S/PDIF Outputs -- All The Same?

I'm confused too. Is AES EBU better than SPDIF? Do different cards outperform others? Are there different versions of AES? If dual AES is better, are there many DACs that accept this input?
 
Thanks.

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 There are several questions here, none of them with absolutely straightforward answers. The AES and SPDIF protocols are very nearly the same - they are the same from the perspective of the digital audio data itself - and you can usually convert between the two using a simple passive transformer adapter without issue. SPDIF uses a coax connection, in which the cable shield is also the "ground" or zero voltage lead. AES/EBU uses a separate shield (which helps protect the signal from RFI/EMI) and two signal leads. AES also is higher voltage, which helps raise the level of the signal relative to any RFI/EMI noise. So, for long cable runs in electromagnetically "noisy" environments, the AES approach is theoretically superior - hence its near universal use in professional audio environments. This is not an academic point: noise in a synchronous digital protocol like AES or SPDIF can contribute to timing errors, or "jitter".
 
In a home audio environment, with short cable lengths, the differences between AES and SPDIF are probably far less important than the quality of the components connected by them. That gets to the question of the built-in SPDIF on a PC. There are two things to consider here: the timing error (or jitter) of the SPDIF output, and the capabilities/limitations of its software drivers. Most built-in SPDIF outputs will not be designed for low jitter. Depending on which DAC you're using, this may or may not be significant. Many well-designed DACS, like the Benchmark DAC-1, are almost impervious to jitter on the input stream. Others are more sensitive.
 
The primary impact of the software drivers is to potentially limit the sampling rates and/or bit depth of the digital audio output. Ideally, you'd want an output which can deliver 44.1, 48, 88.2, 176.4, and 192 khz sampling rates, as well as 16 and 24 bit audio depths. This would allow you to output all of  the various high resolution audio tracks becoming available from Reference Recordings, HDTracks.com, iTrax, and the like. Pro-audio cards like the Lynx support all of these resolutions; built-in SPDIF outputs usually don't.

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Dual AES is a way of splitting the L/R digital signal into two separate (left and right) connections - so yes, this is a two-wire connection. This halves the data rate each connection must carry, reducing transmission jitter, especially over long distances, where cable inductance and capacitance are greater. See:
 
http://www.professorcalle.com/assests/MDCC/MUM2600/2600%20Digital%20recording.pdf
 
Most DACs designed for home use don't support dual AES. If a DAC doesn't have at least two physical AES connections (they're identical in appearance to XLR balanced analog connections), it obviously doesn't support dual AES. If it does have multiple connections, you can only tell by reading specifications.

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