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Digital Sampler/Delay (Part 3)

Part three of this fascinating project from Paul Williams.


In this concluding article, Paul Williams rounds off construction, goes through the setting up procedure and describes how to get the best from this versatile module.

Having detailed the construction of the analogue board last month, it only remains to assemble the digital (logic) board, and the memory board.

Assembly of the these is eased by the use of double sided, plated-through PCBs. For those not familiar with plated-through PCBs, every hole is plated on it's bore so that a circuit is maintained from one side of the board to the other at every pad position. This means that track pins are eliminated, and soldering is not required on the component side of the PCBs.

(Click image for higher resolution version)


Logic Board



Assembly of the logic board may therefore begin with the lowest profile components such as resistors, followed by IC sockets, then capacitors, transistors and the ceramic resonator, X1. Take care with the polarity of the electrolytic capacitor and the IC sockets, but leave the ICs themselves out until later. Insert according to the parts list, and the overlay printed on the PCB itself, a few at a time to prevent crowding, then solder and crop. Solder the buss connector and the 34-way male expansion connector in place, ensuring that they are pressed firmly down onto the PCB. The regulator, IC41 can now be positioned, forming its leads down so that when inserted into the PCB, the tag hole targets on the PCB hole. Using a short spacer over the regulator tag, fix in place using a self-tapping screw from the underside of the PCB. The regulator leads can now be soldered and cropped.

Trim each pot shaft to 8mm from the bush using a hacksaw, whilst holding the pot shaft in a vice, or just use a pair of cable cutters. Fit a PC bracket to each pot and locate into their respective PCB positions, but don't solder at this point. After determining the correct orientation of the LEDs, bend their leads down at right angles to suit the PCB holes, 4mm from the body of the large LED and 6mm from the body of the small one, and locate into the PCB without soldering. Place the shakeproof washers on the pots, then offer the front panel up, feeding the pot bushes and LED domes into the appropriate panel apertures on the right-hand (Trim, Start, Length & Decay control) side. The panel is then fixed in place by means of the pot nuts which should be fully tightened. The pots, brackets and LEDs can now be soldered having made sure that the pots are fully home, and that the panel is square to the PCB. Fit keytops nice and squarely to the two push switches and position them on the PCB. Temporarily tack one of the joints on each switch with solder and check the keytop positioning in the panel apertures. If it is not central, heat the tacked joint to re-position the switch. When all is well, solder all the switch joints.

Spend some time checking over the assembly very carefully since dry joints and solder splashes are all too common in such a dense assembly, even for the experienced constructor. When you're completely satisfied with the job, load the ICs into their sockets, being careful with orientation, and taking some anti-static precautions since most of the devices are static-sensitive. Keep the ICs in their conductive packaging until the last moment and, touching both the packaging and some of the PCB tracks, transfer the ICs from the packaging to the sockets, avoiding touching the pins if possible.

Now, taking the partly assembled and checked analogue board, load it's ICs into position, again taking static precautions as some of these are static-sensitive. Offer the analogue board up to the logic board so that the jumper cables can be fed into the logic board and soldered on the component side. Having done that, fold the analogue board down onto the work surface so that the pots and LEDs can be positioned on the analogue board as described above, folding the assembly back up and securing the pots to the panel before soldering. Fit the push switches as before, centralising them if necessary before soldering. An M3 screw then attaches the logic board to the free end of the long spacer on the analogue board.

(Click image for higher resolution version)


Memory Board



Having completed the main boards, the tiny memory board will seem like childs play. Again the assembly procedure consists of soldering the resistors, then IC sockets and capacitors. This only leaves the 34-way female connector which is applied to the underside of the PCB and soldered on the component side. To keep things tidy, align the 'arrowed' Pin 1 of the connector with the PCB arrow. Again taking static precautions, load the memory board ICs into position being careful with orientation.

After checking the assembly very carefully, offer the memory connector onto the expansion connector on the logic board, ensuring that the arrowed pins line-up, and secure with a self-tapping screw through the memory board into the free end of the short spacer on the logic board.

Finally, fit the knobs and caps so that the marker line of each covers the scale evenly, with equal 'dead-band' at each end.

Setting Up



No test equipment is needed to set the module up, just a sound source of some kind, a monitoring amplifier and a pair of ears!

Before applying power to the module, set the three lower top-access presets on the analogue board (all except the topmost one), and the front-access threshold preset centrally. The topmost preset should be set fully anti-clockwise. Remove the top dust cover from your sub-rack and tape a piece of card over the top of the power unit position to prevent electric shocks. Slide the module into a convenient position in the sub-rack and switch on the power. If the power unit indicator does not illuminate, or if it fades, this would indicate that there is a short-circuit fault and so the power should be switched off immediately. You then have to find a fault which thorough checking should have uncovered in the first place!

If all is well, set the Regen, Mix and Start controls fully anticlockwise and the Output, Trim, Length and Decay controls fully clockwise. Apply an instrument signal to the audio input (the top socket) and adjust the Input control so that the Lim indicator flashes on the loudest peaks. If the output socket (the third down) is now monitored, the signal should appear normally. Advancing the Mix control fully clockwise should reveal an identical, but delayed version of the input signal. Using a preset trimming tool, or a small screwdriver insulated over most of it's length with tape, slowly adjust the uppermost preset clockwise until the signal is heard to start to break-up. Back the preset off slightly to lose any hint of breaking up or crackling. The adjustment must be made slowly since the results are delayed by almost a second and a half.

Since the unit will have powered-up in the delay mode (with all LEDs extinguished), advancing the Regen control will cause echo repeats to be produced. The Length control will set the speed of repeating. With the Regen control fully clockwise adjust VR9, the preset just behind the Regen control so that the repeats sustain almost indefinitely, without building up into a 'mush'. This is best done at a moderately fast repeat rate.

Now set the Length control fully clockwise again, touch the Sample button and play your source instrument. The Sample LED will illuminate, and the Rec indicator will show yellow until the sound source triggers the unit, when red indication shows that a sample is being recorded. After a couple of seconds, the Rec indicator turns green, showing that the sample is ready to be played. The sample can then be heard either by playing the source instrument again, or by touching the Trigger button with no input signal present. Now touch the Loop button so that the associated LED lights. The sample will now repeat continuously, although it will fade unless the unit is kept triggered. The operation of the editing controls can now be proven. Select minimum Length so that the sample repeats rapidly, selecting a suitable Start position so that the repeats are distinct. Now adjust VR7, the preset just in front of the sockets so that the speed of repetition can be heard to alter. Find the band of adjustment within which the sample repeats the fastest, and leave VR7 set at a position at the centre of this band.

If an analogue synthesiser is available, the CV and Gate signals should be routed to the bottom, and second socket down respectively on the Sampler. The pitch of the sample should now be under control of the synthesiser. VR1 at the centre of the analogue board is used to calibrate the span of the CV control input so that octave changes do indeed produce one octave shifts in pitch. If the resultant change is less than one octave, adjust VR1 slightly clockwise and vice-versa. It may be necessary to trim the front access tune preset to scale the keyboard sensibly.

In Use



Once the ultimate position in the Sub-Rack has been established, the Self-adhesive socket label should be applied to the back of the Sub-Rack to cover the unused socket holes. It should be noted that since this module cannot be switched out directly (apart from by turning the Mix control anticlockwise), it doesn't form part of the rack's linking system. The link chain is however maintained around the module position, but without being processed by it.

The mode of the module is changed by touching the Sample/Delay button, above which is an indicator to show the mode selected. The unit will initially power-up in the delay mode.

The Input control is used to adjust the input signal level such that the Lim indicator glows on and off most of the time. If the indicator rarely lights, then the dynamic range of the unit is not being adequately utilised, but too much input drive will cause an almost continuous glow when the limiting action may become noticeable. This is not necessarily disastrous though since the limiter adequately protects against overload distortion, and the resulting compression may sometimes be desirable. The Output control allows for any necessary make-up gain or loss.

The modulation input socket can be used with a modulation oscillator module to achieve vibrato or, in the delay mode, animated ADT and chorus or modulated delay effects. In the sample mode, modulation is best used during sample playback, rather than during recording when the effect would be irreversible.

Delay Mode



When the Sample/Delay indicator is extinguished, the Delay mode is selected allowing ADT, echo and doubling effects to be produced. The Length control sets the delay time, the Trim control being used for fine adjustment. Bear in mind though, that full signal bandwidth is only achieved when the Trim control is fully clockwise. Sustain of echo repeats is determined by the Regen control. Almost infinite echo repeats should be easily attainable. The output can consist of all-dry signal, all-effect, or a mixture of the two as determined by the Mix control. If the effect is to be mixed at a desk, then an all-effect signal is obviously the order of the day.

The Decay and Start controls and the Overdub, Loop and Trigger switches have no effect in the Delay mode.

Sampling



The unit is armed for recording a sample as soon as the Sample mode is entered by touching the Sample/Delay button. This record standby mode is shown by the record status indicator turning yellow. As soon as the input signal amplitude is above a preset threshold level (or immediately on entering the sample mode if the signal is of sufficient amplitude), the record status indicator will turn red indicating that a recording is being made. The threshold level may be adjusted by means of the front panel accessible 'thresh' preset. Recording can also be initiated by applying a trigger pulse to the Gate input from a synthesiser for example; or if a dummy plug is inserted into the gate socket, the trigger button can be used, the dummy plug being necessary to prevent the input signal from auto-tripping.

Figure 5. Sampler Logic.
(Click image for higher resolution version)


When taking a sample, the Trim control would normally be set fully clockwise so as to achieve maximum bandwidth, although a lower setting could be used for very long samples. If the recording was unsuccessful, then touching the Sample/Delay button twice will once again arm the unit ready for recording. When a sample has been successfully recorded, the Sample/Delay button should be left well and truly alone otherwise the sample may be lost forever. So as to make all the memory available for sample storage, the Start control should normally be fully anticlockwise, and the Length control clockwise, which will result in a sampling time of between 1.4 and 8 seconds depending on the position of the trim control, although more on this later.

Playing the Sample



After recording a sample, the record status indicator will be green, showing that a sample is ready to be played. If the input signal is still present and there is no jack in the Gate socket, then the sample may begin to play immediately. This can be stopped by turning down the Input control. The sample can then be played either by touching the Trigger button, by applying a trigger pulse to the gate input, or by applying an electronic drum pad signal to the audio input. The latter will also allow the sample to be played dynamically, in which case the Input control is adjusted so that hard 'whacks' on the pad cause the Lim indicator to flash. The 'thresh' preset plays an important role here since a good dynamic range on the played drum sound will depend on very sensitive triggering. The sensitivity will be limited by the pickup of extraneous sounds or vibrations, especially those from another nearby pad. A drum machine with separate instrument outputs could also be fed to the audio input to trigger percussive samples, orchestral 'stabs' or at least n-n-nineteen other sounds.

Samples would normally be played with the mix control fully clockwise so that any input signal is not passed to the output. However, it may be that some 'live' sound could be usefully mixed with the playing sample, such as when an analogue synthesiser is used to control the sample pitch. By feeding the synthesiser's output into the sampler input, the two could be mixed. This is also true when a drum pad or drum machine is used to trigger a percussion sound, where some of the sound from the pad or machine could be mixed with the sample.

The pitch of the sample can be tuned using the Trim control, or played by an analogue synthesiser via the CV input. The front accessible Tune preset is used to scale the synthesiser keyboard. The controllable pitch extends both above and below the sampled pitch, even if the Trim control is hard over to either extreme. If your synthesiser is not calibrated to precisely one volt per octave, then the sampler response can be trimmed to suit using the screwdriver adjustable preset which is half way down, and at the centre of the left-hand circuit board. The top dust cover will have to be removed for this, although care should be taken to cover the power unit end with card to prevent electric shocks. The adjustment is best made with a proper trimming tool, although a small screwdriver insulated along most of its length with tape will suffice.

A gate connection will usually also be used when using synthesiser control.

Figure 6. Sampler Memory and Control Circuit.
(Click image for higher resolution version)


Editing and Looping



The Start control can be advanced to skip over any unwanted sounds which were recorded just after the trigger point. The Length control then determines when the sample terminates. The Decay control comes in very useful for tidying up the end of an abruptly finishing sample.

Since this module boasts a generous sample time, many sounds can be recorded in their entirety, eliminating the need for looping. Some sounds are difficult or impossible to loop with undetectable edit points, although driving the limiter hard during recording may assist here by flattening out the dynamics to a certain extent. The golden rule is: only loop if you really need to.

When Looping is selected using the Loop button (as shown by the indicator above the button), sample playing commences immediately after a recording has been made, although the looping signal will fade away unless the unit is triggered as previously discussed. Looping is also useful for setting up the editing points, even if the sound is not going to be looped finally. If the Length control is turned anticlockwise, the Start control can be used to rapidly 'search' through the memory for suitable editing points. If the editing controls are set such that the length is greater than the memory left after the start position, then part of the sample at the beginning of the memory will be 'tacked' on to pad the play time out to the selected length.

A suitable start point for looping will usually be immediately after the initial attack period when the harmonics have started to settle down and the amplitude is reasonably stable. The loop length should be as long as possible to avoid 'burbling', although the limit will be determined by how quickly the sound changes its amplitude or harmonic content. Further adjustment of the editing controls may then be necessary to match the butting ends to eliminate clicks generated by steps. The Start control should be used for final trimming since it has slightly better resolution than the Length control.

Each time a looping sample is gated, a length of sample at zero memory will be played before the loop so that the initial attack of the recorded sound is preserved. When the synthesiser key is released, looping continues during decay, the rate of decay being controlled by the Decay control.

Overdubs



To avoid clicks being generated, the overdub button should only be pressed in the non-loop play standby mode (with the record status indicator green). Pressing the Overdub button will arm the unit for recording just as before, except that this time, the signal in memory can be preserved by advancing the Regen control. The mix between the new and old signals put into memory is controlled by Input and Regen respectively. Several overlays can be built-up in this manner. New recordings can be made to appear in a different key even if the recorded pitch is different by adjusting the Trim control. Alternatively, differently tuned instruments could be overdubbed at the correct pitch using the Trim control. Any plug in the CV input should be taken out when tuning overdubbed samples since the sampler does not respond to keyboard control during recording; only during playback.

Several samples can be stored simultaneously by using the editing controls to select different parts of memory. The position to be recorded into is set up on the editing controls whilst playing that position to see that it is vacant. Sounds could also be selectively deleted in this way. The Sample/Delay button must not be used since the memory will become corrupted. Instead the overdub button is used to set the record mode with the Regen control anti-clockwise. Different sounds could be chained together in this way for playing in a string or sequence.

Saving and Loading



To preserve a hard-won sample for future use, it can be saved on any analogue tape machine by playing the entire memory contents. The module should be in the non-loop mode with the Start control anticlockwise and the Length control clockwise. Once the tape machine has been set to record, the Sampler can be triggered to blurt out its memory contents.

To subsequently load the sample back from tape, set the tape machine just before the sample, touch the sampler Sample button and start the tape playing. The auto record trip facility will then start the loading process when the sound begins. For samples which have a slow attack which adjustment of the threshold preset can still not adequately capture, you can either manually trigger loading just before the sound starts, using the tape counter as a guide or, if you can record in reverse by turning the tape over, record a short blip just after the reversed sound so that when playing the tape normally, the blip will appear just before the sample, and will thus trigger the sampler. The blip and any delay can then be edited out using the Start control.

The Modular Effects Rack series takes a short break next month.

The Digital Sampler/Delay module is available from: Paul Williams, Tantek, (Contact Details). The price inclusive of VAT and postage (within the UK) is £219.95 in kit form, or £299.95 ready-made and tested. Further information on the modular effects rack system can be obtained from the above address, or by 'phoning (Contact Details).

Digital Sampler/Delay - Specification

Dry Signal
Frequency response (-3dB) 12Hz to 35kHz
Output noise (unity gain) -98dBm(A)
Treated Signal
Frequency response (Trim clockwise) 15Hz to 15kHz (-3dB)
Delay/sample time (Trim clockwise) 15mS to 1.4 sec
Delay/sample time (Trim anticlockwise) 90mS to 8 sec
CV range 6 octaves
Input dynamic range 87dB
Converter dynamic range 72dB
Headroom above limiting 87dB


Series - "Digital Sampler/Delay"

This is the last part in this series. The first article in this series is:


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Part 1 | Part 2 | Part 3 (Viewing)


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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Apr 1986

Donated & scanned by: Mike Gorman

Topic:

Electronics / Build


Series:

Digital Sampler/Delay

Part 1 | Part 2 | Part 3 (Viewing)


Feature by Paul Williams

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> Analogue Equipment Design

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