i have updated my imac to yosemitte since then my apple tv does not work properly the sound and video appears chopped.

Hi paracelse,

I'm sorry, I'm a bit unclear on exactly what kind of issue or issues you are having with your Apple TV since your update to Yosemite on your iMac. For general Apple TV troubleshooting, you may find the following article helpful:

Apple TV: Basic troubleshooting - Apple Support

Note: If you actually do have a 1st Generation Apple TV (the silver model, not the black one) as noted in the referring article, then this page may be of more use:

Apple TV (1st generation) - Apple Support

Regards,

- Brenden