The following 381 words could not be found in the dictionary of 615 words (including 615 LocalSpellingWords) and are highlighted below:

10k   470n   4watt5   about   abrupt   absence   account   accuracy   accurate   across   adjust   adjusted   Adjusting   adjustment   advantage   all   almost   an   and   another   applied   arrangement   assume   at   attachment   available   back   basic   be   been   being   better   beware   board   Book   buffer   but   by   C0s   C12   C6   can   capacitor   Capacitors   carbonate   certain   Certainly   chain   Chapter   Chapter2   Chapter3   charge   charged   circuit   circuitry   circuits   clarity   cleaned   completes   complex   component   components   Components   compromise   constructed   constructor   contact   contacts   control   conventional   correct   cost   course   crisply   critical   current   depressed   desired   diagram   differs   difficult   divider   does   draw   drawback   drive   drop   each   easily   effect   eight   electrical   enables   encountered   enough   ensure   etc   etched   exact   excessive   expensive   F351   fact   Fig   Fig08   Fig09   Figure   find   five   fixed   for   forward   fraction   from   gear   generate   get   give   given   gives   glide   glissando   good   Good   Greater   hand   having   high   higher   hold   home   horizontal   However   hum   Ideally   If   if   impedance   in   In   include   increased   inevitably   input   instantly   into   intro   its   itself   Keep   key   Keyboard   keyboard   keyboards   keys   kind   knob   known   large   latter   leakage   least   less   like   linear   loading   located   logarithmic   long   longer   low   lowest   M2   made   mains   major   materials   means   might   millimetre   million   millivolts   miniature   minimum   Miscellaneous   more   Most   move   much   must   need   needed   never   new   next   no   noise   normally   note   notes   O81   obtain   obtained   octave   Of   of   ohms   On   on   One   one   only   onto   operated   or   order   original   other   output   over   own   pads   particularly   pattern   Penfold   per   perfectly   period   pick   pitch   played   playing   plays   plug   plus   polyester   portamento   possible   potential   Potentiometers   practice   preset   printed   Printed   problems   prod   properly   prototype   prove   provide   provided   providing   quality   quantity   quite   R10   R17   R18   random   range   rather   reached   readily   real   reality   really   reasonably   reduction   relative   released   replaced   require   required   requirement   resistance   resistor   Resistors   resistors   respectively   result   results   retains   Rl7   rnegohms   sake   same   sample   satisfactory   seconds   seem   seems   Semiconductors   sense   set   setting   several   shortcoming   should   shown   shows   significant   simple   simplicity   small   so   some   something   sophisticated   source   stage   still   storage   stray   stylus   Stylus   subsequent   such   sufficiently   suit   suitable   supply   switch   switches   switching   synthesiser   synthesisers   taken   takes   termed   test   than   that   The   the   them   then   theory   there   These   these   things   this   This   through   time   to   tolerance   too   Touching   true   type   types   ultra   unable   unit   unusual   up   use   used   using   usually   V3   V4   value   values   very   volt   voltage   voltages   we   well   were   when   which   while   will   Wire   With   with   words   you  

Clear message


Chapter 2: Keyboard


Keyboard circuits can be quite complex and difficult to set up, but good results can be obtained using a very basic type such as the one shown in the circuit diagram of Figure 8.

The first requirement is for a potential divider to generate all the voltages for the required notes. These voltages are provided by RV3 plus R10 to R17, but only eight fixed resistors are shown in Figure 8 for the sake of clarity. In practice this resistor chain must include one less fixed resistor than there are notes on the keyboard. Most keyboards available to the home constructor seem to be four or five octave types, which require 48 and 60 resistors respectively. The use of a logarithmic VCO means that all the resistors must be of the same value, and there is no need to use any unusual and difficult to find values. In fact the exact value used is not critical, but it must not be too low or the resistor chain will draw a high supply current. On the other hand, using a high value could result in the keyboard circuit being unable to drive the subsequent circuitry properly, and could also give problems with excessive stray pick up of mains "hum" or other electrical noise. A value of 47 ohms seems to be a good compromise. Ideally the keyboard resistors should have a tolerance of 0.5% or better, but suitable components might prove to be difficult to obtain, and very expensive if a suitable source can be located. Good results should be obtained using resistors having a tolerance of 1%, and these are readily available at quite low cost.

RV3 is adjusted so that there is 83.33 millivolts across each resistor, or 1 volt per 12 resistors in other words. This adjustment does not need to be particularly accurate if the synthesiser is to be used on its own, and RV3 can then be given any setting which enables the VCO to be set up for the correct pitch range. Greater accuracy is needed if the unit is to be used with other synthesisers (which must be of the 1 volt per octave type). In the absence of suitable test gear it is still possible to give RV3 the correct adjustment. First use the keyboard voltage from the other synthesiser to drive the VCO, and set up the VCO for the correct pitch range.

The sample and hold circuit has IC6 as a very high input impedance buffer stage. This is required in order to provide a sufficiently low output voltage to drive one or more VC0s, while providing a high enough input impedance to ensure that there is no significant loading on charge storage capacitor C12. The input impedance of IC6 is about one million rnegohms, and in theory there should be no significant reduction in the charge on C12 over a long period of time. In reality things are less certain, as the leakage resistance of C12 itself plus any leakage through the printed circuit board also have to be taken into account. However, provided C12 is a good quality type such as a carbonate or polyester component, there should be no significant drop in the charge voltage over a period of at least several seconds.

If we assume that RV4 is at minimum resistance, when any keyboard switch is operated C12 is almost instantly charged to the new keyboard voltage, and it retains that charge after the key is released. Adjusting RV4 for increased resistance gives much the same effect, but it takes C12 longer to adjust to each new voltage. This can be used to give a glide from one note to the next, rather than abrupt switching from one note to another. This effect is usually termed "portamento", but it is also known as "glissando". The higher the value of RV4, the longer it takes the synthesiser to move on to new notes. However, beware of using an excessive glide time as this could result in the desired notes never being reached.

With its relative simplicity this keyboard circuit inevitably has a shortcoming. With more sophisticated keyboard circuits the lowest note is normally the one that is obtained if more than one key at a time is depressed. With this circuit you will get a more or less random note if more than one key at a time is operated. Of course, this is not really a major drawback, and is perfectly satisfactory provided the keyboard is played reasonably crisply. Certainly no real problems were encountered when playing the prototype. Keyboard Components (Fig. 8)

R10 to Rl7 47R (1 % or better, quantity to suit keyboard)
R18 100R(1/4watt5%)

RV3 10k miniature horizontal preset
RV4 2M2 linear

C12 470nF miniature polyester

IC6 TLO81CP or LF351

Printed circuit board
Keyboard with materials for key contacts Wire, control knob, etc.

Stylus Keyboard

One advantage of this ultra-simple keyboard arrangement is that it can easily be applied to a stylus type keyboard. This is not a keyboard at all in the true sense, but is a printed circuit board etched with a miniature keyboard type pattern. Touching a stylus of some kind onto one of the "keys" completes an electrical contact and plays the required note. This type of keyboard is more difficult to use than a conventional type, but it does have a very large advantage in that it can be constructed for only a small fraction of the cost of a real keyboard.

Figure 9 shows the stylus keyboard circuit, and this only differs from the original in that the keyboard switches have been replaced by the keyboard pads and the stylus. The latter can be ,something like a test prod or a 4 millimetre plug., Keep the keyboard pads well cleaned so that a good electrical contact can easily be made to them.

PenfoldBookChapter2/KeyBoard (last edited 2007-02-12 10:24:06 by TomArnold)