@ atariage we re-find out that GTIA is generating the keyclick sound which will be mixed into the pokey sound output... in my atari books i read that writing values to the CONSOL register $d01f will cause sound output done by the GTIA (strange... ;)) and atari OS does use this method for doing the keyclick...
is there any software (tracker, game, demo) which use this "5th" channel for sound or digi output???