Millisecond Forums

I am having a few issues with timings for my current task.

The task is a Flash beep illusion.

The following stimuli are presented.

1.    Fixation cross for 1000 ms
2.    Then 2 stimuli are presented simultaneously, (1) a ‘Ball’ for 12ms (covered by the ‘Grey’ picture) and (2) a ‘Beep’ (7                  seconds in duration).
3.    36 seconds after the end of the first ‘Beep’ a second ‘Beep’ should then sound.

Below is the script for a trial with a 36 ms delay between the end of the first ‘Beep’ and the start of the second ‘Beep’.

<trial Delay_36>
/ stimulustimes = [0=Fix; 1000=Ball; 1000=Beep; 1012=Grey; 1012=Fix; 1043=Beep]
/ trialduration = 2000
/ branch = [trial.Agency]
/ recorddata = false
/ ontrialend =[values.delay=36]
</trial>

I have monitored timings via the audit option. Unfortunately, my stimuli times are out by around 10-12 milliseconds. This error varies across trials and machines. I am not currently erasing stimuli.

My question then is this, is my programming approach wrong or am I asking for too much accuracy from a humble PC?
Many thanks for your help
Peter