The aspect ratio is fixed for HD content(we assume a 16:9 HDTV), but variable for some SD content using the aspect ratio button(button on remote shows 2 boxes). The default is auto, which uses broadcast AFDs to determine the aspect ratio(4:3, stretch and zoom). By pressing the aspect ratio key on some SD content, you can toggle through the available options(4:3, stretch and zoom) to suit the TV. Once you have adjusted the aspect ratio for the selected channel it will stay in the adjusted setting until the channel is changed, and the next time you go to the channel you will need to press the aspect ratio button again if you wish to readjust.
NOTE: The TV could also have it's own aspect ratio options so make sure these are not causing issues.
If the SD content is being broadcast in wide format, the CM-7500 aspect ratio button, cannot function. If you wish to change the aspect ratio, you'll need to do that in the TV menu system/GUI.