The dCS Ring DAC

The dCS Ring DAC
An overview of the technology behind the standard-setting performance of the Elgar

While dCS was the first to produce a 24bit/96k DAC (Digital to Analog Converter), the first with 24/192, and now the first with DSD conversion, they were also the first to admit that there’s a lot more to good musical performance than long word lengths and high sampling rates. Over the next few years we expect to see an increasing level of hype about those higher sampling rates. Soon 24/192 DACs will be found in all but the cheapest of audio components. While we realize that the vast majority of people will never spend $15,000 for a dCS Elgar Ring DAC, we do want to make sure that they aren’t blinded by all the hype and think that a 24/192 label on a front panel is necessarily synonymous with high quality musical reproduction.

Coming into the audio arena with a background in digital conversion for military radar, the people at dCS knew that what really counted was true resolution, not a number on a spec sheet. (Either you can resolve an incoming air to air missile a mile further away or you can’t.) In audio, when a manufacturer says he uses a 24 bit DAC, all that really means is the DAC can do 24-bit arithmetic – not that it can resolve 24 bits of musical information. In fact, most 24 bit DACs are lucky to resolve 20 bits of information. Here’s an overview of the problem and the solution:

Multi-bit Nonlinearity — In multi-bit DACs there is a resistor associated with a current source for each bit. Each resistor is half the value of the one before it. So the ratio goes something like this 1 : 0.5 : 0.25 : 0.125 : 0.0625 etc. By the time we divide by two 24 times, the theoretically correct value of the last resistor is 0.000000119209289550781 of the first. Because it is physically impossible to achieve this type of accuracy, all multi-bit DACs suffer from some non-linearity (they distort the signal). This distortion becomes greater as you move from more significant bits to less significant (loud stuff to background detail). Typically, somewhere around the 20^th bit the ability to resolve any additional detail is lost.

One-Bit Noise — In Bitstream (1-bit) DACs the resistor matching problem is eliminated and linearity is very good. However, the signal to noise ratio is terrible (6dB). A technique called oversampling is used to improve the signal to noise ratio to acceptable levels. However, the high oversampling frequencies result in narrow pulse widths. Timing errors now become significant, jitter increases, and the end result is thesame. The signal is distorted and the resolution of low level detail is degraded.

dCS Elgar Ring DAC — The dCS Ring DAC uses a patented 5-bit unitary weighted design (i.e., all the resistor values are the same). Oversampling frequencies are low (i.e., it’s less vulnerable to clock frequency errors). But, even this design isn’t perfect. Small variations in resistor value could still have an adverse effect on performance. Even with the carefully matched resistors used in the Elgar their resistance can change with age or temperature. To address this the Ring DAC, instead of using one resistor per bit, uses a large array of resistors. By using a proprietary algorithm (or is it Elgar-ithm) to continuously vary the number and positions of the selected resistors from sample to sample, as though around a circle (hence the name "Ring DAC"), the inevitable slight variations in the values of the resistors are randomly distributed throughout the quantizing range. This effectively turns any tolerance errors into random white noise, which is far more benign than the distortion that would otherwise have occurred. Finally, sophisticated noise shaping is used to move the bulk of the random noise into the high frequency spectrum above 100 kHz, where it is easily removed with analog filtering.

But the proof is in the final performance. When tested, virtually all other DACs will fail to resolve a full 24-bit signal. The Elgar is the only DAC in the world with genuine linearity down to 24-bits.

In addition to unsurpassed performance, the Elgar, in terms of investment, has no peers. The use of discrete components, software based digital filters, and programmable gate arrays means that the unit can be upgraded should digital audio standards change in the foreseeable future. dCS demonstrated this when they added 192kHz capability to the original Elgar, and again when they added Sony DSD (the carrier used in SACD).