Can Zoom’s new 32‑bit floating‑point interface banish clipping forever?
The benefits of 32‑bit floating‑point audio are most valuable in unpredictable recording environments, so it’s no surprise that this format was first implemented in field recorders. The basic idea is that the input signal is fed to two A‑D converters, which are aligned to different preamp gain ranges. No matter what input signal is fed in, one or other of them will capture it without overloading or introducing noise. In essence, a 32‑bit A‑D converter is ‘unclippable’, which is a very useful quality in a device that might be recording Formula 1 cars one day and birdsong the next.
Zoom are one of several manufacturers whose portable recorders support 32‑bit recording, and they’ve now introduced what they say is “the first dedicated 32‑bit float audio interface”. The UAC‑232 is a portable, bus‑powered stereo audio interface that connects to a macOS, Windows or iOS/iPadOS host over USB, and as long as your DAW supports 32‑bit floating‑point audio, should eliminate converter clipping as a problem in computer audio recording.
Corner Capers
The rectangular form factor of the UAC‑232 is familiar from many other small USB audio interfaces such as the Focusrite Scarlett Solo, but is disrupted by metal bars that stick out parallel to each corner and allow the unit to be stood on its end if that’s more convenient. Zoom also include a couple of Velcro cable ties that can be used to tidy cables, secure the UAC‑232 to something, or hang it from a mic stand. In the latter role, the cable ties will stop it falling off a stand altogether, but won’t prevent it sliding downwards if it spots an opportunity, so it might have been more effective to integrate a threaded mounting socket into the case. Note also that the UAC‑232’s mostly plastic shell is quite lightweight, so a light tug on a cable is enough to pull it over if stood upright — and, unlike on Zoom’s F8, for example, the protruding bars at the corners offer no protection to the front‑panel controls.
On the front panel, you’ll find two combi XLR/jack input sockets. Both can accept mic or line‑level signals, and the leftmost can also be switched to a high‑impedance mode for DI’ing electric guitars. Phantom power is independently switchable for both inputs. A single headphone output on quarter‑inch jack has its own volume control, and there’s a larger one for the main outputs. These are on rear‑panel quarter‑inch jacks, which sit next to 5‑pin DIN sockets for MIDI in and out, a Kensington lock slot, and two USB‑C ports. One of these makes the data connection to the host and carries bus power, while the other allows an ancillary power supply to be added if you’re using the 232 with a tablet or phone. The review unit came with a Type‑C to Type‑A cable but not a C‑C one. Although it uses Type‑C ports, it’s a USB 2 device, as indeed are almost all USB audio interfaces.
The UAC‑232’s simple back panel features twin USB C ports, MIDI I/O sockets and quarter‑inch audio outputs.
Like many other small audio interfaces, the UAC‑232 offers direct monitoring switched from the front panel. At first, this seems extremely basic, with no hardware control over the relative levels of the input and playback signals, or any choice about where they should be panned. However, Zoom make available an optional utility called UAC‑232 Mix Control, which can be downloaded from their website or installed from the App Store if you’re on a tablet or phone, and this increases monitoring flexibility somewhat. It also enables a loopback function, which mixes the DAW return signal back into the input path; this probably isn’t very useful in a recording context, and makes it easy to get into feedback loops, but could be valuable in situations where you want to incorporate backing tracks into a live stream.
No Gain, No Pain
What you won’t find on the front panel of the UAC‑232 are any input gain controls. In principle, these are redundant on a 32‑bit floating‑point interface, since the dynamic range of the dual converters is sufficient to cope with any analogue voltage swing that reaches them. Low‑level signals don’t need to be boosted to make the most of the A‑D converter’s dynamic range, and loud ones don’t need to be attenuated to avoid clipping. In practice, however, it’s useful to be able to trim the signal level so that it’s in the ballpark right off the bat; just because you know you can adjust clip gain later on in your DAW to recover an undistorted signal doesn’t mean that you want to be monitoring the distorted input during tracking.
To this end, the UAC‑232 applies a default level adjustment depending on what type of input you present it with. For example, with an XLR connected, it’ll initially assume you’re using a dynamic microphone and will compensate for an anticipated low‑level input. Enable phantom power and it’ll set a level appropriate to a capacitor mic with a hotter output, and the same applies for line and guitar signals too. In practice, these defaults get it right much of the time, but if you are distracted by clipping in the monitor path, or struggling to hear the input signal, a visit to UAC‑232 Mix Control provides the answers.
The Mix Control software.
The signal coming in at each input is represented on a rather hypnotic, endlessly updating waveform display, drawn by a cursor that takes about eight seconds to move across the window. To the left of this display you’ll see a vertical slider: this allows you to modify the default input level. Any offset you apply here is preserved when you change input types. Each input also has a level and pan control determining how it’s presented in the direct monitor mix. In Music mode, these monitor mix controls affect only what’s heard, not what’s going to disk; only the ‘gain’ slider is reflected in the level being recorded in your DAW. However, the UAC‑232 also supports an alternative Streaming mode, in which both the gain slider and the mixer level and pan controls are in the record path: what’s recorded (or streamed) is, in effect, the output of the monitor mixer.
Unfortunately, the UAC‑232’s direct monitoring has one rather annoying flaw, which is that despite its cleverness in auto‑detecting what type of source is coming in, it doesn’t auto‑mute the inputs when no cable is connected, and defaults to applying the amount of ‘gain’ appropriate to a dynamic mic. Consequently, if either of the inputs is unused, it dumps a fair bit of hiss into the monitor path, unless you visit Mix Control and turn it down.
Clipped Accents
As previously mentioned, setting the gain slider so as to avoid ‘overloads’ is optional if you are recording 32‑bit float files, but not all DAWs support 32‑bit recording. If yours doesn’t, or if it isn’t enabled, overloads will be all too real! The words “32‑bit float” on the front of the unit helpfully illuminate as an indicator, but this can be misleading — on macOS, for example, all it shows is that the Core Audio driver is operating at 32‑bit float, and tells you nothing about whether your DAW is or can.
It should also be pointed out that although 32‑bit recording can eliminate the risk of clipping at the A‑D converter, said converter is preceded by analogue circuitry, and there’s a limit on the voltage swing that this circuitry can handle. If you try to feed the interface a signal that’s too hot, you’ll still get clipping in the analogue domain and there won’t be anything you can do about it. The UAC‑232 can accept levels up to +24dBu on the TRS line inputs or +11.5dBu for a high‑impedance signal, which should be plenty, but the specified maximum input level for the mic preamps is only +6dBu, which doesn’t really leave that much headroom for loud sources. It’s probably adequate for its intended uses, but if you’re getting involved in sound design, you might have to stick to Formula E rather than Formula 1.
From F To U
The UAC‑232 uses the same preamp design as Zoom’s popular F‑series field recorders, and both its specifications and its subjective sound quality are excellent. It’s capable of operating at sample rates up to 192kHz, and the input and output circuitry is pretty much flat to 80kHz. Not many of us are likely to hear anything in that sort of range, but it has value for the aforementioned sound‑design applications, allowing you to pitch down recorded sound and retain a wide bandwidth. The headphone output is also a step up compared with many small bus‑powered interfaces, offering clear sound with more power than I felt I was ever likely to need. I’m not sure how many UAC‑232 buyers will really need the MIDI I/O, but it could be a nice bonus for those who do. The Mix Control utility is simple and effective, not that that should be difficult to achieve for a stereo audio interface, and I must also commend Zoom’s exemplary manual, which is available both as a PDF and as an extensively hyperlinked HTML page.
32‑bit recording on the UAC‑232 is a welcome example of a ‘pro’ feature finding its way on to an affordable interface.
There is no shortage of stereo desktop USB audio interfaces on the market, and the UAC‑232 faces off against the likes of Focusrite’s Scarlett 2i2 4th Gen, PreSonus’ AudioBox iTwo, Audient’s EVO 4, Lewitt’s Connect 6 and Steinberg’s UR22C. By offering 32‑bit recording, Zoom have certainly given it a unique selling point, and a very valuable one, though it should be remembered that this only works with some DAWs, and that the auto‑gain features built into the Audient and Focusrite units have much the same practical benefit of protecting your recordings from converter clipping. Like the digitally controlled preamps on the Scarlett and EVO, 32‑bit recording on the UAC‑232 is a welcome example of a ‘pro’ feature finding its way onto an affordable interface. You won’t be recording gunshots with it, but it effectively eliminates clipping in most circumstances. Recording will never be foolproof, but it gives us one less problem to worry about.
Summary
Not only is the UAC‑232 a simple, effective and good‑sounding stereo audio interface, but its 32‑bit floating‑point option means you’ll never clip the converters on the way in.
