Immersive audio is increasingly important, and requires large multi-channel monitoring systems. Grace Design's m908 is intended to provide complete control over such a system.
DAWs and some interfaces include monitor-control facilities that are sufficient for many, but I prefer an independent system: something I can rely on should the computer crash and generate nasty noises, which I can place within easy reach, and with controls that don't take up valuable screen real-estate. For a majority working in music production the stereo audio format is all that matters, and countless controllers cater for that. However, for broadcast, gaming and film, and even some live-sound and theatre applications, multi-channel sound is a regular requirement. Here, the choices are fewer, and very few controllers cater for 'immersive audio' formats such as Dolby Atmos, Auro 3D and higher-order Ambisonics, which call for 20-plus speaker channels rather than six or eight. But this is exactly the kind of situation for which Grace Design's new m908 multi-channel monitor controller was designed.
Building on the operational concepts of its excellent sibling, the m905 stereo monitor controller (reviewed in SOS July 2014: https://sosm.ag/grace-design-m905), the m908 employs a similar arrangement of three major components: a rackmounting Audio Control Unit (ACU), which performs all signal processing and acts as the connection hub; a desktop Remote Control Unit (RCU); and a Power Supply unit. Whereas the m905 basically employed digitally controlled analogue signal paths, though, the m908's signal processing is entirely in the digital domain. Digital I/O format conversion is handled by an Artix-7 FPGA (Field Programmable Gate Array), while the audio signal processing is performed by a 1GHz 32-bit floating-point DSP. The operating system and user interface run under embedded Linux on an ARM-Cortex A-15 processor.
The compact power-supply box supplies a quartet of DC voltages and some status information across to the ACU via a chunky, locking Amphenol connector. A single IEC inlet accepts 100-240V AC. Grace's description of it as a "dual-redundant power supply" threw me, because my live broadcasting experience suggests that a dual-redundant system is one which can draw power from two entirely separate and independent sources, such that if one supply source fails the other takes over automatically, maintaining normal operation. Grace's implementation is different: the PSU box has a single mains power inlet that feeds two identical universal switch-mode power modules, with automatic switching should one fail. So, rather than guaranteeing resilience of external power sources, this approach seems intended to avoid down time in the event of an internal PSU failure. (These internal power supply modules are apparently "easily field-replaceable".)
The beating heart of the system is the ACU, a 2U rackmounting chassis whose only front-panel facilities are a power-on light and a stereo headphone socket. A variable-speed fan can keep this unit cool but only kicks in when necessary; it never came on during my tests.
As you might expect, the rear panel is awash with all manner of connectors and option-module slots. The core housekeeping facilities comprise a locking DC inlet from the power supply, a 15-way D‑sub to connect the remote control, a single Ethernet port (reserved for factory diagnostics), a USB-A host socket for firmware updates, and a quarter-inch TRS socket for remote control of talkback. There are also four GPIO (General Purpose Input/Output) circuits, which don't have a dedicated socket; they're accessed through unused pins of the multi-channel AES and remote-control D‑sub connectors.
Audio connectivity starts with three D‑sub sockets carrying 24 channels of AES3 I/O. A separate XLR connector provides an extra stereo AES3 input, and an RCA phono socket accepts an S/PDIF input. These are intended for auxiliary monitor sources. Two Toslink optical sockets are configurable for dual stereo S/PDIF (up to 96kHz) or dual eight-channel ADAT input. S/MUX support allows the sockets to be combined for eight ADAT channels at 96kHz.
A USB Type B port is Class-2 compliant for computer audio (there's an ASIO driver for Windows), supporting up to 24 channels of PCM at base sample rates, reducing to eight channels at quad rates. Two BNC sockets cater for word clock in and out, and Grace's fourth-generation s-Lock PLL system is employed to minimise jitter. The system doesn't offer sample-rate conversion, so sources generally need to be synchronised to the m908's internal clock or to an external house clock, but inputs from sources that can't be slaved (such as consumer CD players) can be configured to become the system clock temporarily while selected.
In addition, a horizontal slot accepts an optional plug-in module for alternative digital I/O formats. Capable of handling up to 32 channels in and out, current options include a ProTools Digilink card (with Primary and Expansion ports) and both Dante and Ravenna/AES67 cards (with primary and secondary Ethernet sockets).
Two more slots accept one or two optional eight-channel ADC input cards, accommodating up to 16 analogue line inputs. However, inputs 1 and 2 are shared with a pair of RCA phono sockets (for an unbalanced stereo input) and a pair of XLRs (for a balanced stereo input), so only one of these three sources can be active at a time. An optional, fully configurable stereo phono cartridge preamp module can also be installed.
Other XLRs accept an external talkback mic input (with phantom power and up to 74dB of gain) and provide a line-level talkback output, while another pair provides a stereo Control Room speaker output. Eight channels of analogue outputs, intended for additional monitor speakers, are duplicated across two more 25-pin D‑subs, while a third D‑sub delivers another eight analogue outputs, which can be configured as yet more speaker outputs or as up to eight stereo artist Cue sends. Every input, analogue or digital, can also be delayed by up to 1s to correct for lip-sync errors.
The RCU is the user interface for whole system, and it's very elegant. The design is an evolution and extension of the m905's controller, with the same adjustable base that allows it to be angled by up to 30 degrees. Very conveniently, the RCU has a headphone socket at the rear, duplicating the one on the front of the ACU, and a USB Type A port for importing or exporting setup/configuration files. A built-in talkback mic is calibrated to provide accurate continuous in-room SPL measurements, which are displayed on the LCD screen. Only one slim cable (with DA15 D‑sub connectors) connects the RCU and ACU, so there's minimal clutter on the desktop, and the unit stays where it is placed.
The colour LCD screen is similar to the m905's, with buttons above and below to access menu options, but the most-used functions all have dedicated hardware buttons, most with secondary actions via a long press, and a nicely balanced encoder wheel adjusts the monitor volume. I found operating the system pleasingly obvious and responsive, and configuring many aspects was straightforward. That said, the sheer depth of configurability is astonishing, and the 135-page manual is definitely a 'must-read' when installing and setting up the unit, even though the m908 ships with factory presets for the most commonly used monitoring and connection arrangements.
Monitor control functions are, of course, comprehensive starting with selection of up to three separate control-room monitoring systems. Each of these can be configured individually for any channel count (up to 24), hooked up via any available connector formats, with any level and delay offsets, bass management parameters, room correction EQ, and so on.
Dedicated RCU buttons provide solo/mute of individual speakers up to a 7.1 format, and there are global Mute and (adjustable) Dim buttons. The Mono button becomes a 'stereo difference' button when held down (an elegant solution) and the mono output destination can be specified. Multi-channel sources can be down-mixed as required, with all parameters fully adjustable via the appropriate menus.
Input sources can be configured for any desired number of channels and physical connections. They're normally selected exclusively, but up to three inputs can be mixed together, provided they're clock-synchronous. The current monitor source selection can also be routed to the artist Cue outputs and/or to any available outputs to feed a metering system.
The sheer depth of configurability is astonishing, and the 135-page manual is definitely a must-read.
The home screen presents the available input sources across the top and bottom as soft-buttons, with the active source highlighted in blue when selected exclusively, or green if the input-summing mode is active. As there are 16 inputs and only eight buttons, a second menu page is accessed by pressing and holding the bottom-right button. Each input source can be configured with a name, the physical input connector, channel count and format, any permanent level offset, its clocking mode, and so on. A temporary level offset can also be dialled in for comparing or matching sources, just by holding the input source button and turning the encoder wheel; the offset value is displayed in place of the source name, so you won't forget it's been tweaked!
Filling most of the display's left-hand side is a graphical layout showing the current speaker format, with different icons to indicate the positions of the main channel speakers, subwoofers, and any height or top channels. These icons change colour to indicate the signal level at each speaker output. The Mute/Solo status of each speaker is also displayed here, while the system's sample rate and clock source are displayed below the graphic.
On the right-hand side is a large, green, numerical display showing the current volume setting, either on a notional 0-100 scale or relative to an SPL reference level. The selected speaker-layout preset's name is shown above, and the current in-room SPL level below (as dBC and dBPeak values), along with the current headphone volume and cross-feed settings.
Comprehensive bass management and speaker placement options are provided, of course, starting with five different high- and low-pass cross-over filter order (slope) options (including a bypass), a switchable 10dB boost for the LFE signal, and low-pass summations for mono or stereo subwoofers. Each individual speaker output has ±20dB of level adjustment, 12 bands of parametric EQ (important for some Atmos room-correction scenarios), and up to 250ms of delay to compensate for different physical distances from the listening position.
This is, necessarily, a brief overview of the m908's facilities and features; an in-depth investigation would be as large as the manual, because it's such a versatile and expansive system, with seemingly every possible option and requirement fully catered for. While the software configurability is very impressive, the quality of the hardware is crucial, and the m908 has the wonderfully well-engineered character we've come to expect from Grace Design. The remote control exudes well-built solidity, the buttons and encoder wheel feel reliable and sturdy, and the display is crisp, bright and easy to read. I really enjoyed using it.
I couldn't fault the digital signal path processing in any way, and the analogue I/O achieved an AES17 dynamic range figure of around 125dB (A‑wtd), which is very respectable. Round-trip latency from analogue input to analogue speaker output is around 5ms at base sample rates, but under 1ms on the Cue outputs — the difference presumably due to the bass management and room EQ on speaker channels. I was surprised to find the D‑A converter alignment delivers +18dBu at full scale, while the A‑Ds accept the SMPTE standard of +24dbu, but +18dBu is still far more than most active speakers or amplifiers ever require in practice.
Obviously, not everyone needs a monitoring controller that can handle the most elaborate immersive speaker systems, but for some, installing a controller that can accommodate such requirements with ease in the future could be a wise investment. The m908 is just as 'at home' and easy to use with stereo or standard 5.1/7.1 monitoring formats as it is with a Dolby Atmos 9.2.4 system.
I can't think of any other monitor controller that is capable of managing a 24-channel speaker system!
- Superb sonics throughout.
- Extraordinarily configurable.
- Comprehensive speaker-conditioning facilities.
- Excellent build quality and elegant user interface.
- Unusual interpretation of a 'dual-redundant' power supply.
A unique digital hardware monitor controller supporting speaker formats with up to 24 channels.
£7679 including VAT.
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