What USB driver has to do with the volume control of the interface itself, on its digital outputs used as the main output bus?
I should have been more clear in my statement. BLA use a version of the Thesycon driver, which like with other similar XMOS-based audio interfaces on the market, includes a mixer app that reveals an onboard routing matrix and mini mixer. My understanding is that, at least with the 6x6, this takes place on the device itself wherein the latency of the built-in routing and mixing of i/o, for example, is
not impacted by the buffer size of the audio interface driver settings. Now I don't know what the inherent routing latency of the built-in mixer/router is by itself, BUT in the demos of it, it can be seen clearly passing audio from the ports in standalone mode with very low latency essentially in near real-time,
without a driver, without any routing performed by a computer. So for example, in standalone mode, the OTG port is automatically routed to the analog outs, etc., but while connected to a computer, these channels can be freely routed and mixed, which is great set of flexible options for live streamers, for example. This kind of functionality is similar to other Thesycon-driven XMOS-based interfaces like those made by Audient, etc. For example, the Audient id44mkii has a standalone mode too, but on that product, it uses stored settings you have to previously set in the driver. Nethertheless, in both those examples, the routing and basic mixing itself is taking place on the device itself,
not within the computer in the driver itself. Hope that makes more sense. While they don't have complex DSP functions, they do have limited DSP capabilities suitable for built-in routing matrices and mini mixers.
The 6x6 is obviously the most flexible (comparing the 2x2 and 6x6), as it has more i/o and a standalone mode that automatically re-routes itself to a practical configuration for streamers (which is pretty neat IMO, especially with the OTG port). The 2x2 also uses the same driver and mixing feature, but the design and capabilities of the device are far more simiplistic, and it's possible the latency is higher too. I have not confirmed this. However, as I understand it, even the 2x2 has basic built-in routing that can be configured to route/mix streams, including a loopback feature.
This is different than what the Zoom U-44 offers, which you rightly pointed out, since it has no accessible onboard mini-mixer, and no advanced driver with a routing matrix, etc. However, it should be noted that it does work in a standalone mode too, where it can act as an AD/DA device (even running on battery power!), and you can manually toggle between optical and coaxial S/PDIF. So it's still quite a nice little swiss army knife device IMO in its own way.
And BTW, all the above mentioned devices work in class-compliant mode, so the driver is not necessary to install. However, you do lose access to the built-in routing capability in the case of the BLA stuff.
But you are correct about the volume knobs on the BLA and Zoom devices mentioned being analog only, which is a disappointment for this workflow, as it obviously reduces the potential flexibility of the devices, since you can't attentuate a digital signal without the drivers installed to access the mini-mixer. In the case of the U-44, you'd have to adjust the digital output via some software application in the computer (i.e., DAW, etc.), which would then be hit with the
total RTL
including device buffer size. This is a big downside of the more simplistic Zoom for this workflow. In the case of the BLA stuff, you can use the router/mini-mixer which I understand to be on the device itself, and not subject to
additional latency caused by the round trip of the driver buffer size settings... so while there would still be some latency (converters plus inherent built-in router/mixer), it would be significantly better than the Zoom U-44 in this example, which has to go through something like a DAW, which is subject to buffer size too.
Hope that's more clear, as I was responding to your comment that the BLA stuff "don't have DSP and so no digital volume control" which is technically incorrect, as they do have a digital volume control on their built-in router/mini-mixer. To your larger point, I agree with you that it's not a huge distinction in real life since you can't do anything with the router/mini-mixer without installing the driver package, though (except standlone mode on the 6x6). But technically, it is on the device, not on the computer. Some Thesycon/XMOS audio interfaces have digital controls as opposed to analog controls, and might be more suitable for the OP's use case, but then again I can't think of one off hand that has the i/o he wants... BTW in theory even the Audient devices like the id44mkii have some programmable function buttons on the interface, but Audient doesn't do much special with them... but if Audient cared to do something special with them for standalone mode, for example, they could trigger more advanced configurable states or other routing/mixing features in the Audient onboard mixer/router, again, without a driver. But it too falls short of its potential in that regard.
