please help me understand....SPDIF on motherboard.

[Wynand]

Right....

I've got a P4M800-M7A motherboard from Biostar. It has a ACL655 audio chip from realtek. On the datasheet of the ACL655 it says it supports 6 channels out, this is verified by the software that gets installed. You can choose whether you want 2 or 6 channels out.

Here comes my problem.

I can't seem to find anywhere where it gets mentioned if all 6 channels will be output on the spdif header and what its enco.

Also, can I tell the PC to decode the data from say, AC3 to PCM and put that out on the spdif?

I'm pasting a list of features from realtek's site to show what it CAN do.


ALC655
AC'97 Rev 2.3 Audio CODEC

General Description
The ALC655 is a 16-bit, full-duplex AC'97 Rev. 2.3 compatible six-channel audio CODEC designed for PC multimedia systems, including host/soft audio and AMR/CNR-based designs.

The ALC655 incorporates proprietary converter technology to meet performance requirements for PC99/2001 systems. The ALC655 CODEC provides three pairs of stereo outputs with 5-bit volume control, a mono output, and multiple stereo and mono inputs, along with flexible mixing, gain, and mute functions to provide a complete integrated audio solution for PCs.

The digital interface circuitry of the ALC655 CODEC operates from a 3.3V power supply for use in notebook and PC applications. An integrated 50mW/20ohm headset audio amplifier for Front-Out and Surround-Out, a 14.318MHz -> 24.576MHz PLL, and a PCBEEP generator cut BOM costs. The ALC655 also supports S/PDIF input and output for easy connection of PCs to consumer electronic products, such as AC3 decoders/speakers and mini-disk devices.

The ALC655 CODEC supports host/soft audio from Intel ICHx chipsets as well as audio controller-based VIA/SIS/ALI/AMD/nVIDIA/ATI chipsets. Bundled Windows series drivers (98/ME/NT/2000/XP), EAX/Direct Sound 3D/ I3DL2/A3D compatible sound effect utilities (supporting Karaoke, 26 types of environment sound emulation, 10-band equalizer), HRTF 3D positional audio and Sensaura™ 3DPA (optional) provide an excellent entertainment package and game experience for PC users. In addition, the ALC655 is embedded with an impedance sensing capability to detect when a device has been connected to input or output jacks.


Features
Meets performance requirements for audio on PC99/2001 systems
Meets Microsoft WHQL/WLP 2.0 audio requirements
16-bit Stereo full-duplex CODEC with 48KHz sampling rate
Compliant with AC'97 Rev 2.3 specifications
Front-Out, Surround-Out, MIC-In and LINE-In Jack Sensing
14.318MHz -> 24.576MHz PLL to eliminate crystal
12.288MHz BITCLK input
Integrated PCBEEP generator to save buzzer
Interrupt capability
Three analog line-level stereo inputs with 5-bit volume control, LINE_IN, CD, AUX
High-quality differential CD input
Two analog line-level mono inputs: PCBEEP, PHONE-IN
Two software selectable MIC inputs
Dedicated Front-MIC input for front panel applications (software selectable)
Boost preamplifier for MIC input
LINE input shared with surround output; MIC input shared with Center and LFE output
Built-in 50mW/20ohm amplifier for both Front-out and Surround-Out
External Amplifier Power Down (EAPD) capability
Power management and enhanced power saving features
Supports Power-Off CD function
Adjustable VREFOUT control
Supports 48KHz S/PDIF output, complying with AC'97 Rev 2.3 specifications
Supports 32K/44.1K/48KHz S/PDIF input
Power support: Digital: 3.3V; Analog: 3.3V/5V
Standard 48-pin LQFP package
EAX™ 1.0 & 2.0 compatible
Direct Sound 3D™ compatible
A3D™ compatible
I3DL2 compatible
HRTF 3D positional audio
Sensaura™ 3DPA enhancement (optional)
10-band software equalizer
Voice cancellation and key shifting in Karaoke mode
AVRack® Media Player
Configuration Panel for improved user convenience

[preiter]

More than two channels out on SP/DIF usually means Dolby Digital aka AC3. The problem is that encoding Dolby Digital requires special licensing from Dolby and almost no sound cards that are available support it.

NVidia's NForce2 (but not the later NForce chipsets) and the Mystique are the only sound cards I know of that support Dolby Digital Live (that's what they call realtime DD encoding).

So basicially, unless you buy that one sound card, you are stuck with two channels out of SP/DIF. If you have the correct software, you can use SP/DIF passthru to output DD from sources which are already DD encoded (e.g. DVDs), but I have had trouble getting that to work.

[Wynand]

The guy I work for uses a P4 for his multimedia and he uses a on-board soundcard. Using the spdif into a commercial a/v receiver he gets dolby, no problem. The a/v receiver recognized dolby and shows it on the lcd. When he forces the output to PCM, the amp says stereo. Thus I have come to the conclusion that.

1. Buy a commercial a/v receiver ( too expensive ) or
2. Use stereo spdif out or
3. Bugger digital and use the 6 analog outs and don't worry about all the noise on the signal.

Maybe a combo of 2 and 3

SPdif for music and analog for games/movies.