How fast is electricity when compared to say USB, ADAT, firewire, thunderbolt?
And, maybe 'electricity' is not the correct word as I'm referring to the flow of audio signals. 
 
For the past 18 years or so my rig has been a digital mixer via adat to the computer. I chose this route back then basically because I figured that there would only be one transformation of the signal as it entered the mixer (A/D). As apposed to analog where you would have A/A then A/A out into the A/D of the interface. (A/A meaning analog/analog).
 
I've been thinking about downsizing my rig (and upgrading the convertor quality) and have some stumbling blocks in my head about the whole thing about what is the best way to go? 
 
Kind of a weird question I guess. 
Anyway. Thanks.

An electric signal travels through a wire at a significant fraction of the speed of light.

timidi
How fast is electricity when compared to say USB, ADAT, firewire, thunderbolt?
And, maybe 'electricity' is not the correct word as I'm referring to the flow of audio signals. 
 
For the past 18 years or so my rig has been a digital mixer via adat to the computer. I chose this route back then basically because I figured that there would only be one transformation of the signal as it entered the mixer (A/D). As apposed to analog where you would have A/A then A/A out into the A/D of the interface. (A/A meaning analog/analog).
 
I've been thinking about downsizing my rig (and upgrading the convertor quality) and have some stumbling blocks in my head about the whole thing about what is the best way to go? 
 
Kind of a weird question I guess. 
Anyway. Thanks.

Your question conflates several concepts.
 
The speed of electric charges in a wire is about 2/3 the speed of light in a vacuum. That 2/3-of-vacuum speed is the same for the LED/laser photons in fiber optic cable.
 
USB, Firewire, and copper Thunderbolt have various encoding schemes for the electric signals. These schemes make for a range of data rates. Data rates are not to be confused with propagation speeds of the electric charges in the wire. In fiber optic cable, for ADAT and optical Thunderbolt (and MADI) the same rules apply; the encoding scheme determines the data rate, number of channels, and bit depth, while the propagation speed is 2/3rds of the vacuum speed of light. 
 
In both cases, this speed in the copper or fiber is a little more than 500,000 the speed of sound in air.  That's why it's a waste of time to listen to Hi-Fi salespersons who push you to match the lengths of your left and right channel cables.  
 
The time required for an A/D conversion is on the order of a few microseconds. It's the device driver and buffering in a computer that takes up (relatively) lots of time, causing many milliseconds of delay that we experience as the annoying computer latency. 
 
HTH

I think the question timidi is asking is simply whether there's a downside to using a digital mixer in the studio due to its latency.
 
Generally speaking, no, there is not - as long as the mixer output isn't recombined with the direct analog signal, which would result in comb filtering.
 
We usually needn't worry about mixer latency because it's small enough to (normally) not matter. Mixer latency is going to vary from one model to the next but is normally inconsequential.  
 
Worst-case scenario would be a drummer monitoring through the mixer. Drummers are the most likely to complain about latency in their headphones. Consider that the normal propagation latency in air for a drummer is about 3 milliseconds, the time between hitting a drum and hearing it. Given that 3 ms is about the threshold of perception, the mixer would have to delay the headphone signal by double that - 6 ms -before even the most timing-attuned drummer could perceive it. And 6 ms would be a very, very slow digital mixer.
 
 

GaryMedia
The time required for an A/D conversion is on the order of a few microseconds. 

The latency due to the filters in an ADC/DAC can be a couple of ms or so.

I'm basically just trying to figure out what is the best signal flow for a new setup.
The digital mixer has been great and I'm kind of on the fence about the whole thing. The mixer is a DA7 circa 2000ish so I'm figuring convertors are much better nowadays. 
 
That said, I've always been hesitant regarding anything USB. Maybe because I get occasional hiccups with attached USB devices. I have no electrical engineering background or knowledge of any sort really. Grew up plugging into an analog mixer and THAT to me was 'electricity'. Everything else including USB, ADAT, Firewire, thunderbolt etc is not :)
 
But most of the front ends I see are somehow USB related. So, it leaves me in a quandary. 
I have not had one issue with my digital setup in 16 years. DA7 via ADAT to an RME AIO card. 
 
So, the RME is like circa 2009 and actually I've never heard it's convertors. Been meaning to have a listen on the output side straight to speakers. Anyway, A lot of the front end USB, firewire etc interfaces also have adat connectivity as an extra I/O.  I'm wondering if I can tap the ADAT I/O of the interface and go into my RME PCIe card and still retain all the features of the particular interface/preamp etc. Like for example a preamp or interface is marketed as a USB device but also has firewire and ADAT. Would all those different I/Os have the same functionality as the USB. I know for the ADAT that it would be limited to 48hz on an 8 channel setup. But, with some kind of voodoo, can be made to work with 4 channels at 96hz. (My RME has only one ADAT in/out).
 
Anyway, just thinking.
Thanks for the responses. 
 
 

drewfx1

GaryMedia
The time required for an A/D conversion is on the order of a few microseconds. 

The latency due to the filters in an ADC/DAC can be a couple of ms or so.

Bonnie Baker from Texas Instruments has written a series of articles on this and similar topics. That's where I got my info.  An example of the sort of info is in this PDF:  http://www.ti.com/lit/an/slyt264/slyt264.pdf
 
My audio interface (a Behringer X32) has analog-to-analog latency of 0.8-milliseconds (800-microseconds) with all its internal digital glory. At a minimum that includes an AD and DA.  However, when the audio path has to go through computer via USB or firewire, all the normal computer driver, serialization/deserialization, etc., issues arise, and the latency numbers now have a lower boundary >3 milliseconds, and the provided drivers don't reliably do much better than 8-milliseconds round-trip. 
 
 
 
 

timidi
...I have not had one issue with my digital setup in 16 years. DA7 via ADAT to an RME AIO card. 
...I'm wondering if I can tap the ADAT I/O of the interface and go into my RME PCIe card and still retain all the features of the particular interface/preamp etc.  I know for the ADAT that it would be limited to 48hz on an 8 channel setup. But, with some kind of voodoo, can be made to work with 4 channels at 96hz. (My RME has only one ADAT in/out).

I think I'm getting that you want to keep your RME AIO in place, and choose an ADAT-included interface from the zillions that are in the marketplace, and connect it to the RME that you have.   The answer is yes, you can do that. Many of the interfaces on the market can take their analog-in, run it to ADAT out, and also route it to the embedded USB or Firewire interface whether there's a computer there or not (although configuring it will usually need a computer there to set up the routing). 
 
Many of them also support SMUX, which is the 4-channel-at-96kHz-sampling capability that you'd need.  So, now at least you can confidently shop, knowing that what you want it out there.

GaryMedia

drewfx1

GaryMedia
The time required for an A/D conversion is on the order of a few microseconds. 

The latency due to the filters in an ADC/DAC can be a couple of ms or so.

Bonnie Baker from Texas Instruments has written a series of articles on this and similar topics. That's where I got my info.  An example of the sort of info is in this PDF:  http://www.ti.com/lit/an/slyt264/slyt264.pdf

We're getting off topic, but that paper is not discussing audio ADC's, but rather specialized converters built for minimal latency (at a cost of other factors).
 
Audio ADC's/DAC's can have filter latency anywhere from a fraction of a ms to at least a couple of ms or so, depending on a number of factors.

Wow, my participation in this topic has been mighty wobbly!
 
First, I was guessing on the OP's 'real' question, guessed wrongly, and was too specific by answering about propagation speeds when the OP was really inquiring about conversion latency.
 
Then, I posted a PDF about conversion latency in its minimalist form, and was told that I wasn't specific to audio conversion A/D converter latency, even though the rest of my post pointed out that my Behringer X32 analog-to-analog latency is sub-millisecond. 
 
It's a good thing that I replaced the vacuum tubes in my time machine, anticipated the direction this thread would take, and two weeks ago posted an Echo AudioFire Pre8 on eBay that meets the needs of the OP.  It (the Pre8, not the time machine) has ADAT capability (both 'normal' and S/MUX) that is remembered between power cycles and, once configured, will pass signal without a computer on the firewire interface.  It's still there for sale.
 
I'll bow out for now, and let everything else flow as needed.