Some clever lateral thinking from Fostex has resulted in a neatly-integrated mixing system which is easy to use and doesn't break the bank. Martin Russ finds out that mixing with MIDI is more fun than you might have imagined...

TV and hi-fi remote controllers are mass-market consumer items, so the cost of the components is kept low by the huge quantities used. The important part of a TV or hi-fi system from a mixing point of view is the audio system; the remote controller usually controls volume, muting, treble, bass and perhaps even the stereo balance in Nicam stereo TVs. Remote controllers are digital, so it comes as no surprise to learn that the audio part of a TV is controlled digitally — and usually a single chip does all the hard work.

We now have the two essential ingredients of innovation: a low-cost, digitally-controlled audio chip designed for use in TVs and midi systems, and the home studio market for affordable MIDI controlled mix automation. Enter the Fostex MixTab and DCM100.

The Split

The DCM100 and MixTab provide an 8-channel stereo mixer, with two separate auxiliary send/returns, and with complete automation of all controls, stored in 100 scene memories. The DCM100 is a 1U-high, 19-inch rackmounting module with a front-to-back depth of 350 mm, which means that you may need to provide it with additional support when racked. It contains all the audio inputs and outputs, as well as the gain trim controls and the LED output metering.

The MixTab is 250mm square and about 40mm deep, and looks much like any compact mixer, except that the only sockets on the rear panel are for MIDI and the mains adaptor power lead. The other give-aways are the 2-digit LED display, Store and Recall buttons, and the 65 LEDS — which flash away like a Christmas tree for a couple of seconds when you first switch it on!

The units are split so that the DCM100 deals with the audio signals, and so can be placed in a convenient location to minimise the cable lengths, whilst the MixTab can be placed where its controls can be easily reached whilst playing. A normal mixer forces a compromise between these two, which either makes the faders difficult to reach, or else results in induced mains hum because of close proximity to other equipment. Separating the control part from the audio part also means that you can use the DCM100 without the MixTab, and it then behaves like a MIDI-controlled box of VCAs. A single MixTab can be used to control up to three DCM100s, which provides 24 channels in total.

There are eight stereo inputs, which may be considered as 16 mono inputs arranged in eight pairs, depending on your application. The two auxiliary sends are stereo too, which means that you can use a 'stereo input' effects processor with the system. The auxiliary returns have the same two shelving EQ controls as the main channels: 100Hz and 10kHz, with +/- 15dB of control. All of the controls (except the input trims on the DCM100) are automated: faders, mutes, EQ, auxiliary sends and returns — and the settings may be stored in the 100 scene memories. This is rather like using a stage lighting desk, where you set up scenes during the technical rehearsals and then just recall them in sequence for the actual performances.

Front Panels

The DCM100's front panel has only nine rotary controls: eight channel trim controls (0 to 30dB) and the headphone level control. Two five-segment (three green, two red) LED displays provide limited monitoring of the output level, but no provision is made for input level metering, not even a simple overload LED. This is a pity, because setting up the channel gain properly is important to maximise audio quality.

On the MixTab (Mixer Tablet), each channel has a slider (marked from 0 to 10, but actually only effective from 1 to 9) for volume, a mute button, pan control, hi and lo EQ controls, auxiliary send control and the auxiliary send selector button. There are two auxiliary send level controls, two returns and hi and lo EQ controls. A master volume completes the 'mixer' controls.

The MixTab automation controls comprise the scene selection buttons: Up, Down and Tens/Units selection, as well as the Save and Recall buttons. The DCM select switch allows one MixTab to control up to three DCM100s. The Mode select switch and 'Smoothing' control complete the user controls.

Despite appearances, the scenes are stored in the DCM100, not in the MixTab. The two devices use the MIDI cables to communicate, so although the MixTab is a very convenient way of controlling the DCM100, you can achieve the same results via MIDI. I imagine that special drivers for controlling DCM100s from popular sequencing packages will quickly become available, but it should not be beyond the capabilities of computer-literate owners to define their own MIDI Controller setups.

When you recall a scene, the DCM100 changes its audio switching and levels accordingly, but the MixTab controls do not move to reflect this — you don't get motorised controls in this price range! The MixTab has three operational modes used to control, examine and edit the DCM100. The three modes are called: Direct; Preview; and Enable.

- Direct mode sends MIDI messages only when you alter a control. The LED associated with the control flashes yellow to indicate when a message is sent. This lets you make quick changes to a control when you don't care about what it is currently set to — and so you can get sudden jumps in volume of EQ as the DCM100 reflects the change.

- Preview mode lets you find out what the current setting of any control in the DCM100 is. When you recall a scene, the LEDs associated with the controls that have changed in value will start to flash — green if the control is lower than the DCM setting, and red if the control is higher than the DCM setting. The flashing gets faster as you move the control nearer to a matching setting, and then stops when the DCM setting and the MixTab setting are the same. In this mode, no MIDI messages are sent, so you can see how the DCM100 is set up without changing anything.

- Enable mode is like Preview mode, except that once you have reached the match setting, any further movement of the control will cause MIDI messages to be sent to the DCM 100, with the yellow flashes of the LED showing when you are actually changing settings. It could be regarded a mix between the Direct and Preview modes, except that the Preview mode lets you home in on the exact setting by moving the control between red and green until the LED goes out. In Enable mode, the control becomes active as soon as you reach the match setting.

The user interface is easier to use than describe. I would have preferred a system where the flashing gets slower as you approach the correct value, and faster as you move away, but I quickly became accustomed to the Fostex method. After a while I left the MixTab in Enable mode almost all the time, and discovered that if you flick the mode switch to Direct and then back to Enable after recalling a scene, the LEDs stop flashing, and all those controls send MIDI messages reflecting their current position — useful, and not mentioned in the manual!

MIDI

Although the MixTab and DCM100 are obviously intended to work together, you can use the DCM100 on its own as a MIDI-controlled mixer. (You could use the MixTab on its own as a general-purpose MIDI Controller, but since the MIDI Controllers it uses are on only one MIDI channel and have special meanings in the MixTab context, you would need some sort of extra processing to make it really useful — conversely, Fostex could rework the software inside the MixTab, but that would be another product...)

The MIDI messages that the two devices use to communicate are quite simple: control changes and program changes. Since almost all MIDI sequencers can cope with control and program changes, the potential for use with a wide range of hardware and software sequencers is very good. Using MIDI Controllers also has the advantage that many sequencers will chase certain commonly used Controllers, and the allocation of the lower numbered Controllers to the most critical functions (the volumes and pans occupy the first 32 controllers) is obviously designed to make the most of this.

System Exclusive dumps are not used at all, which can make the storage of sets of scenes awkward since you can't use ordinary SysEx librarian programs. One hundred scenes can be quite a lot of mix changes, and some sort of long-term storage via SysEx or a memory card slot would be useful. It is possible to record the scenes into a sequencer by using the misleadingly named Dump Request or Power-Up Dump facilities. You can set the units so that they will automatically transfer controller messages when you first power up, or whenever you change a scene, or by sending a Dump Request message. The request message is, again, not a System Exclusive message, but another MIDI Controller message, which makes sending requests from a sequencer awkward. By setting the DIL switches on the rear of the two units, you can control the MIDI channel they use, the dump requests, and the volume curve for the sliders (linear or exponential).

Digital Control

The obvious way to control audio signals digitally is with a Digital Signal Processor chip, but DSPs are often too expensive for use in TVs and midi hi-fi, despite looking impressive on spec sheets. Going digital also means converting from analogue to digital, and this can be costly for lots of channels. Conventional mix automation systems use VCAs (Voltage Controlled Amplifiers — as used in analogue synthesizers to control volume), but high-quality VCAs can be expensive, and converting digital signals into suitable control signals is not simple. A more pragmatic approach steps back and looks at the problem from a different perspective, which is where the TV chips come in.

Using digital to control analogue circuits is a good mix of the best of both worlds. High-quality analogue processing chips are relatively simple to produce, while digital circuitry is well suited to the precise numerical control which is required. The chip used in the DCM100 uses a control method very similar to that found in some of the Digital-to-Analogue Converters found in CD and DAT players.

Digital signals are used to switch resistor values in and out of circuit. The resistance controls the gain or equalisation which is applied to the audio signal, so the changes are made in discrete steps. The size of the steps is usually designed to be small, but not too small. For TV and hi-fi remote control purposes you need to have steps which are just audible, and which cover the available range of volume in about 30 steps, which gives steps of about 1dB. The result is the characteristic 'zipper' noise clicks as the resistor values are changed. VCA systems use filtering circuits to try and ensure smooth transitions in the control voltages, but the resistors in the DCM100 chip are either in or out of circuit, which means that there is always a possibility of some noise as the volume abruptly changes.

The DCM100 does have some problems with clicks as the settings are changed, but this is very dependent on the signal level at the input and the slider position — changing a slider position from 9 to 8 was much more prone to pops than from 6 to 5, for example. High audio levels make things considerably worse, which emphasises the lack of any input overload indication. Quite a lot of my equipment was easily capable of overdriving the inputs to the DCM100, and a little less sensitivity would have been useful. The 30dB of gain which was available on the trim pot was turned down to zero for all my 'line level' equipment. This suggests that some input attenuation, rather than gain, is required.

Conclusion

There have been several attempts to use MIDI to provide control over mixers in the recent past. The one from Simmons suffered from noise, while the more up-to-date Mark Of The Unicorn 7S sadly seems to have been overlooked by many potential users. Fostex have provided a new twist — a very familiar user interface in the form of the MixTab, which looks and feels very much like a compact mixer, but without the cluster of audio cables at the back. Using just a pair of MIDI cables (and the power cable) frees the tablet from many of the constraints of cabling up conventional mixing consoles, and so makes the product attractive even before you consider the other facilities that are provided.

For a good quality 8-channel mixer without automation, you could expect to pay about half of the cost of a DCM100/MixTab combination. The automation and convenience of the tablet make the extra cost well worth considering. As a closed system, the pair give an effective low-cost solution, whilst for control via MIDI, the DCM100 works with non-MIDI sound sources, and so provides a way to integrate a complete studio without the expense of a direct to disk digital audio sequencer like Cubase Audio, Digital Performer or Studio Vision.

The audio quality is generally good, though some 'zipper noise' is occasionally evident when changing gain settings while a signal is present. Some users have suggested that Fostex should have included a Mid EQ control, but the chosen arrangement was dictated by the facilities offered by the special consumer chips around which the design was based — if there is enough demand for TV and hi-fi remote control with mid tone controls, then I guess future generations of this product will have them too.

I can envisage the DCM100 and MixTab finding a wide range of uses, provided that prospective users are willing to show a little imagination in their applications. The use of the word 'scene' makes me suspect that one of the major uses Fostex had in mind when they designed it was as a way of storing and retrieving lots of complete 'snapshots' of a mixer panel — and not as a way of reproducing complex slider movements in realtime. It certainly changed my way of working with a mixer — after a couple of days, I began to use it almost like a patchbay!

This could well be the product that finally changes the way in which people view mixers. I can remember the way sound synthesis changed when monosynths changed from having one memory (the control knobs!) to having lots of instantly accessible memories. Because of this one innovation, the synthesizer rapidly evolved, from an experimenter's fun machine, into a serious and usable musical instrument. Fostex may have just done the same for the mixer — in a couple of years you may well wonder how anyone ever managed with a 'memoryless' mixer!

Further Information
Fostex DCM100 £449; Mixtab £299. Prices include VAT and are correct at time of going to press.

Fostex UK Ltd, (Contact Details).

The inside of the DCM 100 is dominated by the large single-sided PCB which holds the majority of the circuitry. The input and output jack sockets have their own smaller PCBs, and these are connected to the main board by cables. There is a large number of cabled interconnections: more than 60 in total. A flexible metal shield is used to try and separate some of these wires from the main PCB, and this has been removed for the photograph. There is also a large number of electrolytic capacitors — over 200 in all. Hi-fi and audio design purists tend to insist on DC coupling, but I suspect that the chip which is used has forced the use of capacitive coupling between stages.

The digitally-controlled audio chip is the TDA7314, one of a family of special-purpose chips designed for use in TV and hi-fi equipment as the stage just before the output amplifier. The chip is made by Toshiba in Japan, and SGS-Thomson in the EC — surprisingly for a product made in Japan, the 7314s in the review equipment were made by SGS-Thomson. There are 13 of these chips used altogether: one for each channel, and a further five for functions like the auxiliary sends, returns and master volume. Each chip provides simple EQ facilities and the volume control for a stereo channel. Additional switching is provided for each channel by 4066 CMOS switches — presumably for the auxiliary send switching. The 7314s are controlled from a proprietary serial bus designed and licensed by Philips, called the I2C bus.

The main processor chip is surface mounted underneath the main PCB, and the operating system is contained in a 27C256 EPROM, marked V1.02. A NiCd battery-backed 6264 type static RAM provides the on-board memory. Most of the chips used were made by Hitachi, including the SIL BA4560N amplifier chips used to provide the input gain.

The PCBs used were SRBP — a paper-based product which is a lower-cost alternative to the expensive fibreglass based boards typically found in more professional products. All the components were through-hole mounted, with the exception of the main processor. An additional PCB contained additional chips for interfacing to the I2C bus, and looked like an afterthought. The large number of wire interconnections were also unusual — they seem to be designed to bring the incoming audio signals from the rear panel to the front panel input gain controls. I would have preferred long spindles on the front panel controls, with the PCB rotated through 180 degrees, so that the input sockets on the rear panel were near the input gain controls.

The MIDI sockets had some strain relief, but the holes on the PCB intended for EMI suppression ferrite beads had been linked with wire instead. The jack sockets on the rear panel were mounted onto two smaller PCBs, and were probably the best engineered part of the design.

Overall, I found the build quality to be slightly disappointing, particularly the lack of any provision for ease of servicing: lots of untidy audio cabling; the awkward to remove (and replace!) flexible shielding; the main processor inaccessible on the underside of the main board, the lack of EMI protection on the MIDI sockets; and the add-on 'DCA Interface' PCB which half covers the operating system EPROM. However, viewed in the same context as consumer hi-fi equipment, rather than professional audio equipment, the standard was comparable with many of the mid-range CD players, cassette decks and amplifiers that you would find in any high street electrical store, which explains the remarkable price-to-functionality ratio.