Yes, i agree that using a digital output is a good idea if possible.
It 'protects' the sound stream from electrical noise induced by the sound card DAC's, the cars electrical system, cable runs... etc.
So I was just curious if having such a high SNR is worthwhile, as the item with the lowest SNR in the system (HU, CarPC, Amps, processor) will ultimately determine the overall maximum SNR of the system...
Having said that, I think I can save myself some money in some areas, and still get good sound as long as I look for a card that gives good (low) THD, (high) Dynamic range and (high) Channel Crosstalk rejection figures.

Basically, I just want to ensure I'm matching my devices as best as possible (without any overkill)... no point putting F1 status HU with Dicksmith 4" coaxials

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Ascension> I believe the SNR is a function of "the re-construction of sound data by the processing device", not a function of the transport medium. The medium may determine how much additional noise is induced/rejected during transport to another device (ie: from the cabling). Also additional noise is added during conversion from D to A - This is why the analogue outs "should" have a worse SNR.
So I dont think that just because something has digital out, thats means its 'got' to be good, it might just be 'better' than the analogue signal from the same chipset.
The Digital Out should have its own SNR, which should hopefully be higher than the analogue output, due to not having to go through another set of processors/converters.
(as can be seen in some www.digit-life.com reviews).