When tracks or albums are analyzed, the perceived volume of the track is computed, using the Replaygain algorithm. The perceived and maximum volumes are stored in the audio file's tags. The volume of the file is not changed during analysis.
When gain is applied, the volume of the file is changed. The amount of the gain adjustment is stored in the audio file's tags. This allows the gain adjustment to be undone without any loss of quality.
Each audio sample of the track is given the same relative adjustment. This means that the relative volumes of the loudest part compared to the softest part of a track are not changed during Gain Application.
If Gain is applied without doing Analysis first, the Analysis is done automatically for you.
During Gain Adjustment, iGain will attempt to adjust the volume of tracks or albums to the Target Volume. The target volume is user-adjustable.
When Track is selected from the View Menu, each track is analyzed independently. If the different tracks of an album have varying volumes, their volumes will be equalized after applying gain changes.
When Album is selected from the View Menu, the tracks of each album are analyzed and adjusted together, as if they were a single large track. If the different tracks of an album have varying volumes, the relative differences will be preserved after applying gain changes. Album View is recommended for classical music albums, in order to preserve the volume differences between movements of a symphony. Track View is recommended for a pop album of greatest hits.
When an audio CD is encoded as an MP3 or AAC file, the amplitude of its individual audio samples are represented by fractional numbers between 0 and 1.0. If a track whose maximum amplitude was 1.0 is made louder by iGain, its resulting maximum amplitude may exceed 1.0. This is referred to as clipping.
iTunes and iPod can tolerate small amounts of clipping with no problems. However, severe amounts of clipping will result in distortion when playing back adjusted tracks.
iGain may be set to limit gain adjustments so that clipping does not occur.
The audio samples in MP3 or AAC files are grouped into small blocks known as frames. Each frame's header contains a Global Gain which applies to all audio samples in that frame. Adding 1 to or subtracting 1 from Global Gain results in a volume increase or decrease of about 1.5dB.
iGain operates by modifying the global gain fields of all frames of a track by the same amount. This allows the gain changes to be reversed, with no loss of signal quality. There are no digital to analogue conversions done during gain adjustment.
The limitation of this approach is that volume adjustments can only be one in units of 1.5dB. The required gain changes during analysis are rounded to the nearest 1.5dB. This means that the actual adjusted volume will be within 0.75dB of the desired volume. In practice, most humans can not detect volume differences of less than 1.0dB, so this limitation is not serious.
MP3 and AAC file formats include fields known as tags, which are used to store data such as title, artist, album, comments, etc. iGain adds additional tags which contain the results of analysis, and the cumulative volume adjustment.
These additional tags do not appear to interfere with iTunes or iPod. Users of these players should never have to remove these tags. The tags may interfere with the operation of other players. Therefore, iGain provides a command to remove the tags it has added. Since these tags are used to undo gain changes, you will not be able to undo changes after removing tags. It is recommended that you undo gain changes prior to removing tags.