When working in studios with an automated mixing desk, it's easy to be mesmerised by the sight of faders moving up and down without any human intervention, almost like some Great Producer in the Sky trying to taunt us to do better with some celestial dance of the faders. That impression shouldn't detract from the fact that an automix facility is one of the most important developments in studio technology since the multi-track tape recorder. This is particularly true in the case of much recent electro-music, where a complex interplay of sounds and textures can only be achieved with non realtime techniques of sound manipulation. However, the cost of a typical automix facility has generally put it in the realms of fantasy for all but the top-flight 24-track plus studios who can afford the £100,000 or so for a Neve computer-mix console. That's until now, for the CPE-800 Compu-Editor really provides an economic alternative.

A Break With The Past

Roland's studio equipment is a recent addition to their range, but hitherto has been confined to 'effects' (the Roland Rack) or microprocessor control of analogue synthesisers (the MC-8 Micro-Composer), rather than units directly concerned with the recording and playback chain itself. So, the Compu-Editor is something of a break with the past, and its existence reflects initiative on the part of Roland in America rather than Roland in Japan. In fact, the original American design was conceived for computer-controlled mixing of a multi-keyboard set-up, but the gestation period resulted in a much more generally applicable unit. A valuable aspect of the Compu-Editor is that it is designed as an update for any existing studio. With a RSD 16 into 4 desk and a TEAC 8-track machine, the Compu-Editor instantly adds the sophistication of computer-mixing.

The unit has been out for about 9 months, but it has only been available in this country for the past few months. At the moment, the Compu-Editor is being sold by only Roland themselves and Turnkey. Whilst it normally retails for around £3,500, Turnkey are offering it at a considerably reduced 'special price', and will also set it up in the studio of an interested customer, in order to give them an in situ demonstration of its capabilities.

Design

The Compu-Editor is split into two units: firstly, the CPE-800 control unit which houses the 15 faders, the microprocessor, 32K of RAM, function controls, time and memory displays and controls, and a SMPTE time code generating program with read and write capabilities; secondly, there's the VCA-800 unit which consists of 15 VCAs receiving control voltages from the control unit and, typically, interfaces with a mixing desk via individual channel outputs and monitor returns. The faders operate by changing the DC voltage fed to opto-isolators. The light-variable resistance in turn adjusts the gain of a subsequent amplifier. Each resultant voltage is fed to a demultiplexing A/D converter which translates the 0-10 V fader setting into binary code, representing 128 steps from +9 dB (10V DC) to -36 dB (1V DC). 110 steps of 0.39 dB provide more resolution in the working area of the faders.

Since brightness changes in an opto-isolator are not instantaneous (off/on takes 10 msec, on/off takes 100 msec), control of the amplifier passes from the opto-isolator to a FET circuit for instantaneous gain switching. The fader position code is stored in the onboard RAM and this can be read, edited, or saved onto tape as the situation dictates.

The CPE-800 generates the SMPTE time code used world-wide and this enables the control unit to lock onto and synchronise fader movements. The data resulting from addressing pages of RAM relevant to a particular mix is a multiplexed after D/A conversion and the 15 control voltages outputted via multi-core to the VCA-800 unit. The VCAs in this unit also work around opto-isolators, and, unlike many other VCAs that demonstrate audible level clipping, these modules are really quiet. The excellent specifications for the VCA-800 attest to this: -95 dBm for noise, 80 dB (10 kHz) for crosstalk, and 15 Hz to 30 kHz - 1 dB for frequency response.

Use

To start the automix, it's necessary to transfer the SMPTE code from the Compu-Editor to a spare track on the multi-track machine that you're using. The timing track is then fed back to the Compu-Editor, and, if the two machines are started at about the same time, the Compu-Editor locks onto the SMPTE code ensuring sync with the audio tracks on the multi-track. On selecting 'start', the time display shows the time read from the tape track in hours, minutes and seconds.

In addition, there's a memory percentage display that registers the amount of available memory left for mix storage. In order to program a mix, the 'write' mode is selected on whatever channels you wish to work with. Engaging the 'start' button results in the SMPTE code clocking the display upwards and puts the operator in the real-time mix record mode. As the faders are brought up, so the memory display starts to count down. The rate at which the memory is used up is totally dependent on the number of channels that you're using and the activity of the faders. Thus, if you were doing a manic mix-down for a piece of post-Stockhausen electronic music, you'd probably find yourself (or, rather, the Compu-Editor) running out of mix time pretty rapidly. On the other hand, if you elected just to have a simple fade-in and fade-out, and very little else moving during the course of a mix, then you could theoretically go on for about 100 hours!

The mix can either be done in one sitting (true real-time mixing) or as a number of mixes spread over the 15 channels (overdub mixing). In the latter case, the tape motion is halted after working with an initial set of faders, 'stop' engaged to process data and reset the CPE-800, and the tape rewound. The next group of faders is then selected with 'write', and the tape recorder and Compu-Editor started again. In addition to fader data, the 'write' mode will also allow a 'mute' instruction selected on particular channels to be sent to RAM. The 'mute' function switches off the channel FET and the resultant 0V DC is converted to the lowest fader code.

Having finished a mix, the unit is readied for playback by engaging 'read' on all channels. Pressing 'start' results in data being read from RAM and relevant control voltages being outputted to the VCA-800. Since the faders don't actually move when a stored mix is read, a couple of level comparators are built into each channel for LED indication of whether the fader position is higher or lower than the setting being recalled from memory. This facility also enables the operator to rewrite mix data by using the comparator LEDs to match levels with a particular point on the previous mix and then 'punch in' a new fader movement.

The contents of onboard RAM can be saved onto cassette and loaded back, though, with a normal mix, it's possible to get about 4 different mixes in the internal memory without having to dump onto tape.

By connecting an oscilloscope to X and Y gain output sockets on the back of the CPE-800, it's possible to see a fader position display that graphically shows the setting of all 15 control voltages at once. Although not required in the 'manual' or 'write' modes, this facility is fairly essential when reading a mix from memory, especially if one wants some idea of whether an increase in dynamic level is due to an increase in performer dynamics or the action of a fader. During playback, it may often be necessary to override a particular fader, i.e., returning to real-time control, and a 'manual' key results in the position of a particular fader setting the gain signal to the VCA-800, regardless of the computer mode.

As well as the SMPTE mode, there are also two other modes of timing: firstly, the 'internal clock' mode, which results in the timing sync being derived from the Compu-Editor itself, rather than from a timing code on the multi-track, and also allows the use of a 'halt' function for event recording of a complex mix; secondly, there's a 'scene' mode, which allows scenes or patches to be set up and recalled in sequence. The three modes, 'SMPTE', 'internal clock' and 'scene', are selected by a timing mode select switch, which, for some curious reason, is situated on the back panel of the CPE-800.

Aside from the automation update application of the Compu-Editor in budget studios, there's also no reason why it shouldn't find favour with larger studios wishing to expand computer mixing facilities. Furthermore, the fact that the CPE-800 provides control voltage outputs means that it is equally suitable for, say, providing a computerised lighting board in theatres.

Thanks to Andrew Stirling of Turnkey for demonstrating the Compu-Editor. Anyone interested in knowing more about this excellent unit, or seeing it in action, should contact him on (Contact Details).