Powertran MCS1 (Part 5)
Part 5. Tidying Up
Co-designer Tim Orr takes us through the mechanical construction and front panel controls of the MIDI Controlled Sampler.
We conclude our coverage of the build-it-yourself MIDI Controlled Sampler with mechanical assembly details and a quick run-down of the unit's front panel controls.
We've now covered most aspects of the MCS1's design, performance and construction, but one element of the last-mentioned topic that hasn't yet been dealt with is the mechanical assembly which, though not in itself particularly demanding, has to be done with care if you're to have a unit that works as it should do and, just as important to many, looks the part as well.
E&MM's over-worked Technical Illustrator, Len Huxter, has done such a good job on these particular drawings that, to a large extent, the diagrams speak for themselves, and no additional explanation is necessary. Figure 1 is a case in point. This shows a plan view of the interboard wiring that has to be undertaken before the various PCBs within the MCS1 will get to be on speaking terms with each other: the picture says it all.
The front panel wiring is a little bit more involved, as a quick glance at Figure 2 would suggest. The problem here is that what was originally a multicolour (one colour for each connecting wire) illustration is now a decidedly monochrome one, so it isn't perhaps as easy to read as it could be. Note also that VR3, labelled on the diagram as Pan, carries the name Mix on production Samplers: the change in nomenclature is due simply to a change of heart over how the pot's function could best be described, as the panel description on the pages that follow should explain.
The transfer of Figures 3(a) and 3(b), which together show the mechanics of how to wire up the unit's power supply, from original colour sketch to printable diagram also posed problems, in that the two drawings you see here form one super-diagram in the author's original. The split is a logical one, though, with (a) showing the connections between the transformer and the PCB and (b) illustrating the wiring between transformer and the MCS1's back panel. With luck, the printed results should be even clearer than they need to be.
Figures 4 and 5 are detail illustrations of mounting procedures for the front panel and main PCB (DV2) respectively. These are pretty self-explanatory, but the drawings that follow aren't quite so easy. Figure 6, for instance, is a view of the inside of the front panel, with the display filter as the focus of attention: don't forget to peel off the protective covers, from both sides of the filter before you attempt to fit it, and once you've done that, the device should be installed with its matt side pointing outwards and its gloss side in, with a 3mm gap left at the top of the panel. Trim off any excess double-sided tape with a sharp knife to complete the job.
The installation of the spin-wheel controller (RC300) poses similar difficulties, though like he display there's only one of it, so things aren't nearly as serious as they might be.
Figure 7 is obviously a help here, but there are a few more points worth making concerning the exact sequence of operations. The first task is to pack out the inside of the controller with a surplus jack-socket washer, after which the wheel can be trial-fitted to the front panel. Mark the controller's shaft as close to the panel as possible once it's fitted, remove the controller and cut to the appropriate length - you can now fit it for real. One last mechanical detail is the fitting of four rubber feet to the underside of the MCS1 cabinet, with a further two beneath the main PCB: Figure 8 shows the approximate positioning of the latter.
Our last port of call is an explanation of the MCS1's controls and connections - what they do and why they're there. We'll start, logically enough, with the Audio In connection, which is made via a standard quarter-inch jack socket. A pushbutton is used to select low (1K5) or high (28K5) impedance levels, which should cover most operational eventualities. Audio Out is similarly connected, its socket being echoed (no pun intended) on the rear panel - another pushbutton selects zero or 20dB attenuation.
The four rotary pots at the left of the Sampler's front panel are easily explained. Level is a continuous gain control operating on the input signal, Repeat controls the amount of internal feedback around the delay line and hence the number of repeats produced, Mix alters the proportion of direct signal in relation to the output of the delay line, and Tune is a fine overall pitch control. The Controller on the panel's extreme right is a bit more complicated, as it's used to alter the value of whichever parameter has been selected. It has a 360-degree rotation, and is capable of incrementing or decrementing up to 50 steps per revolution, depending on the parameter in question. Just left of centre on the panel layout is a four-digit (0.56", seven-segment LED) display, and this shows both parameter values and other information of importance to the user.
The remainder of the MCS1's front panel is occupied by no fewer than 24 selector switches, and I can think of no better way of explaining these than going through each of them one by one.
The Freeze button is what you use when you want to prevent any further data being written into the unit's memory, so that any sound already in there is frozen. The switch operates only in Delay Line mode and works in conjunction with the Freeze input on the rear panel. Like all the selector switches, it incorporates its own LED, and this is used in Voice mode to indicate when a recording is in progress. The light remains off while the recording is actually taking place, but illuminates once the sound has been successfully stored in memory. Moving to the right along the front panel, Click Track selects the unit's built-in metronome, capable of superimposing 16 beats onto the audio output for every complete trip around a full memory length, though obviously the shorter that length, the smaller the number of clicks the circuit will produce.
The two blue switches that follow are used to select Delay Line and Voice modes respectively. In the former, the MCS1 reads and writes from memory continuously, thereby generating a continuous cycle of sounds/echoes. As we've just seen, these sounds can be frozen or, should you so desire, transferred to Voice mode operation, in which their pitch and duration can be played from a connected keyboard. The red Record button comes into play in Voice mode, and its operation is worth describing in some detail. Once your finger (or whatever part of the body you happen to be using) has made contact with the button, the associated LED starts to flash at a rate of about four times a second, indicating that the MCS1 is ready to begin recording. Said recording commences at the onset either of an audio trigger or the Record button being pressed for a second time, and it's at this point that the Record LED turns continuously on and the Freeze LED continuously off. And when the recording is complete, the status of these indicators is reversed and the stored sound can be replayed via either a keyboard or the Play switch that lives next-door, as it were.
The BBC and Down Load pushbuttons are both concerned with add-on hardware options that'll be available in the near future. The former is so called because a soon-to-be-available interface will allow MCS1 sounds to be stored within the memory of a BBC Micro, and the pushbutton will be used to access this facility. Moving on, the NR switch is already fully operational and is used to activate the MCS1's built-in noise reduction circuitry, which brings about a useful audible removal of otherwise bothersome quantisation noise.
The Coarse, Medium and Fine switches are used to select the sensitivity of the spin wheel controller, something that will obviously vary depending on the parameter whose value is being modified. The operation of the Sample Speed selector isn't quite as straightforward as its name might suggest. On the MCS1, the speed is expressed as a variable number with a value between 128 and 4095, which represents the value needed to program the internal divider chain. Thus, contrary to all expectations, 128 is a fast sample speed and 4095 a rather slower one. Whichever speed you select, it's indicated on the LED panel display, so you always know precisely where you stand.
Like the Sample Speed selector, the RAM Size button functions in Delay Line mode, and in this case the display shows the parameter as a number between zero and 6553 (actually, 65,536, but the display can only show a maximum of four digits, so you'll have to use your imagination a little). And once you've discovered what the current RAM size is, you can modify it using the Controller. Another somewhat self-evident selector is Bypass, which simply routes the input signal to the audio output, thereby ignoring the antics of the MCS1's processing entirely.
If you intend connecting the Sampler to a controlling keyboard, then a working knowledge of the Gate* Trig and MIDI* CV selectors is essential. Both induce Voice mode functions, and as its name might imply, the first switch is used to select whether the sound stored in memory is gated (* = LED on) or triggered. Two gate modes are available, one that turns off abruptly and a second that has a half-second decay tail on its end. The display shows S for a short decay time and L for a long one. An unilluminated LED tells you the sound will be triggered, which means that although it's started by the gate signal, it'll play itself automatically to the end of the memory. For the non-synthetically literate, a gated signal can be put through a looping process, but a triggered one cannot. The second switch selects whether MIDI (LED on) or CV connections will be used to relay pitch information to the MCS1. In fact, pitch, sound duration, pitch-bend and vibrato are all controllable via MIDI, the note(s) played being indicated in MIDI code on the LED display. On switch-on the MCS1's MIDI channel number will default to 01, but all you do to alter this to any of 16 channels is hold the selector switch down and rotate the Controller accordingly.
We've already seen that the sample speed of the unit can be varied continuously as a value of N from 128 to 4095, but there is in fact an alternative way of changing that speed. The Pitch Shift button lets that same value be altered in semitone increments, from 12 (low end) to 80 (high end), and again, it's the spin wheel controller that's used to do the value changing. Looping is quickly and simply achieved on the MCS1, using the Loop Start and Loop Length selectors. The former defines the point in memory where the loop begins, while the latter determines the 'jump-back' length. If you're foolish enough to set a loop length greater than the loop start (or to put it in layman's terms, you ask a loop to jump back to a position that's actually in front of its starting point), the display will show all its decimal points lit up at once, and the unit will automatically reset the jump-back address to zero.
The four remaining selectors (yes, the end is well and truly in sight) operate in conjunction with the MCS1's internal tracking filters and sweep oscillator. Not surprisingly, Filter Offset allows said filters to be offset by an octave less or an octave more than the system sample speed. The chosen figure is displayed by the LED network as a number between zero and 12. Sweep On-Off is used to, er, turn the sweep on and off, and Sweep Range acts as a level control for the oscillator's sinewave modulation. This is shown on the display as a figure between zero and 100, while the value set by the Sweep Speed pushbutton (in conjunction with the good ol' Controller, of course) is displayed as a figure between 1 and 100.
In addition to the somewhat insignificant Power switch, the MCS1's derriere houses a fair complement of connecting sockets, a couple of which - the Audio Out and Freeze In jacks - have already been mentioned. As for the rest, the CV and Gate In connections are also quarter-inch jacks: the latter carries a high TTL (+4V) signal to generate a gate command, and can be used either in conjunction with the CV socket for control via a one volt per octave keyboard, or on its own as a trigger input from a drum unit of some description.
Five-pin DIN connectors take care of MIDI In and Out (note that the latter does not function as a MIDI Thru connection) and the BBC In-Out socket that connects the MCS1 to a BBC Micro via the custom-designed interface unit. Finally, the Master-Slave and Serial Bus multiway connecting points are both intended for use with future options, so you needn't worry about them for the time being.
Gear in this article:
Feature by Tim Orr
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