RCA vs optical cable?

Are there any diferences between a regular RCA interconect cable and an aoptical cable? Can I use a regular RCA interconect cable to hoop up a transport and a dac? I understand that both cables have a 75 ohms resistance, so I suppose there is no difference between these cables. I am completely wrong? Any comments. Thanks in advance for your help.
First, optical cables do not have a 75ohm impedance. That is a spec for electrical cables but standard RCA audio cables are not 75ohms. Only digital audio cables are or should be.

Second, you cannot connect these two types of cables to the same jacks. What input and output jacks do you have?

Third, assuming that you can use either, there is continual debate over which one is preferred, so I do not think you can go wrong either way.

what i think you meant to ask is whether you can use a standard analog (red/white) rca audio cable as a digital coaxial (spdif) cable. (digital optical cables have a different type of connector). as kal notes, an analog rca cable has a different impedance and won't work well as a digital coaxial cable. however, a standard analog (yellow) composite video cable is 75ohm and should also work as digital coaxial cable.
Loomishohnson you got my question right. Thank you Kal for pointing out that RCA cables do no have an impedance of 75 ohms. I just wanted to see if I could get away not expending some money in a digital cable. Thank you for your responses!
I would suggest trying a regular analog RCA to hear what it sounds like first before purchasing a digital RCA. If you like what you hear, great, if not, demo some different digital cables.

I am using an analog RCA from my SBT to my Audio Note 2.1 DAC and it sounds great to me and that is really all that matters. Right?

People are always asking if this can be done or what will this sound like if I do this, etc. I like to experiment with different set ups. In my opinion, an individual is in the wrong hobby and is lazy if they don't want to hear what different configurations sound like. It takes a lot of time and effort to experiment with what one is hearing.
Depending on many system-dependent variables, intentionally introducing what will probably be a significant impedance mismatch by using an analog interconnect to conduct a digital signal might in some cases, by happenstance, produce good results. In past threads here several members have in fact reported good results doing that.

However IMO it is poor practice, and if the results are sonically pleasing chances are that what is happening (at least in situations where the DAC does not provide jitter rejection that is near perfect, such as by means of ASRC technology) is that the mismatch is resulting in the introduction of jitter that happens to be euphonic in the particular system. From this paper by Steve Nugent of Empirical Audio:
Another interesting thing about audibility of jitter is it's ability to mask other sibilance in a system. Sometimes, when the jitter is reduced in a system, other component sibilance is now obvious and even more objectionable than the original jitter was. Removing the jitter is the right thing to do however, and then replace the objectionable component. The end result will be much more enjoyable.

Jitter can even be euphonic in nature if it has the right frequency content. Some audiophiles like the effect of even-order harmonics in tubes, and like tubes, jitter distortion can in some systems "smooth" vocals. Again, the right thing to do is reduce the jitter and replace the objectionable components. It is fairly easy to become convinced that reducing jitter is not necessarily a positive step, however this is definitely going down the garden path and will ultimately limit your achievement of audio nirvana.
-- Al
"Jitter can even be euphonic in nature if it has the right frequency content. " This is beyond my jargonistic experience and may be "jitterish b.s."
04-09-12: Ribit
"Jitter can even be euphonic in nature if it has the right frequency content. " This is beyond my jargonistic experience and may be "jitterish b.s."
The statement is not b.s. in any way, although it may sound like it is to someone who does not have relevant technical background.

Also, the paper from which the statement is quoted, which I linked to above, IMO provides some very good background on the subject of jitter.

Disclaimer: I have no affiliation with the author of the paper or his products, not even as a user.

-- Al
someone dares to disrespect almarg? does he not fear the wrath of the audio gods?
Distortion can be euphonic or detrimental. How it's done is practically infinite.
Jitter is just another way.
Not too hard to wrap your head around once you really think about it.

All the best,
The first public 'gon stoning will ensue!
"Puny Audio Gods"