The team around the new audiophile German online magazine LowBeats put the MC-3+ Smart Clock through its paces in an in-depth review that includes a tutorial on the hot topics re-clocking and jitter. Thanks to MUTEC’s proprietary 1G-Clock technology the MC-3+ impressed with its high precision clock and re-clocking capabilities.
Jürgen Schröder August 30, 2015
The Time Machine
The presented Mutec MC-3 + is in the first place a high-precision Wordclock generator. When it comes to linking multiple digital audio components in recording studios, it is absolutely necessary as a central clock. Thus, it fulfills a function that will probably play an increasingly important role in the coming hi-fi technology. That alone makes it not really interesting for listening at home as currently only very few consumer devices are equipped with a Wordclock input that is mandatory for this purpose. Far more exciting for HiFi fans, however, is the second important function of this dinky box from Berlin: the Mutec MC-3 + is able to fully "re-clock" incoming digital AES3 or S/P-DIF signals - and that means nothing less than getting rid of sound spoiling clock jitter.
This seems to be an easy task but technically it is anything but trivial. Many devices eliminate a jitter by simply shaping a pure pulse but that really does not do the trick. It is not a surprise that even in extremely expensive hi-fi equipment a complete re-clocking of the digital signal rarely takes place.
Reclocking - for what? Whenever digital appliances are connected to each other, the master-slave principle rules: the master, such as a CD drive, not only offers the audio data but also the beat by which the D/A converter’s digital samples are re-converted to analog. It is true that the D/A converter provides its own clock but this has to be synchronized with the master’s clock to avoid a buffer over- or underflow which either causes cracks or dropouts in the audio signal.
This synchronization is usually taken over by a so-called Phase Locked Loop (PLL) circuit: by comparing the phases, it determines the differences between the master and the slave clock and controls the D/A converter’s system clock accordingly. And here lies one of the essential causes of the jitter problem: the PLL control loop, derived from the phase comparator and the clock oscillator, generates a more or less strong phase noise - i.e. time-wise, the clock frequency is not stable, it jitters. Thus, the PLL loop is a sound determining element in the D/A converter since the clock accuracy directly affects the sound quality.
Clock Around The Rock
All these problems are unknown to the Mutec MC-3 + as it has no PLL - at least not in the form as just described. Certainly, while in re-clocking mode, it also has to adjust itself to the clock frequency of the external reference signal but it does it in a completely different way: the MC-3 +’s synchronization is achieved by a DDS-process (direct digital synthesis). DDS generators (described in chart 1) have many benefits: they not only provide exactly adjustable, extremely stable fixed frequencies in a large range, but - what is even more interesting for audio applications - they stand out by very low phase noise. There is a rule for DDS generators: the higher their internal clock speeds are, the less noise is in the output signal.
Chart 1: By direct digital synthesis (DDS), signals of any shape and frequency can be generated from a stored curve. These individual samples are selectively retrieved from a memory and then re-compiled. The DDS process allows for clocks that are extremely frequency-precise with very low phase noise. (Source: National Instruments)
To achieve a maximum degree of noiselessness, the DDS generator in Mutec’s MC-3 + works with a very high system clock of 1 GHz which is converted internally to a special frequency that was previously elicited by a number of tests to be the best for audio clock generation. The actual synchronization process runs in Mutec as follows: while the digital audio data, supplied by the source, is read into a memory, the MC 3 + determines the current, average clock frequency. The DDS generator is now synchronized very accurately, i.e. in phase, on it. Thus the MC-3 + can accurately follow an eventual, slow drifting of the reference signal while reliably blocking the input signal’s higher frequency clock jitter like a low-pass filter. Unlike conventional PLLs, this process is also not accompanied by jitter gravid phase noise. Hereby the audio data is of course not at all altered but jitterless sent to the output with a new clean clock.
Everything just theory? So much for - how excellent the re-clocking function of the Mutec MC-3 + works in practice, shows the comparison of the measurement diagrams (gallery below) in an almost dramatic way: for the measurements I connected an external D/A converter (AMI Music DDH-1) via the usual S/P-DIF interface with a coaxial cable to a budget CD player (NAD C 542) functioning as a digital master device. By means of a special test signal (see diagram 1) I first determined the jitter spectrum of the CD player at the analog output which showed very decent values (diagram 2). Then I started the same measurement at the analog output of the coaxial-fed AMI Music DDH-1: obvious, jitter-induced noise components appeared here especially in the close vicinity of the 11025-Hz test tone that were partly even 30 decibels higher than the CD player’s analog output (diagram 3).
Picture 1: The Mutec MC-3 +’s casing measures exactly half of the 19-inch standard width. It is accompanied here by two of our test D/A converters - the AMI Music DDH-1 (top) and the Arcam airDAC (right). The NAD C 542 CD player (below) served as source. (Photo: J. Schroeder)
I finally inserted the MC-3 + which was set to re-clocking mode into the coaxial connection between the CD player and the D/A converter and measured again the jitter spectrum at the AMI Music DDH-1’s analog output. The results were so dramatic that I could hardly believe it and checked the measuring arrangement again and again. It remained: the MC-3 + succeeded in completely eliminating all jitter-induced noise components as diagram 4 impressively proves. And not only that: a closer look at the low frequencies reveals that the harmonic structure of the modulated 229.7 Hertz square wave signal was even closer to the original signal (diagram 1) than the CD player. As you can see, while the Mutec MC-3 + had the AMI converter’s rather jitter sensitive PLL well in hand, it improved its jitter performance significantly.
Diagram 1: The 16 bit/44.1kHz-J test signal in its purest digital form - 11.025 kHz square with -6dBFS; modulated with a square wave 229.7 Hz 1 LSB. The spectral lines in an interval of 229.7 Hertz stem from the modulation signal’s finite 16 bit resolution. (Diagram: LowBeats)
Diagram 3: J-test signal from diagram 1 reproduced through the AMI DDH-1’s D/A converter analog output which was fed by the S/P-DIF coaxial input of the NAD C 542 CD player. Compared with the CD player’s output, the clearly increased jitter components, especially in the range 8 to 14 kilohertz, can be perceived. (Diagram: LowBeats)
Diagram 4: The same configuration as in diagram 3 but with Mutec MC-3 + in re-clocking mode inserted in the S/P-DIF coaxial transmission line. The output of the AMI DDH-1 comes out now completely jitterless. In addition, the original harmonic structure of the 229.7 Hz modulation signal is almost perfectly maintained. (Diagram: LowBeats)
The MC-3 + in the listening test: Correct timing is all you need
In the listening test, the Mutec MC-3 + re-clocking did not emerge as an exaggerated effect but in a more musical way: instruments got a tighter tonal structure and were presented in a cleaner and dynamically more accentuated way. Regardless of the musical style, the sound became more concise and moving but was still flowing. The lower frequencies sounded tighter and more transparent with the MC-3 + re-clocking, especially noticeable with long sustain instruments such as low-tuned drums or deep piano chords. The overall musical performance was given more drive and charisma, without appearing thereby artificially brightened. The kind of muddiness in the sound that you usually hear only in direct comparison disappeared completely. In short: the Mutec MC-3 + took the grey haze off the music. Once heard, there was indeed no turning back.
Especially with jitter-prone digital output equipped digital sources, the difference was clearly noticeable: in my case it were a DVB-S PC card for satellite radio reception and a DJ CD player with speed setting. In such cases, the MC-3 + actually worked like a tonal autofocus.
In addition, there were also converters which reacted very little to the re-clocking of Mutec: e.g. the D/A converter pre-amplifier Nubert nuControl, which is anyway completely decoupled from the master clock with its asynchronous sample rate converter (ASRC). The same was true of the more than 13000 Euros costing Analog DAC of MSB Technology. Although not using an ASRC chip, it has its own crafty MSB synchronization process.
Mutec MC-3+: (Not only) For Professionals
However, the profound aural impact of the re-clocking function should not detract from the fact that the Mutec MC-3 + offers further useful applications. The second most important for HiFi friends is certainly the digital format converter. It sounds complicated but it is very simple: e.g. S/P-DIF in (be it coaxial or optical) - AES3 (often referred to as AES/EBU) out -which of course also works vice versa. Similarly, the re-clocking mode is always active in the digital format conversion.
Elementary for the studio use is of course the function as a high precision Wordclock generator for which the MC-3 +, thanks to its 1-gigahertz DDS technology, has all the attributes: in this mode, no audio data is transferred but "blank frames" of exactly the length of a sample word for synchronizing all involved digital components. These work in exact sample locksteps and do not have to extract the system clock individually (as in HiFi technology) in a cumbersome and jitter-prone way out of the audio stream.
For the Wordclock distribution, the Mutec MC-3 + has six back-panel BNC outputs in the form of three pairs. Thus, each of these jacks can supply more than three with T-adapters connected devices in a chain, allowing a total of 18 digital components to be connected via BNC. In addition, also the AES3 output and the S/P-DIF connectors (coaxial and optical) put Wordclock signals out. This makes it possible to overclock semi-professional equipment without Wordclock inputs via S/P-DIF: the target device must only have an S/P-DIF input for this. Musicians and home recording people can so improve their audio interface sound elegantly: in this way, e.g. sample clock jitter can be avoided during a recording when the interface’s PLL is less stable than the Mutec timing guide.
Photo 3: The Mutec MC-3 +’s connector panel combines fittings of professional and consumer standards. In addition to the internal clock generator, also an external clock reference can be connected via a combined Wordclock/10MHz input. In addition to Wordclock sources, it can also be so-called atomic clocks or GPS receivers. (Photo: Mutec)
The outputs of the MC-3 + are divided into five groups: BNC (1 - 3), S/P-DIF (4) and AES3 (5). Each group can be assigned via a touch key to different output clock frequencies which can be either the simple or even multiples of the selected basic sampling rate (32-192 kHz). While the AES3 and S/P-DIF outputs are limited to a maximum of 192 kilohertz, even up to 768 kilohertz are possible via BNC - in addition there is still the so-called 256-fold "Super Clock" available for clocking older Digidesign ProTools converters with 11.29 and 12.29 megahertz. However, a sample rate conversion does not take place in the frequency multiplication since no audio data is transferred during the Wordclock generator operation.
Except its function as Wordclock generator, the Mutec MC-3 + can be used as a clock distributor, too. Next to the Wordclock signals, any other digital sources, e.g. the optical S/P-DIF input, can serve as references. As the MC-3+ only issues Wordclock signals without audio information, it can not be used as an active S/P-DIF signal distributor.
In addition to all the features, the Mutec MC-3 + also offers some very helpful detail solutions: while operated by an external reference signal, the MC-3+ retains the reference signal’s clock frequency even when it fails - preserving you from unpleasant synchronization margins in Live and in studio situations. Also very useful: the re-clocking function can be activated if the Mutec uses an external clock reference - in this way, for example, the output of a CD player can be synchronized to a given studio clock.
The operation of the MC-3 + is solved in an elegant way: besides the rear power switch, just only two tap buttons can be found on the front. With one it will toggle through the menus, with the other one various options can be selected that are automatically activated after a few seconds. Very practical: pressing a tap button again always recalls the last chosen menu, so during a setting you do not always have to start from scratch. As a matter of course, all your settings are retained even after a power off or a power failure.
Conclusion: Mutec MC-3+
The MC-3 + is a very precise and flexible Wordclock generator, format converter and a clock distributor and it can provably even eliminate jitter by re-clocking. For all this, its price of 750 Euros is more than justified. For the studio scene it is an absolute Best Buy - no wonder that it already is a hot tip for renowned mastering luminaries such as Dave Kutch (themasteringpalace.com) or Maor Appelbaum (maorappelbaum.com/STUDIO).For HiFi friends, however, it grants countless opportunities to upgrade their existing equipment sonically - be it for stabilizing jittering program sources or drives, to fix the D/A converter or digital active monitors for a sound improvement more to the clock or serving just as a digital format converter. How much the sound will improve during the re-clocking is directly related, though not visible from the outside, to the design of the PLL circuit in the D/A converter. Roughly estimated, about 80 to 90 percent of all devices will benefit sonically - sometimes significantly - from the MC-3 + as a re-clocker. This raises the question why you should not invest your 750 Euros instead in a better D/A converter: however, this does not guarantee you that its PLL then reacts as jitter-insensitive as the existing device in conjunction with the Mutec MC-3 +. So here an exciting listening test at your dealer is advisable.
Moreover, this test shows yet another aspect that is not only for HiFi friends extremely exciting: as the jitter measurements clearly demonstrate, the ‘pimping’ of an existing CD player with an external converter does not automatically lead to better sound results - not even if the external D/A converter offers better conditions in terms of signal processing than the player. What is lost by any jitter can not be compensated by even the finest transducer electronics. Seen from the sonic point of view, integrated CD players have so far not lost their right to exist - a fact that some manufacturers, such as Naim, have repeatedly emphasized. This also means: if you want to find out how much sound potential actually lies still dormant in your digital equipment, there is definitely no way around the Mutec MC-3 +. However, in future tests of digital equipment we will always perform a sound comparison - with or without the MC-3 +’s re-clocking: a service that can only be found at LowBeats.
Mutec MC-3+ 2015/08