bars at the same level and make sure it does not overshoot.

audacity-volume-indicator

Related:
How to increase audio volume of a video file
Normalize an MP3 with MP3Gain

5 comments:

  1. Compressing an already compressed mp3 file is a no no for sound quality reasons. Better to save the adjusted file to a lossless format (Wave, Flac etc.)

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  2. This method doesn't compresses the MP3 file, it only increases the gain or volume of the file and then re-encodes the file using the same bitrate. If you don't change the original bitrate or sampling frequency, you can re-encode any MP3 file a hundred times over and no quality will be lost.

    This is the basic difference between Analog and Digital processing.

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  3. Thanks for the feedback. I ran a test following your instructions above. I used an mp3 file @ 256kbps bitrate, made some gain changes then exported to mp3. The resulting file was only 128kbps bitrate and had therefore been re-encoded. There was no option to select the bitrate to match the original before saving the file. The fact the Gain slider goes up in increments of +/-1 dB is strange to me having for many years used other mp3 tools that do not re-encode such as mp3Gain and mp3Trim. With both tools the volume can only be changed in increments of 1.5 dB which is due to the nature of the mp3 format. MP3 and AAC files can only be losslessly modified in 1.5 dB steps. This article explains it:-

    http://wiki.hydrogenaudio.org/index.php?title=Replay_Gain

    Can you shed any more light on this for me.
    Thanks again

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  4. **UPDATE**
    Found the mp3 bitrate options button when exporting to mp3. By default it is set to 128kbps so altering it to the same setting as the original file is obviously possible but very awkward considering the bewildering array of options for quality, speed and mode. It all adds up to making me think the file is going to be re-encoded regardless of the settings you select. If not, then the program should select for you the exact credentials of the original file.

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  5. Thanks for enlightening me on this matter. I thought increasing the gain will be like normalizing, where only the gain is increased leaving the audio data untouched. I apologize for assuming things without looking into the Audacity settings.

    I believe you do have a valid point. Converting to a lossless format is a preferable solution, although I also believe the human ear is incapable of distinguishing the difference between two MP3 files encoded with different settings.

    ReplyDelete