Surround sound formats


All this TV setup makes me curious; am I getting proper surround sound? I have 5.1 speakers connected to a Denon AVR-2309CI and everything HDMI, is it actually working?

The Denon receiver can decode the following formats via HDMI. I barely know what most of these are, but here’s a useful reference, also wikipedia.

  • 2 channel linear PCM (16, 20, or 24  bit samples, 32-192 kHz), also multi-channel.
  • Dolby Digital (5.1), DD Plus (7.1), Dolby TrueHD (7.1 high quality). Also known as AC3.
  • DTS (5.1) and DTS-HD (7.1)

When playing back a 2 channel audio source, my Denon supports a bunch of playback modes to fake out surround sound (“matrixed“). I believe the sound has to have been mixed this way for this to actually work.

  • Dolby Pro Logic II (fake 5ch) and IIx (fake 7.1)
  • DTS Neo:6 (fake 6.1) and “Neural” (XM HD Surround)

Looking at the random video files I have on my hard drive, most have audio in MP3 2.0 or AAC 2.0. One is DTS 5.1 and a couple are Dolby Digital 5.1. I don’t think there’s any way to tell if any of those 2.0 files are matrix encoded fake surround, probably not.


That’s all the theory, what happens in practice?

The good news is my new AppleTV seems to correctly play back 5.1 sources, via both Netflix and PLEX. In particular the receiver seems to autodetect DTS or Dolby Digital and if I stand next to a rear speaker it does sound differentiated. Yay! It may help I told PLEX my receiver can decode AC3 (aka Dolby Digital).

The confusing news is for all these stereo sources I don’t really know what receiver setting is best to use. I have no idea how common this matrixed audio stuff is or whether it’s worth the effort to decode it. I’ve got a button on my remote labelled “Movie Sound” that cycles the various matrix modes but can’t really hear much difference in a quick test.


After a bit of looking I found a test suite, the charmingly named “AC3 Dolby digital 5 1ch Sound Test”. It’s available around the net in various places, like filestube. (Or watch it in 2.0 on Youtube.)

This file plays back fine via PLEX on my Apple TV. I also played with using Handbrake to encode it in various ways. Sounds great in true Dolby Digital 5.1. Also sounded surprisingly good in matrixed Dolby PLII; the rear channel separation worked well, although the subwoofer was silent. HandBrake also supports a “6-channel discrete” mode which didn’t really work right on my A/V system.