RME ADI-1

High‑performance digital converters are becoming more affordable every day, and this new offering from RME certainly proves that good and cheap are not necessarily mutually exclusive. Hugh Robjohns undergoes complete conversion...

One weak spot of any 'computer + soundcard' recording system has always been the susceptibility to interference of the A‑D/D‑A converters when they are located on the soundcard inside the computer. By removing the converters to an external unit (ideally connected via optical lines), conversion quality improves dramatically and true 20‑bit performance is readily achievable. Alternative uses for external converter units include providing additional high‑resolution analogue inputs to digital mixers, and extra monitoring or recording outputs.

The ADI‑1 is one such unit, a very compact (half‑rack, 1U) box which contains a pair of independent 20‑bit A‑D and D‑A converters (this is the baby of the ADI series; an eight‑channel ADAT/DTRS unit and a professional two‑channel 96kHz variant are also available). The ADI‑1's deceptively simple front panel is dominated by two input gain controls and a large collection of LEDs. The level controls provide a calibrated 0dB gain point when fully counter‑clockwise, +20dB when fully clockwise — the overall input sensitivity being determined by a rear‑panel push switch to select +4dBu or ‑10dBV operating levels.

To the right of the input level controls is a pair of horizontal LED bar‑graph meters with seven LEDs showing signal levels between ‑60 and 'Over'. The Over lights are red, those at ‑3dB yellow, and the remaining five sets are green. Although hardly sufficient for really accurate level calibration, they are considerably better than the 'signal present' type of indicators often found on budget converters, and would presumably be used in conjunction with higher‑resolution virtual metering provided by the computer audio system. Also, the meters are true digital meters, their display being derived from the output data stream.

The analogue‑to‑digital converter can be switched to operate at either 32, 44.1 or 48kHz sample rates by repeated presses of a front‑panel push switch — the selected rate being shown on one of three green LEDs. The remaining four LEDs are associated with the digital‑to‑analogue converter and show the incoming sample rate (32,44.1 or 48kHz) together with an error lamp which illuminates if it fails to lock to the digital input. In fact, the D‑A will happily accept any input rate between 20 and 55kHz, which allows tracking of varispeeded sources, for example.

Rear Panel

The rear panel of the tiny RME box provides a wealth of flexibility, far beyond all expectations. The inputs to the A‑D are on the right‑hand side and are catered for with a pair of Neutrik combi‑jack XLRs. These are electronically servo‑balanced and can be switched to operate at either +4dBu or ‑10dBV levels via a small adjacent push switch. If used in an unbalanced installation, the servo‑balancing circuit design automatically compensates the input level accordingly. The S/PDIF digital output is provided on both gold‑plated phono and TOSlink optical connectors.

The input to the D‑A stage is via either another gold‑plated phono socket or a TOSlink optical connector, the selection being made by another small grey push button between the relevant connectors. The analogue output signal appears, electronically balanced, on both XLRs and quarter‑inch jack sockets.

Understandably, there is little room in the box for a properly screened internal power supply, and so a small co‑axial connector accepts a 12V AC power input from a wall‑wart supply.

Technology

The converters are based around the latest 128‑times oversampling chip sets with a 'guaranteed' 100dB dynamic range. The A‑D uses a Crystal CS5335 converter linked to a CS8402 to format the data for the S/PDIF digital outputs. The status flag is set to 'consumer' but without copy protection enabled, so that most professional AES‑EBU inputs would happily accept the signal via a suitable adaptor cable if required.

The specifications give the dynamic range of the A‑D as 102dB (A‑weighted) and the D‑A as 108dB (A‑weighted) — both achievable figures in practice. Noise and distortion are a commendable 0.003 percent (‑89dB), and frequency response is flat within a decibel between 20Hz and 19kHz. The A‑D will accept a maximum input signal of up to +20dBu, but the D‑A can output a maximum level of only +10dBu — the EBU specification recommends +18dBu for a 0dBFS digital signal.

The D‑A side of the unit contains a CS8412 to receive, decode and reclock the incoming data stream before handing it over to an AKM4320 for conversion. The output buffer is based around a common 4580 op‑amp and uses the same servo‑balanced idea to ensure that levels remain consistent whether the output is wired for balanced or unbalanced operation.

In Use

The ADI‑1 offers high‑quality outboard conversion, at an affordable price. It is easy to use with virtually any digital system; I tried it in conjunction with a variety of computer cards, budget digital consoles and domestic CD and DAT players. In all cases, it bettered or matched the performance of the existing converters.

The only real drawback with this unit is that it cannot be used as an input device for CD‑R, Minidisc or DAT machines because there is no option for correct dithering down to 16‑bit resolution. I realise this was not what it was primarily designed to do, but it would have been useful. However, as an output converter it performed very well indeed, making a dramatic improvement over an early DAT machine's internal D‑A, for example.

In its intended role, linked optically to a digital soundcard (such as RME's own offerings) it was excellent and, once plugged up, effectively became transparent in the complete system. If you are looking to maximise the quality of an internal computer audio card, or have a card with only digital I/Os, this is a very attractive and pragmatic solution indeed.