I cannot get my Apple TV to output Dolby digital audio

I got my new Apple TV and hooked it up. The HDMI cable goes to my Pioneer surround sound Dolby 5.1 receiver. I have the Apple TV audio settings set for Dolby 5.1. Every source on the Apple TV I've tried is only outputting stereo. I've tried HBO movies that I *know* are Dolby, but my Pioneer receiver says "stereo" on the front panel. Every other device I have which includes an Amazon Firestick, Xfinity X1 cable box, Sony DVD player will output Dolby. When I select one of these sources my Pioneer will display "Digital" on the front panel and I will indeed hear surround sound. When I switch the input on the Pioneer receiver to Apple TV the Pioneer display switches to "stereo" and I only hear audio out of two front speakers! This isn't right! Note: I replaced an Apple TV 3rd generation with this newer 4th generation unit. When I hook back up the 3rd generation Apple TV I get surround sound, the display on the Pioneer receiver says "digital". So, What is going on???? HELP!!