The most important criterion for a converter between digital music data and analogue music is a time base that is as exact as possible. The analogue vibrations are stored digitally as many points in a coordinate system, so to speak. The Y-axis is the amplitude (volume), the X-axis is the time. No matter how precisely the amplitude is defined, if the time base wobbles, errors will inevitably occur. A great deal of development time has therefore gone into this clock section, which generates a high-precision reference clock and can be found in all our converters.
Internal clock generation
Two oscillators generate the clock frequencies for the two sampling rate families: 44.1kHz multiples and 48kHz multiples. This assembly is crucial for our AD converters, which are also valued in many studios as clock generators for other digital devices precisely because of their high precision.
If the source sending the signal – e.g. the DAW – is synchronised to the ADC during music playback, then the DAC naturally also has the best framework conditions. The clock is then not subject to any external influences. Only the clock generated internally and distributed directly to the converters is decisive for the quality of the conversion.
We also take a more complex approach to clock distribution than is usually found in the audio sector. We use components and electrical formats that are otherwise only used in the IT sector for very high clocked applications with simultaneously high required 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 devices also benefit from the maximum precision achieved in this way.
Clock recovery from external sources
The AD converter is the crucial start of the digital signal chain. What is lost here is lost for all time. For this reason, it is generally advisable to use an internal clock with every AD converter, as this means that no external factors can influence the clock quality and therefore the sonic result. The only exceptions are AD converters with such a moderate clock section that external master clocks can achieve better results.
However, there are also areas of application where synchronisation to an external clock is required. This is the case with a pure DAC, but also in a larger network or with a general house clock in the studio complex. The big challenge here is to synchronise to the required system clock and at the same time keep the clock precision as unaffected as possible by the external source. This is usually achieved using a form of phase-locked loop (PLL). The PLL ensures that the source and DA converter run synchronously in the larger time range and that a stable clock signal is output in the smaller time range. 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 here runs in the range of 4-5GHz and is suitably divided down to 256fs, i.e. 49.152MHz for 192kHz audio, for example. The sound level thus reaches a similar order of magnitude as with the ADC’s internal clocks.