dCS Elgar D/A processor
By John Atkinson   •   July, 1997 Things are changing rapidly in the world of professional digital audio. After a decade of stability, with slow but steady improvement in the quality of 16-bit, 44.1kHz audio, the cry among audio engineers is now "24/96!"—meaning 24-bit data sampled at 96kHz. Not coincidentally, DVD offers audiophiles a medium with the potential for playing back music encoded at this new mastering standard.

Originally a defense and space contractor offering high-performance (and presumably high-cost) analog/digital converters (ADCs), British-based Data Conversion Systems is one of a small number of companies pioneering the new standard. Its 24-bit dCS 900-series ADCs are well-respected in the classical audio engineering community, as is its dCS 952 D/A processor. Now dCS has entered the consumer arena, applying its pro audio and military experience to the needs of the High End.

Enter the Elgar
The solid-state dCS Elgar is a low-profile unit finished in two-tone gray with a central green LED matrix on its front panel. Chrome pushbuttons select Standby—dCS recommends leaving the unit powered all the time—Display brightness, Phase invert, De-emphasis choice, Input select, Mute, and Volume/Balance. The last governs the function of the right-hand control knob, presumably a shaft encoder as it is capable of continual rotation. The volume can be controlled digitally in 0.5dB steps down to a minimum of -60dB; the balance is controlled in -1% steps down to each channel being turned off. Pressing the Display and Volume/Balance buttons brings up a menu of subsidiary functions.

The rear panel has the usual selection of data inputs, including two AES/EBU inputs on XLR jacks, which can be used in double-speed mode and parallel mode for high-speed (96kHz or 88.2kHz) decoding. The input receiver can be switched between wide-window mode (called "Analogue") for data sources with poor clock stability and/or accuracy, and a narrow-window, jitter-rejecting "Digital" mode for closer-tolerance sources. Missing is a data output, something that those of us with DAT recorders and AES/EBU-input level meters find useful. Both balanced and unbalanced analog outputs are provided, both capable of being switched between 3V RMS MOL and 1V (now 6V/2V).

But it is inside where the true beauty of the Elgar resides. The necessary oversampling, decimation, and low-pass filtering, along with control of volume and balance, are achieved with DSP and Programmable Gate Array chips. Usually, a D/A processor uses a conventional, binary-weighted, multi-bit DAC running at 8 times the sampling frequency (8fs), or a single-bit DAC running at 256fs or higher. Each topology has limitations in different areas: multi-bit DACs get very complex if you want better than 20-bit performance; single-bit DACs have to use high-order noise-shaping to get effective 20-bit resolution, and are more prone to data jitter.

The Elgar features neither. Instead, dCS uses what they call a "Ring DAC." This proprietary design features a 5-bit, unitary-weighted, resistor-ladder DAC in a 64fs oversampling topology. dCS is careful not to claim that it has 24-bit performance—this implies an unweighted RMS noise floor of -144dBFS—only that the Elgar will accept 24-bit data. However, they do specify an unweighted noise floor lower than -110dB in the audio band, which implies a true resolution close to 19 bits.