Sunday, April 2, 2017

MOTU UltraLite AVB: Hybrid Stereo + 5.1 Setup

I use MOTU UltraLite AVB as my primary sound interface. It's a versatile and easy to use device, with lots of audio inputs, outputs, and excellent DSP-based routing and mixing capabilities. Once you have created a certain audio setup, UltraLite AVB offers a way to save it and restore it later. For example, I had a setup for 2.1 speaker configuration, and a setup for 5.1 surround configuration (why they have to be different?—see below), and I was switching between them depending on the material playing.

But switching between setups isn't something that my kids or wife can do easily. So I decided to create a hybrid configuration that can be applied to all my use cases. Here they are:

Use Case 1: This one is active when kids play games. They sit next to the computer, way behind the left and right monitors, so they can't hear them properly. The only speaker that can deliver sound to them is the Cambridge Audio Minx Go located below the computer monitor.

Use Case 2: This one is for playing stereo content. The primary speakers is a pair of JBL LSR305 supported by KRK 10s sub, comprising a 2.1 setup. But since there are also rear KRK RPG2 5 speakers set up for the surround use case, and the center channel, these can be optionally engaged for widening the soundstage and enhancing dialog clarity in movies.

Use Case 3: This is the real 5.1 surround setup where each of the 6 speakers has its own channel to play. However, since the speakers are not full range, bass parts of their channels need also to be routed to the subwoofer, in addition to the LFE content.

The presence of additional speakers in the 2.1 setup doesn't allow it to be used for the 5.1 case. Take for example, the left channel in the 2.1 setup--it needs to be routed into the front left speaker, as well as into the center, and into the rear left speaker, and this routing is incompatible with the 5.1 setup, where the front left channel only goes into the front left speaker.

On the computer side, achieving a hybrid setup is pretty easy. On Mac, in Audio MIDI Setup app it's possible to assign different input channels of a multi-channel audio interface to different configurations, e.g. for stereo use input channels 1 & 2, while for multichannel 5.1 setup, use channels from 3 to 8. Now, the question is, how to configure MOTU to route the channels accordingly.

This actually has turned out to be non-trivial. Mostly because the approach for controlling routing and mixing used in MOTU products is modeled after classical mixing boards used in studios. In practice that means there are restrictions on what can be connected to what, and at which stages effects can be applied.

The main effect I need is the equalizer—to perform some basic room correction. Another important thing is digital attenuation which allows aligning speaker output levels precisely, as knobs on inexpensive powered monitors usually lack required precision—you can do basic alignment with the knobs, but then if you need to make one of the monitors softer, say by 1 dB, the only way to achieve that is by attenuating the corresponding channel on the sound card.

In order to visualize for myself all the allowed connections between mixing stages of MOTU card, I've created the following diagram:

See, it's actually not that simple. Each processing block can be characterized with the following attributes:

  • how many inputs (and outputs) does it allow; typical values are 0, 1, and many. E.g. a sound card input can only output audio data to the DSP, so it have zero mixer inputs, but it can be used as a source to any number of other mixer blocks;
  • whether the block has effects; that's easy to figure out--only blocks that provide effects appear on the Mixing tab of MOTU control UI;
  • what is the way to route the output of the block back to DSP; E.g. the "Mix Aux" block—a plentiful resource, can't output to other mixing blocks, its output can only be chained via another "Mixer Input" block, and this connection is done using the Routing tab;
  • and finally, some stereo blocks can be split into independent mono channels, and some don't.

Note on the Reverb group: it's a special Mix Group because, first, it is the only that contains a reverb effect, and second, other Mix Groups can send to it directly from the Mixing tab, but not to each other. This feature is expressed on the diagram as a special input marked "R".

After figuring out the rules, and having the use cases in mind I've came up with the following diagram of how the blocks should be connected:

Here, "L" / "R" letters on inputs and outputs designate left and right channel. I had to only use one channel on "Sub L" and "Sub R" groups because equalizer settings are different for left and right channels, and unfortunately a Mix Group can't be split into a pair of monos.

This is how the mixing configuration looks on MOTU UI (note that Mix Aux strips didn't fit):

Having a diagram at hand was really helpful to set everything up.

The Reverb group is used for the real channels in order to add a delay. Unfortunately, there is no direct way to set up a delay on MOTU (that's a big deficiency to my view, compared to miniDSP products). The trick was to use a "Pre-delay" setting on Reverb effect, set all other parameters of Reverb to minimum, and compensate with an EQ for a high frequency shelving that Reverb creates. This restores the frequency response, but not the phase, resulting in a non-uniform group delay. But this is hardly noticeable.

As a conclusion, I would say that I greatly appreciate robustness of MOTU configuration abilities, but I would really like to have some "DYI" mode for DSP that would offer the following:

  1. Input bi-quad (or better, multi-pole multi-zero) coefficients directly.
  2. Remove the processing block "specializations".
  3. Input delays directly, not as part of the Reverb effect.

5 comments:

  1. How did you mix down the Sub L and Sub R into one mono output?

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    1. Hello Champ! On the "Mixing" tab you need to split into 2 mono channels one of the mixer inputs and one of the "Aux" busses, by clicking on the two blue dots under the channel name (note that "Group" busses can't be split). Then you set up an "Aux Send" from input L and from input R into the same channel of the Aux bus you've just splitted.

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    2. Ah got it, thanks for the help on that, and then are you eqing Sub L and Sub R separately to avoid phasing? Sorry I am somewhat new to this but trying to setup my audio interface for 5.1 monitor while I edit

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    3. Frankly, I don't remember why I was using different EQ on Sub L and Sub R back then. I don't do that anymore.

      As for phasing, I think you anyway need to have low frequency part in your L and R channels to be in phase or close to that. This is to avoid problems when summing for the sub and for mono playback.

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  2. Got it, thank you for your help

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