MusicNet AD24 & DA24

You've got a digital desk and a high‑resolution computer workstation, but how do you input 24‑bit source material? Hugh Robjohns checks out MusicNet's answer.

MusicNet is a Swiss company who mainly specialise in the distribution of music technology equipment, including products from JL Cooper, MOTU, TLA, Friendchip, Emagic and others. However, in response to increasing customer requests, the company decided to produce their own low‑cost, high‑resolution 24‑bit digital converters for use with the growing number of PC and Mac‑based audio/music workstations.

The AD24 analogue to digital (A‑D) and DA24 digital to analogue (D‑A) converter are both 8‑channel, 24‑bit units designed to connect with any ADAT‑compatible PCI soundcard such as Korg's 1212I/O, the RME DIGI32/8, Alesis' ADAT Card, the Sonorus STUDI/O, and the MOTU 2408. The interface between the soundcard and the converters uses the ADAT‑format TOSlink optical fibre (electrically isolating the analogue audio completely from hostile computer environments). All the analogue connections in and out of the units are standard, balanced, professional line‑level signals on XLRs. External wall‑wart transformer units are supplied to power the converters.

Both the AD24 and DA24 are half‑width rackmounting units, allowing a complete 8‑in, 8‑out installation to occupy 1U of rack space. The converter technology is based around Crystal Semiconductors' 24‑bit chip sets, which seem to be becoming the industry standard for top‑end hi‑fi products, as well as for a wide range of professional and semi‑pro digital audio equipment.

AD24

The front panel of the A‑D converter features three buttons, a power switch and a collection of LEDs. The left half of the panel carries the signal present (green) and peak (red) LEDs for each of the eight inputs. The assumption here is that as accurate digital metering is provided on the workstation there's no need to duplicate it on the converter, which would only add to the cost. To the right, the three push buttons all have associated status LEDs, the first providing a Calibrate function which optimises the linearity of the A‑D converter chips. The second button selects the sample rate (48 or 44.1kHz), and the third selects clock source (internal or external).

On the rear panel eight XLR sockets are crammed as close together as they could possibly be, making for a tight fit with the slightly fatter style of XLRs produced by some manufacturers. Squeezed in on the left‑hand side are the 15 VAC low‑voltage co‑axial power socket, the optical ADAT‑format output port, a BNC word clock connector, and a chassis earthing terminal. The unit is not grounded through the power supply or its optical link to the computer so the metal chassis could float (representing a possible electric shock hazard), unless it is properly bonded to a safety ground.

The use of the ADAT optical pipe to connect to the computer workstation is not only very convenient, it avoids both the nightmare of grounding and the problem of digital noise habitually encountered in computers of all types. The four‑metre light pipe supplied with the unit allows the converter to be mounted well away from the computer frame, but within reach for the Calibrate button!

As already mentioned, the AD24 employs Crystal 24‑bit converters with 64‑times oversampling delta‑sigma architectures providing better than 117dB(A) dynamic range. The specifications are most impressive with total harmonic distortion below 0.001 per cent, channel matching to 0.09dB, and common mode rejection of the balanced inputs better than 60dB at 20kHz thanks to the specialist symmetrical input stages used. The unit is unusual in that the input stages are DC‑coupled, but a digital filter in the sigma‑delta converter removes unwanted DC and subsonic signals below 20Hz.

When the unit is powered it automatically enters into its calibration mode which takes a few moments to complete. This process optimises the linearity of the converters, minimising noise and distortion. The manual recommends additional user calibration after the unit has reached a stable operating temperature or whenever the clocking is altered (either between the internal 48 and 44.1kHz clocks or when an external word clock source is used).

There are no provisions to adjust analogue input sensitivity and the unit has been designed such that a 0dBu input corresponds to ‑20dBFs — the professional SMPTE standard. The front panel LED meters are driven from the digital output signal so are completely accurate and it's assumed that the workstation metering displays would be used to optimise signal levels. Since a full 117dB of dynamic range is available, it is less critical for signals to be peaked close to the digital maximum, so a greater headroom can be afforded without significant noise penalties.

The manual suggests, not unreasonably, that the unit performs best when running from its internal clock source, although the specifications do not provide any figures on its accuracy, stability, or jitter. Thus, in most typical configurations, the AD24 would be running on its internal clock and the workstation and other connected digital equipment would be slaved to the AD24 via its ADAT output. However, although the panel does not mark it as such, the rear BNC word clock connector also outputs the internal word clock when the machine is operating on its internal clock source. This allows other AD24 units, or other equipment, to be clocked directly from the converter via a simple BNC lead.

A more flexible and professional way of working would be to use a dedicated external word clock generator to clock all equipment within the system. In this situation, the AD24 is simply switched to accept an external word clock connected to its BNC socket (in the range 40 to 50kHz). I was surprised to discover that when running on an external clock the sample rate LEDs do not indicate the rate of the incoming word clock, but merely reflect the position of the sample rate switch! I feel there's opportunity for considerable confusion here. However, when the phase‑locked loops have successfully locked on to the incoming clock signal, an orange LED next to the power switch illuminates.

DA24

The D‑A converter has been designed to visually match the A‑D unit with the same arrangement of metering LEDs, the same three push buttons, rocker switch and status LEDs.

The first push button activates a de‑emphasis filter corresponding to the standard 50μ/S15curve written into the specifications of all CD and DAT players/recorders. The pre‑emphasis/de‑emphasis facility was used in the early days of digital recording to conceal the poor noise performance of the first‑ and second‑ generation converters, although it is rarely used today. However, there is still a lot of media around with pre‑emphasised recordings, so providing the means to decode it is useful.

The second button tells the unit what the sample rate is (44.1 or 48kHz) so that its de‑emphasis filter has the correct turnover frequencies. The third button reduces the input resolution to the converters from the normal 24‑bit word lengths to 16‑bits. This allows the unit to operate with devices such as Type I ADATs without converting eight LSBs

worth of unwanted noise! Again, a Lock LED is provided next to the power switch to indicate when the device has successfully decoded and extracted the word clock from the ADAT optical interface. Like the AD24, the unit is calibrated internally so that a 0dBFS digital signal equates to +20dBu.

Using the DA24 is as simple as plugging it all in and switching on. Most users will never require the de‑emphasis filter and will rarely operate at 16‑bits (why invest in a 24‑bit converter to operate at 16 bits?) so I suspect that the front panel will hardly ever be touched. Unlike the AD24, there is no requirement (or provision) to recalibrate the system and the LED metering only confirms what you already know from the meters of the workstation.

The handbooks for both the AD24 and DA24 include a couple of very useful and well‑written technical papers discussing hum and how to remove it (by paying attention to system grounding techniques), and jitter in digital audio. Both of these are also available on the company web site too (don't be put off by the German language welcome page — the news pages and papers are all available in English).

In Practice

The MusicNet converters are well‑built, good quality units which fit the expanding niche market of low‑cost computer audio workstations extremely well. They are very simple to use (apart from the tedium of having to recalibrate the A‑D converter) and easy to integrate with the typical PCI interface cards likely to be encountered in this application. Sound quality is certainly very acceptable and although I had no 24‑bit A‑Ds to perform a direct comparison with, the D‑A unit compared very favourably indeed with other hi‑fi and semi‑pro two‑channel converters in my possession — most of which were priced far higher than the extremely modest MusicNet equipment. In fact, if you do the sums you will discover the MusicNet AD24 costs a mere £62 per channel and the DA24 only £50 a channel.

Having established the abilities of the DA24, and using familiar equipment at the front of the chain, my entirely subjective opinion was that the AD24 was easily capable of matching the abilities of the D‑A converter in the performance stakes. I did, however, find it advisable to recalibrate the AD24 on a fairly regular basis as the noise and distortion occasionally became obtrusive. However, a large part of this was almost certainly because I was continually changing clock rates and sources during my auditioning which is known to upset the converters.

I really can't fault the DA24 at this price and the only real criticism I would level at the AD24 is that its output resolution can't be reduced to 20 bits. This would have increased its flexibility by allowing it to be used with Type II ADAT recorders as an alternative to the internal converters — increasing the market for the unit considerably. However, adding extra provisions such as these would inevitably add to the cost, which would be inappropriate.