Amplify or normalize

Amplify and Normalize are very similar effects, but they have some subtle differences. The first difference that you will notice is that the Normalize effect has an option for correcting DC offset.


Remove any DC offset
DC-offset.png
In this image you can see that the upper track is not centred vertically. It is “off-set”. We can correct this fault by using the Normalization effect with “Remove any DC offset” selected. Using Normalization set with this selected, but with “Normalize maximum amplitude to” not selected, will correct this off-set without making any other changes to the amplitude - the result will look like the lower track.


“Normalize maximum amplitude to” vs. “Amplification (dB)” vs. “New Peak Amplitude (dB)”

The Normalize effect offers the option to “Normalize maximum amplitude to” a chosen level.

The Amplify effect offers two options: either to set “Amplification (dB)”, or to set New Peak Amplitude (dB)".

For mono tracks, these are essentially different ways of saying the same thing. If we have a track and the peak amplitude is -6dB (+0.5 to -0.5 on the vertical scale), then we have three ways to bring the peak up to 0dB (+1.0 to -1.0 on the vertical scale).

  1. We can “Normalize the maximum amplitude” to -0.0dB
  2. We can set “Amplification (dB)” to 6dB
  3. We can set “New Peak Amplitude (dB)” to -0.0dB

If we are not using the DC offset correction, then each of these effects will do exactly the same thing and our processed audio will be exactly the same whichever we use.


When to Use Normalize with DC off-set correction
If DC offset is present in the audio track, then you should use DC offset correction.

A very small amount of off-set is very common, but large amounts (such as shown in fig 1) should not normally happen. If your recordings regularly produce noticeable DC offset then this indicates that there is a fault in your system (usually with either the sound card or the sound card drivers, though connection of faulty sound sources can also cause this problem).
amplify-normalize.png
In fig.2 we have applied the Amplify effect to a waveform that had DC offset (upper track), and in the lower track we can see the same waveform after the Normalize effect has been used with DC offset correction.

In both cases the maximum peak amplitude was set to 0.0dB, but note that in the upper (Amplified) track, the resulting waveform is noticeably smaller than in the lower (Normalized) track. Also, you may notice that the DC offset has become worse when the Amplify effect has been used - the DC offset has also been amplified.

The reason that the Amplified waveform is smaller is that the positive peaks have been shifted up vertically by the DC offset, so although the positive peak is at 0.0dB, the negative peak is far away from the -1.0 level.


Differences between Amplify and Normalize on multiple audio channels

This is the issue that has sparked off several lively debates as Audacity’s Normalize effect breaks with the convention of most other audio programs.

Amplify multiple audio channels
If you select multiple tracks, or tracks with more than one channel (stereo tracks) and apply the Amplify effect, then all audio channels will be amplified by the same amount.
If you select “Amplify by 3dB”, then all audio channels will be amplified by 3dB, irrespective of their original level.

Normalize multiple audio channels
If you select multiple tracks, or tracks with more than one channel (stereo tracks) and apply the Normalize effect, then all audio channels will be independently normalised to the same peak level.
If you select “Normalize to 0.0 dB”, then every selected audio channel will be normalized to 0.0 dB, irrespective of its original level.


Let us see this effect on a stereo track. In fig 3 we can see two stereo tracks - they each have 2 audio channels - a left channel and a right channel. Note that the peak amplitude of the Left (upper) channel of each stereo pair is quite a bit greater than the peak amplitude of the Right (lower) channel.
stereo tracks.png
We will now Amplify track 1 (the upper track) to 0.0dB and we will Normalize track 2 to 0.0dB
Amplify-vs-Normalize.png
Note that the in the upper stereo track, both Left and Right channels have been Amplified by the same amount (about 1.0dB).
Note that in the lower track, each channel has been processed independently so that each channel has a peak value of 0.0dB - they have each been Normalized to 0.0 dB


Advantages of using the Amplify effect
You can quickly and easily amplify one or more tracks by the same amount. For example if you want to reduce the level of several tracks by 3dB, then use the Amplify effect and set it to “Amplification = 3.0dB”. This is very useful if you want to change all of the channels in your mix by the same amount as it will maintain the same relative balance of
the tracks.


Advantages of using the Normalize effect
You can correct DC offset errors with this effect.
You can quickly and easily maximise multiple audio tracks at the same time.
If you have accidentally recorded one channel quieter than the other in a stereo recording, the Normalize effect will go a long way toward restoring a correct balance as it normalises each channel independently.


When should you not use the Amplify effect
If your audio has DC offset present, you should use the Normalize effect to correct the DC offset.

When should you not use the Normalize effect
If one or more channels intentionally have different peak levels to other audio channels, then you should not normalise them together. Normalize works on audio channels independently and will bring the peak levels of each audio channel to the same level.

In particular, if you have a stereo track and one channel has a higher peak than the other channel, but the overall Left/Right balance is correct, then you should not use the normalize effect as it looks for the peak level in each channel and will change the stereo balance. You can still use the DC offset correction in this case with the “Normalize to” de-selected.

Great thought-piece here Steve - great to have this documented here. Are you planning to add this article to the Wiki and/or the 1.4 manual?

There is a subtle variation on Steve’s methods that you can do if you have DC offset and still want to use the Amplify effect

  1. Use the Normalize effect with the DC offset remeoval selected and with no normalization selected.

  2. Then use the amplify effect (to keep the relative levels of the tracks in step as before)

WC

Thanks, and yes, I’ve just finished adding it in the “For Users” category of the wiki: http://audacityteam.org/wiki/index.php?title=Amplify_and_Normalize

I thought I had covered that with the last sentence (“You can still use the DC offset correction in this case with the “Normalize to” de-selected.”) but perhaps I was not explicit enough. Feel free to add that to the wiki version if you think it should be stated more clearly.

It’s what happens when I don’t read thoroughly enough :blush:

Good to see it in the Wiki.

WC