Each 922 Decoding Computer is comprised of four major stages: the Digital Input Section, the Digital to Analog Conversion Section, the Variable Level Output Stage, and the Power Supply system. Digital Input Section The Digital Input Section functions as a receiver of information from the 931 Digital Controller via the Dual Fiber Interface, performs the DigiMaster interpolation filtering and volume control, and sends the digital audio signal to the D>A section.

DigiMaster 1.4 Interpolation Filter

The Wadia 922 uses the latest generation of Wadia’s proprietary interpolating digital filtering system, first introduced in 1991. The DigiMaster 1.4 Waveform Algorithm has been optimized for a wide range of performance parameters. Incoming data samples are reconstructed with special emphasis placed on time and phase accuracy. In contrast, typical off-the-shelf digital filtering is optimized for flat frequency response and fails to recreate the subtle musical presence and details that are preserved only with accurate time-based algorithms. The DigiMaster 1.4 interpolator uses a 12th-order, curve fitting, spline interpolation algorithm to precisely reconstruct the original analog waveform. Using digital signal processing (DSP), a curve is fitted that conforms to the current sample plus future and prior samples calculated at 48 bit resolution and output at 24-bit precision. The DigiMaster 1.4 operates at the rate of 64-times re-sampling of 44.1 kHz, that is, 63 interpolated values are calculated for each original sample from the CD. Although the interpolated samples are calculated by DigiMaster 1.4 to 48-bit precision, the original data samples from the compact disc are not altered in any way. A conventional 8x oversampling filter provides interpolated samples resulting in an approximated waveform with discrete stair steps. The DigiMaster 1.4 algorithm, operating at 64x oversampling, provides dramatically improved resolution in the waveform. Hence, the DigiMaster 1.4 algorithm more precisely reconstructs the original waveform.

Digital to Analog Conversion Section
The 922 D>A section converts the digital audio data into an analog signal. As described above, the DigiMaster 1.4 algorithm produces digital audio data at a very high sampling rate. For example, using the standard CD format, the DigiMaster 1.4 algorithm calculates 63 interpolated values for each original sample. The output to the D>A section is 2.8224 million samples per second, each sample with 24-bit resolution. This number gains even greater significance when compared to the DSD (SACD) system, which decodes 1-bit data at this rate. The 922 D>A section utilizes eight Burr Brown PCM 1704 DAC chips to convert the positive and inverted halves of the signal with a theoretical resolution beyond 24-bits; four for the positive half of the signal, and four for the inverted half. The individual circuit blocks are described in greater detail below.

Isolated Digital Coupler Array
The first element in the D>A section is an array of Burr Brown ISO150 Isolated Digital Couplers. These devices are placed between each of the incoming digital signal lines and the DAC chips to provide complete electrical isolation of all signal and ground lines. This prevents any digital noise and ground currents from the digital circuitry from adversely affecting the performance of sensitive analog signals in the 922 D>A section. Some digital products use optically-coupled isolators to isolate the digital inputs. The ISO150 avoids problems commonly associated with optocouplers, as optically isolated couplers require high current pulses and allowance must be made for LED aging. Optical isolators can exhibit a 50% reduction in light output from their internal LED within one year. In a worst-case scenario, this degradation could cause a performance or reliability issue. Since the ISO150 uses no LEDs, aging is not a factor and reliability and lifespan are greatly increased. The ISO 150 devices used in the 922 Decoding Module utilize a tiny internal air gap that acts as a capacitor by transferring high frequency signals. These devices allow data up to 80 Mbits/sec to be transmitted, while still achieving a very high degree of DC and low frequency isolation. Although the air gap is small, each device can withstand peak voltages up to 2400 volts.

Time Shifted DAC Array
To handle the 2.8224Mhz, 24-bit data stream from the DigiMaster 1.4 interpolation filter, the 922 D>A section uses a total of eight each of the Burr-Brown PCM 1704 24-bit DAC chips per channel configured in a balanced, time-shifted array. The Time-Shifted DAC array increases the speed at which digital information is converted, as well as increasing system resolution. In most designs, digital-to-analog conversion rates are limited by the settling time of the DAC chips and their current-to-voltage (I/V) amplifiers. When the conversion rates exceed the typical 8-times at 44.1 kHz (or 352 kHz) rate, insufficient settling time between samples degrades sonic performance. Wadia’s patented time-shifted DAC array overcomes this problem by feeding data to the DAC pairs sequentially. As a result, each DAC only has to decode a fraction of the total samples, allowing ample settling time between successive data values. As mentioned above, the Time Shifted DAC array provides higher resolution, e.g., when the current output of each DAC in the array is summed, the resulting signal has higher resolution than would be possible using individual DAC chips due to a lower level of noise in relation to the audio signal. Each DAC puts out a LSB (Least Significant Bit, the bit with the smallest value or representing the smallest portion of audio data) signal, representing the smallest amplitude signal possible for that DAC. When the currents from each DAC are summed, the resulting summed output signal has an improvement in signal-to-noise ratio (SNR): the noise floor remains the same, yet the overall output has increased, yielding an improvement in the theoretical resolution. This improvement is measured by the square root of the number of devices that are employed. For example, using two DAC chips would increase SNR by the square root of two, which is 1.41. By employing four DAC chips, the SNR is improved by a factor of two (equivalent to 1 bit). By using a total of eight DAC chips, the Wadia 922 Decoding Module achieves a resolution that is 2.82 times greater than a single DAC could provide.