3 min read

Howto play an AAC-5.1 over SPDIF

I'm preparing, at least psycologically, to HDTV on my PC. My current CPU isn't so great, but I have an external amplifier 120w x 6 that I don't what to throw in the trash soon.
When the 1920x1200 1080p monitors, and the rest, will decrease the price I'll be able to do the big step without any bad surprises (I hope), until now I try to taste how my last generation home cinema could be.

Some technical considerations

Dvd audio is AC3-5.1 and external digital receiver plays it with no problem via SPDIF.
Modern videoes 1080p, 720p, MP4, mov, Matroska (mkv) often use audio compressed in AAC-5.1, this is good because AAC is more efficient than AC3, but that is bad for my receiver that, as commonly for a receiver, it supports only the AC3 format.

So, how to use 5.1 with new formats?

You have to buy an AAC capable hardware receiver or convert AAC to AC3 at runtime.
This is theorically doable on Linux, and practically doable on Windows right now, thanks to a wonderful tool called AC3Filter.



If you hears some strange beeps probably you should try to enable/disable PCM.
I advice to keep AC3Filter everytime on system tray to see if the encoding works well.

If everything works the output should be something like:

 Input format: PCM16 3/2.1 (5.1) 48000 
User format: PCM Float 3/2.1 (5.1) 0 
 Output format: SPDIF 3/2.1 (5.1) 48000 
  SPDIF status: AC3 encode
  SPDIF passthrough for: MPA AC3 DTS
  Use AC3 encoder (encode stereo PCM)
  Check SPDIF sample rate (allow: 48kHz 44.1kHz)
  Query for SPDIF output support
Decoding chain:
(PCM16 3/2.1 (5.1) 48000) -> Processor -> (Linear PCM 3/2.1 (5.1) 48000) -> Encoder -> (AC3 3/2.1 (5.1) 48000) ->
Spdifer -> (SPDIF 3/2.1 (5.1) 48000) -> Dejitter -> (SPDIF 3/2.1 (5.1) 48000)
Filters info (in order of processing):
(PCM16 3/2.1 (5.1) 48000) -> PCM->Linear converter -> (Linear PCM 3/2.1 (5.1) 48000) -> Input levels ->
(Linear PCM 3/2.1 (5.1) 48000) -> Mixer -> (Linear PCM 3/2.1 (5.1) 48000) -> Bass redirection ->
(Linear PCM 3/2.1 (5.1) 48000) -> AGC -> (Linear PCM 3/2.1 (5.1) 48000) -> Delay -> (Linear PCM 3/2.1 (5.1) 48000)
-> Output levels -> (Linear PCM 3/2.1 (5.1) 48000)
Stream format: AC3 3/2.1 (5.1) 48000Hz
Bitstream type: byte stream
Frame size: 2560
Samples: 1536
Bitrate: 640kbps
SPDIF stream type: 0x1
Frame interval: 2560
Actual bitrate: 640kbps

Players and other tools

I tried several players with modern videos and AC3Filter an here is my chart:
Windows Media Player, althoght every time it tries to connect to find the correct codec, wmp is the player that worked better for me, even if in the mkv the texual subtitles (ttxt subs) didn't appere.
The Core Media Player</a> is quite good too, and it supports subs.
Videolan (VLC) worked in 2.0, I didn't try Mplayer but in Linux it is the best media player so I think it isn't bad on windows too.

I you miss some codec try Nero Video Decoder for video and Morgan Stream Switcher for audio.

Some demo in AAC-5.1 to test

mov Trailer from Apple 1080p


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