The most important criterion for a converter between digital music data and analog music is a time base that is as exact as possible. The analog oscillations are digitally stored as many data points in a coordinate system. Y-axis is the amplitude (volume), X-axis is the time. The amplitude can therefore be defined as precisely as possible, but if the time base wavers, there will still inevitably be errors. Consequently, a lot of development time has gone into this clock section, which generates an exact reference clock and is found in all our converters.
Internal clock generation
Two oscillators generate the clock frequencies for the two sample rate families: 44.1kHz multiples and 48kHz multiples. This module is crucial for our AD converters, which are also valued in many studios as clock generators for other digital devices because of their high precision.
But also, if the source sending the signal – e.g. the DAW – is synchronised to the ADC during music playback, then the DAC has the best general conditions, too. The clock is then not subject to external influences. Only the internally generated clock that is distributed directly to the converters determines the quality of the conversion.
We also take a more complex approach to clock distribution than usually found in the audio sector. For this purpose, we use components and electrical formats that are otherwise only used in the IT sector for very high-frequency clock applications with simultaneously required high clock quality. This technology provides the basis for being able to supply 32 or more channels with the same exact clock everywhere in our large modular systems for professional studio applications. And our smaller units also benefit from the highest precision achieved in this way.
Clock recovery from external sources
The AD converter is the all-important start of the digital signal chain. What is lost here is lost for all time. Therefore, it is generally recommended to use an AD-converter with an internal clock. This way no external factors can influence the clock quality and thus the sound result. The only exception are AD-converters with such a poor clock section that external master clocks can achieve better results.
However, there are also applications where synchronisation to an external clock is necessary. With a pure DAC there is no different way, but also in a larger network or with a general house clock in the studio complex. Here, the big challenge is to synchronise to the required system clock while still allowing the clock precision to be influenced as little as possible by the external source. This is usually done by a form of phase-locked loop (PLL). In the larger time domain, the PLL ensures that the source and the DA converter run synchronously. In the smaller time domain it ensures, that a stable clock signal is output. In our Femto-Clock-II, several PLL stages are connected in series, with the last stage achieving stability in the femtosecond range. The actual clock base runs here in the range of 4-5GHz and is suitably divided down to 256fs, for example 49.152MHz for 192kHz audio. The sound level thus reaches a similar order of magnitude as with the internal clocks of the ADC.