Two Audio Interfaces

Mac computer allow audio interfaces aggregations so yes you could use more than one at the condition that they both support the resolution of the project in your DAW..
For exemple, if one interface doesn't do 192khz or 24bit....
Patch bay are usefull when you have more gear than inputs/outputs in your audio interface.
For exemple 8 input interface but 5 stereo keyboards. You could plug all the keyboards to the patch bay and all the inputs of the interface.
So you could use a patch cable (usually short cable) to connect what ever keyboard to what ever inputs of your interface.

Adat card ? well Adat is a type of connexion. Usually connected with digital wires. It allows to connect 2 units together.
For exemple a stand alone preamp unit with 8 channels could have an adat port output which could go to your interface adat input and then you get to record the preamps via your interface in digital without quality lost. Note than 1 port can handle 8 mono signal up to 48khz or 4 mono signals at 96khz. To get 8 channels at 96khz you would need 2 adat ports (ins and outs)

If you aggregate two sound cards that don't have their clocks synced (via word clock or SPDIF or ADAT optical or sync cable), one of them will have its signals resampled.

A traditional use of a patch bay is to bring all the rear panel connections of a mixer and rack of gear to the front of the rack in order to allow repatching without the need to go behind the racks.

bouldersound, post: 462467, member: 38959 wrote: If you aggregate two sound cards that don't have their clocks synced (via word clock or SPDIF or ADAT optical or sync cable), one of them will have its signals resampled.

Im my mind, running stuff at different sampling rates in one project, is ludicrous.

If you aggregate 2 interfaces running at the same sampling rate, it doesn't involve resampling anywhere. Why would it?

16 or 24 bit is also better the same for the entire set-up. If it's not, the missing audio will be filled in with zeroes or truncated by Core audio on MacOS. Your DAW will do the exact same thing, if needed. There's no way this process could result in artefacts of any kind. Of course, there are bugs...

Just like RME, fi, does, the clocks will be re-synced by Core audio. External clocking brings nothing but confusion. As long as everything is connected to one computer, there's no need for external clocking. If you're in a set-up with several AD stages and need to keep sync, like location film, you might want to check your clocking options.

It's simple enough to set upaggregated devices in Audio/Midi setup. No fancy tricks needed. Just try it out.

Note that it works with most, but not all interfaces. Most noticeable, Tascam. Tascam doesn't officially support it. Some Tascams seem to aggregate flawlessly, others don't.

It works best with two comparable interfaces. I would expect an old 2-channel M-Audio aggregated with a 56 channel RME to fail, but you need to test it to see if it works. It depends on the drivers. Solid drivers work, messy drivers usually fail.

Just for the hec of it, I once aggregated 7 USB audio sticks (stereo I/O). Worked pretty well when used with the right USB ports. Before my hair turned grey, I used a 2/6 and a 6/2 interface aggregated on a daily basis. Gave me 8 in, 8 out @24 bit, 48 kHz, over USB 1.1. And that's about the most you can expect from USB 1.1. Note that both of these were 'driverless", meaning they used the USB audio class driver built-in to MacOS.

I'm trying to use 2 Focusrite ( Scarlet 18i18)

That should work. And FR have good support.

These boxes also have ADAT. You could use that to slave one to the other and use 16 input channels over 1 USB connection. Need to check how routing is affected in the Focusrite app. I'm not really familiar with the latest version, I'm afraid.

Could be a bit simpler to set up than aggregation. Hardware usually works more reliable than software.

You'd need two short optical cables to link them over ADAT.

Norman Fernandez, post: 462474, member: 51653 wrote: I'm trying to use 2 Focusrite ( Scarlet 18i18)

Only the Scarletts that allow you to route the converted analogue inputs to the ADAT output would work in that way. I don't know an 18i18, so can't tell you whether it is possible on that model.

cyrano, post: 462472, member: 51139 wrote: If you aggregate 2 interfaces running at the same sampling rate, it doesn't involve resampling anywhere. Why would it?

Because clocks drift if they're not synced. That's why we have word clock sync connectors.

From what I've learned with a quick search, the 18i18 only has ADAT input, so you can't use that to link two of them for a single USB connection to the computer. You also probably can't use that to sync the clocks, but they do have SPDIF I/O on RCA connectors, which would likely let you sync the clocks.

In the specs

• Digital Inputs:1 x Coax (S/PDIF), 1 x Optical
  
• Digital Outputs:1 x Coax (S/PDIF)
  

class="xf-ul"> No ADAT...

So the question is could the clock be transmitted via Spdif.. I doubt it..

pcrecord, post: 462485, member: 46460 wrote: In the specs

• Digital Inputs:1 x Coax (S/PDIF), 1 x Optical
  
• Digital Outputs:1 x Coax (S/PDIF)
  

class="xf-ul"> No ADAT...

So the question is could the clock be transmitted via Spdif.. I doubt it..

When you slave a SPDIF device to incoming signal it gets the clock from the signal. If that weren't true then SPDIF would not work without a separate clock connection like word clock. Word clock becomes necessary when you have multiple digital sources that aren't being clocked from incoming signal.

pcrecord, post: 462485, member: 46460 wrote: In the specs

• Digital Inputs:1 x Coax (S/PDIF), 1 x Optical
  
• Digital Outputs:1 x Coax (S/PDIF)
  

class="xf-ul"> No ADAT...So the question is could the clock be transmitted via Spdif.. I doubt it..

Ar you sure that spec is for an 18i18 and not an 18i8? The numbers in the name are the clue. If an 18i18 ever existed, the first number would be the number of inputs: 8 analogue, 8 ADAT and 2 S/PDIF. The second would be for the number of outputs, i.e. the same mix. The actual 18i8 has the same inputs (8 + 8 + 2), but only 8 analogue outputs - no ADAT output. The 18i20 is the nearest to that spec.

Boulder is completely correct about S/PDIF carrying sync. Every packet has a sync sequence preceding the header + data to ensure correct unpacking, and the interval between sync sequences gives the sampling frequency.

Boswell, post: 462489, member: 29034 wrote: Ar you sure that spec is for an 18i18 and not an 18i8? The numbers in the name are the clue. If an 18i18 ever existed, the first number would be the number of inputs: 8 analogue, 8 ADAT and 2 S/PDIF. The second would be for the number of outputs, i.e. the same mix. The actual 18i8 has the same inputs (8 + 8 + 2), but only 8 analogue outputs - no ADAT output. The 18i20 is the nearest to that spec.

Boulder is completely correct about S/PDIF carrying sync. Every packet has a sync sequence preceding the header + data to ensure correct unpacking, and the interval between sync sequences gives the sampling frequency.

It's 18I20,
I realized too late,
So I just left it.

Norman Fernandez, post: 462490, member: 51653 wrote: It's 18I20,
I realized too late,
So I just left it.

There's an edit option.

Norman Fernandez, post: 462490, member: 51653 wrote: It's 18I20

The Scarlett 18i20 has ADAT input and output, and it has word clock output. But since it doesn't have word clock input you can't use that to sync the clocks. But you can use either the SPDIF or ADAT connections to sync the clocks. I think it's a little more finicky than word clock, but I've done it before. Even better, you should be able to pass signal via ADAT from one to the other, and have only one USB connection so your computer sees it as one device and you don't have to aggregate multiple devices. That's what I would do. If you're running 44.1k or 48k audio you can use just one optical cable. You'll need two to run higher (up to 96k) audio.

Norman Fernandez, post: 462490, member: 51653 wrote: It's 18I20,
I realized too late,
So I just left it.

This changes everything !! :)

Here are the instructions to connecte them both.
https://support.focusrite.com/hc/en-gb/articles/360001374139-Connecting-Two-Scarlett-18i20-s-via-ADAT
They can sync via ADAT

bouldersound, post: 462487, member: 38959 wrote: When you slave a SPDIF device to incoming signal it gets the clock from the signal. If that weren't true then SPDIF would not work without a separate clock connection like word clock. Word clock becomes necessary when you have multiple digital sources that aren't being clocked from incoming signal.

That's the thing, not every units can be slaves via SPDIF. For exemple, if I take my spdif (rca) from my FF800 to my Yamaha home theater amp, it works but there is a big lag about 200ms. If this would be the behaviour of a recording unit, it would make a mess in the DAW.
Also my Mitek AD96 is connected to my FF800 via spdif.. but the clock is syncked with worldclock BNC cables.
In the case of 18i8, not sure it can.. .but now we are talking of the 18i20 ;)

pcrecord, post: 462494, member: 46460 wrote: That's the thing, not every units can be slaves via SPDIF. For exemple, if I take my spdif (rca) from my FF800 to my Yamaha home theater amp, it works but there is a big lag about 200ms. If this would be the behaviour of a recording unit, it would make a mess in the DAW.
Also my Mitek AD96 is connected to my FF800 via spdif.. but the clock is syncked with worldclock BNC cables.
In the case of 18i8, not sure it can.. .but now we are talking of the 18i20 ;)

A home theater device is probably running the signal through a surround processor. But your home theater device is automatically slaving to the input.

But latency is a different thing from clock sync. Typical symptoms of sync errors if you connect SPDIF devices but fail to set the receiving device as slave are clicks and pops. If you have two unsynced devices connected by separate USB cables to a computer and record the signals (without using something like aggregate device) you can get drift, dropouts or other effects. You don't get a consistent offset.

I bet you could sync your FF800 to your Mitek through SPDIF if you wanted, even though word clock sync is more reliable. It's only when you have multiple digital sources that word clock becomes absolutely necessary.

Generally speaking, you get the least jitter on an A/D device when using its internal clock. So if I were tracking through the Mitek I might sync from it to the FF800. If I were using both to track with at the same time I might decide which one had the most important content and make that the master. But that's getting a bit more picky than is really necessary.

bouldersound, post: 462495, member: 38959 wrote: I bet you could sync your FF800 to your Mitek through SPDIF if you wanted, even though word clock sync is more reliable.

My mytek is actually my master clock (via bnc) .. going to a UA 4-710 then a Focusrite Saffire 56 (only for headphones) then the Fireface 800
I'll check if the FF800 can slave sync to spdif..

pcrecord, post: 462496, member: 46460 wrote: I'll check if the FF800 can slave sync to spdif..

Yes, it can, either coax or optical.