I have finally made some improvement in  my tetriz game; here's the changelog:

The most obvious change is the music during the gameplay... now I wonder if it could be annoying...

The most challenging improvement was optimizing the collision detection system. Now it should be accurate AND faster than before (otherwise the music during the gameplay could not be implemented). This results in a gameplay slightly more fluid; as you can see by the screenshot, I've managed to reach level 27 with 130 lines cleared.

Another small change is the UDG of the blocks from which tetromines are made.

...last but not least, I've fixed the random function, so the tetromines are not always in the same sequence as before.

For now I have released only the keyboard version of the game: I need more time to test the joystick version...

GLDM