"All DD+ streams must contain at least one independent substream or legacy substream, which contains the first (or only) 5.1 channels of the primary audio program."
But Dale, Dolby in their own literature specifically cites that it is backwards compatible. Since they are the author of their own specifications I'm going with what they say over Wikipedia. I know ATV sure sucks the 5.1 out of higher encodes. I am not certain about the technical aspects but it works as Dolby describes. The Wikipedia also says DD+ can be sent down TosLink and the Dolby Standard says HDMI 1.3 is required.
For maximum compatibility with the
worldwide installed base of home theater
systems that incorporate Dolby technologies,
and for connectivity over S/PDIF interfaces,
the decoder also generates a standard Dolby
Digital bitstream output (up to 5.1 channels)
from any incoming audio coding format,
which can also be an encode of the 5.1 mix
from dual 5.1 decoders, all contained within
the one Multistream Decoder package.
Apple TV, commonly referred to as ATV. DD+ streams are as they should be out the HDMI port, DD streams are sent the TosLink connector. Been doing it for years and the Audio issues with Roku is why I bought an ATV, so I could watch movies in 5.1 surround from my old and very expensive Onkyo receiver.
DECODING AUDIO STREAMS
Apple TV includes a built-in decoder for Dolby Digital Plus and Dolby Digital audio. This means that the Apple TV is able to decode Dolby audio rather than simply pass the compressed bitstream to another device (such as an audio/video receiver [AVR] or a sound bar).
Decoding the audio inside of the Apple TV enables Apple to support several features such as mixing of other sounds from the system, Siri® voice input, and so on.
When a user plays media with stereo or multichannel audio (including Dolby Digital Plus and Dolby Digital audio), the audio soundtrack is decoded to two or more PCM channels (uncompressed digital audio) before it is sent to the AVR or other downstream device.
Dolby Surround – This setting is intended to force the Apple TV to output a Dolby Digital (also known as “AC3”) compressed bitstream. This may be required with certain legacy devices or sound bars that support the older Dolby Digital format but do not yet support Dolby Digital Plus or multichannel PCM audio. Remember that Dolby Digital is only capable of up to 5.1 channels of audio, so this option will never be able to send 7.1 channel audio to the connected HDMI device.
When this setting is selected, the audio is first decoded (although constrained to 5.1 channels) and mixed with other system sounds, and is then reencoded to the legacy Dolby Digital format just prior to the HDMI output. The connected device (TV, sound bar or AVR) will then receive a Dolby Digital bitstream.