Wadia Digital 1000 Decoding Computer
The WD1000 is a single-box decoder with a single BNC digital input which can accept 32kHz satellite, 44.1kHz CD, 48kHz R-DAT, and other similar digital audio sources. Switching between sampling frequencies is automatic, and indicated on the front panel. Audio output is fixed, while absolute-phase invert is provided by a front-panel button. It is a substantial, heavy unit built to full rack size, with a fine finish and good build quality.
In addition to its high resampling rate, which allows for the merest degree of vestigial low-pass filtering, other special features include the proprietary programmable digital filter, built using discrete digital signal processing (DSP) integrated circuits, and specially built EPROMS (Erasable Programmable Read-Only Memories).
This is my first view of custom audiophile digital-audio technology. The filters can, in fact, be designed on a PC, burnt on EPROMS, and installed as required in the decoder. This technique is expensive in terms of processor hardware, but liberates the designer from the need to use mass-production LSI filters whose technical and sonic parameters have been determined elsewhere, perhaps with the wrong priorities in mind.
To use the WD1000, digital audio sources are required, namely appropriate CD players with a wired digital output. I have found sonic differences to be apparent between transports feeding the same decoder despite the previously claimed incorruptibility of digital-audio data codes. Best results were obtained with Philips-technology transports, including the Marantz CD12 two-box machine, and the French Micromega design.
Build and technology
Fabricated from heavy-gauge machined aluminum-alloy plates, the case has 3mm-thick covers with no provision for ventilation. Perimeter casework is in 6mm material, while the front panel is an extravagant 15mm thick. Wadia could not be accused of compromising on the case, which is to "mil"-grade battleship standard! Circuitry is present above and below a central horizontal platform, the underside devoted to the power supplies, the topside to the main decoder sections. A total of six printed-circuit boards are used, linked by commercial-grade plug-in strip connectors. Interestingly, the two Swedish power-supply toroids, generously rated at 63VA, are each bolted in beneath the D/A convertor cards, with some shielding provided by the alloy interpanel. This cannot be 100% unless a ferromagnetic shield is also employed.
Working through the supplies, the AC mains input is a detachable IEC card, integrally fused; the supply line is filtered by a Corcom unit, a symmetrical balanced C-L-C filter design. The power-supply arrangements are comprehensive, with full segregation shown between the various operating sections, plus discrete preregulation and a decent reservoir capacity achieved with seven main reservoir capacitors.
On the processing board, fixed-voltage chip regulators are liberally distributed, these 7915/7815 CT and other similar types. I counted a total of 17, though I could have missed one or two. Ceramic multilayer and tantalum capacitors are used extensively for regulator and local decoupling, both for digital and audio sections.
On my test sample, the digital input is via a single BNC terminal (optical is optional), and data recognition and extraction are accomplished by the well-established Sony CX 23053 LSI. Until an input signal is detected, a separate oscillator scans the 3050kHz bandwidth; on detection, the input signal is then synchronized to the sample clock via a phase-locked loop, and this signal appears together with the digital audio data on the I2s bus output from the CX 23053.
In a conventional decoder, this data would normally feed a one-chip oversampled digital filter, then on to the DACs, but in the case of the Wadia, this is where the custom technology takes over.
The processor technology is 16-bit, 16x-oversampled, digitally filtered, with the converters selected Burr-Brown PCM56 serial input devices, run without deglitchers but with OPA606 ICs used for I/V conversion and buffering. Filtering and de-emphasis are both entirely passive, and are relay-controlled. The output lines pass through large ferrite toroids to reduce RF interference in the output. With pre-set control of the low-level resolution and a low DC offset, the WD1000 is direct-coupled to its output terminals, these gold-plated Tiffanys.
Custom digital filtering is the raison d'ëtre of this model, which employs two, 36-bit wide, AT&T DSP16 high-speed DSP chips per channel. These processors are programmed by a unique code to generate a custom filter characteristic optimized by the Wadia designer to achieve the best compromise between sound quality and filter technical performance.
The designer condemns conventional brickwall filtering, though I feel some of his criticism is directed at the early analog 11th- and 13th-order CD filters, not the modern high-order (+200th) linear-phase filters popular now.
The Wadia digital filter algorithm is designed to "minimize curve matching error in the time domain to reduce intertransient ripple and harmonic envelope distortion." A suitable decode algorithm is appropriately implemented in Wadia's "Frenchcurve" set of combination interpolation theorems for music reproduction.