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Digital Sampler/Delay (Part 2)

More from Paul Williams on this exciting new addition to the Tantek range.


Having generally described this exciting new module for the Modular Effects Rack, last month, Paul Williams now discloses more of the intricacies of the design, and takes a preliminary look at construction.


The audio signal presented to the unit remains in it's analogue form within the confines of the signal conditioning circuit shown in Figure 4. The incoming signal is buffered with variable gain by IC16a and presented to IC16b via the pre-emphasis network C12, R35 and R36. The signal then passes via IC17: a fourth order low pass switched capacitor filter which removes any ultrasonic component in the signal which might otherwise lead to aliasing with the VCO frequency. IC18a then mixes some treated (delayed) signal to the, as yet 'dry' signal under the control of VR10 (Regen). It's the output of this stage which is fed to the converter electronics for analogue-to-digital conversion. The output level of this stage is watched over by IC20a and IC19c and d forming a full wave rectifier. Any excessively high levels cause TR2 to conduct, pushing current into the control pin of IC19a, an Operational Transconductance Amplifier (OTA). The increased transconductance of this device then increases the negative feedback around IC16b, reducing its gain and thus bringing the output of IC18a down to an acceptable level. This limiting action is indicated by the LED D9, driven by TR1.

Treated signal from the DAC is received by IC21a, a sixth order low pass switched capacitor filter. Both the filters are under control of the VCO multiplier via the FCLK signal so that the maximum bandwidth possible is available for any given delay or sample time. The OTA IC19b, along with IC20b forms a Current Controlled Amplifier (CCA) with a deemphasis network C20, R59 and R60 to negate the effects of the pre-emphasis network, and at the same time reduce quantisation noise. The CCA is used to produce velocity sensitivity as we will see shortly. VR13 allows the mixture between the dry and treated signals to be varied, the composite signal being buffered by ICb with variable gain, and passed to the output.

The output of IC16a is precision rectified by IC21b, producing a DC voltage on C18 which is proportional to the input signal amplitude. When the signal ceases, the control voltage leaks away at a rate determined by the Decay control, VR12. This control voltage is only allowed to pass to the voltage-to-current converter, IC20c and d and TR3 when the unit is playing a sample, as dictated by the analogue switch, IC10c. IC10d otherwise forces the V-C converter to produce maximum current (ie. no attenuation). IC21c acts as a comparator to detect when the input signal level is above a preset triggering level, variable by means of the Threshold preset: VR11. A plug inserted into the gate socket overrides the precision rectifier, giving control of triggering over to the gate socket.

Figure 4. Signal Conditioning Circuit.
(Click image for higher resolution version)


Control Logic



The operation of the module is timed and controlled by the logic shown in Figure 5 (next month). The Sample/Delay mode is set by the flip-flop IC24a, this being toggled by SW2 and indicated by the LED D15. This flip-flop controls the manner in which many sequences take place within the logic. The other flip-flops under manual control are IC25a for looping and IC23a for overdubbing, the latter can only be set when the sample mode is selected. Timing is predominantly controlled by the VCO, although the 6MHz clock and associated divider IC36 provide the other timings necessary for conversion and memory control. The address counters, IC37, 38 and 39 select the current memory address, this being updated at every VCO cycle. At each new address the VCO fires the monostables IC26a and b. Since the memory devices used are dynamic, they have to be refreshed every couple of milliseconds in order to keep the data intact. So that refreshing and read/write cycles do not clash, IC26b produces a BUSY signal a couple of microseconds before reading to indicate to the memory control when refreshing must be suspended. The process is similar when a write operation takes place at the end of a conversion, when the EOC signal fires the monostables IC27a and b.

When the preselected length of memory comes to an end, IC40 produces an END signal which sets the HOLD flip-flop IC24b. During the Hold period, conversions take place to digitise the Start and Length pots. The first conversion during Hold is for the Length pot, during which time the LNSG flip-flop IC28a is set. The digital word generated from the conversion is used to preset the down counter, IC40. The STAFF flip-flop IC28b then initialises the Start pot digitisation, the result of which presets the counters IC38 and 39. After this, the Hold mode is terminated, allowing address counting to continue from the start address loaded into IC38 and 39. Since IC40 counts down along with the address counters, it produces an END signal when the equivalent of the length pot digitisation has been counted through.

In the delay mode, no start conversion is produced since address counting always starts at zero. In the sample non-loop mode, the Hold state remains true until IC28a is triggered either by the TRIG signal becoming true with SAMOD true, or vice versa. The flip-flop IC25b causes sample playing to start at zero memory address when the unit is triggered and looping is selected. Subsequent loops start at the edited start point.

When the mode is toggled from Delay to Sample, the REC flip-flop, IC23b becomes set. Once IC28a becomes triggered, recording will start at the edited start point and continue for the selected length, when the END signal resets IC23b so that the recorded sample can be played. During recording, reading of the data already in memory is normally inhibited so that the setting of the Regen control does not effect the recording. Pressing SW4, the overdub switch sets IC23a along with IC23b for overdub recording. The only difference now is that the sample already in memory is played back simultaneously so that it may be mixed with the newly recorded sample using the Regen control.

Memory



Since the latest 256K DRAMs are used, the entire 64K byte of storage is contained within just two chips. The devices used, IC47 and 48, as shown in Figure 6 (to be published next month) have internal refresh address counters, so they simply have to be 'reminded' to refresh a new location at least every 16 microseconds. The RFRQ signal is raised by the control logic every ten microseconds or so, causing the memory strobing flip-flops IC44 and 45 to produce the appropriate refresh strobing sequence on each RFRQ pulse. The strobe pulses are timed using the master 6MHz clock.

Just before the memory is required to perform a read or write operation, the BUSY signal from the control logic prevents any new refreshes being initiated, whilst allowing time for any current refresh to be completed. To reduce the number of pins on the memory devices, the 16 address lines have to be multiplexed onto the eight address pins by means of IC42 & 43. The RWP signal causes a memory strobe sequence to be produced which strobes firstly the row address via IC42, then the column address via IC43 into the memory. The memory will then either read data onto, or write data from the data buss, depending on the state of the MWRITE signal.

(Click image for higher resolution version)


Construction



Although everything has been done to ease construction and setting up, the kit is not recommended for the novice kit constructor due to the obvious complexity of the unit. Although a backup 'get-it-going' service is provided by the kit manufacturers in case of problems, would-be constructors in any doubt are advised to purchase the unit ready assembled and tested.

The double-width module consists of two main PCB assemblies, namely one analogue and one digital, onto which the front panel is attached, and between which a short multiway jumper ribbon cable runs. The digital PCB also carries a 'piggy-back' memory board which at present occupies an expansion socket. Memory expansion would then be affected either by replacing the memory board, or by extending the expansion connector to an expansion module.

Construction of the Digital Sampler/Delay module is, however greatly eased by the use of double sided PCBs, and since exclusive use is made of PC mounting connectors, switches and potentiometers, there is no interwiring to do. All the constructional notes should be carefully read and understood before you pick up the soldering iron. Also, since most of the ICs are MOS devices which are static sensitive, they should be left in their protective packaging until the PCB assemblies are ready for them.

The analogue board assembly should be tackled first. Looking at the screen printed component side, insert a track pin into each of the square pad positions, being careful not to miss any. The pins are best inserted while still in 'stick' form, breaking the stick away from the inserted leading pin each time. Solder the pins on both sides of the PCB. The PCB should then be populated according to the parts list, and the overlay printed on the PCB itself. Start with the lowest profile components first such as the diodes, gradually working up in height through resistors, IC sockets, transistors and capacitors to presets. Bending the leads of wire-ended components outward at 45 degrees prior to soldering will hold the components in place without running the risk of shorting together a pair of pads. Only insert ten or so components at a time to prevent crowding, soldering and cropping the leads at each stage. The ICs, pots, LEDs and switches should be left out until later. IC22 can however be soldered in at this point. Take great care when inserting any polarised components such as diodes and electrolytic capacitors to ensure they are orientated correctly. The buss connector and the five jack sockets can then be soldered whilst holding them firmly down onto the PCB. A piece of foam rubber laid on the bench comes in handy for holding connectors and the like in place on upturned PCBs during soldering.

Now insert one end of each of the two short ribbon cable jumpers into the component side of the PCB at the appropriate positions and solder. Fix the long spacer to the component side of the PCB using an M3 screw. Check the assembly so far very carefully, preferably with an eyeglass, being on the lookout for wrongly polarised components, dry joints and solder splashes. Once you are entirely happy with the assembly, carefully load the ICs, one at a time into their sockets. Some precautions should be taken to prevent static damage, although there's no need to be frightened of them; just make sure that that the devices are left in their protective packaging until the last moment. Touch both the conductive packaging and the component side foil on the PCB whilst transferring them, and avoid, if possible touching the the IC leads. It would be preferable not to wear a nylon sweater at the time, nor is it advisable to go for a walk around on a nylon carpet prior to the transfer!

Next month, we conclude construction with the assembly of the other two PCBs.

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

Digital Sampler/Delay - Pants List

Main Assembly
Knob 8 off
Knob cap 8 off
Keytop 4 off
Front panel (punched & printed) 15-way jumper 2 off
Long spacer
Short spacer
M3 screw 2 off
Self tapping screw 2 off
Black M2,5 screw 4 off
Solder


Analogue Board
Resistors- ⅓W 5% CF unless stated
R1,68 1M 2 off
R2 51K
R3 330K
R4,49,52,61,63 10K 5 off
R5 560K
R6,9,24 1K 3 off
R7,20,21,32,33,56 22K 6 off
R8,34 150K 2 off
R10 120
R11,12,14 1K8 3 off
R13 3K9
R15,28 4K7 2 off
R22,23 2K2 2 off
R25 8K2 1% Metal film
R26,30,31 3K0 1% Metal film 3 off
R27 2K2 1% Metal film
R29 3K3
R35 27K
R36,60 5K6 2 off
R37 180K
R38,39 82 2 off
R40,57 56K 2 off
R41 39K
R42 33K
R43 120K
R44,45,50,65 47K 4 off
R46 47
R47,51,58,62 270 4 off
R48 15K
R53,54 220K 2 off
R59 1K2
R64 100K
VR1,4,7,11 10K vertical preset 4 off
VR2 10K multiturn preset
VR10 100K lin PC pot
VR8 470K log PC pot
VR9 47K vertical preset
VR13 10K lin PC pot
VR14 47K log PC pot

Capacitors
C1,2,10,11,17,24,26,28,29 100nF polyester 9 off
C3,9,21 1000pF polystyrene 3 off
C6,7,14,19 470nF polyester 4 off
C8 4.7nF polyester
C12 10nF polyester
C13,15 33pF ceramic 2 off
C16,25,27 47uF 16v radial electrolytic 3 off
C18 2.2uF 63v electrolytic
C20 47nF polyester
C22 100pF ceramic
C23 10uF 25v electrolytic
C31 10nF polyester

Semiconductors
D1-8,10-14 1N4148 13 off
D9 5mm red LED
D15 3mm green LED
TR1 BC182
TR2,3 BC212 2 off
IC1,9,16,18 LF353 4 off
IC2 CA3096
IC3,8 LM311 2 off
IC6 DG308
IC7 LF398
IC10 4066
IC11 AM6072
IC17 MF4-50
IC19 LM13600
IC20 LM324
IC21 MF6-50
IC22 79L05

Miscellaneous
JK1-5 ¼" PC jack socket 5 off
SW1,2 PC push switch SPDT 2 off
Buss connector
Track pin 100 off
8-way DIL socket 8 off
14-way DIL socket 3 off
16-way DIL socket 3 off
18-way DIL socket Pot PC bracket 4 off
PCB (with printed overlay)


Logic Board
Resistors - ⅓W 5% CF
R16 150K
R17,18 10K 2 off
R19,94 2K2 2 off
R66 47K
R67,69,70,72,74,82,83,90 22K 8 off
R71,73 1M 2 off
R75,76,77,78 270 4 off
R79,85 4K7 2 off
R80 68K
R81,87 18K 2 off
R84,55 100K 2 off
R86 33K
R88 10M
R89 390
R91 2R7
VR3 100K lin PC pot
VR5,6 47K log PC pot 2 off
VR12 2M2 log PC pot

Capacitors
C30,36,37,38,42,47 470pF ceramic 6 off
C31,33 10nF polyester 2 off
C32,34,35,48,50,51,5 100nF polyester 7 off
C39,40,43,44 100pF ceramic 4 off
C41,45,46,4 33pF ceramic 4 off
C49 47uF 16V radial elecrolytic

Semiconductors
D15 3mm green LED
D17 Tricolour LED
TR4-7 BC182 4 off
IC4 4046
IC12 MC14559
IC13 74HC244
IC15 74HC86
IC23,24,25,28,14 4013 5 off
IC26,27 4528 2 off
IC29,31 74HC00 2 off
IC30,32,33,34 74HC02 4 off
IC35 74HC04
IC36,5 74HC4024 2 off
IC37 4520
IC38,39 4516 2 off
IC40 40103
IC41 7805
X1 6MHz ceramic resonator

Miscellaneous
SW3,4 PC push switch SPDT 2 off
Buss connector
Track pin 100 off
14-way DIL socket 15 off
16-way DIL socket 8 off
20-way DIL socket
Pot PC bracket 4 off
32-way male connector
PCB (with printed overlay)


Memory Board
Resistors - ⅓W 5% CF
R92,93 4K7 2 off

Capacitors
C52 100nF polyester
C53,54 33pF ceramic 2 off

Semiconductors
IC42,43 74HC244 2 off
IC44,45 74HC74 2 off
IC46 74HC00
IC47,48 TMS4464 2 off

Miscellaneous
32-way female connector
14-way DIL socket 3 off
18-way DIL socket 2 off
20-way DIL socket 2 off


Series - "Digital Sampler/Delay"

Read the next part in this series:


All parts in this series:

Part 1 | Part 2 (Viewing) | Part 3


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Analogue Equipment Design

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Equipment Guide


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

 

Home & Studio Recording - Mar 1986

Donated & scanned by: Mike Gorman

Topic:

Electronics / Build


Series:

Digital Sampler/Delay

Part 1 | Part 2 (Viewing) | Part 3


Feature by Paul Williams

Previous article in this issue:

> Analogue Equipment Design

Next article in this issue:

> Equipment Guide


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