Drawmer's affordable new combination of master clock and sample rate converters provides an ingenious solution for high-quality digital interfacing in the home studio.
Most audio engineers who work regularly with digital audio equipment will already have grasped the necessity to ensure that everything within a digital studio system is sample-rate synchronous. Digital signals can only be mixed together if their samples arrive at the mixer at the same instant in time, and at the same average rate. In large professional digital studios this is usually achieved by using a master word-clock generator, the outputs of which are distributed to all the digital equipment: the digital desk, the various replay machines, A-D converters, sample-rate converters, and so on. Where the area also handles audio for video production this master clock generator may be referenced from a video sync-pulse generator (or may generate video syncs itself) to ensure that the digital sample rate is locked precisely to the video frame rate — an essential condition where digital audio and video cohabit.
Another important facet of a digital system is that the quality of the analogue-to-digital and digital-to-analogue conversion process is heavily dependent on the stability of the reference word clock. An unstable clock (or poor-quality interfaces and cables) creates 'jitter' — the digital equivalent of 'flutter' in analogue replay systems. Instead of hearing the rapid pitch variations known as flutter, though, jitter causes the sampling instants to occur slightly earlier or later than they should. This 'sampling error' manifests itself either as high-frequency noise in extreme cases, or more often as an imprecise stereo image. Often budget converters employ relatively low-quality internal clocks, and a significant performance improvement can be obtained by driving such converters from a high quality master word clock, which typically has far greater stability and much less jitter.
However, high-quality word-clock generators are usually expensive beasts — especially if external video referencing is required. Many AES-EBU Grade One reference clocks (and there is absolutely no point in acquiring a master clock which has a lower specification) cost in the region of a thousand pounds, for example. This may not present a problem for well-equipped professional studios, but it is a substantial outlay for any home studio. Furthermore, a lot of semi-professional or low-budget digital equipment is not designed to accept an external clock reference anyway, and so cannot easily be referenced to a central master clock. This particular problem can be overcome if the sound desk is equipped with asynchronous inputs — sample rate converters built into the digital inputs — but this is normally a feature of high-cost professional equipment, and is not commonly found on the budget digital desks favoured in home studios. Catch 22!
Fortunately, there is now a rather ingenious and remarkably cost-effective solution to these problems: the revolutionary new M-Clock recently introduced by Drawmer. This 1U rackmounting digital master clock unit is intended for stand-alone digital audio installations and provides Grade One stability on its reference outputs. An important point to note is that the M-Clock cannot itself be synchronised to an external reference clock or video sync, so it is unsuitable for use in a video environment, or as part of a larger digital studio complex where multiple areas require synchronisation to a central house master clock. Nevertheless, there are a great many audio-only production environments where an affordable, stand-alone master clock would prove a major asset, and that is the market for which the M-Clock has been designed.
Master word-clock units are not particularly interesting products. There are few controls, and once configured they rarely need to be adjusted. So the front panel of the M-CLock is not terribly exciting, with just five buttons and fifteen LEDs. Two of those buttons are used to select the output sample rate from 44.1, 48, 88.2 or 96kHz, the current rate being shown by an associated LED. The master word-clock outputs are presented on the rear panel of the unit through an array of eight BNC terminals. In addition there are four 'digital black' (AES11) clock reference outputs, two configured as AES outputs (on XLRs) and two as S/PDIF outputs on coaxial phono sockets.
To extend the flexibility of the unit, two of the buttons on the front panel enable some of the BNC clock outputs to have overclocked sample rates. Outputs five and six can be switched as a pair to provide clock outputs at twice the main selected rate, while outputs seven and eight can be selected to output Super Clock — a clock signal at 256 times the normal rate used by Digidesign's Pro Tools hardware interfaces.
The master clock signal produced by the M-Clock is derived from one of two very high-quality crystals (one for 44.1 and 88.2kHz, and the other for 48 and 96kHz). In fact, these crystals make up a significant proportion of the unit's total cost, and a great deal of care has been lavished over the associated oscillator circuitry and the board layout to ensure the clock retains the maximum stability and the minimum jitter.
There are several manufacturers producing master word clocks, but none have taken the extra, innovative step which Drawmer has done with the M-Clock. As well as producing word clock and digital black signals, the M-Clock does something quite unique: integrated within the unit are four high-resolution sample rate converters (SRCs), each referenced internally to the master clock. The combination offers several advantages to the user, not least in saving rack space! Since the SRCs receive their reference clock signals directly, all the M-Clock's outputs remain available to clock external equipment — you do not have to use any of the outputs to synchronise the sample rate converters. Secondly, as the SRC reference clocks are confined within the unit, their precision is assured and the reclocked outputs are guaranteed to be synchronous with the master clock outputs. More importantly, from an operational point of view, up to four semi-pro or domestic digital devices which lack external clock reference facilities can be connected through the M-Clock's internal SRCs and presented as synchronous sources to a digital mixer, simply and without additional expensive converters or complex wiring.
Of the four SRCs built into the M-Clock, three have their inputs on the rear panel, with the fourth input connected on the front. The latter provides a convenient means of attaching occasional or portable equipment. The first two SRCs are equipped with balanced AES inputs on XLRs, with the third accepting S/PDIF on a coaxial (phono) socket. The fourth SRC input can be selected between a phono socket and a Toslink optical connector using the fifth and final button on the front panel, mounted between the two sockets. A pair of LEDs indicate the current input selection, and a separate row of four LEDs show the status of each SRC channel, illuminating when valid input signals are detected.
Each output from each SRC is made available simultaneously on three separate connectors, enabling output distribution and interface format conversion, if required. Four Toslink optical outputs are provided on the front panel, and both AES (XLR) and S/PDIF (coaxial) sockets are available on the rear.
Within the AES-EBU and S/PDIF data streams are many helpful bits of information, and one of these is a flag which can be used to indicate the sample rate of the material. There are various standard possibilities, such as 44.1kHz or 48kHz, and another option is to say "I'm not telling!", known as 'sample rate not indicated'. During the initial testing of the M-Clock, this little flag caused some rather odd problems. Everything seemed to work perfectly, except that a Tascam DAT machine would not accept a sample-rate converted signal — almost every other manufacturer's equipment would.
The reason was soon discovered, however. In their default, stand-alone state, the SRC chips pass through the channel and user status data unchanged. This was handy from the point of view of passing ID markers, and so forth, but it also meant that if the source was indicating a sample rate of 44.1kHz, say, it would still say 44.1 even after sample rate conversion to 48kHz! Most current equipment seems to ignore the indicated rate, and simply measures the incoming data rate for itself, hence no problems. However, it appears that Tascam do the job properly, and when their machine detected a disparity between actual and indicated sample rates, it refused to play ball!
Fortunately, a solution was easy to find. A very simple wiring change on the M-Clock circuit board enables the SRC chip to blank the incoming channel and user status bits, and so the output signal simply states 'sample rate not indicated'. Everything seems to be happy with this condition, and simply measures the rate for itself. Problem solved: Drawmer one, Gremlins nil.
Plugging the M-Clock into a digital system is very straightforward and logical, but does require several word-clock and AES or S/PDIF cables to convey the clock signals to the various digital devices. This is no place to skimp on quality — inferior cables will introduce jitter and will completely undermine the performance benefits which might be achieved by using a master clock in the first place! So, make sure you use 75Ω digital or video coax (not aerial downlead) cable for the word-clock signals, terminated with the correct type of BNC connectors, and either 75Ω or 110Ω cables respectively for the S/PDIF and AES digital black references (if used). Likewise, use good-quality cables to hook up any free-running equipment through the sample rate converters and on to the digital mixer's inputs.
Once plumbed in, all you have to do is select the appropriate clock rate on the M-Clock and configure the various items of digital equipment to reference to their external clock inputs. You should then have a stable, accurate and completely synchronised digital system with significantly reduced jitter artefacts.
For my editing and mastering work I use a lot of digital equipment which is normally all referenced to an Aardvark Aardsync II master clock. Comparing the audible and measured results after substituting this for the M-Clock suggested that the latter's reference stability and jitter performance is only marginally below that of the far more expensive Aardvark unit. In fact the M-Clock gave very little away in terms of a poorer image stability, which is a very impressive result indeed given the M-Clock's significantly lower price. I also compared the performance of the M-Clock with the clock output of my Apogee PSX100 converter — a unit known to have a particularly good internal reference clock — and found it impossible to distinguish between the two units.
Given the UK retail price of the M-Clock, it represents remarkable value in comparison with the other master clocks on the market — and even more so given the built-in quartet of sample rate converters. Anyone using a digital sound desk and more than a couple of digital sources will almost certainly benefit from installing this intriguing little unit.