Is All SPDIF Audio Output the Same?

CyberSpaZtiK asks: "I am going to build a Linux audio appliance to hold my music collection in various formats and for output to my stereo system. Because of a probable lack of Linux availability or support for audio cards with high quality D/A converters and low-noise electronics (or am I mistaken?), I want to keep the output path completelely digital by using a card with SPDIF output. However, it occurs to me that I actually know very little about SPDIF - are all SPDIF outputs made equal? Can I expect every SPDIF interface to emit the exact PCM data of the source audio, or are there over/under-sampling/aliasing, etc. issues that you sometimes get with digital signal processing? What do I need to understand about SPDIF and/or other digital output interfaces to make an informed decision?"

Comments

Use TOSLINK instead

[RzUpAnmsCwrds]

SPDIF outputs are usually pretty consistent at passing the PCM data or the DD/DTS sountrack if you have them configured right.

Some cards, however, such as Creative's Audigy series, are notorious for resampling inputs/outputs, so you might want to check.

Even a cheap card, like the $15 cards on Newegg, should provide a clean output. Don't buy the garbage about "jitter" that I'm sure someone will bring up - so long as your card and cabling are operating within the specification, you won't have any problems.

Do consider TOSLINK instead, however. TOSLINK uses fiber-optics, so your audio equipment and PC are electrically isolated. This reduces the chance of creating a ground loop or introducing RF noise into your reciever/amp. Moreover, it protects your equipment in the event of an electrical mishap.

Re:Use TOSLINK instead

[sffubs]

Well, my Audigy is probably doing all kinds of crap to SPDIF audio signal, but to tell you the truth I can't find fault with it by listening. (My hearing isn't below average, before you ask :) ).

I would just like to confirm what you say about TOSLINK. My PC is currently too far from my receiver for my optical cables to stretch, so I have to use the SPDIF connection. Every time there is an electrical event in my house (heating, fridge, freezer, kettle switching) the audio cuts out for a second or so.

Re:Use TOSLINK instead

[FlexAgain]

RzUpAnmsCwrds (262647) wrote
Do consider TOSLINK instead, however. TOSLINK uses fiber-optics, so your audio equipment and PC are electrically isolated. This reduces the chance of creating a ground loop or introducing RF noise into your reciever/amp. Moreover, it protects your equipment in the event of an electrical mishap.

One slight clarification, TOSLINK normally does carry SPDIF. TOSLINK is primarily just detailing the physical medium, the data is still encoded as SPDIF (which can also be carried on wire). The original author didn't specify how he was intending to use SPDIF, it may have been over either medium.

Re:Use TOSLINK instead

[Anonymous Coward]

Ugh, Audigy. For games, feel free to use the Audigy, but that thing WILL resample everything, which is probably not noticeable if you're going straight to speakers after, but you start getting lovely second-and-third-generation artifacts if you pass it to something ELSE that resamples.

Most games use DirectSound3D or OpenAL these days, so you shouldn't even notice if you don't have the Audigy, since they do it in software mode that's quite indistinguishable from the hardware -- audio's simply less demanding than video that way. The soundcard built into your motherboard is probably better than an audigy because of its resampling shenanigans alone.

Jitter is a bunch of crap

[Hidyman]

We are talking digital signals here.
Any self respecting DAC circuit will not be affected by jitter.
I use toslink all the time and there is no problem with "jitter".

Jitter is marketing hype.

Re:Jitter is a bunch of crap

[chriso11]

I spend a lot of time dealing with jitter in my job. For example, if you want to capture a TV signal, then jitter is quite important (even more if it is HDTV). But for generating audio signals over a serial link, then jitter should not be a problem, provided the digital data is properly latched, and the clocking method is not absolute crap (jitter of 1ns on the clock would not be an issue, and that is a crappy level of jitter - it gives a SNR of ~93dB at 20KHz - jitter requirements for WLAN is more like 50ps, and you see how cheap the cards are).

What is amazing is the "high quality" TOSLINK and SPDIF cables! It is digital data folks - as long as it is not a crappy cable, then you will be fine!

$25 TOSLINK card

[hab136]

I've been using a Chaintech AV-710 with my linux home theater PC for a long time now (a year?), outputs to my surround sound receiver. Fully supported under ALSA. mplayer, xine, and ogle all pass through the AC3 5.1 sound for my receiver to decode. I went for fiber optic, mainly because I didn't want to worry about grounding effects.

Chaintech's product page

There are Consumer and Pro modes

[Marillion]

The S/PDIF protocol has a consumer mode and a professional mode. I do some professional audio work and my DiO-2496 will emit both. My MD player will only accept the consumer mode which includes Serial Copy Management System (SCMS) flags which indicates if the source is first generation (allowed) or second generation (not allowed). The other nice thing about this card, it is completely ignores inbound SCMS and can re-code a second generation stream as a first generation consumer stream or a professional stream. Haven't needed it, but cool. I've connected it to professional DAT units, consumer MD units and DVD players.

MAudio Delta 44

[medgooroo]

Your mistaken. the MAudio cards all work beautifully... and sound it too... *currently listening to music in superb definition on said card* together with jack and assorted other toys, audio on linux is not something that is lagging.

One thing to be aware of!

[amliebsch]

I didn't realize this before going to SPDIF: as far as I know, there are no sound cards and only one chipset that will output more than 2 channels through the digital link. Even if the card supports 5.1 surround by analog jacks, e.g., the SB Audigy, it will not encode your digital signal in anything other than 2-channel PCM; except when you are directly passing it raw AC3 or DTS digital data (say, from a DVD or an AC3 encoded file.) You will not be able to get, for example, surround sound over SPDIF from games that support multi-channel surround sound.

If anybody know of sound cards available for purchase that actually support this, (the feature is called DICE), let me know.