The old claim of digital being “pure, perfect sound forever” is actually true – just so long as the signal remains in the digital domain and is held in a stable storage medium. Actually listening to it? That’s another matter altogether!
As soon as you attempt to convert a digital signal to analog via a DAC, or even move it between multiple digital devices that require synchronization, you face significant problems and degradation. In either case, the accuracy and synchronization of the data clocking is a critical factor in avoiding induced and cumulative errors, both in terms of leading edge definition and decay or the actual placement of notes. These errors can have a catastrophic impact on the credible reproduction of recorded music, making the difference between a performance that is believable and one that is merely recognisable. Because the clocking of data has such a vital role to play in digital music reproduction, it is something that CH Precision has expended enormous time and effort to advance.
There are a number of established approaches to the challenge, each offering its own advantages and disadvantages. The most common are the Analog Phase Locked Loop (PLL) and Asynchronous Sample Rate Conversion (ASRC) often selected because they are easy to implement and relatively cost-effective for cost-conscious designs.
Based on a local clock with variable frequency, the analog PLL offers good phase alignment between input and output but imports (and potentially adds to) inherent jitter levels in the incoming signal.
The ASRC approach re-maps incoming audio data to a local fixed frequency clock. This operation involves a complete re-computation of all audio sample values. This not only means that new points are computed between existing samples, but even incoming samples get modified. The fact that it is not a bit-perfect system also means that it is unsuitable for DSP over PCM (DoP) transmission. Jitter rejection using this technique is better than with the analog PLL approach (high frequency phase noise is lower), but low frequency jitter remains problematic.
CH Precision has developed an advanced hybrid solution that combines a short FIFO buffer with the two most sophisticated approaches to data transfer. In the first instance, both the data output from the FIFO and the clock frequencies employed by the DAC are derived from a pair of locally located VCXO (voltage controlled crystal oscillators) one dedicated to 44.1kHz based data streams and the other to 48kHz multiples. By placing the clocks as close as possible to the DAC, we prevent post generational error being introduced through differential track-lengths and other distortion mechanisms.
These clock frequencies are then precisely tuned to match the incoming data stream’s rate through a proprietary advanced control loop, resulting in extremely low jitter levels and excellent phase alignment between input and output signals, ideal for multi-DAC applications. Although complex and costly to implement, this approach delivers consistently excellent performance, especially when combined with a distributed clock signal.
For further improved performance, CH Precision allows users to implement Wordclock Synchronization by designating an individual unit (normally the DAC) as the clock master and sending its clock signal to other units in the chain via the BNC sockets on the SYNC-IO boards (optional on the D1). This system generates a readily appreciated improvement in musical performance, rhythmic and dynamic integrity. But musical and spatial performance can be further improved through the use of the T1, temperature stable, external master clock, a superbly accurate clock source that can even reference its output frequency to the atomic clock that controls the GPS satellite system.
Digital systems are totally dependent on the accuracy and implementation of the clocks that control their data streams, with timing errors and jitter producing direct degradation of the musical signal. CH Precision goes to extraordinary lengths to ensure that the entire digital conversion process is controlled by a single clock and, that that clock delivers the most accurate reference frequencies possible. Most digital technology is essentially invisible, its claims having to be taken on trust. But in this case, the fact that users can reconfigure the clocking within the system and even upgrade the Master Clock itself means that they can hear directly just how critical clock accuracy and the engineering that goes into it is to the final musical performance.