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.
Internal clock generation
Two oscillators generate the clock frequencies for the two sample rate families: 44.1kHz multiples and 48kHz multiples. This section was actually developed for our AD converter, which is valued in many studios for its high precision as a clock generator for other digital devices. If external source devices such as a music server are connected to the dac-pre via ddUSB, these oscillators are also active during music playback. The DAC then has the best possible framework conditions, as the reference clock is generated internally and distributed to the DA converters via the shortest possible route. The clock is therefore not subject to any external influences. Only the clock generated internally in the DAC 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
In the case of conventional digital source devices such as CD/DVD players, televisions, computers or music servers/streamers with AES/EBU or SPDIF outputs, internal clock generators cannot be used. The major challenge here is to synchronise to the system clock of the source device so that the data does not arrive too quickly or too slowly for the DAC. At the same time, the clock precision should be influenced as little as possible by the external source. This is usually achieved by 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 sound level thus reaches a similar order of magnitude as with the DAC’s internal clock generators. However, there is always a residual dependency on the source. This is completely eliminated with playback via ddUSB.