Why are there two Dolby Headphone technology logos: one saying Dolby Digital and the other Dolby Pro Logic® II?

Dolby Headphone is designed to process a 5.1 signal. It can also process stereo sources, but the stereo signal must first be converted to a 5.1 signal. Products can feature either of these logos, depending on the manufacturer. So, products with a Dolby Pro Logic II/Dolby Headphone logo use the Dolby Pro Logic II technology to expand stereo content to 5.1-channels, which are then processed through Dolby Headphone. Listeners are able to get a 5.1-channel surround sound experience from a stereo source with any pair of stereo headphones. Products with a Dolby Digital/Dolby Headphone logo decode 5.1-channel Dolby Digital soundtracks and apply Dolby Headphone processing to the multiple channels. Because the material is already encoded with 5.1 Dolby Digital, the multiple channels are discrete. Dolby Headphone then gives the listener a 5.1 Dolby Digital surround sound experience from any pair of headphones.  more