Once the data stream has been re-clocked within its dedicated, native frequency clock domain, the next stage is to synchronously up-sample it (by integer factor) to the DACs’ operating frequency. The 705.6/768kHz D/A conversion frequencies have been chosen as they are well above the threshold of human hearing, ensuring that the shift in time constant associated with its roll-off is imperceptible. This is in direct contrast to non-oversampling DAC designs that, in order to eliminate digital artifacts, rely on brick-wall, low-pass analog filters that damage the time and phase information across almost the entire audible band. CH Precision’s oversampling design allows the use of our fine-tuned third order Bessel analog filter, that works at such a high frequency that it leaves the precious audio-band unaltered. The result is improved phase coherence allied to reduced coloration, creating more natural dynamic transitions, perspectives and proportions within the music.
To further increase the accuracy of our up-sampling, it is carried out by our proprietary Polynomial Equations to Enhance Resolution (PEtER) algorithm, introduced with our HD-input card. Where most up-sampling DACs use zero-padding followed by a low-pass filtering stage to perform this task, we chose a more advanced approach to better mimic natural sound propagation. Instead of ensuring that the up-sampled signal presents no trace of energy above an arbitrary chosen frequency, the spline interpolation technique implemented in our DACs minimizes the spurious energy produced by the up-sampling operation as a whole. This method ensures the retention of all original sample values, which is not the case with most other techniques. The CH Precision approach produces more natural and convincing sound, with greater low-level resolution and instrumental texture, improved harmonic structure and tonality by eliminating the exaggeration or unnatural smoothing that so many high-resolution digital systems exhibit.
High resolution PCM material follows the same path as the red-book CD stream. Only the spline’s interpolation factor varies, from 16x to 8x, 4x or 2x (respectively for CD quality, 88.2/96kHz, 176.4/192kHz or DXD original sampling rate), in order to reach the DAC’s 705.6/768kHz operating frequency.1-bit DSD streams are converted by a proprietary algorithm to an ultra-high information density 24 bit / 705.6 kHz PCM format in a single operation. This means that the exceptional temporal resolution associated with the DSD format is preserved, while the high frequency modulation noise necessary to its operation gets filtered in the process.
Finally, the up-sampled data is fed to a multiple array of R-2R DAC chips, via a proprietary signal conditioning stage. Because each converter has a slightly different gain and offset, it is necessary to calibrate this advanced conversion system in order to harmonize the operation of each individual DAC chip to achieve maximum performance.
