DigitalOut

These instructions apply to digital connections using either electrical coax or optical connections.

• 1 Special terms

• 2 Overview

  • 2.1 Sound Data Encoding
  • 2.2 Digital Sound Media
  • 2.3 Side notes

• 3 Check your mixer

• 4 Find your device

  • 4.1 Separate playback device
  • 4.2 Device aliases
  • 4.3 Alternate method

• 5 Use your device

  • 5.1 PCM output

  • 5.2 Digital surround passthrough

    • 5.2.1 Test your setup
    • 5.2.2 Solve problems

  • 5.3 Configure your media player

    • 5.3.1 mplayer
    • 5.3.2 MythTV
    • 5.3.3 xine

• 6 Set digital out as default

Special terms

Terms used when dealing with digital outputs are:

• IEC958 - The name of the IEC standards document and often seen in the alsamixer controls
• S/PDIF or SPDIF - Stands for Sony/Philips Digital Interface Format, a specification for digital audio communication
• TOSLINK - Optical connectors/cables used for digital audio signal transmission
• HDMI - Connectors/cables used for digital audio and video signal transmission

Overview

When dealing with audio signal transmission, several standards exist for the media AND the encoding. Here is a quick overview of what exists. More information can be found on this very good article

Sound Data Encoding

• PCM signal - values of a channel at each time step. Usually, values are coded on 16, 24 or 32 bits and classical frequencies are 44.1 (mainly CD) or 48 (mainly DVD and BD) kHz. One PCM signal is required for each channel (ie 6 channels for 5.1 sound). The PCM signal can have two forms:

  • analog form: the PCM signal is transmitted by the sound card via analog output (generally through jack plugs). In this case, the receiver has only to amplify the signal.
  • digital form: the PCM signal is transmitted by sending the series of numerical values. In this case, the receiver must convert it (with a DAC: Digital-to-Analog Converter) to analog before amplification.

• Encoded BitStream - numerical data that encode one or more audio channels. In this case, the receiver must be able to decode the data, then to convert each channels (with DAC) to analog, and eventually to amplify them. Some well known encoded bitstream formats are Dolby Digital (sometimes known as AC-3), DTS, etc. The compression used for an Encoded BitStream can be:

  • "lossy" compression: as with MP3 (but with better results), some parts of the original audio data are lost in order to archive a better compression of the data. Examples: Dolby Digital, DTS, etc.
  • "lossless" compression: when decoded, the values of PCM channels are identical to the original at the expense of a lower compression. All "lossless" compressions have the same quality: they decode to the same (original) PCM values. Examples: Dolby TrueHD, DTS-HD Master Audio.

Digital Sound Media

Digital signals (PCM or BitStream) can be sent across different media. We can distinguish:

• Coaxial S/PDIF - a coaxial wire is used to send data in S/PDIF format. This connection has a throughput that allows at most two raw PCM channels or some "lossy" compression codecs. No "lossless" compression codec can be used on this media. This means that in order to have 5.1 sound with a coaxial S/PDIF connection, the sound must be transmitted in a (supported) encoded bit stream and decoded by the receiver.
• TOSLINK S/PDIF - same as coaxial S/PDIF but the information is transmitted through an optical link. The throughput is the same as for coaxial S/PDIF, hence the same limitations apply.
• HDMI - audio information can be transmitted together with video through an HDMI connection. The throughput of an HDMI connection is sufficient to allow 8 raw PCM channels or a "lossless" compression codec bit stream [perhaps HDMI before version 1.3 allows a little bit less]. That means that, with a HDMI connection, decoding of 5.1 or 7.1 sources can be done either before sending the data (i.e. on the computer and sending data as raw PCM channels) or after sending the data (i.e. by the receiver).

Side notes

On some systems with e.g. an HDA Intel sound card, HDMI sound transmission only works if the video is also transmitted (xrandr can be used to enable the video output).

Check your mixer

Mixer settings will affect your testing later on, so check them now. Run alsamixer. Check that Playback or All is selected for View (top of screen). Look for the following channels and take action as noted.

• IEC958 Output: Press M to unmute. File:Alsamixer pcmout.png alsamixer with IEC958 channels set to PCM out.
• IEC958, IEC958 1 or similarly named channels: Set to "PCM Out". \<-- HOW do you do this?? Added sample image, but thumbnail creation doesn't work... For now: [1] This doesn't apply to all hardware.

When referring to a digital output, setting it to "PCM Out" instructs the card to pass through the audio bytestream exactly as it is received from an application without applying any mixer settings. This is important, since a main point of having a digital audio output is to extract raw data from the PC to a device that may be more capable. Many people, apparently including hardware designers, my regard digital outputs purely as a noise-free version of analog outputs. However, the interface is really a separate (if unidirectional) interface. This is important to keep in mind when passing AC3 surround data streams through the card to an external decoder. In that case, modifications of the stream would make it completely invalid.

TODO: Tell people how to set "PCM out" mode. Add more common mixer settings, try to keep it simple and as general as possible.

Find your device

There are different ways by which the digital output may be exposed to the system by ALSA: as an independent device of a soundcard, or as an alias. The ways to find either one are described below.

Separate playback device

On most cards the digital out is a specific audio playback device. To see which ALSA playback devices you have, you can use this command:

aplay -l

Sample output:

[[email protected] ~]$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: T71Space [Terratec Aureon 7.1-Space], device 0: ICE1724 [ICE1724]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: T71Space [Terratec Aureon 7.1-Space], device 1: IEC1724 IEC958 [IEC1724 IEC958]
  Subdevices: 0/1
  Subdevice #0: subdevice #0
card 0: T71Space [Terratec Aureon 7.1-Space], device 2: ICE1724 Surrounds [ICE1724 Surround PCM]
  Subdevices: 3/3
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2

The device is specified as hw:(card),(device). The sample above shows device 1 is labeled "IEC1724 IEC958". That is the digital output device on this card, so it would be called hw:0,1. If your card does not show an IEC958 device, look for S/PDIF or digital.

Device aliases

On some cards digital out is already -- or even exclusively -- defined as an alias. See aplay -L for available playback devices. On a system with nForce 4 chipset onboard audio and an M-Audio Delta Audiophile 2496, this results in:

$ aplay -L
default:CARD=CK804
    NVidia CK804, NVidia CK804
    Default Audio Device
front:CARD=CK804,DEV=0
    NVidia CK804, NVidia CK804
    Front speakers
surround40:CARD=CK804,DEV=0
    NVidia CK804, NVidia CK804
    4.0 Surround output to Front and Rear speakers
surround41:CARD=CK804,DEV=0
    NVidia CK804, NVidia CK804
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=CK804,DEV=0
    NVidia CK804, NVidia CK804
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=CK804,DEV=0
    NVidia CK804, NVidia CK804
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
null
    Discard all samples (playback) or generate zero samples (capture)
default:CARD=M2496
    M Audio Audiophile 24/96, ICE1712 multi
    Default Audio Device
front:CARD=M2496,DEV=0
    M Audio Audiophile 24/96, ICE1712 multi
    Front speakers
surround40:CARD=M2496,DEV=0
    M Audio Audiophile 24/96, ICE1712 multi
    4.0 Surround output to Front and Rear speakers
surround41:CARD=M2496,DEV=0
    M Audio Audiophile 24/96, ICE1712 multi
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=M2496,DEV=0
    M Audio Audiophile 24/96, ICE1712 multi
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=M2496,DEV=0
    M Audio Audiophile 24/96, ICE1712 multi
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
iec958:CARD=M2496,DEV=0
    M Audio Audiophile 24/96, ICE1712 multi
    IEC958 (S/PDIF) Digital Audio Output

If you see IEC958 or S/PDIF in that list you can use the complete identification including the CARD and DEV parameters as a playback device instead of ```hw:x,y```.

Important Note: The OSSEmulation uses devices ```hw:0,0``` and ```hw:0,1``` for /dev/dsp and /dev/adsp. See the page OSSEmulation.

Alternate method

If you did not find your digital device with the above commands, keep reading. Otherwise, skip to the Use Your Device section.

This is another command to show ALSA devices on your sound card:

cat /proc/asound/devices

Here is an example:

[email protected]:~$ cat /proc/asound/devices 
  1:       : sequencer
  0: [0- 0]: ctl
  8: [0- 0]: raw midi
 18: [0- 2]: digital audio playback
 17: [0- 1]: digital audio playback
 16: [0- 0]: digital audio playback
 24: [0- 0]: digital audio capture
 33:       : timer

There are three audio playback devices:

 18: [0- 2]: digital audio playback
 17: [0- 1]: digital audio playback
 16: [0- 0]: digital audio playback

One of these is the digital out. We can test all outputs using the aplay command, or alternatively alsaplayer or mplayer. If you want to use aplay for this test, you need to have a .wav file. The numbers inside the square brackets ("[x- y]") are indexes for the soundcard (x) and the device (y) on that soundcard. So to play using the first playback device on the first card ("[0- 0]") you could use

aplay -D hw:0,0 file.wav

For the second and third you could use

aplay -D hw:0,1 file.wav
aplay -D hw:0,2 file.wav

If you receive a cryptic error message about "Channels count non available" then the wav file uses a sample rate different to the hardware. You can add a resampling layer with the 'plug' plugin, e.g.

aplay -D plughw:0,0 file.wav

Or with mplayer on [0- 0]:

mplayer file.wav -ao alsa:device=hw=0.0

If you hear sound when running one of these commands, you have found your digital device. If not, check your connection, receiver volume, and mixer settings described in the section above.

On some hardware sound does not seem to come out reliably unless you specify a subdevice explicitly, e.g. "hw:0,1,1".

Use your device

To utilize the digital output, connect your sound card to something with a digital input (possibly a stereo or surround receiver). Tell your program to send sound to 'hw:x,y', where x is the card number and y is the device number shown by 'aplay -l'. For my card, this is hw:0,1.

PCM output

If you want to use your digital output for sounds and music you simply tell the application of your choice to use the IEC958 device.

• MPlayer lets you use the device via '-ao alsa:device=hw=0.1' (beware it's a dot, not a comma)
• XMMS can be set in Preferences->Audio I/O Plugins->Output Plugin by choosing ALSA, then Configure and setting Audio device to 'hw:0,1'
• MythTV can be configured in Utilities/Setup->Setup->General, on the third page, by typing 'ALSA:hw:0,1' into Audio output device
• xine can be set on the audio tab in setup by setting any output you like to 'pcm.hw:0,1' and the audio driver to alsa
• Phonon can be configured in the KDE system settings, after checking the advanced devices box. If you use a custom .asoundrc file you need to add a "hint" block on the pcm to list.

If you want to test separate PCM channels over an HDMI connection then speaker-test is your friend. Keep in mind that multichannel PCM only works with an HDMI connection or with analog output.

Digital surround passthrough

While the setup thus far allows you to use the digital connection, it doesn't enable Dolby Digital or dts passthrough, where the complete signal is sent to your receiver for decoding. Without that, you don't get surround. Please note, not all sources have digital surround in them. DVDs and digital TV signals commonly do.

Also note that when you start watching a DVD movie you might not get surround sound until you play the actual movie (i.e. previews or the menu often have no digital surround).

Test your setup

A good test for "bit-perfect" digital passthrough (or S/PDIF passthrough) is to play these example wav files to your receiver decoder through a single TOSLINK or analog digital connection. First you should turn down the volume on your amplifier all the way to prevent damage to your speakers in case of failure. Then you can use aplay for playback, since that is rather easy to use if you just want to dump data into the card. The DD or DTS light should come on and you should hear surround sound when raising the volume. The command to play one of the files may look like this:

aplay -D iec958:CARD=M2496,DEV=0 Norrlanda.wav

or

aplay -D hw:0,1 Norrlanda.wav

The parameters:

• -D iec958:CARD=M2496,DEV=0: Set the output device. You can use the device names from aplay -L.
• The files provided at the site have proper headers that set up the data stream. If you get garbled results with other files you try to play, you may need to use -f dat to force 48kHz sample rate, 16 bit little endian sample format, which is often used on DVDs.

Solve problems

If you hear screeching, modem-like sounds or noise with digital pass-through then that means your digital output isn't set up correctly and the AC3/dts decoder can't make sense of the data it receives. If this happens there are some things you can try:

• IEC958 knows two modes: audio (PCM) and non-audio. AC3 and dts require non-audio to be selected. ALSA should do this automatically but you can force non-audio mode with iecset audio 0 and audio mode with iecset audio 1.
• Try using 'plughw' instead of 'hw'. (Shouldn't work but for some people it apparently does)

If neither of these work for you, it may be due to the driver and card's support of sample rates and formats. This might be the case if you see errors about "invalid arguments". This is one reason to use the dmix mixer plugin. Look for user notes on your card at http://alsa-project.org/alsa-doc/, ALSA modules and Sound cards. Read about dmix and .asoundrc (asound.conf).

Configure your media player

mplayer

In MPlayer, use -ac hwac3 or -ac hwdts as the audio codec. Do not specify channels, because history dictates channels be set to 2, the default. Make sure channels is not in your mplayer config files. MPlayer uses the first audio track it finds so if you want to use another, run it with -v and look for 'aid' (English Dolby Digital 5.1 is sometimes 128). Then run mplayer with '-aid 128'. If the audio track is dts, use hwdts. As for the device, use -ao and keep in mind that the comma is replaced by a dot:

 mplayer -ac hwac3 -ao alsa:device=hw=0.1 dvd://

MythTV

MythTV can pass through digital surround from digital TV. Go to Utilities/Setup->Setup->General, on the third page, and select "Enable AC3 to S/PDIF passthrough".

xine

In xine, go to the audio tab in setup and set speaker arrangement to Pass Through and 5.1 output (alsa passthrough) to your digital output, like 'pcm.hw:0,1'. If that doesn't work, try 'plug:iec958' instead.

The settings below work for me with an M-Audio Delta Audiophile (ICE1712 chip) to get passthrough to work in xine. Note the syntax for card specification, which is taken straight from aplay -L. The card only offers the IEC958 link as an alias, while only exposing a single device to the system. Those lines are supposed to go in the ~/.xine/config file:

audio.device.alsa_passthrough_device:iec958:CARD=M2496,DEV=0
audio.device.alsa_surround40_device:surround40:CARD=M2496,DEV=0
audio.device.alsa_surround51_device:surround51:CARD=M2496,DEV=0
audio.output.speaker_arrangement:Pass Through

Set digital out as default

On most soundcards the digital output emits the same sound signal as the analog output. If this is not the case for you then you can set the default playback device by using .asoundrc or /etc/asound.conf. If you want your digital output to be the default you might need to add this:

pcm.!default {
        type hw
        card <the card number you worked out above>
        device <the device number you worked out above>
}