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Metric Halo ULN2+DSP

Firewire Audio Interface [Mac OS] By Derek Johnson
Published August 2004

Metric Halo ULN2+DSPPhoto: Mike Cameron

The latest addition to Metric Halo's Mobile I/O range is, like the existing MIO 2882, now available with a powerful DSP option for running audio processing plug-ins.

When it was released, Metric Halo's MIO 1882 Mobile I/O Firewire-equipped audio interface was in the vanguard of such devices. At the time of SOS's review in November 2002, the only other manufacturer to have entered the fray were Mark Of The Unicorn, against whose popular 828 and 896 interfaces the Mobile I/O was pitched. It's about 18 months later as I write, and though the market has grown, it hasn't done so explosively. Different manufacturers may have differing ideas about how to engineer a Firewire audio interface, but there is now a range of audio devices using this high-bandwidth standard, from affordable to rather expensive.

Metric Halo's products sit somewhere in the middle of the market price-wise, offering good value rather than rock-bottom retail. This is especially the case with their ULN2 — a limited-input partner to the original, equally compact, MIO 2882. It may not be awash with inputs and outputs, but it does make up for this lack in other ways.

Internally, for instance, the ULN2 is equipped with a powerful, user-configurable digital mixer that offers very low-latency monitoring. And what's more, since SOS 's original review of the Mobile I/O, both the MIO 2882 and the ULN2 have become available in '+DSP' versions equipped with an Analog Devices SHARC 21065 chip, 2MB of flash memory and 8MB of SDRAM, and it's this version of the ULN2 that's under review here. This +DSP model is noticeably more expensive, but is equipped with a significant chunk of configurable processing power that takes a big load off your computer. As a taster, a handful of pretty useful plug-ins are included as part of the package, with the potential for other developers to repurpose their plug-ins to run on this DSP. And that's a very attractive prospect.

Getting Physical

Both basic and +DSP units have the same physical controls and interfacing. The interface offers six audio inputs, only four of which can be used simultaneously, and in addition to the resulting four audio channels that can be routed to your audio application, up to six audio streams can be routed back to the hardware. That's a fraction of the 2882's 18 ins and outs, and is unlikely to overtax the Firewire connection. Neither unit offers MIDI interfacing, which seems de rigeur on USB audio interfaces.

The ULN2's digital I/O is available in both professional AES-EBU and domestic S/PDIF formats, and there's also word clock I/O. The ULN2's digital I/O is available in both professional AES-EBU and domestic S/PDIF formats, and there's also word clock I/O. Photo: Mike Cameron

But if it lacks in audio channels, the ULN2 makes up for this in facilities and quality of hardware. First of all, this is a Mobile I/O product, with the emphasis on mobile. It can draw power from the Firewire connection to your computer, a supplied international 9V power supply, or optional broadcast battery pack (a special four-pin connector can be found on the rear panel). This flexibility of powering makes ULN2 an ideal compact companion to an Apple computer in any static or mobile situation (currently, there are no Windows drivers). If location recording is your thing, knowing you won't have to worry about mains problems will be a bonus point.

Starting with the analogue inputs, we find a pair of Neutrik combi jacks which can accommodate balanced/unbalanced jacks or XLR connections, the choice of input made by a front-panel switch. Making the choice switches in different circuitry for the mic amp or the DI amp. Amazingly, each also offers a send/return insert point. This isn't addressable by software in any way (shame), but is nevertheless a refreshing addition: no matter how good software signal processing becomes, there will still be those of us who have favourite hardware that we'd like to use early on in the audio acquisition process. Nice touch.

The input electronics have been designed to be 'ultra low noise' (as in ULN), and feed 24-bit, 96kHz A-D converters. The unit can operate at 44.1, 48 or 88.2 kHz, but the bit rate is fixed, so your application will need some way to dither down if you work with 16-bit audio in your sessions.

Phantom power can be applied independently at either analogue input, and there's a flexible gain stage: a 12-position stepped main gain control on the front panel is joined by a passive trim pot that can be switched into circuit independently; the trim controls can be linked in order to change the settings for both channels simultaneously, for stereo use.

Digital interfacing is similar to that of the 2882, though lacking the multi-channel ADAT interface: your choice is a professional bi-directional AES connection on XLRs, or co-axial S/PDIF I/O on a pair of phono jacks. The pro/consumer choice is great to have, though only one input can be used at any given time alongside the analogue ins, hence just four audio channels can be fed to your computer. Both outs can be used simultaneously, though. Metric Halo go the extra mile, especially for pro situations, by providing word clock in and out sockets, so the ULN2 can lock to house sync, or be itself the master digital clock in your studio. Another nice touch.

Outputs are limited, though thoughtfully implemented: a main stereo out (with -10dB and +4dB switch, for semi-pro or pro operation) is joined by a monitor out, and a front-side headphone socket. Monitor and headphones have their own separate level controls, which is more thoughtful design. All that remain on the rear panel are the dual Firewire connections — it's possible to chain further devices, including more ULN2s or 2882s, from this connection, and in some circumstances, it'll even provide power to devices such as hard drives.

Up to 10 snapshots of the ULN2's internal settings can be stored, and snapshots can be recalled from the front panel.Up to 10 snapshots of the ULN2's internal settings can be stored, and snapshots can be recalled from the front panel.Photo: Mike Cameron

Returning to the front panel, we find snapshot selection buttons, four 10-segment level meters — a pair each for stereo in and out streams, with clipping indication — and ULN2 status LEDs. The rest of the package includes rackmounting ears and a CD-ROM containing documentation, drivers, and other software. There is, however, no paper manual — not even a printed installation guide. It's all on the CD, and there isn't that much concrete coverage for the +DSP side of things, which was very much still under development as I wrote this review. In fact, as soon as I received the ULN2+DSP, it was necessary to download a package of new software, including an updated Console utility and new firmware for the interface itself.

Into The DSP

Users of the +DSP version of the ULN2 get a fourth window in the Console utility, which provides access to the DSP-based routing, processing and parameter editing features. Essentially, this graph-like interface provides you with a place in which to enable plug-ins from the 16 examples supplied, chain them however you like and place them in the signal path, again pretty much where you like, and edit them. It's here that the ULN2+DSP comes into its own. The inputs, in a column down one side, offer the analogue and digital ins, returns from your audio software, and a number of internal sources. Connections are made by mouse click, producing virtual patch cables between connection points. Each input can be 'multed' to feed as many processor inputs as you like, and outputs always go to the column of 16 'Process' busses, which turn up as destination options in the Mix/Output Routing page. This system is really flexible: you can process audio on its way to your software, while it's in the software, or as a final sweetening effect on its way out to a mastering machine or monitor system. The facilities are in place to make this a handy interface — and digital mixer — in a variety of studio and live mixing and recording situations.

Basic signal path routing and virtual mixer configuration is undertaken in the Mix/Output Routing window. Basic signal path routing and virtual mixer configuration is undertaken in the Mix/Output Routing window.

It's possible to save +DSP setups to disk, and a load meter lets you keep track of your DSP resources. There are actually two windows in which to create DSP setups, one labelled 'Virtual DSP'. Nothing happens here — there is no communication with the ULN2 if attached — but seems to be a handy tool for people to tinker with processing ideas when the interface isn't available. Anything you save from the Virtual window can be loaded in the real window when the ULN2 is plugged in.

Prime amongst the included plug-ins is Metric Halo's mono or stereo MIO Strip channel strip. This combination of gate, compressor and six-band EQ is spectacularly configurable, and seems to be descended from Metric Halo's multi-platform Channel Strip plug-in. The +DSP version is a more highly developed beast, however. Also available are a compressor, six- and 12-band EQs, limiter, and delay (all in mono or stereo options), plus channel summer, channel difference, channel sum/difference and a mid/sides processor.

You'll notice some esoteric tools in that list — and the delay, by the way, is not a creative effect. It provides up to 255 samples of delay compensation, even though there are no delays inherent in the +DSP system, and can be used to compensate for the delay from mics that are long distances apart, and to create such neat toys as a look-ahead compressor. The summer module acts a unity-gain mixer; adding two identical inputs, multing a connection from a source to the summer's two inputs, creates a signal 6dB louder than the input. The difference module phase-inverts one input before adding it to the other (which, with identical inputs, would result in silence). The sum/difference module combines both functions in one. How these modules are employed depends on the user — a wide range of applications is possible.

Side-chain access is provided for compressors, limiters and channel strips, allowing external keying and frequency-conscious processing. EQs are particularly good, each band having high and low cut, high and low shelving, band-pass and parametric options. There are comprehensive controls, which you can ignore by simply tweaking the graph with your mouse: each band is colour-coded, and tweaked by handy little on-screen dots. The way bandwidth is managed, when parametric or band-pass options are selected, is particularly elegant. All processors have a library option, and a bypass switch.

There is some serious signal processing potential here, and the only thing that'll make this an even more attractive feature will be more plug-ins, ideally with third-party developers utilising this DSP to take the load off host-based software.

Consoling Thoughts

The front-panel status LEDs reveal a side to the ULN2 that doesn't immediately seem accessible — which is because we haven't yet explored the supplied MIO Console utility. This software is really an integral part of the Mobile I/O package: it unlocks the unit's mix routing, and in the case of the +DSP model, provides on-screen editing for the supplied plug-ins that run on the extra DSP hardware (see box). Without Console, the ULN2 is not terribly useful: the internal mixer needs to be configured for audio to be routed anywhere. However, the ULN2 isn't quite as reliant on the Console application as the 2882: the latter has no front-panel controls or switches at all, and even basic operations such as gain and phantom power switching are managed in that unit's version of the software.

MIO Console's Analog I/O Control window, showing level meters for currenly active audio streams, plus the signal path chart and system settings. MIO Console's Analog I/O Control window, showing level meters for currenly active audio streams, plus the signal path chart and system settings.

If this sounds like the start of a negative comment, I'll speedily say that it's not. The ULN2, especially in its +DSP form, is a powerful interface, in spite of its limited number of audio channels. One can't imagine the actual hardware that would be required to replicate the flexible signal path emulated by Metric Halo's DSP expertise. And once you've created a mix setup — the internal signal path is essentially several virtual digital mixers, with comprehensive bussing — you can save it as a snapshot that can be recalled from the hardware front panel. An unlabelled column of LEDs next to the meters, plus a couple of buttons, let the user quickly access 10 snapshots, which save all ULN2 settings. There are memories that can be stored from within Console to your computer, but these aren't accessible from the hardware.

The software itself is logically laid out, though a little on the obtuse side, and one needs to take time to become familiar with its operation. The first of four windows, labelled Analog I/O Control, is more concerned with providing information than control: it offers a handy flow chart of the mixer inside the ULN2, plus comprehensive metering for all four input and all six output streams, and digital locking status. Various system controls — clock source, sample rate, digital input source, and digital input sample-rate converter — are chosen or enabled here.In the Mixer window of the MIO Console software, you're provided with control over the ULN2's virtual mixer, plus duplicate flow chart and system settings. In the Mixer window of the MIO Console software, you're provided with control over the ULN2's virtual mixer, plus duplicate flow chart and system settings.

These system controls are also available on the second window, which controls the virtual mixer, with standard level faders, pan pots, mute and solo buttons and a master fader. This mixer grows depending on how you set up the matrix and patchbay on the next page. Here we find the Mix/Output Routing window, where the ULN2's internal digital mixer is configured. This takes the form of a mix matrix, output patchbay, and a handful of pop-up menus. Two of these, labelled Matrix Parameters and Patchbay Parameters, let you save settings for recall later. Helpfully, a number of settings are provided to help your software communicate with the ULN2 while you figure out how to make the patchbay and matrix do more specifically what you require. Adding links in the mix matrix creates on-screen faders and mix controls in the 'mixer' window.

System Requirements

  • Firewire-equipped Mac with 128MB of RAM and 1024x768 or better monitor resolution.
  • Mac OS 9.1 or higher or Mac OS X.
  • ASIO or Core Audio-compatible host software (such as Cubase, Nuendo, Logic Audio, Digital Performer or Deck).

Putting It Together

Under Mac OS X, any Core Audio-compatible application will happily talk to the ULN2+DSP. It certainly worked without a hitch with Cubase SX — and I can vouch for the near-zero-latency operation of the interface! The ULN2+DSP also took over system audio duties, and provided an audio output for Propellerhead Reason. In Cubase, the ins and outs appear as normal, and can be selected for recording and output routing. Adding the Console-based processing takes a little getting used to — Console obviously needs to be open at the same time as Cubase SX — but when you get the hang of it, the +DSP's plug-ins become an easily accessible part of your Cubase session. Actually treating audio from within Cubase requires a little ingenuity, though. The issue may be addressed in future updates, but currently, it is possible to patch ULN2+DSP outputs back into its inputs, which if you route the digital out to the digital in, effectively turns the digital input into a return for the processed signal.

The +DSP graph page. Here, you can a stereo MIO Strip plug-in routed between the analogue ins and two of the 16 process busses, and a stereo compressor routed between two CAW channels (from the audio software running on your Mac) and another pair of process busses. MIO Strip's comprehensive, and highly graphic, editing window takes centre stage. The +DSP graph page. Here, you can a stereo MIO Strip plug-in routed between the analogue ins and two of the 16 process busses, and a stereo compressor routed between two CAW channels (from the audio software running on your Mac) and another pair of process busses. MIO Strip's comprehensive, and highly graphic, editing window takes centre stage. I would have thought there are not many users left running audio applications under Mac OS 9, but apparently the security of a stable setup developed over a number of years is appreciated by pros and home recordists alike. Metric Halo are to be congratulated for continuing support for Mac OS 9, and ASIO under that platform. I tested Console and the hardware under Mac OS 9, and they worked just fine, though the software revision is a little behind that for Mac OS X.

I'll note that I had a little instability with Console, but that this was fixed by placing it in Mac OS X's Applications folder. It's worth remembering this tip if you experience any strange behaviour from applications that you're trying to run from another location other than this folder — Mac OS X doesn't always like it if you don't.

A certain amount of brow-furrowing is inevitable with this software/hardware package, especially as it seems to be under continuous development. The situation will become a little easier when Metric Halo sort out their support documentation — something in depth for the +DSP side is needed, and a basic paper-based primer would allow those of us who quickly lose patience with PDF manuals to get up and running a little more quickly. It seems that Mobile I/O technology has been slow in development — the +DSP technology had been announced at the time of the original review of the 2882, but is only now properly available, and still essentially in beta form as I write — but the results of this careful progress are worth it.

Overall, this is a fine audio interface for any application where the input/output count is all that's needed, and the fact that something that sounds this good can be used outside the studio is icing on the cake. The quality of the analogue inputs is really excellent — ultra low noise indeed — and the outputs (feeding a Genelec monitoring system) had me noticing grunge and noise on samples and some existing digital audio that hadn't been apparent with my normal audio hardware. The hardware/software combo also makes for a powerful system, in terms of moving audio around, in spite of the small number of audio ins and outs. The +DSP version especially becomes an extension of your computer and audio software, and having plug-ins of this quality that require no overheads of your computer is the cherry on the icing. Reaching the end of a review and never having had latency issues is also a big surprise, of the most pleasant kind. Candles, perhaps!

Regarding price, it's a hard call: the full retail listed below, which seems a little high, could be seen as nominal. Audiostate, Metric Halo's UK distributors, have been pushing some very favourable deals recently, and street prices are likely to be significantly less than list. But beware that if shopping around actually lets you have a play with either the plain or the +DSP versions of the ULN2, you'll be very tempted to pass the plastic...


  • Great sound quality.
  • Feature-packed in spite of limited number of inputs.
  • Very flexible signal routing.
  • Brilliant built-in DSP promises much, especially if third parties exploit it.
  • Still works with Mac OS 9.


  • Currently Mac-only.
  • More expensive than many other units with a similar I/O count.
  • +DSP Console software still essentially beta, and tricky to master.
  • No paper documentation, and limited docs for +DSP extras.


Great sound, great facilities, and a hugely flexible hidden mixer equate to a pretty good choice for the mobile Mac audio artist. Some kinks in the software do little to detract from a great product.


ULN2 £999; ULN2+DSP £1499. Prices include VAT.

Audiostate Distribution +44 (0)1933 227228.