The HDMI ports that you mention on your various devices are unlikely to be input ports, and even if they are they are less likely to be able to convert the HDMI signal from the Apple TV to the type of connection you are connecting to the TV with.
Short of changing your TV, you may wish to consider a converter, however converters come with their own problems.
There are hardware converters that will convert HDMI to various other types of output, however there are some issues with doing so that you should be aware of.
Resolution and aspect ratio
I'm not aware of any converters that will scale the output from the Apple TV, any TV or projector which is used will need to be widescreen and support resolutions of 720p (Apple TV 2).
The HDMI on the Exporer 8300HD is an output.
Review your bill with comcast. Your set-top box is "too" much and you are likely over paying.
The Apple TV won't work with your TV
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