~Proceedings ICMCISMCI2014 14-20 September 2014, Athens, Greece
tone
Vct trigger attack EG
Vid logic smooth #3
env. gen. #3 - - - - - - clip
- Osc -diviy #2VCAdiode
Osc dividerf o env. gen. #2 #3 clip H3()
I c-swing-type VCA #3 highpass #2
EG EG hihas#
1 Osc divider L - - - -- - bis3 #2- - emitter
1 #5 #51 I11 op-amp Ht(z
# 2n semitter EG#c2ip
O Ot divider follower #1.2
4 4 sn elopegen etor #1 s VC # Hh2(z)
d#3 de#3 I I I "op-mpswing-type VCA #2 highIpass #2 I
1 1 ~b2(z) op-ap Hdc(z) - ---1
Osc 4 divider 1 - -ba psfi r-.. bias #2 I emitter Hlz
1 #2 #2 I-ba-p-s -il-r -2follower 1 1 Hlz
I I I 1 1 I tone control
Osc H divider -I Hbp1(z) H op-amp Hdc(z) -- -- VCA diode I Hhl(z)
Schiamtt tigger oscillators ba nd pass filter #1. bas# swing-ty pe VCA #1 highlpass #1
level
Hie(z)
op-amp output
clip
output buffer
and level control
Figure 2. TR-808 cymbal emulation block diagram.
able in terms of passive component values and the device
properties of the Schmitt trigger inverter.
Plugging in Equations (1)-(2) to the definition of duty
cycle, the time constant RC drops out-the duty cycle depends only on the inverter's device properties VT-, VT+,
VOL, and VOH
D in (OH- T- (VOH-VT-)(VOL-7T+)) 3
- OH - T+ )/n (VOH -VT+) (VOL -VT-)
The six oscillators have nominal frequencies (or ranges)
of 205.3, 369.6, 304.4, 522.7, 359.4-1149.9, and 254.3 -627.2 Hz. Oscillators #5-6 have internal trimpots (TM2
and TM1) in series with their resistor, for factory tuning to
specific frequencies (800 and 540 Hz, respectively). 9
By design, the HD14584 is powered with 5 volts, yielding a duty cycle of D = 47.98% and an amplitude of
A 5 volts for each oscillator. 10
All six oscillators are summed via voltage dividers in
a passive mixing network. The output of this network is
applied to the input of the band pass filters. Respecting
superposition (grounding the other oscillator outputs), an
example calculation of this attenuation (for oscillator #1)
is:
Vsum _R53
V0,1 R53 + (R37 R39 R46 R48 R50) ()
Fig. 3 shows a comparison between the sum of all six oscillators (the signal applied to each band pass filter) and
tabulated data from a SPICE 11 simulation of the circuit. 12
9 Early manufacturing runs (prior to serial #000300) used different values for the resistors and capacitors in oscillators 5 and 6 [1].
10 using the "typical" values from the device's datasheet [8]
" Simulation Program with Integrated Circuit Emphasis, a widelyused family of general purpose analog electronic circuit simulators.
12 No attempt was made to align the phase of the model to the tabulated SPICE data. Fig. 3 just shows, qualitatively, that the model and the
reference SPICE simulation have similar features.
- 0.2 (SPICE
model
0o.1 J 2J
E 0
0.07 0.072 0.074 0.076 0.078 0.08
time (seconds)
Figure 3. Sum of all oscillators (Vsum).
4. BAND PASS FILTERS
The two active band pass filters 13 each filter the combination of the six rectangular waves. What follows is a representative analysis and simulation of band pass filter #1.
Identical methods were used for #2.
Assuming ideal op-amp behavior, nodal analysis of band
pass filter #1 yields a transfer function of the form:
Vbp#1 _ 22 + _1s
H,1 (s) =Vsum a s3 + a2s2 + a1s + ao (5)
with coefficients:
/32 = -R56R57 (C13 + C14) C10
13= -R57C10
R3 = R56R57R52C13C14C10
a2 = R56R57C13C14 + R56R52 (C13 + C14) C10
al = R56 C13 + R56 C14 + R52 C10
ao=1.
Fig. 4 shows the magnitude response of each band pass
filter. Band pass filter #1 has a center frequency around
3440 Hz and band pass filter #2 has a center frequency
around 7100 Hz. These band pass filters strongly accentuate the upper overtones of the square waves, while deemphasizing their fundamental frequencies.
13 whose topologies are closely related to the so-called "bridged-T network", a bridged-T Zobel network in the negative-feedback path of an
op-amp, that is used in each of the 808's voice circuits [3]
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