The most important criterion for a converter between digital music data and analog music is a time base as exact as possible. Be aware that in a digital environment 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. Actually, this section was developed for our AD converter, which is appreciated in many studios because of its high precision as a clock generator for other digital devices. If external source devices like a music server are connected to the dac-pre via our USB interfaces with arfi-optical interface, these oscillators are also active during music playback. The DAC then has the best general conditions because the reference clock is generated internally and distributed to the DA-converters via the shortest possible path. The clock is therefore not subject to any external influences. Instead, the reference clock is first sent to the external USB module or our music server, and these source devices then send the music data back to the DA converter in precisely the right time. But beware: The clock quality is still not influenced by the external source devices in this way. Only the clock generated internally in the DAC and distributed directly to the converters is decisive for the conversion quality.
We also take a more elaborate approach to clock distribution than usually found in the audio industry. For this purpose, we use components and electrical formats that are otherwise only used in the IT sector for very high frequency clocked applications with simultaneously required high clock quality. This technology provides the basis for us to supply 32 or more channels with the same exact clock everywhere in our large module systems for professional studio applications. And our smaller devices also benefit from the highest precision achieved in this way.
Clock recovery from external sources
For conventional digital source devices such as CD/DVD players, televisions, computers, or music servers/streamers with AES/EBU or SPDIF outputs, internal clocks cannot be used. Here the big challenge is to synchronize to the system clock of the source device so that the data does not arrive too fast or too slow for the DAC, while the clock precision should be affected as little as possible by the external source at the same time. This is usually done by some form of a 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 then 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 sonic level thus reaches a similar order of magnitude as with the internal clocks of the DAC. However, due to the principle, there is always a residual dependence on the source. With playback via arfi-optical, this is completely excluded.