The following 269 words could not be found in the dictionary of 615 words (including 615 LocalSpellingWords) and are highlighted below:

100k   100u   10k   10u   2u2   about   accurately   across   additional   adequate   advanced   all   Alternative   alternative   although   amount   amplification   amplified   amplifier   an   An   and   Another   attachment   audio   avalanche   average   avoid   back   band   base   based   basic   be   because   been   being   bias   biased   board   Book   break   breakdown   but   By   by   C101   C102   C103   C104   can   Capacitors   cause   causing   certain   certainly   Chapter   Chapter2   Chapter3   Circuit   circuit   circuits   closed   combination   component   Components   conjunction   connected   context   control   conventional   coupled   cymbal   D101   diagram   dictated   difficult   diode   diodes   disadvantage   Dl   down   effects   electrolytic   electronic   emitter   enables   envelope   etc   exceeded   expensive   F351   fairly   fed   Fig   Fig21   Fig22   Figure   filter   filtered   for   forward   frequency   from   Fs   gain   generated   generates   Generator   generator   give   gives   good   great   handclap   high   higher   hissing   holder   in   In   incorporate   inexpensive   interesting   intro   junction   keyboard   leaves   less   level   like   likely   lines   load   loop   lot   melodies   Miscellaneous   mix   more   most   much   necessary   needed   never   no   Noise   noise   normally   obtain   obtained   Od   of   omit   on   one   Only   organ   Os   output   peak   Penfold   percussive   perfectly   pick   pin   pipe   pitch   played   potential   produce   proper   provide   provided   provides   purity   purposes   quality   quite   R101   R102   R103   R104   R105   radial   rails   range   relatively   resistor   Resistors   resonance   results   reverse   Semiconductors   set   shaped   shaper   shown   shows   signal   signals   significant   silicon   similar   slightly   smoothing   so   some   sound   sounds   Source   still   stray   strong   such   sufficient   suitably   supply   sweeping   synthesised   synthesisers   talking   terminals   than   that   The   the   these   This   this   though   times   to   total   track   transistor   try   two   type   typical   unusual   up   use   used   useful   Using   using   versatile   very   via   volt   voltage   volts   vulnerable   watt   we   well   When   when   which   white   wire   with   within   woodblock   worthwhile   would   Z5   zener  

Clear message


Chapter 2: Noise Generator

The Noise Generator

Noise Generator

Not all synthesisers incorporate a noise generator, but it is certainly a circuit which I would never omit from one. In this context the noise we are talking about is of the "white" noise type, which is the "hissing" sound that we normally try to avoid in electronic circuits. When suitably envelope shaped this basic noise sound can produce some interesting effects, but it is much more useful and versatile when used in conjunction with a VCF as well. A lot of percussive sounds can be synthesised, such as cymbal, handclap, and woodblock type sounds. By sweeping the filter from the envelope shaper some unusual but very worthwhile effects can be obtained. Another use of noise is in conjunction with a VCF which is set up to accurately track the pitch dictated by the keyboard. This enables melodies to be played on the noise generator/YCF combination provided the VCFs resonance control has been well advanced. A more conventional but similar use would be to mix the filtered noise with the VCO's output signal, and this can give some good pipe organ type sounds.

Figure 21 shows the circuit diagram of a noise generator based on a Z5J noise diode. This is fed from the two 12 volt supply lines via load resistor R101, and the total supply potential of 24 volts is sufficient to cause D101 to break down and avalanche like a zener diode. Also like a zener diode, a certain amount of noise is generated across the component, and this is coupled by C101 to a high gain amplifier based on IC101. This provides a voltage gain of about 4OdB (100 times), and this is needed because the noise. voltage generated across Dl, although higher than from an average zener diode, is still not very great. The amplified output from IC101 is about 7 volts peak to peak. Using a noise diode gives a good quality output with no significant bias to any frequency band within the audio range. R104, R105, C103, and C104 are used to provide additional smoothing of the two supply rails. This is necessary as the high closed loop gain of 1C101 leaves the circuit vulnerable to stray pick up of signals from the VCOs.

Noise diodes have the disadvantage of being relatively expensive and difficult to obtain. An inexpensive alternative is a silicon transistor connected as shown in Figure 22. Only the base and emitter terminals are connected, and these are connected so that the base - emitter junction is reverse biased. This results in the reverse breakdown voltage being exceeded, causing the junction to avalanche. It generates a fairly strong audio noise signal, but the typical output level is slightly less than that of a Z5J noise diode. It is adequate though, and it is not necessary to use any additional amplification. The noise purity is also likely to be not quite as good as when using a proper noise diode, but it is still perfectly adequate for most purposes.

Noise Generator Components (Fig. 21)

Resistors (all 1/4 watt 5%)     
R101    100k    
R102    10k     
R103    1M      
R104,105        120R

C101    2u2 63V radial electrolytic     
C102    10uF 25V radial electrolytic    
C103,104        100uF 16V radial electrolytic

IC101   LF351   
D101    Z5J noise diode

Circuit board   
8 pin DIL IC holder,    wire, etc.      

Alternative Noise Source (Fig. 22)

PenfoldBookChapter2/NoiseGen (last edited 2007-02-13 07:50:17 by TomArnold)