I have Scarlett 18i16 and Fast Track Ultras. Can I connect Ultras via coaxial SPDIF to Scarlett in a daisy chain? If I'm not mistaken, Ultras can work independently in standalone mode, but not fully functionally probably. Probably I can use VB Audio Voicemeeter but I have no experience either.

Either way, I want to connect them primarily at the hardware level.

 

What are you trying to achieve? Do you want to record 8 audio tracks simultaneously, or something like that? If so, the devices don't need to be connected to each other at the “hardware level.” Connect them both to your computer via USB and set up an “Aggregate Device.” For Windows, you will need to use “ASIO4ALL.” For Mac, you will need to use “Audio MIDI Setup.”

 

I want to record 5 synths to DAW, simultaneously if possible or one by one. Synths are set with different MIDI channel and Ableton will send MIDI. I don’t want to use a bit noisy mixer with stereo out. Two Ultranovas have SPDIF OUT.

 

a single coax cable can only carry a stereo signal. If your goal is recording more than 2 channels, S/PDIF alone will never do it. You need multichannel capable interfaces so USB, ADAT, MADI, Dante, etc. Aggregate Devices just combine them in software, and they don’t increase S/PDIF’s hardware limit. The S/PDIF protocol is for stereo PCM. Two interfaces need to both be cabled, and so on.

like this:
Ultranova 1 S/PDIF → Interface 1 Stereo stream = 2 channels
Ultranova 2 S/PDIF → Interface 2 Stereo stream = 2 channels

The sample rates have to match, and your computer/daw is the master clock source. Otherwise you will either get drift or sync issues. The two separate interfaces is where setting up an aggregate device comes into play.

 

Last edited: Mar 13, 2026

You seem to have one or two synthesizers too many; why not get yourself a small mixer as well?
Behringer Xenyx 802S - 74 EUR --> www.thomann.de/de/behringer_xenyx_802s.htm
Then, place a Noise Gate FX for noise reduction first in the mixer chain!

DeeGate by Dotec-Audio --> https://plugins4free.com/plugin/3931/
- This is a very simple noise gate that mutes sounds smaller than the volume you set.
- The threshold can be set from -60 to 0 dB.
- Muting of noise in mic & line recording, cut the decay as an effector,
- Extraction of sound beyond a certain volume, etc., the application range is so many and convenient.

18i16 - Key Features

- Four ultra-low-noise microphone/line preamps
- Instrument preamps on inputs 1 and 2
- Four additional line inputs
- Two Air for classic console sound
- Auto Gain Clip Safe Auto Gain
- Alternative speaker switching
- MIDI, S/PDIF, and ADAT

ADAT is most commonly used to easily integrate additional audio channels into your recording setup, either as extra inputs or outputs. For example, if your audio interface only offers two analog inputs and outputs, you can add additional digital ins/outs via ADAT – provided the interface has the necessary connections.

Spoiler: Behringer ADA8200 Ultragain - 159 €

Behringer ADA8200 Ultragain - 159 €
https://www.thomann.de/de/behringer_ada_8200_ultragain.htm

1. AD/DA Converter
2. Successor to the Behringer ADA8000 Ultragain
3. 8-channel AD/DA converter, 24-bit at 44.1/48 kHz
4. 8 integrated microphone preamps (MIDAS designed)
5. Sample Rate Sync
6. All mic/line inputs routable to ADAT outputs and all ADAT inputs routable to line outputs

Typical Synth Studio with Your Interface Using Scarlett 18i16 + ADA8200 You get approximately: 8 Inputs (Interface) +8 Inputs (ADAT) = 16 Synth Inputs Perfect for: multiple synths drum machines modular setups samplers Very important (common mistake): ADAT is digital → synth must first be connected to a preamp/converter. Connecting the synth directly → ADAT will not work.

Optical input and output

The two Optical ports (input and output) give you eight channels of digital ADAT I/O, to connect to other audio equipment with ADAT I/O such an eight-channel mic preamps. There are many ways to connect and clock your Scarlett 18i16 when you're using an external device connected via it's optical ports. For information on clocking and digital IO setups, please see the Sync Status Indicator [25] section. The Sync Status Indicator on your Scarlett 18i16 should light green.

https://fael-downloads-prod.focusri...scarlett_18i16_4th_gen_user_guide_v1.1_en.pdf

 

Last edited: Mar 13, 2026

I never tried using more than one device in a4a, I know that it is possible though...
I'm just thinking: These devices are not samplerate locked, they are not slaved to a wordclock master or something like that. They drift. I don't know how a4a would handle that other than by dropping samples every now and then?
OP could solve this by using one spdif line to sync the devices I guess.
@nmkera
I'd say your idea to use the fast track as an AD converter and feeding spdif into your main interface is a very legit way doing it.
I use some steinberg UR interfaces just as ADAT AD/DA interfaces

 

Last edited: Mar 13, 2026

Thanks mates. I thought that SPDIF can transport more channels. So Fast Track sums four analog inputs to a stereo SPDIF output.

I want to use interfaces for A/D conversion with the least analog distortion. The analog mixers’ A/D converters are not that good as interfaces’ ones, I think.

Aggregation makes sense in this scenario as long as the laptop has enough USB ports.

It is the very informative thread.

 

Last edited: Mar 27, 2026 at 10:50 AM

Aggregate devices are great if you have 2 of the same interface. Mismatched latency from different interfaces can be a pain.

 

Hey @nmkeraj, how many and which external devices do you want to connect and use? Just so we can get a better idea! You either need a larger USB audio interface with more ports or an analog or digital mixer. How much money do you have available?

 

The OP clearly stated what gear HAS and what he want s to do, did you reply to the wrong post?

 

@nmkeraj, take your Focusrite Scarlett 18i16 and forget the M-Audio Fast Track Ultra !

5 synthesizers = 10 mono or 5 stereo 6.3 mm jacks.
Bzw. 5 Synthesizers = 4 Synths = 8 Mono and 1 Synth = 1 SPDF.

Still missing a connection for the 5th synthesizer!!

INPUTS
- Combo microphone/line inputs: 4x XLR/6.3 mm jack - 2 of which can be used as instrument inputs
- Line inputs: 4x 6.3 mm jack
- S/PDIF input and output: coaxial

 

Last edited: Mar 27, 2026 at 2:50 PM

5-6 devices. One or two Ultranovas, X-station, Xio, Roland D110, Akai SG01V.

 

The Behringer Xenyx 802S costs 74 EUR and is an 8-channel analog mixer with a USB streaming interface.
It has two 6.3 mm jack line outs. You would connect one devices there and connect the line out cable to your Focusrite.
www.thomann.de/de/behringer_xenyx_802s.htm

or Behringer Xenyx 502S - 5-Kanal Analogmixer = 54 € --> www.thomann.de/de/behringer_xenyx_502s.htm
or the t.mix mix 502 - 5-channel mixing console = 39 EUR --> www.thomann.de/de/the_t_mix_mix_502.htm

5 x Instruments:
Ultranovas 1x ----------> Behringer Xenyx 802S
Utranovas 1x
X-station 1x
Roland D110 1x
Akai SG01V 1x

@nmkeraj, You'll have to decide the rest...!
There are noise reduction plugins available for the Behringer, for example from Waves, to combat its inherent noise.

 

Last edited: Mar 27, 2026 at 3:10 PM

Voicemeeter limits you to using only one ASIO device driver.

Since both of your interfaces can use ASIO drivers, I would connect each of them via USB to your computer and then use VB Audio Matrix to route ASIO using separate channels into my DAW. In your DAW, you just use the VB Audio virtual ASIO driver and set up enough input channels.

Scarlett 18i16 ASIO --> VB Audio Matrix
Fast Track Ultra ASIO --> VB Audio Matrix
VB Audio Matrix --> Virtual ASIO driver --> DAW

 

It's not a very difficult question to answer once you realize the limitations of the SPDIF protocol. You need to know sample rate, digital lock, clock sources, and so on. It's relatively simple after that. But the ideal answer here would be that if you have this amount of stuff and want it all connected simultaneously, you are already well into "get a mixer" territory. I only have two devices connected via digital anything, one sampler via ADAT and one synth via S/PDIF, but even though they are digital connected I still rather send them through my analog mixer anyway.
You can come up with all the "hacks" and workarounds to get stuff working, but that usually results in a fragile setup in the long run. "I can't do this because this is...." explanations of stuff that should not be barriers in the first place.