Wadia 850 CD player
When you experience something completely different from the norm, it is possible that its instigators have an inside track on the truth and everyone else is wrong; or it is, of course, possible that they're more like Corbett's Uncle Charlie. In the world of CD replay, for example, mid-western manufacturer Wadia uses an idiosyncratic digital filter unlike anyone else's, so when I agreed to take a listen to the Wadia 850, I was curious as to which of these conjectures would prove correct.
The $4950 850...
...is a fairly large, squarish CD player with a light-sealed front-loading disc tray on its left and a display panel on its right, this showing the track/time information in fluorescent blue and the setting of the volume control in LED red. Superficially identical to the more expensive Wadia 860 (reviewed for Stereophile by Martin Colloms in February '98), the 850 uses a more economical TEAC transport mechanism, the CMK4. This still features the VRDS full-disc clamping system, however, made from green plastic. And whereas the 860 uses a digital transport servo, the 850 uses a conventional analog servo circuit.
Inside, the 850 features a Faraday-shielded AC transformer (the 860 uses two transformers, sprung to isolate them from vibration) and extensive AC filtering. Like the 860, the 850 places the DAC/digital filter and analog circuitry inside a separate shielded compartment on the player's right. As with the 860, the master clock crystal for the transport is placed adjacent to the DACs to minimize jitter.
Looking at the four-layer printed circuit board from the top, at the front are connections to the vertical control and display board mounted behind the front panel. Behind that is a Motorola DSP chip. This both implements the digital-domain volume control—which features 0.5dB steps—and runs Wadia's patented DigiMaster reconstruction filter algorithm, operating at a 32x rate. While this currently is implemented for 44.1 and 48kHz sample rates, it can be field-upgraded to run at 96kHz and 192kHz rates should this become necessary.
The digital filter is claimed to operate with 24-bit precision, with an effective resolution of 21 bits. It feeds two pairs of Burr-Brown PCM 1702K D/A converter chips, and from there on the reconstructed analog signal is handled in balanced form, with high-quality, surface-mount Burr-Brown op-amp chips used throughout the signal chain. The balanced analog output is from XLR sockets; the single-ended output is derived by feeding the balanced output to a differential amplifier, thus preserving the advantages of the balanced-DAC operation. Four DIP switches are used to set the analog output level, in four steps from 250mV to 4.4V RMS, allowing the digital volume control to be used in its optimum range in a variety of systems.
Optional boards are available to provide the 850 with digital inputs (two BNC coaxial, one ST-optical, one TosLink optical) and digital outputs (one BNC, one ST-optical). The input board uses Wadia's Rocklok jitter-rejection circuitry, and unused inputs are turned off to prevent interference. One operational idiosyncrasy: the remote control needs to be used pretty much face-on to the player. With my system, whose components are positioned obliquely along a sidewall, this bugged the heck out me. Your mileage, of course, may vary.
Connecting the dots...
One of the big misconceptions regarding digital audio is that the output of a D/A converter consists of a stairstep waveform with discrete levels instead of the continuous waveform typical of an analog system. In fact, Harry Nyquist, a researcher at Bell Labs, proved mathematically many years ago that, as long as the sampling frequency (Fs) is at least twice the highest frequency of interest in the signal to be sampled—ie, that the signal be bandlimited to half the sampling frequency—the analog waveform will be accurately preserved.
The task of reconstructing the original analog waveform is the job of what in a CD player or processor is termed the "reconstruction" filter. This is almost always a digital low-pass filter that, in a conventional player, has an impulse response that looks like fig.1. There appears to be symmetrical ringing both before and after the impulse. The actual shape of the impulse is called a (sine x)/x or sinc curve. As long as the points where the amplitude of the oscillations are zero are spaced exactly 1/Fs seconds apart, the sum of the ringing exactly reconstructs the shape of the original wave between the sampling points. Other than the Wadia designs and some models from Pioneer, all the world's CD players use a digital filter with a sinc impulse response.