I have this CMI8330 ISA Soundcard
Info: Sound Blaster 16 Clones

Funny enough it has S/PDIF digital output, which outputs digital and FM. Since the Analog output is not exactly noise free I try to use the S/PDIF where possible; e.g. I use a PCI soundcard as a DAC (digital to analog converter) in windows. But in pure Dos the PCI card won't read the S/PDIF input... I don't think I will ever find a way to activate that part of the PCI soundcard in dos. So maybe using an external S/PDIF to analog device will allow me to have quality output in any case.

On ebay there is this cheap little thing: Ebay item no. 120606767231
But it is new to me, so maybe someone here can tell me whether this is gonna work or not. I wonder if it is able to mix two digital signals?

HI-FI Mini DAC TDA1543 DIR9001 NOS DAC Incl power ADP S 1.This is an enter lever DAC, DAC Chip for Philip TDA1543 2.This DAC can […]
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HI-FI Mini DAC TDA1543 DIR9001 NOS DAC Incl power ADP S
1.This is an enter lever DAC, DAC Chip for Philip TDA1543
2.This DAC can use for multimedia system, satellite radio, CD player and DVD
player(two channel)
3.Using DIR9001 as digital input receiver
The receiver used is more expensive and of better quality than the popular CS8412/4/6
used in all other DACs. It has many more music details and accurate sound you obtain.
4.Coaxial / Optical Input , Analog Output
5.Short signal path
6.Power Supply: DC 12V,1A (Positive Center) Including
7.Dimension:100mm * 71mm * 25mm ˜ 4in * 2.5 in * 1in
US $35.99 ex shipping

Another idea of mine was to try a Yamaha YMF744 or YMF754 based PCI soundcard and see if I can use its S/PDIF input in pure Dos. Currently I use a Montego II Home studio, which is an Aureal Vortex-2 based card with digital I/O Bracket.

Another thought: How can that ISA card output a 44.1 kHz S/PDIF Signal when FM normally is 49.7 kHz? Suspicious, maybe it is better to stick to to a card with genuine OPL3....