RME ADI2 & ADI4 DD

Photo: Mark Ewing

Two new half-rack boxes offer considerable operational flexibility and pristine sound in both the analogue and digital domains.

I have reviewed several RME hardware products in the past, and have always been impressed with the high audio quality they seem able to provide despite their very moderate pricing. Recent technological developments have made this quality/price ratio even more favourable, with some impressive advances in word-clock stability which are claimed to put the latest generation of RME converters in Apogee territory in this regard — an intriguing thought!

One of the latest RME offerings is the ADI2, a relatively straightforward two-channel A-D/D-A converter housed in a half-width 1U rack case. In essence, it is a development of the previous ADI1 which I reviewed way back in SOS August 1999, sharing a similar (but updated and extended) feature set and mechanical footprint. Technological advances have been incorporated to support 24-bit conversion and all standard sample rates up to 192kHz, with significantly improved jitter rejection.

Case & Connections

The case of the ADI2 is constructed from three folded-steel panels to provide a very strong and fully screened enclosure for the internal circuitry. The front panel is made from aluminium sections, using blue-painted panel areas and clear silver legends to highlight the various controls. The rear panel is painted black with large white legends, and the connections are all very obvious. The analogue line-level inputs to the A-D are catered for with Neutrik combi jack/XLR sockets, accepting either XLR or TRS connectors. The input circuitry employs clever 'servo amplifiers' that can accommodate balanced or unbalanced signals equally well, automatically compensating for the 6dB lower signal level that can otherwise occur when connecting an unbalanced signal to an electronically balanced input.

The unit can be configured from the front panel for one of three operating levels, where 0dBFS (digital peak) equates to +19dBu, +13dBu, or +4dBu. These are designed for compatibility with professional high-level, standard +4dBu, and semi-professional -10dBV equipment respectively. The first two options effectively just change the available headroom from 15dB to 9dB (relative to +4dBu), while the third reduces the sensitivity to accommodate the lower semi-pro operating levels. In this case, the stated +4dBu peak level equates to +2dBV, thus offering a nominal 12dB of headroom above -10dBV.

The A-D's digital output is presented on both a coaxial phono connector and a Toslink optical port, although you can choose different data formats — a front-panel selector enables the output format to be switched between Consumer (S/PDIF), Pro (AES), and ADAT. The phono connector is used to output S/PDIF or AES data: changing the format from Consumer to Pro increases the signal level (from 0.8V to 2.3V peak-to-peak) and changes the status flags to comply with the relevant standards. The phono socket is electrically isolated from the chassis and connected via a transformer so that it is fully floating. Thus, a properly balanced AES output can be obtained simply by wiring the tip and sleeve of a phono plug to pins two and three of an XLR plug respectively. Valid audio data appears to be present at this output regardless of whether the optical socket is configured for Pro/Consumer or ADAT operation. All sample rates are catered for up to 192kHz, using the single-wire double- or quad-speed transmission method which is now more or less the standard.

The optical output can be switched between S/PDIF or ADAT modes from the front panel, but at standard sample rates only channels one and two carry the data, the other six channels being mute. However, at the doubled sample rates the widely supported SMux data format is used, so that the first two channel pairs are used to convey the A-D's stereo signal. Surprisingly, no audio output is provided at all at the quad rates, although a standard-rate embedded word clock is still conveyed in case the ADAT signal is being used for clocking purposes, which is a thoughtful feature.

Moving over to the D-A side of things, digital inputs are catered for once again with a phono socket and a Toslink optical input. The data format arriving at each connector is detected and decoded automatically, but a front-panel selection determines which input is used to feed the D-A (and act as external clock reference, when appropriate). Again, the phono socket is electrically isolated from the chassis and connected via a transformer so that balanced AES signals can be accommodated with a suitable converter lead wired in the same way as described earlier for the AES output.

Rear panels of the ADI4 DD (top) and RME ADI2.Photo: Mark Ewing

The line-level analogue outputs are provided on both XLRs and TRS sockets, featuring the same kind of servo amplifier idea so that the correct signal level is maintained regardless of whether the output is connected to balanced or unbalanced equipment. As with the input-sensitivity selection, a front-panel control allows the output level to be switched between the same three settings, giving peak signal levels of +19dBu, +13dBu, or +4dBu (+2dBV).

The ADI2, like its predecessor, is too compact to allow a mains power supply to be incorporated, so a coaxial power connector is present on the rear panel, complete with a metal hoop through which the power lead can be threaded to prevent it falling out if accidentally tugged. Like many other RME products, this unit is remarkably flexible in its powering options. It can accept DC supplies between 8V and 28V DC (of either polarity), or low-voltage AC supplies between 8V and 20V AC. So battery powering from a range of sources is perfectly viable. A compact third-party mains power unit is also included in the package, and this can accept mains voltages between 100V and 240V AC at 50Hz or 60Hz, to produce a 12V DC output suitable for powering the ADI2.

Front-panel Controls

Having completed the geek's tour of the connectors, the front panel needs little explanation, as most of the facilities have already been mentioned. There are only five buttons, a rotary control, and a quarter-inch stereo headphone socket to master — although the 31 LEDs make it look a little more complex than it really is! Starting at the left-hand side, the first button cycles through the three input-level options, each with a green LED to indicate the current setting. Next up is a vertical bar graph meter with two columns of six LEDs, showing the A-D's digital output level. The bottom four lights are green, followed by a yellow and a red LED, and the ensemble is scaled -60, -30, -12, -6, -3 and 'Over' (although, strictly, this is not an 'over' warning at all since it can only indicate digital signals reaching 0dBFS.) The meter is actually a lot more informative than this description implies, as each LED has several intensities which help to bridge the gaps between adjacent LED levels. AT the bottom end, signals as low as -76dBFS can be identified, while, at the top, peaks of -2dBFS or -1dBFS are easily distinguished. Only signal peaks that actually hit 0dBFS cause the peak-hold function to activate, maintaining the red LED's illumination for one second to attract attention.

The central section of the front panel is concerned with clock-rate and I/O selections. The first button cycles through the clocking functions, with six LEDs to indicate the status. There are three internal crystal-based sample-rate options — 32kHz, 44.1kHz, and 48kHz — followed by an external sample rate derived from the selected digital input. Further pushes of the button cycle through these four options again, but with a 'x2' multiplier for the internal rates, and then again with a 'x4' multiplier. So getting to 192kHz is a tedious process, but simple enough and with clear indications along the way.

The next two buttons configure the digital I/O, the first selecting the coaxial or optical digital input, and the second configuring the digital outputs for ADAT, Pro, or Consumer formats, as described earlier. Once again, LEDs indicate which input connectors and output modes have been chosen. The last button selects one of three analogue output levels, again with LEDs to show the current status.

The final control is a rotary volume knob for the built-in headphone monitor which auditions the output from the D-A converter. The quarter-inch socket sits alongside the control for easy access, and there is sufficient 'oompf' from the amplifier to enable this output to be used to drive unbalanced line inputs, should that ever become necessary.

Technology & Specifications

The earlier ADI1 employed 20-bit Crystal and AKM converters and operated at only the standard sample rates — although this was the state of the art for 'budget' converters back then. Such limitations wouldn't be countenanced these days, of course, and it is interesting to see just how far RME have evolved in the intervening six years. The current unit uses the latest high-resolution AKM chips: the 5385 A-D and 4395 D-A converters (the latter with built-in automatic de-emphasis facilities in the unlikely event that they are needed).

Three different pin-out configurations for the ADI4 DD's digital D-Sub connector are supported, as used by Tascam, Yamaha, and Euphonix. You can change the format by re-plugging an internal ribbon cable.Photo: Mark Ewing

The analogue side of things appears to be handled by the ubiquitous 4580 op amps, in the same circuit topology as used in several other RME products including the previous ADI1 and the flagship ADI8DS (reviewed back in SOS September 2003). All of the unit's control functions, as well as the audio data manipulation and formatting, are handled by a Xilinx Spartan IIE FPGA (Field Programmable Gata Array). Everything has been put together very nicely, on one fairly dense printed circuit board using mainly surface-mount components. The front-panel LEDs and buttons are mounted on a second vertical card, linked via a ribbon cable. All the rear-panel connectors are soldered directly to the main board, but are fixed to the rear panel to help minimise any mechanical strain being conveyed to the PCB connections.

The published specs make impressive reading, with an A-D signal-to-noise ratio of 113dBA and distortion below 0.0003 percent. The D-A boasts a signal-to-noise figure of 119dBA and distortion below 0.0007 percent. Compared to the ADI1, the signal-to-noise ratios are nearly four times better, and the distortion has improved by an order of magnitude! Perhaps even more impressive are the jitter figures, which are roughly half that of any comparable product — less than 0.8ns on the internal crystal and only 1ns when slaved to an external clock reference, with around 30dB of jitter suppression from wobbly external sources! Apparently the Steady Clock technology that provides these impressive results was developed originally to recover stable clocks from the multi-channel version of the AES interface — MADI — which typically suffers a monumental 80ns of jitter!

Audible Benefits

The ADI2 is a very straightforward and compact unit, the simplicity (and comparatively low UK price) of which belies its superb quality. This really is a star performer which compared favourably against my trusty Apogee PSX100, as well as the rather nice converters in the Drawmer DC2476 mastering processor. Noise and distortion are completely inaudible, even when working with generous headroom; the audio spectrum is open and airy (especially at the double and quadruple sample rates); and the stereo imaging is three-dimensional and completely stable — always the sign of a good clock — with wide and deep sound stages. The only relevant negative comment I could raise is that the low bass seems a little thin when compared to some more expensive converters — but you have to move a very long way up the price scale before the RME's inevitable shortcomings start to become noticeable at all.

Leaving the sonic merits to one side, the ADI2 has some practical limitations compared to many state-of-the-art converters. For example, the A-D and D-A stages can't be used as completely separate units, running at different clock rates; the output cannot be dithered down to lower word lengths to suit CD-R and DAT recorders; there is no external word-clock input (separate from that embedded in the digital inputs); and there is no 'soft-limiting' facility to minimise peak overloads when working with minimal headroom. However, these features are not important — let alone necessary — to the vast majority of semi-pro users in our modern 24-bit world.

On the plus side, the I/O flexibility in terms of connections, formats, and operating levels is excellent, the simplicity of configuration a joy, the built-in headphone monitoring a useful facility, and the cost-performance ratio astounding. I'm not a fan of in-line or wall-wart power supplies, but given the sonic and fiscal benefits of this unit, I feel pleasantly disposed to overlook such a minor issue!