How to playback multi-channel (AC3/DTS) AudioCD
If you try to play a multi-channel disc on a computer in usual way (using Windows Media Player for example) without AC3Filter, you'll get just a terrible noise. Why? And how can we playback it?
 What is multi-channel disc?
First of all, what is multi-channel AudioCD and how it works? Brief description can be found in AC3Filter & SPDIF:
- 1.1 General info about SPDIF
- SPDIF (Sony/Phillips Digital Interconnect Format) is a digital audio interface. SPDIF was first used only to transmit stereo 16-bit PCM data (CD player was the only source of digital audio). It is very straightforward: audio samples are transmitted at constant frequency, one-by-one (and bit-by-bit) in 32-bit blocks where 8 bits are used for synchronization and some side info and 24 bits are used for sample. In most cases, only 16-bit transmission is supported (CD carries only 16-bit sound) and low 8 bits of a sample are always zeroed. This interface is very simple and cheap and so it became widespread.
- Therefore, when multi-channel sound era came to home theaters, SPDIF was an ideal candidate for digital interface to transmit multi-channel sound. However, it is a problem: current interface implementations work only with stereo sound, but now it is required to transmit up to 6 channels (or 8 channels at present). It was decided not to change the interface but to transmit compressed multi-channel sound instead of PCM data (for digital interface it does not matter what to transmit). So receiver must recognize compressed data and decode it. For this purpose a new standard was introduced (IEC 61937) that describes how compressed data must be transmitted and how receivers can distinguish PCM and compressed data.
- So SPDIF interface has two modes: PCM data transmission mode and compressed data transmission mode. Unless noted we well use SPDIF transmission, SPDIF output mode and SPDIF stream terms only for encoded streams afterwards.
- 1.2 What is multi-channel AudioCD
- Compressed data may be transmitted over SPDIF instead of PCM data. Therefore, we can prepare a CD disk and place compressed AC3 or DTS data instead of usual PCM tracks. When we playback this disk with a CD player connected to a receiver we will get true multi-channel sound!
- But this trick does not work with analog connection and portable CD players: we will get terribly loud noise instead of nice music. Because of this CD disks with AC3/DTS tracks are relatively rare.
When a media player tries to playback a multi-channel disk it does not know that it contains compressed data. It's just because it is no simple method to distinguish usual and multi-channel disks. We have to read some data from the disk and have some method to know what kind of data we got. Hardware receivers have a special detection circuit, but software media players (most of it) have no. That's the point why media player cannot playback such discs right even when some AC3 or DTS decoder is installed.
 How to playback?
AC3Filter since v1.20b has built-in auto compressed data detector. First of all we should enable AC3Filter to process data from AudioCD to decode. This could be done with Use AC3Filter for... PCM option at System page of AC3Filter settings. This will enable AC3Filter to process any audio:
Also we must enable Detect SPDIF stream in PCM data. This option enables auto-detection of compressed data.
We may want to be notified when AC3Filter is used and see what happening during playback. For this reasons we can use Show tray icon option. With this option enabled tray icon will appear at notification area when AC3Filter starts working. Double-click on this icon to call filter settings page to monitor current filter activity:
Now we're ready to go! Insert your disk and enjoy the music! If auto-play is disabled, just run Windows Media Player and go to menu Play -> DVD, VCD or CD Audio.
You can use another media player. For example in Media player Classics you can playback the disc by calling menu File->Open CD-ROM.
Also this method allows you to playback .wav with AC3/DTS content without any conversion!
This method does not work with Winamp! and some other players that use its own decoders and does not utilize AC3Filter.
 SPDIF output issue
If you have an external receiver/decoder you can send compressed data directly to the receiver so it will decode the audio. Such option is called SPDIF passthrough.
But it is a big problem with SPDIF passthrough mode: most sound cards cannot work with 44100Hz SPDIF sample rate (used by AudioCD). Currently it is no workaround for this. The only solution possible is to resample audio from 44100Hz to 48000Hz. But this feature currently is only in plans.
 How to create a multi-channel disc
If you're interested to try multi-channel music, you may find some music and instructions here: www.sr.se. You may write a disc from these files or just open it in your media player.