High definition audio wiring guide

Oct 29 2008

receiver.jpgSo folks you're sold on Blu-ray, and you want to connect your new Blu-ray enabled home theater PC to your killer sound system, but which way is best? what are the trade-offs? 

Let's chart the Blu-ray audio formats first so you get an idea of what formats are used on Blu-ray, which formats are mandatory, and so on. Then we'll discuss the ways to connect your HTPC's audio to your home theater. As you can guess there are pluses and minuses with each method, and some require a bit of discussion.

Blu-ray Audio Format Support Chart

 Audio Codec

Mandatory 

S/PDIF compatible 

Notes 

Dolby Digital

X

X

Blu-ray allows full spec bitrate of 640Kbit/sec versus 448Kbit/sec on DVD. So even good old Dolby Digital will show improvement over the DVD version of the same movie soundtrack.
Dolby Digital Plus     An improved lossy codec from Dolby. Pretty much never used on Blu-ray discs.
DTS

X

X

DTS has a maximum bitrate of 1.5Mbit/sec. However, DTS was typically only 768Kbit/sec on standard DVDs, so as with Dolby Digital, DTS tracks tend to be full rate.
DTS-HD High Resolution    1 An improved and less compressed cousin to standard DTS. Used on early Blu-ray discs, especially on European releases. Rarely used now.

Dolby TrueHD
(lossless)

    A defacto standard for Blu-ray thanks to widespread studio and player support. Sony, Warner, and Paramount are the biggies here.
DTS-HD Master Audio
(lossless)
   1 Not as widespread as TrueHD, but is fast becoming a new defacto standard thanks to widespread use by Fox, Universal, and New Line.
Multi-channel LPCM
(lossless)

X

 2-channel only

Linear Pulse Code Modulation: in essence this is raw audio (like a WAV file). LPCM sound tracks are quite common.
       Note 1: The DTS-HD formats contain a standard DTS 'core' for backward compatibility. Playing a DTS-HD track will result in the core DTS 5.1 track being sent over S/PDIF.
Your Audio Wiring Options 

S/PDIF

[Also known as Toslink/optical, and coaxial digital audio]

spdif_inputs.jpg

The convenience of having one digital audio cable is hard to ignore, and because every receiver for the last 5+ years supports S/PDIF, it works with a wide variety of equipment. 

S/PDIF of course has bandwidth limitations and the new high resolution audio on Blu-ray discs won't fit down the pipe.

What happens when I play a Dolby TrueHD or DTS-HD Master Audio soundtrack via S/PDIF?

If you choose a Dolby TrueHD track it will be decoded into LPCM and sent over S/PDIF. As noted above, S/PDIF can only carry 2 channels of LPCM, so you'll get stereo audio.

Because TrueHD is an optional format the Blu-ray specs say that the disc must also include a soundtrack that is in one of the mandatory formats (LPCM, DTS or DD). In most cases the TrueHD soundtrack actually has a hidden Dolby Digital version that is supposed to be used automatically by players which don't support TrueHD and/or when a player is set to output bitstreams over S/PDIF.
NOTE: It seems in practice PC player software doesn't always follow this guideline.

Also note that any of the mandatory codecs can be offered on the disc to satisfy the specs. This means that in some cases you'll end up with two formats that don't send multi-channel data over S/PDIF. The most widespread example of this is SpiderMan 3, which contains a 24-bit/48KHz TrueHD track, and an LPCM 16-bit/48KHz track, thereby fulfilling the mandatory format requirement, but also screwing over S/PDIF users who want a 5.1 soundtrack.

DTS-HD has a rather nifty solution for backwards compatibility, all high resolution data is built up around a standard lossy DTS "core". This satisfies the mandatory audio format requirement and lets users of more advanced equipment benefit from the lossless version of the soundtrack. For legacy users this means that when a player is connected by S/PDIF the lossy DTS core of the DTS-HD Master Audio track is sent over S/PDIF giving the end user a 5.1 channel experience.


So what's an audio loving HTPC user to do about these limitations? You have a few options, and they're outlined below:

Player Software Mixing with S/PDIF

One option is to enable Audio Mixing, both Cyberlink and ArcSoft offer a software encoder that converts the decoded audio into Dolby Digital or DTS for S/PDIF compatibility. This way all formats "just work" and you get to save money for audio upgrades down the line. The quality of such implimentations is debatable, but it gets the job done and is free with the full retail versions (i.e. not an OEM bundle) of both players.

NOTE: The early version of Cyberlink's solution (PowerDVD 7.3) was known to cause a "tin can" effect, where things sounded odd, like everything was recorded in tin can or an echoed bathroom. Based on feedback from users of PowerDVD 8 Ultra things seem to be fine now.

Player Setup

PowerDVD Ultra

Right-click and choose "Configuration", then the "Audio" tab, Make sure "Use SPDIF" is selected. Check "Enable S/PDIF Mixing" and then choose the format you'd like to re-encode to.

TotalMedia Theatre

Make sure any disc is stopped and then open the main menu by right-clicking or with the "i" -- Information button -- on the Media Center remote, choose "Setup", make sure "S/PDIF" is selected for "Speaker Output:", then check "Audio Mixing." Your audio will now be re-encoded into DTS.

Dolby Digital Live or DTS Connect with S/PDIF

Dolby Labs has a real-time encoding technique that can take any audio your PC can play back, including 3D sound from games, and encode into a Dolby Digital signal. This technology is known as a "Dolby Digital Live" (DDL). This ability can be found in either a discrete sound card or with select motherboard audio chipsets.

The software implementation of Dolby Digital Live is available with select Intel branded motherboards as part of Intel's Dolby Control Center suite. There are quite a few stand alone sound cards with Dolby Digital Live, these include AuzenTech's product line, Diamond XS71DDL, ASUS Xonar family, HT Omega Claro and Striker, Razer Barracuda, Bluegears b-Enspirer 7.1 and a few others.

Not to be out done, DTS has their own take on real-time encoding called "DTS Interactive", which is part of the "DTS Connect" suite of technologies. The AuzenTech X-Plosion 7.1 DTS Connect, ASUS Xonar D2, HT Omega Claro and Striker, and Bluegears b-Enspirer 7.1 all support DTS Connect. Also the Gigabyte GA-MA78GM-S2H AMD 780G motherboard includes DTS Connect.

Caveats

This first method is just fine for handling gaming and other miscellaneous audio formats that aren't in need of real high resolution output. Because you're still using S/PDIF the bitrates are still limited to the maximum of standard Dolby Digital or DTS, so you aren't getting any of the high resolution audio benefits.

However, this isn't an ideal solution and it gets hard to recommend that you go out and buy a sound card that supports one of these methods given that many of the sound cards listed above are ~$100 and DDL/DTS Connect are really just stop-gap solutions on the way to full high resolution audio. But, if you already have a motherboard or sound card that supports one of these technologies, give it a shot!

Player Setup

Set your Blu-ray player software to "5.1 speakers" -- the sound card will handle the rest, if you set it to "S/PDIF" you'll actually be disabling Dolby Digital Live/DTS Connect. More information on Vista audio setup is here on AuzenTech's site.

Also note that if you use Vista Media Center you'll need to re-setup the audio settings to match your speaker setup. This is not necessary under Windows XP.

Multi-channel Analog

This means you hook your PC up to your receiver like you were hooking up to a PC speaker set. Get three mini-jack to stereo RCA jack adapters and wire them up to your receiver's analog 6-channel input. Just about every receiver has a 6-channel direct input and since your PC is doing the audio decoding work this method allows for any audio your PC plays to "just work."

 mc_analog_input.jpgminijack-rca.jpg

Caveats

When you go analog the speaker management is done by the source (the PC) not the receiver. The receiver's DSP isn't being used, since that operates in the digital domain, in this case the receiver acts simply as an analog amplifier and thus your speaker calibration and/or setup may need to be tweaked. This means that your sound card's DACs (Digital to Analog Converters) need to be taken into account, cheap basic DACs won't sound as good as a higher quality card.

Also because of the output being in the analog domain from a PC the copy protection specs require that the audio data be capped at 16-bit/48KHz. This isn't the end of the world as most Blu-ray sound tracks are 16-bit, and nearly all are 48KHz (which is the standard for Hollywood movies), even with these limitations the expanded audio information of the high resolution codecs still makes a difference.

Player Setup

Set your Blu-ray player software to "5.1 speakers" if you're using a 6-channel direct input, if your receiver supports 8-channel analog input and you have 7.1 speakers, set the player accordingly.

Under Windows Vista you may need to change the default audio output to "Speakers" if you were using S/PDIF previously.

Also note that if you use Windows Media Center you'll need to re-setup the audio settings to match your speaker setup.

HDMI Audio

HDMI is where it's at, it brings the convenience of having one cable for high resolution video and audio. Unfortunately HDMI on the PC is still a bit immature, and HDMI enabled surround sound receivers aren't as commonplace.

hdmi_inputs.jpg

Currently the only high resolution HDMI audio format a PC can send is LPCM, this is fine in theory because all formats can be decoded into LPCM (remember LPCM is raw audio, so decoding into LPCM is lossless) and sent to a receiver just fine. Because LPCM over HDMI is raw digital audio transport it allows for any audio your PC can play to "just work."

Caveats

As with analog, due to mistrust of the PC platform all audio from Blu-ray discs is capped at 16-bit/48KHz, unlike analog output, once a proper protected audio path has been enabled on the PC (the foundation of which is in Windows Vista) this limitation should be removed for hardware that supports PAVP (Protected Audio/Video Path). Hardware that does not support the protected path will still be subject to the cap.

Currently only one shipping product claims to have PAVP support ready to be used once the software support catches up: the Intel G45 chipset. In the future, and by definition, the perpetually delayed (and buggy) HDMI sound cards from AuzenTech and ASUS will have to support PAVP to be able to bitstream Blu-ray audio to compatible receivers.

Also note that some older HDMI receivers treat LPCM pretty much like analog input in that it is considered raw audio to be amplified, and no speaker management is used. This has changed with nearly all recent (2007 and newer) models.

For more check out this very nice thread on the subject over at AVSForum: 5.1/7.1 PCM, HDMI, and DSP - An Explaination of the Future-Proof receiver

Following the tier structure outlined in the linked thread, you want at least a "Level 5" receiver for proper speaker/bass management of LPCM over HDMI.

Player Setup

Set your Blu-ray player software to "5.1 speakers" or "7.1 speakers" if you have the extra speakers in your surround sound setup.

In Windows you will need to change the default audio device to the HDMI audio device. The name varies by manufacturer but the word "HDMI" is in all of the device names, so it's pretty clear which one to set as default. Example instructions with images (based on ATI cards) are available here.


More resources:

Connect your Media Center PC to your surround sound system 

This is a very good write up made with Windows XP Media Center Edition 2005 in mind, but has useful information about hook up options and illustrations for both S/PDIF and Multi-channel analog.

Website design by Yammm Software
Powered by Drupal