It's not automatically made... each song has it's own preview start and preview length defined by the StepMaker. If your songs are with wrong previews, it means that the person who did the steps for the song did not adjust the time.
Originally Posted by Grizz
If you have several files that the preview is wrong, look for other simfiles packages that contains the right times with other people. I think there are correctly made simfiles with correct preview times...
But if you want to adjust the previews yourself, you can do it using one of these two ways described below.
[I] Within Stepmania
1. Go to Song Edit
2. Select the song you want to adjust
3. Enter the edit screen mode (at any difficulty)
4. Hit "M". the preview music will start playing.
5. Use the "[" and "]" keys to adjust the preview start. There's a number at the right part of the screen that shows the time the preview starts.
6. Repeat 4 to 6 until the preview beginning is right.
7. To adjust the preview length, use "shift+[" and "shift+]".
8. Hit "M" and repeat until the preview length is right.
9. After everything is fine, hit ESC, select SAVE, then hit ESC again and EXIT.
10. Your song preview is fine now.
[II] Outside Stepmania, using a text editor
1. Open Notepad
2. Open your song folder. Example: "C:\Program Files\Stepmania\Songs\MySong\"
3. Open the .sm file. Example: "my song.sm"
4. Look for the following lines:
5. Replace the <number>s with the time (in seconds) where the preview should start and the length of the preview.
7. Your song preview is fine now.