Viewers of digital television services have the ability to receive digital audio along with the digital pictures. In many cases, this audio is Dolby Digital (AC-3) and is capable of up to 5.1 channels, but in some cases such as with satellite broadcasting, the audio may be delivered using another coding format such as MPEG-1 Layer 2 that is decoded back to baseband PCM before output. In an over-the-air or cable set-top box, the only format for digital audio is Dolby Digital (AC-3), but it can be downmixed to stereo PCM. Therefore, in most all set-top boxes a choice can be made regarding how to output digital audio. The output can be set to be PCM only, PCM or Dolby Digital (AC-3), or sometimes Dolby Digital (AC-3) only. Although somewhat self-explanatory, it is worth pointing out that each selection will produce very different results. In the case of PCM, all audio of any format is converted back to two channel PCM prior to output.
The most popular out-of-box default for these set-top boxes seems to be PCM only as it is the most compatible with very old legacy home theater receivers that might not contain Dolby Digital (AC-3) decoding which although rare, do exist. If Dolby Digital (AC-3) was output to home theater receiver without a decoder, an obnoxiously loud sound could result, so it seems manufacturers have taken the safe path in many cases.
Consumers and broadcasters must be careful as this PCM-only mode is of course also completely compatible with home theater receivers that do contain 5.1 channel Dolby Digital (AC-3) decoding. The result will be that 5.1 channel broadcasts will be downmixed to stereo and decoded via Dolby Pro Logic, PLII, DTS Neo:6 matrix decoders. The audio will still fill all the speakers, but it will not be truly discrete 5.1 and it will not match the intent of the program producer. Again, when troubleshooting with a customer reporting audio issues, make sure to check this setting, as it could be the heart of their trouble.
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