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RME OctaMic XTC

Mic Preamp & A-D/D-A Converter
Published October 2013
By Hugh Robjohns

RME OctaMic XTC

The latest addition to RME's OctaMic range of multi-channel preamps is designed to interface with digital recording systems of all stripes.

Building on the success of their OctaMic II and OctaMic D preamp/converters, RME have introduced a new model called the OctaMic XTC. This latest variation shares the blue front panel of its siblings, but possesses far fewer physical controls. Instead, all of its parameters are adjusted with the aid of a bright, colour LCD, where menus and options are accessed very easily using two encoder knobs.

The first four of the unit's eight high-quality, digitally controlled mic preamps are also usable as line inputs, and the last four as instrument inputs. Unlike other OctaMic models, the XTC doesn't provide balanced analogue line outputs for each preamp, only digital outputs. These cater for AES3, ADAT, MADI and USB formats, and sample rates up to 192kHz. On-board monitoring is via two separate headphone amplifiers, with fully independent source selection. A comprehensive internal routing matrix allows any preamp output or digital input to be auditioned, as well as patched freely to any other digital output, providing extremely flexible signal routing and format conversion. The XTC's rear panel also includes a pair of five-pin DIN sockets, which allow MIDI data, including SysEx, to be transported via the MADI and USB ports.

USB & MIDI

The USB 2.0 port is included primarily to make firmware updates easy, rather than to turn the device into a full audio interface (RME already make several of those), but as the interface is fully class-compliant, MIDI and audio can be passed to other class-compliant devices. This means that as well as being natively compatible with Macs, the XTC can be used with Apple's Camera Connection Kit to pass up to 24 audio channels, along with MIDI, to and from multi-channel audio recording apps in iOS systems. Indeed, RME are actively promoting the XTC as a high-quality multi-channel interface for the iPad. Sadly, all Windows OSs lack native support for USB 2.0 class-compliant devices, so while RME offer a bespoke driver and program for Windows that allows firmware updates to be flashed into the XTC, there's no support for audio or MIDI over USB, and none is planned; Windows recognises the XTC as an audio interface, but it's unavailable for WDM or ASIO operation, and neither RME nor I are aware of any third-party driver that provides a remedy to this situation.

The inclusion of MIDI, though, does bestow another useful feature for both Mac and PC users, which is that the XTC's preamps can be remote controlled via MIDI commands that are generated from the input strips of RME's TotalMix software. This is particularly handy if the XTC is used in conjunction with another RME audio interface running TotalMix FX. For example, coupling the XTC with a Fireface UFX interface over ADAT and MIDI would add eight analogue inputs and two stereo headphone outputs to the UFX.

Operation

The OctaMic XTC's generous complement of digital I/O makes its internal routing options particularly useful.The OctaMic XTC's generous complement of digital I/O makes its internal routing options particularly useful.

Rear-panel analogue connectivity comprises eight Neutrik Combo sockets for the mic/line and mic/instrument inputs. The AES3 connections use an AES59 (Tascam standard) 25-pin D-sub connector, with four AES3 pairs in each direction supporting eight channels at up to 192kHz. Dual ADAT optical outputs conform to the S/MUX protocol, which enables them to accommodate higher sample rates (eight channels at 96kHz, or four at 192kHz), and there's a single ADAT optical input too. The MADI I/O employs the standard optical connections and supports the full 64 channels in each direction at base sample rates, with the commensurate channel reduction for higher sample rates. The XTC can be clocked from its internal crystal at all standard rates up to 192kHz, from any digital input, or an external word clock. There's a word clock output for sync'ing other equipment, too. RME's Intelligent Clock Control (ICC), SyncCheck and SteadyClock technologies are all included, to simplify clocking and minimise jitter. An internal universal power supply accepts mains voltages between 100V and 240V AC via a standard IEC inlet.

The front panel is very simple indeed. Just three LEDs are associated with each preamp stage, to indicate signal presence/overload, phantom power and pad. The status and configuration of each pair of input channels can be shown on the LCD screen by pressing a shared Select push-button under the LEDs. The two headphone outputs are located to the right of the preamp section, and their levels and sources are again controlled through the LCD. An array of eight more LEDs indicates the clock source and sync status, and MIDI data activity.

To the right of the front panel are four push-buttons and two encoder knobs, arranged around the crisp LCD. The buttons access the Headphone, Groups, Channel and Setup menus, and the various menu options and parameters are selected and adjusted with the encoders, which also have press-switch functions. A mains on/off rocker switch completes the panel controls.

The LCD screen always indicates the levels of both headphone outputs and both the clock source and rate, in a strip along the bottom. When a channel Select button is pressed, the LCD instantly shows the current gain settings of the two preamps (both as bar-graphs and dB values) along with a pair of horizontal green bar-graph level meters (with peak-hold dB readouts). If the Select button is held down, the two channels can be toggled between independent and ganged adjustment, and the gain of the selected preamps is controlled directly by turning the two encoder knobs (the display making clear which knob controls which channel). This means that adjusting a preamp's gain is as fast as pushing one button and turning one knob.

RME's AutoSet function is activated for the corresponding channel by pressing the relevant encoder: if the channel gain is set to maximum and the loudest chord or note played into the preamp during a rehearsal, the XTC will adjust the channel gain automatically, to ensure the peak reaches no higher than -6dBFS. The gain will not change again unless the input level rises higher still. This should prove a very handy feature for musician-engineers!

Pressing the Channel button by the LCD accesses additional channel functions, such as the input pad (channels 1-4), instrument input (channels 5-8), phantom power (which has a soft-start to avoid thumps), polarity inversion and mute. The rotary encoders are used to select the required channel, move up or down through the menu, and change the settings. An interesting feature is the ability to group channels together, so that gain adjustments can be applied across multiple channels simultaneously. Channels can be assigned to one of four independent groups, if required, through the Groups button. The Phones button recalls a screen to access the overall level (high/low), headphone volume (from -64 to +6dB in 1dB increments), mute, balance, polarity inversion and source independently for each headphone output.

General system configurations are accessed through the Setup button's five submenus. The Setups menu stores or recalls all configuration settings from six memory locations, while the Options screen deals with clock source, sample rate and word-clock termination. General Settings administers MIDI device ID, control pass-through on/off and LCD contrast, and reveals the software version, while the MADI screen sets delay compensation and ID, 56- or 64-channel format, and 48 or 96k frame. Digital Routing determines the signals dispatched to each digital output in blocks of eight contiguous channels. So ADAT 1-8 can receive signals from Mic 1-8, ADAT in, AES in, MADI in (all channels in blocks of eight) and USB, for example. The USB inputs are labelled 'Playback' and the routing is a little more flexible, with options for allocating channels 1-8, 5-12, 9-16, 13-20, or 17-24. The USB outputs are labelled Recording and the preamps always feed channels 1-8, but the rest are assignable in blocks of eight.

Quiet XTC

Physical control of this preamp is via the two digital encoders, which provide speedy access to parameters via a series of menus.Physical control of this preamp is via the two digital encoders, which provide speedy access to parameters via a series of menus.

The above description might make the OctaMic XTC seem complex, but it's actually reassuringly simple to set up and use, even without referring to the excellent manual, which contains a wealth of detailed information that makes it easy to make use of the XTC to its fullest capabilities. The mic preamps and converters are very capable indeed (see the 'Tech Talk' box). The noise floors are smooth and low, the sound is neutral and transparent ('clean', rather than 'characterful'), and there are good headroom margins. Gain matching between channels is precise and easy to set up in stereo or multi-channel groups. The instrument and line inputs work exactly as intended, with good gain ranges, and the headphone outputs proved happy to drive a range of common high- and low-impedance headphones to generous levels quite effortlessly, with good bass and accurate sound stages. The ability to send the preamp outputs to multiple destinations simultaneously allows backup recordings to be made easily on location, and the option to convert between digital formats and to monitor digital returns adds to the flexibility enormously. The audio and MIDI over USB facility should, as intended, appeal to Mac users, or those wanting a compact but comprehensive front end for an iPad.

Overall, then, the OctaMic XTC is a welcome addition to RME's range, with an impressive blend of features and excellent technical performance — and although it costs more than previous OctaMic units, the added functionality and impressive technical specifications more than justify the price tag.   

Alternatives

There are many eight-channel preamps with digital outputs on the market across a broad range of prices, but few that better the XTC's technical performance and none with its comprehensive digital I/O, routing matrix for format conversion and local monitoring, or remote control over MIDI.

Tech Talk

The OctaMic XTC's digitally controlled, low-noise mic preamps are based around the same Texas Instruments PGA2500 mic-preamp chip that RME use in their Fireface 400, Fireface UC, Fireface UFX and Micstasy, and can accommodate a range of input levels spanning an impressive-sounding 85dB, although the controllable preamp gain range is actually only 65dB. The gain-staging arrangement is unusual in that, although it starts at unity gain (0dB), the first step jumps straight to +10dB and then increments in 1dB steps all the way up to +65dB. At the maximum gain setting, a mic input signal of -63dBu will produce a practical digital recording level of -10dBFS. To put that in everyday terms, its equivalent to an 84dB SPL source in front of a Shure SM57 mic, or 66dB SPL in front of a Rode NT2. In other words, there's plenty of usable gain on hand here, and even at maximum gain, the power-supply mains hum components all measure below -105dBFS.

At minimum gain and with the relay-switched 20dB pad engaged, the mic input XLR would need +32dBu to make it clip, and that would require something like a Neumann M149 tube mic exposed to 150dB SPL — so it's not likely to happen in practice! The TRS line inputs can cope with an even more extreme +39dBu with the pad engaged, while the instrument inputs clip above +21dBu. Presumably because of the high-impedance buffer stage, the minimum gain for the instrument mode is +10dB and the overall gain range is reduced to 55dB. Nevertheless, the maximum gain setting will deliver -10dBFS from a -45dBu input signal, which should cater for most guitar pickups very comfortably.

As with so many modern preamp designs, the line inputs are padded down to increase the impedance and decrease the sensitivity, before being fed into the corresponding channel's mic preamp. In theory this approach risks unwanted distortion and noise, and would have been frowned upon by high-end electronics designers 20 years ago. However, RME's design introduces less than 1dB of extra noise and distortion, which is insignificant compared with the practical advantage of precise, repeatable and recallable gain control.

RME's flagship Fireface UFX interface employs 'Advanced Parallel Conversion' technology (combining two converters per channel) to maximise the signal-to-noise ratio, but advances in converter performance have overtaken that approach, and the AKM AK5388 chips used here (and in all of RME's products since the ADI-8 DS MkIII) delivers about 2dB less noise overall than APC. My own AES17 dynamic range measurements gave a figure of 116dB (A-weighted) and distortion below 0.0003% via the line input at +4dBu (with the mildest tendency towards odd-order harmonics). The new converters also feature lower latency than many, typically introducing less than 0.28ms at 44.1kHz sample rates and proportionately less for higher sample rates.

The mic input's EIN figure was a smidgen over 126dB at maximum gain, and 125dB at 30dB gain, and channel crosstalk was below -100dB at 10kHz, which is very impressive indeed. The input frequency response is flat to below 10Hz, reaching -1dB at 8Hz. Headphone outputs were capable of delivering +17dBu in the high setting, and about +4dBu (+2dBV) in low mode, and the D-A stage delivered an AES17 dynamic range figure of 117dB (A-weighted) with distortion of 0.0004% and a slight tendency towards second-harmonic dominance.

Published October 2013

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