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

[farnz]

Bullshit. Complete and utter bullshit. In decent equipment, when you reclock, you use a PLL to lock to the existing signal, and create a binary bitstream in a buffer; you then use your own clock to pump bits out of the buffer. So long as the jitter isn't too bad, there will be bits going in often enough that the buffer never drains. All that then matters is the quality of your clock.

Jitter is an issue for equipment designers; it is not an issue for equipment users. With the aid of a decent lab, you can verify this in double-blind tests; jitter either makes no difference at all, or causes the sound to glitch.

Just google for jitter.

[Kickasso]

It will occupy you for years to come.

Re:Answer

[Anonymous Coward]

1. Slashdot actually is a news site, not a support number.

Then why do they have an "Ask Slashdot" section?

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

[stienman]

Jitter is a problem for electrical engineers and programmers. By the time we're done with the system, you won't be able to tell whether there is any jitter or not, nevermind how much. Regardless, there WILL be jitter.

Unless, of course, all your units have synchronized clocks, or each have their own atomic clock.

Unlikely, to say the least.

Jitter is not a problem the average prosumer really needs to worry about, nevermind the average consumer.

The audiophiles who care about it care the same way about their tubes, oxygen-free cables, and green highlighters. Whatever gives you a warm fuzzy feeling, man.

But, technically, it does exist, and it is a problem that results in either doubling up on samples, skipping samples, or some sort of macabre clock synchronization scheme that only ends in tears.

Only, technically, that's not jitter either.

-Adam

Re:Jitter is a bunch of crap

[Andy_R]

Actually, Jitter is a real, audible and nasty issue in certain specific applications... but not this one.

It's not a problem you are ever likely to come across outside a big recording studio where several devices are talking to each other digitally with DAC clocks drifiting compared to each other (oh, and it's easliy solved, the solution is to slave everything to a master clock).

The problem of sending a 44.1kHz signal from one end of your house to another is trivial compared to feeding a broadband signal from your ISP to your computer, and you never have jitter issues there.

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.

Re:Way to go

[sneakers563]

Well, I should say that I wanted to build a living room "jukebox" and DVR for parties, so my requirements might be a bit different from yours. I've used Mserv because I wanted a kiosk-type jukebox that would act like a real jukebox. That is, if no songs were selected, it would start picking songs based on ratings and how long it had been since they had last been played. I don't know of any other jukeboxes, Windows or Mac (perhaps someone can enlighten me) that will weight it's random selections like that. I wrote my own kiosk-style frontend using Python, but it appears that someone else has done the same thing with Shrill, complete with album art. I have a friend who's doing something similar with MPD, but I haven't used it myself. I've also played around with MythTV, which was nice because of the DVR features, but it didn't have the random feature that I wanted.

Re:Answer

[IKillYou]

Hey shithead - Slashdot discussions answer plenty of questions like this. YOUR answer is entirely useless, and poorly worded to boot.

"read errors and the like" - Brilliant, thanks. Why don't you post under your member name so I can come kick your ass.

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.