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Digital Sound FAQ

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Apr 1, 2002

The most important thing to remember is that speakers are analog. That's just the way it is. So at some point your digital connection will have to be converted into analog using DACs.

The 2nd thing is that the DACs tend to have the biggest influence on the quality of the sound.

The 3rd thing to remember is a few key acronyms:
APU = Audio Processing Unit (named by the same folks who named the GPU, nVidia)
DAC = Digital to Analog Converter
DD = Dolby Digital
AC3 = Audio Coding version 3 - lossy encoded audio format developed by Dolby
DTS = Lossy encoded audio format developed by Digital Theater Systems
PCM = Pulse Code Modulation (an uncompressed digital sound signal)
S/PDIF = Sony/Philips Digital Interface (standard digital audio interface using RCA jacks and 75ohm coax cable)
TosLink = Toshiba Link (standard digital audio interface using fiber optic cable)

The 4th thing to keep in mind is that digital sound comes in many different formats. There are three main digital sound formats (each with sub-formats) that we need to worry about. PCM is an uncompressed stereo format. Dolby Digital (AC3) and DTS are competing multichannel formats. DACs will convert PCM, but DD and DTS need to be decoded first.

So, first a bit of history:

In a typical analog computer setup, your sound follows the following path (simplified, but you get the idea): APU -> DAC -> Mini-Amplifier -> Analog audio cable -> Amplifier -> Speaker

The APU generates and processes the audio on your computer - different APUs are capable of doing different things (DTS decoding, real time effects, etc). Audio is typically output from the APU in PCM format, usually at 16bit, 44.1khz (CD quality).

This PCM bit-stream is then fed to the DAC where it is converted into analog sound.

From the DAC the sound is passed to a small amplifier which boosts the output level so you can use headphones or small unpowered speakers. Often these small amps are included in the same chips as the DACs.

The sound then travels down the analog audio cable to the amplifier. This amplifier can either be a standalone unit (ie: a receiver) or built into the speaker (ie: typical computer speakers). Different amplifiers offer different controls, but most offer at least volume and some kind of tone adjustment.

From the amplifier the sound is passed to the speaker.

This is how it stayed for a long time. People slowly started adding channels, but all you had to do is add more DACs, amps, and speakers. Generally people were happy.

But then came CDs together with consumer level Dolby Digital and DTS, and the sound card people thought "Hey, we can make a lot of money off of this!".

Creative Labs was pretty much the first company off the block with their digital speakers. They would pass the stereo PCM signals from the APU down a proprietary digital DIN connection to the speakers. The speakers would contain the DACs and amps, as well as the speakers themselves. Because the DACs in the speakers weren't really any better then the DACs on their soundcards (you had to use their cards because of the proprietary speaker connections) there wasn't really much difference in sound quality. But the marketing people got to put the words "digital" on the boxes.

The other thing that appeared around this period was the S/PDIF and/or TosLink connectors. Most cards would allow two things with these connectors - output the front speaker's stereo PCM signal and allow "S/PDIF passthrough". S/PDIF passthrough just passes a digital bit-stream from the source (ie: computer DVD drive) directly to the S/PDIF and/or Toslink connector without the APU (or anything else) doing anything to the signal. Most of the cheaper sound solutions to this day still only do these two things with the digital out connectors.

For a while companies (mainly Creative Labs) continued to muck around with their proprietary digital speaker connections. Creative Labs was eager to keep this arrangement because they had a monopoly on sound cards. If you wanted a soundcard you pretty much had to get a SoundBlaster. If you wanted digital speakers for your SoundBlaster you had to talk to Creative Labs.

Then came some real digital speaker packages that would not only decode stereo PCM, but also AC3 and DTS. Companies like Logitech started producing great setups like the Z-680s and people flocked out to buy them in droves. They eagerly plugged them into their waiting S/PDIF and TosLink ports and then started screaming and swearing when they only got stereo sound out of them. When playing movies they could activate S/PDIF passthrough and use the decoders and DACs in their digital speaker packages, but when playing games they had to use the soundcard DACs and analog cables to get multi-channel sound.

And then nVidia brought out nF1 with Soundstorm. This introduced Dolby Digital Live! realtime encoding and allowed the APU to pass a multi-channel AC3 bit-stream to the speakers instead of being limited to a stereo PCM bit-stream. Suddenly people noticed that their digital speaker packages worked like they were supposed to. Although nVidia backed out of the DDLive! encoding after the nF2, other companies like C-Media, and Realtek started picking it up.

Not to be left out, DTS recently started offering their version of DDLive!, called DTS Connect. DTS Connect does exactly the same thing as DDLive!, except it encodes the audio into a multichannel DTS bitstream instead of a Dolby Digital AC3 bitstream.

So which is better? DTS tends to have higher bitrates (less lossy compression) and is generaly regarded to be higher quality. We'll have to wait for some reviews of DTS Connect computer audio chips to be sure 'tho.

DDlive! homepage
DTS Connect homepage

Ready to take the plunge?

So that's where we are today. Stuck in a big mess. Ideally we want a setup that looks like: APU -> DDLive!/DTS encoder -> Digital cable -> AC3/DTS Decoder -> DAC -> Amplifier -> Speaker

For some reason people like to start at the speaker end, so that's where I'll start. There are two routes you can take. You can get a digital computer speaker package (like the Logitech Z-5500) or you can go for a home theater receiver (like the Yamaha HTR-5840). Both have pros and cons. The computer packages tend to be smaller and cheaper. The receivers offer better sound and better connectivity. Either way you'll want to make sure that the setup you choose has the connections you need, as well as supporting both Dolby Digital and DTS decoding (all digital receivers should support PCM).

When you're looking at self powered speakers and receivers remember that there are many ways to measure power. One 120W amp may only provide half the output as another 120W amp. http://www.bcae1.com/voltages.htm

Next up is the digital interface. There are many threads full of people swearing that TosLink is clearer, or S/PDIF is crisper but when it comes down to it remember it's all 1s and 0s, and as long as they get to the other end of the cable it'll all sound the same. S/PDIF tends to be cheaper and more flexible then TosLink. You can use the composite video cable that probably came with your last video card purchase and save some $$$. Toslink is better for long distance runs (ie: more then 5m (15').

Some older S/PDIF equipment is not correctly grounded and you may get a ground loop problem. This will result in your audio popping/crackling/popping or cutting out, no sound at all, or a hum. If you get any of these symptoms try a TosLink cable. Because a TosLink cable is not electrically conductive ground loops will be eliminated.

The final thing you'll need is the soundcard - unless you get something with DDLive! and/or DTS Connect encoding you'll get stuck with the old stereo PCM sound for everything except DVD movies. See a few posts down for a list of DDLive! and DTS Connect soundcard & motherboards.

Once you get your soundcard installed just enable DDLive! encoding, turn on S/PDIF passthrough in everything you can find, and you're good to go.
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S/PDIF coax on the left, optical TosLink on the right (notice the red glow):

Both output exactly the same data. Coax is cheaper, but doesn't have the same "cool" factor as TosLink.


Want to run dgital out from your Live!, Audigy, or X-Fi card? You'll need a 3.5mm (1/8th inch) mono to RCA adaptor like this:

Plug it into the "Digital Out" jack. You can also use a longer one (with a cable in between the two ends), but they are prone to interference unless they use coax cable. You'll also need a 75ohm coax cable to go between the adaptor and your receiver. Remember you'll only get stereo PCM from these cards 'tho, except when watching DVDs (or other pre-encoded AC3/DTS sources).


I should thank ATi and nV for all the free 75Ω coax cables they've provided over the years with their video cards:

They are perfect for connecting your receiver via S/PDIF coax.


Proprietary Creative Labs / Cambridge Soundworks digital DIN connection (right side):

These would pass either two stereo PCM bitstreams (for 4.0 speaker setups) or on later cards three stereo PCM bitstreams (for 5.1 setups). The proprietary speakers only work (in Digital mode) on Live! or Audigy cards. Early cards had the DIN connections on them, some provided a seperate PCI bracket with the DIN, and later ones just used an adaptor (like the one above) that pluged into the "Digital Out" jack.

You can't use these adaptors on the X-Fi cards, because of the FlexiJack design. This made many people unhappy.


This is what you should see on your receiver if you've got your digital PCM bitstream set up correctly:

Notice the PCM logo on the left, and the two channels listed on the right.


This is what you should see on your receiver if you've got DDLive! set up correctly:

Notice the Dolby Digital logo on the left, and the 5.1 channels listed on the right.


This is what you should see on your receiver if you've got DTS Connect set up correctly:

Notice the DTS logo on the left, and the 6.1 channels (in this case) listed on the right.
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DDLive! Realtek ALC882D control panel:

Note how you can pick between DDLive! and PCM output for the digital out.


DDLive! C-Media CMI9880 control panel:


DDlive! and DTS Connect soundcard & motherboards:

Currently, everything will be based on one of a few basic chips. They can be either PCI based (designed to be used on a PCI card) or intel HDA (High Definition Audio or also known as Azalia) based which are soldered onto the motherboard. intel HDA is currently supported by the ICH6 and ICH7 southbridges, the ATi SB450 southbridge, and the ULi 1573 southbridge.

Here's a list of the more popular chips and some suggested boards/cards:

nVidia Soundstorm (info). Proprietary DDLive! chip.
DFI LanPart NFII Ultra (AMD sA)
Abit AN7 (AMD sA)
Asus A7N8X-E Deluxe (AMD sA)

C-Media CMI8788/CMI8770 (info). PCI based DTS Connect & DDLive! chip.
Bluegears X-Raider (PCI)
Bluegears X-Plosion (PCI)

C-Media CMI8768+ (info). PCI based DDLive! chip.
Bluegears X-Mystique (PCI)
Turtle Beach Motego (PCI)

C-Media CMI9880 (info). HDA based DDLive! chip.
Asus P5GDC Deluxe (intel i915P s755)

Realtek ALC882D (info). HDA DDLive! chip.
Asus P5WD2 Premium (intel i955X s755)

Realtek ALC882DTS (info). HDA DTS Connect chip.
MSI 945P Platinum (intel i945P s775)
Gigabyte GA-G1975X (intel i975X s775) - also includes onboard Soundblaster Live!, which is pointless if you're using DTS Connect.

Realtek ALC883D (info). HDA DDLive! chip. Cut down version of the ALC 882D (lower SNR, no DVD-Audio support...).

Realtek ALC883DTS (info). HDA DTS Connect chip. DTS version of the ALC883D.
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Good guide, as I just upgraded from an nForce2 board and am surprised that only a few other boards out there do the Dolby Digital Live. Is there a reason Creative doesn't put DDL in its products, and would you still get digital output (5.1 in 5.1 enabled games or stuck in 2.1?) if you had a non-DDL card hooked up to a receiver using digital out?
They'd have to pay Dolby $$$ if they used DDLive!

You need a DDLive! card to get 5.1 out of games using digital out, because the APU is generating the sounds in real time and would need to encode them into AC3 to pass them to the receiver. You'd only get 2.0 otherwise.

2.1 isn't really a valid sound format, even though the speaker manufacturers just like to call their sub + two satellite systems that. The sound they receive is only two channel, but the 2.1 speakers systems have an internal crossover that sends the low frequencies to the sub, and the highs to the sats.

A pre-encoded AC3 (or DTS) bitstream from a DVD, for example, can just be passed to the digital output jack without the APU doing any decoding or encoding.
Inositle said:
So if I bought a AV710 for example, would all sounds be passed over the digital output, or only DD encoded stuff such as DVD's?
You'd get 2 channel stereo for everything, except DVDs, over the digital out.
What about class-D digital amplification? I know it works on a PWM principle, but does it amplify the analog sound or use a digital signal (either from an ADC or digital input) to generate the pulse? I remember reading something about them bypassing DACs because they can just generate the switched pulse from the digital signal. But then switching noise has to be removed from the output so you lose a lot anyway. I've been told they're not "the best" (perhaps far from) but are still damn good and very power efficient.
Good find! Interesting - they are using the Realtek ALC882. Maybe it's just a driver update? I'm going to go investigate before I change the FAQ.

Here's the DTS page about it: http://www.dts.com/consumer/pc/dtsconnect.php\

edit: Their spec page says they are using the ALC882, but according to Realtek it's the ALC883 that supports DTS...

edit2: Ahhh... The ALC882D does DDLive!, the ACL882DTS does DTS. Both have been replaced with the (pin compatible) ALC883_.
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WOW, very good info indeed. This really helps clear up a lot of the confusion. But I still have one question: Does DTS still do the same thing that the DDLive! real time digital encoding? As in, will DTS still output 5.1 digital sound with music, DVD's, as well as games or Does the DDLive! only do that? Now what about the ALC883?

This is definately sticky worthy! :clap: