Is a simple sequencer for the Atari 2600 created by Paul Slocum. He also have other neat project running in his website like the synthcart and sequencer kit and few others.
LoopCart is a simple sequencer who use 2 channels. each channel have 6 subpatterns and each pattern have 8 notes.
Since the atari 2600 joystick don't have many buttons... well just have 1... to edit a value you have to press THE ONE buttom and move up and down, so you can modify the value it had. Also Paul implement the buttons of the console, that mean the right difficultly switch is the play/stop control. Hard/Expert = PLAY and Easy/Novice = STOP the song.
If you want to use this rom into a REAL CART, you will need an AtariVox
to storage the data.
Check the manual here
More information here