Basic Operation

Before you begin using MP3 Trimmer, we recommend that you read at least this section. The most basic functionality - trimming an MP3 file - is described here. When you feel comfortable with the basic functionality you may head on to the other sections.



Loading an MP3 file

Launch MP3 Trimmer and select 'File->Open new MP3' (Cmd-O), or drop the file onto the MP3 Trimmer main window (if you have registered MP3 Trimmer, you can also drop the MP3 file on the MP3 Trimmer icon in the OSX Dock).

You may also drag an MP3 file
directly from the iTunes window to MP3 Trimmer window.

MP3 Trimmer will validate all frames in the MP3 file (with a large MP3 file this can take a while).

If the file is damaged, an alert pops up and offers to repair it (if possible). A new, repaired file will be created.

About MP3s recorded from internet radio stations
MP3/ID3 standards suggest that ID3-tags should be placed at the beginning or the end of the MP3 file, but never inside. MP3 streams recorded from internet radio station may sometimes contain several embedded ID3-tags in the same file, as well as minor bit errors due to connection flaws. Make sure to set MP3 Trimmers import routine to Relaxed-mode (in the Preferences) when working with such files.

About the AAC-format
MP3 Trimmer edits MP3's and will not accept AAC-files (.m4a or .m4p).



Main window overview:




MP3 Information area  1

This shows useful Information on the file:

Audio mode Mono, Stereo or Joint Stereo encoding
Frequency The sampling frequency (Hz)
Bitrate Sample bitrate (kbit/sec)
Offset Number of bytes before the audio data begins
ID3 tag ID3 tag version (if present)
Orig. duration Original duration of the file
New duration Duration of the trim selection
Frames Number of MPEG frames in the file
Frame No. Current frame position in the file



Playback controls  2

Playback controls (middle of the main window) let you listen to your MP3 file.
Locator bar  3 (top of the main window) lets you move to any position in the file.
Time display (in the middle) will show the current playback position.
Audio playback level (above the time display) indicates the audio level during playback only. This does NOT affect the audio level of your trimmed MP3 file.
During playback:
Play button changes to a Pause button - clicking it will pause playback
Fast Forward/Rewind buttons scan the audio in either direction
A Waveform*/VU/equalizer display for visual feedback
* waveform mode only available for MP3 files
During pause:
Fast Forward/Rewind buttons step one audio frame at a time in either direction.



Trim points

The trim points are used to select the beginning and end of the part you wish to keep in the MP3-file.

There are two different trim points; the
In-point and the Out-point. When you first load an MP3 file, the In-point and Out-points are automatically set to the beginning and end of the file.



Setting the In-point  4

Drag the locator to the In-point (beginning) of the piece you wish keep.

The In-point controls are at the left side of the main window.

Click the "
Set" button to mark the In-point. The In-point time display will show the In-point time, the "Set"-button changes to "Clear", and the locator changes to green indicating it now marks the In-point.



Auditioning/adjusting the In-point

You can test the trim points before you save the finished file. MP3 Trimmer is now 100% frame accurate. A small test clip is created and immediately played back. You can change the test clip length in Preferences.

Click the "
Try" button to listen to the current trim point. If the "Try" button is disabled (greyed out), no In-point has been set (see above). You can also initiate the audition window by pressing Cmd-T when you are near a trim point (the closest trim point will be the one auditioned).

The test clip in the audition window will let you listen to the trim point (100% frame accurate) as well as audio fades and/or gain adjustments.

When the audition window is up, the following keycommands are available:
Spacebar Start / Stop playback of the test clip
Enter Restart playback of the test clip
Return Close the audition window

An equalizer display allows you to monitor gain adjustments visually.

If you wish to relisten to your clip, just drag the slider to the beginning, or press the Enter key.

You can adjust the trim point. Stop the test and press the
+ or - buttons located at each side of the "Try" button. Each click moves the trim point one frame forward or backward.



Setting the Out-point  5

The Out-point marks where the clip you wish to keep will end. The procedure is identical to the way you set the In-point. When the Out-point is set, the locator will change to red, indicating that it now marks the Out-point.



Applying fade-in and fade-out  6

At the lowest part of the main window you'll find a checkbox to enable audio fading. If this checkbox is disabled (greyed out) audio fading is not possible*.
By clicking this button you enable the buttons to control the length of the fade-in and fade-out respectively. The length can be set in intervals of
half a second (or 0.1 sec by holding the Shift-key down while clicking) and the longest possible fade time is 45 seconds. To listen to the effect of the fade-in or fade-out just press the desired Try-button. If the Try-button is disabled, no trim point has been set.

You may easily
select the type of fade curve to use by clicking on the fade curve symbols. The fade curves determines how fast and intense the fade action should be - you may try them out with the Audition window to find the most suitable one.




Adjusting the over all gain  7

To the left of the audio fade controls you'll find a checkbox to enable gain adjustments. If this button is disabled ('grayed out') Gain Adjust is not possible*. By clicking this button you enable the buttons to control the amount of Gain Adjust. Adjustments are set in relative percentage values.

The Gain Adjust operation actually adjust the output gain of each frame, whichh is nothing like the 'adjust gain for this song' parameters in many audio playback devices/applications (they may only work for that specific device/application). The gain adjust performed by MP3 Trimmer is written to each and every frame of the song, which will allow it to work on all playback devices (hardware / software). It may seem pretty permanent, but you can always change the gain in opposite direction with no quality-loss (we're dealing with it binary here).

A word of advice: It's easy to distort the audio if too much gain is applied. Use the audition feature to listen to the effect. In other words - use your ears....

The audio fading and gain adjustment routines will not work on MP3 files with CRC checksum protection. However, CRC is not widely used since it slightly degrades the sound quality. On MP3 files not intended for streaming, CRC is of no use at all.



Clearing a trim point

If you move the locator away from the trim point, the color of the locator will switch back to normal (light-blue), and the Clear-button will change back to a Set button.

To clear a trim point, simply position the locator at the trim point again (drag the locator until it stops in either direction). Now, the color of the locator will change and the Set-button will change to a Clear-button. Pressing it will remove the trim point.



Saving your trim selection

When you're satisfied with your trim settings - and have auditioned both the In- and Out-point - you can save your selection as a new MP3 file. This process is done by copying the selected frames, which means no re-encoding is done. Not even the fade parts are re-encoded, instead this is done by changing specific gain values inside the audio frames.

Select 'File->Save trim selection' (
Cmd-S) to bring up the Save dialog. Choose the desired location and name your file.

If you have checked 'Create filename' in the Preferences, MP3 Trimmer will create a filename for you, based on the filenaming template chosen and the ID3 tag information in the file (if present).

In addition, if you checked the option 'Keep ID3 tag'
 8 (enabled if ID3 tag data is present) the ID3 tag data will be preserved in the saved file.

After a few seconds (on an average sized MP3) your new, edited MP3 file has been saved! And with exactly the same audio quality as the original file...

Note that saving is a lot slower when gain adjust is activated. This is normal, since each frame is being gain adjusted, which will take some more time.

You may also save an inverse trim selection (Cmd-Alt-S). This means that the part BEFORE the IN-trimpoint and the part AFTER the OUT-trimpoint will be saved instead. This could be useful if you wish to 'crop out' a section within a song. Please note, however, that audio fades will not be applied when you save this way.



Move the In-point to the Out-point

With both In-point and Out-point set, you can easily exchange the current In-point with the Out-point (the old Out-point will be cleared) by pressing Cmd-M (in the Tools menu). This is a handy short-cut when you're extracting multiple songs from one large MP3 file.