Media Players and Downmixing
Generally, a Dolby Digital Plus bitstream can only be transported over an HDMI 1.3 or greater link. Older receivers support earlier versions of HDMI, or only have support for the S/PDIF system for digital audio, or analog inputs.
For non-HDMI 1.3 links, the player can decode the audio and then transmit it via a variety of different methods.
- Earlier versions of HDMI, such as HDMI 1.1, support PCM audio, where the player decodes the audio and transmits it losslessly as PCM over HDMI to the receiver.
- Some receivers and players support analog surround sound, and the player can decode the audio, and transmit it to the receiver as analog audio.
Most receivers and players support S/PDIF. This lower bandwidth digital connection is not capable of transmitting lossless audio with more than two channels, but a player can transmit a S/PDIF compatible audio stream to the receiver in one of the following ways:
- Blu-ray Disc players can take advantage of the legacy 5.1 AC-3 bitstream embedded in the E-AC-3 bitstream, transmitting just the AC-3 bitstream with no modifications.
- Players supporting the HD DVD standard can transcode the decoded audio into another format. Depending upon the method and options available to the player, this can be done with relatively little quality loss. Dolby's reference decoder, available to all licensees, exploits the common heritage between AC-3 and DD+ by performing the operation in the frequency domain. Hybrid re-compression avoids unnecessary end-to-end decompression and subsequent recompression (DD+ → LPCM → AC-3.) In addition to AC-3, some HD DVD players transcode audio compatible with S/PDIF into 1.5 Mbit/s DTS audio.
Should the player need to decode the audio for a non-HDMI 1.3 receiver, the results should be predictable. The DD+ specification explicitly defines downmixing modes and mechanics, so any source soundfield (up to 14.1) can be reproduced predictably for any listening environment (down to a single channel).
Read more about this topic: Dolby Digital Plus
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