Understanding digital I/O

[dongna]

Looking to possibly expand my setup by adding a second keyboard and a computer/audio interface.

 

One possible combination I'm investigating is a PC361 as a second keyboard and a MOTU Audio Express. In researching both these items, I notice that both have digital inputs and/or outputs. I have never used such I/O. My question is, can-- and should-- I use these I/O connections to connect the PC361 to the Audio Express? Would there be any "issues" in doing so?

 

Also, while I'm asking about this sort of thing... I notice the Audio Express has for analog inputs 2 which they label MIC/INST, and 2 which they label LINE. Obviously the MIC designation is for microphones, but is the INST designation is intended for instruments like electric guitars/bases, or could it also be used for a keyboard instrument? In other words, is there any difference in the signal provided by a guitar and a keyboard?

[J. Dan]

One thing to be aware of is that using a digital connection, your master volume knob will have no effect.

 

I once had the output card die on my old Alesis Fusion and had a gig. I brought my laptop and M-Audio Fast Track Pro to run the SPDIF out from the Fusion through the PC and to the analog out as a workaround. Luckily the Audio interface had a volume knob I could use.

[johnchop]

One thing to be aware of is that using a digital connection, your master volume knob will have no effect.

 

Actually, not true for the PC3 series. There is an option to allow volume control from the master fader.

 

I've used the PC3 + digital I/O with my Focusrite Pro 14. Works fine. Just make sure you match sample rates correctly! Also, the PC3 programs can be a bit all over the place in terms of volume, so be sure you match levels as you build your live sound palette.

 

As for the mic/line, yes, there's a difference in terms of how hot the signal is. I'm not familiar with the MOTU software, but if it's anything like the Focusrite, you can switch the level there.

 

 

[J. Dan]

One thing to be aware of is that using a digital connection, your master volume knob will have no effect.

 

Actually, not true for the PC3 series. There is an option to allow volume control from the master fader.

 

That's a handy feature, however, I wouldn't recommend using it, especially if you're doing anything very dynamic. A digital signal has a limited bit depth. When you "turn down" the signal, you are essentially limiting it to a lower resolution. So for instead of 16 bit as an example, you may have to squeeze your entire dynamic range into 8 bits of resolution, which can create some artifacts.

 

I have a real world example where this happened to me. I was in a band that bought one of the early digital EQ's with RTA analyzer. We mistakenly ran our Main outs into it, then to the amps. At lower volumes, there was audible distortion. We figured out it was because we were essentially only using a very limited amount of the bit depth. The "proper" way to do it was to use the main INSERT, so that the full range was used prior to being attenuated by the Main out faders.

[johnchop]

Agreed. Just because you can doesn't mean you should.

[shambler]

As john says, just match the sample rate of your I/O and you will be OK.

 

I always use the digital output on any synth I've owned to ensure the signal path is interference free.

[JeffLearman]

Short story: go for it (answering both questions).

 

Long stories:

 

One thing to be aware of is that using a digital connection, your master volume knob will have no effect.

Actually, not true for the PC3 series. There is an option to allow volume control from the master fader.

That's a handy feature, however, I wouldn't recommend using it, especially if you're doing anything very dynamic. A digital signal has a limited bit depth. When you "turn down" the signal, you are essentially limiting it to a lower resolution. [example deleted]

Yes, but no. For 16-bit signals, this would be a big issue (as your example demonstrates), but S/PDIF is a 24-bit protocol, so you can attenuate by 50 dB before loss of precision is much of an issue (i.e., at the point where it "degrades" to CD quality. Even then, does a classical CD sound like crap during the quieter passages? No, so you can go 12 or more dB further, to 14 bits, and still have remarkably good fidelity.)

 

The biggest problem is when using multiple digital sources. Only one of them can be the clock master, and with keyboards, you generally can't slave one's DAC clock to the other's, so you can get bit-slip. I mention this because you said "adding" a PC361 keyboard. If you have only one keyboard with digital outputs, no worries. (And if you have one that has an input and an ouptput, and can daisy-chain, no worries on this bit.)

However, Wiki says:

The receiver does not control the data rate, so it must avoid bit slip by synchronising its conversion with the source clock. This means that S/PDIF cannot fully decouple the final signal from influence by the analogue characteristics of the source or the interconnect, even though the digital audio data can normally be transmitted without loss. The source clock may carry inherent jitter or wander, and noise or distortion introduced in the data cable may further influence the process of clock recovery.[5][6][7] If the DAC does not have a stable clock reference then noise will be introduced into the resulting analogue signal. However, receivers can implement various strategies which limit this influence

What this means is that if you slave to your keyboard's clock and your keyboard has a crappy clock, that'll affect the quality of D/A conversion of anything coming from your computer.

 

But what does THAT mean? It means that if you're using your computer and your PC361 live, the PC361 (if it has a flaky clock) might degrade the sound coming from your computer. But if you're using it in the studio, just for recording, it wouldn't be an issue -- instead, it would be ideal to skip the digital-to-analog-to-digital round trip.

 

Even for live use, I bet the PC361's clock is good enough not to worry here. If it was a Keyboards-R-Us brand, who knows.

 

Also, while I'm asking about this sort of thing... I notice the Audio Express has for analog inputs 2 which they label MIC/INST, and 2 which they label LINE. Obviously the MIC designation is for microphones, but is the INST designation is intended for instruments like electric guitars/bases, or could it also be used for a keyboard instrument? In other words, is there any difference in the signal provided by a guitar and a keyboard?

Go ahead and plug your keyboards into the MIC/INST jacks; just set the switch for "INST" and not "MIC".

 

While there are differences between a guitar's output and a line output, and there *can be* differences between an input intended for guitar versus a line input, they're pretty super technical, and you're in the sweet spot where it doesn't matter: you can plug a line output into a guitar amp, for example. (Any stompbox has line outputs, and you can plug them into line inputs or an amp or another stompbox.)

 

It gets trickier when you want to plug a passive electric guitar into a line input, but even that can work, though it's not ideal.