AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Analogue Vocoder – Part 2
L
ast month, in Part 1, we
Pretty curves
introduced our brand-new design
for an Analogue Vocoder, which
creates a unique and fascinating vocal
effect – human speech is superimposed
on a musical instrument.
Before we look in more detail at the
design, first a quick conclusion to the
discussion of the ‘analysis’ (multiple
channel) filters discussed at the end of
last month’s article.
A plot of the speech (or ‘analysis’) filters
is shown in Fig.13. The output/synthesis filters should have exactly the same
response, but are harder to measure (and
hence produce plots for) because there are
VCAs in-between both sections. The highpass and low-pass curves have a much
wider bandwidth with 24dB/oct slopes.
This is because nearly all speech information is in the midrange from around
200Hz to 5kHz.
In general, fil20 Hz 50
100 200
500
1k
2k
5k
10k 20k
ters should always
0dB
use 1% tolerance
–10
resistors, which fortunately are now
–20
cheap. The prob–30
lem components
–40
are the capacitors,
Fig.13. Plot of the analysis filters of the prototype. A little uneven which are often
only available in
due to the use of 5% capacitors. For musical applications this
5% tolerance. Ocis of little consequence – we are not building precision test gear.
c a s i o n a l l y, 2 %
Often the upper and lower band-pass filters are made with a
polystyrene and
wider bandwidth than the ones in the middle, where the speech
silvered-mica types
information is most concentrated.
so
In p u t
cke
ts
so
O u tp u t
cke
ts
1 9 - i n ch
r a ck
B u s b o a rd
e n cl o su
r e , co
Fig.14. The rough photocopied board
layouts in J M Hawkes’ student project
report were smoothed into a form
suitable for etching using Photoshop.
are available. These are the best capacitors
for filters, but they tend to be larger and
much pricier compared to the cheaper
and easy-to-find polyester types.
Board stiff
Life is too short to make a 14-channel
vocoder prototype on Veroboard! So, to
make the prototype shown in Fig.5 (see
last month’s Audio Out) I started off with
IE C
n ta in in g 1 4 b o a r d s
M o l e x,
r i g h t - a n g l e co
cke
t
n n e ct o r s
P h a n to m
b o a rd
( o p tio n a l)
2 x d r i ve
a m p lifie r
b o a rd s
2 x d r i ve
a m p lifie r
b o a rd s
T r a n sf o r m e r *
T o o th e r
b o a rd s
M i cr o p h o n e
p r e a m p lifie r
b o a r d ( o p tio n a l)
L P /H P
B P 1 /2
B P 3 /4
B P 5 /6
B P 7 /8
B P 9 /1 0
B P 1 1 /1 2
P S U
b o a rd
* T o m i n i m i se h u m , p l a ce
a s fa r a w a y a s p o s ib le
f r o m a m p l i f i e r b o a r d s.
F i l t e r b o a r d s x1
C o n tr o ls
so
4
P o w e r s w i t ch
Fig.15. The Analogue Vocoder consists of many interconnected boards. The microphone preamplifier, driver, summer/output and
power supply boards are all useful audio building blocks in themselves. Putting all the circuits together on one big board is not a
good idea since it inhibits experimentation and reduces versatility. (Commercial production would of course dictate much greater
integration to reduce labour intensive wiring, but we don’t face that pressure.)
60
Practical Electronics | December | 2021
n
microphone preamplifier, two driver
boards, 12 band-pass channels on six
boards, an optional low-pass/high-pass
channel board and two summing/output
amplifiers. These are shown in Fig.15.
The vocoder channels are all plugged
into a bus board using right-angle connectors, as shown in Fig.16.
Control signals
The output of the filters are fed into precision full-wave rectifiers (Fig.17) which
are smoothed by second-order low-pass
filters (Fig.18) whose cut off frequency
is proportional to each frequency band.
If the smoothing frequency is too high,
the effect is ‘spittiness’ and distortion. If
too slow, the intelligibility of the speech
is impaired. The idea is to get a general
trend of the energy fluctuations in each
particular frequency band. The cut-off
frequency should be around 5 to 20-times
less than the lower frequency of the
band-pass filter, with a Q of about 1. The
Fig.16. Since all the channel connections are wired in parallel (the ‘bus’) a long piece of
tolerances for these filters are uncritical
stripboard was originally used to connect them all together on the prototype. However,
and 10% tolerance capacitors are fine.
in the design we will be building over the next few months, a proper PCB bus board will
A linear voltage-to-current converter
be used. It has plated-through holes for extra strength.
using an op amp and transistor (shown
in Fig.19) is used to feed the control pin of the VCA’s
an edited version of JM Hawkes’ PCBs. Unfortunately, all I had were
transconductance amplifiers. This ensures better tracking
scratchy photocopies of taped artwork. These were soon cleaned
at low levels because the (PNP) transistor provides a much
up in Photoshop (shown in Fig.14) and printed out for etching.
higher drive impedance than just a resistor.
Once I had made my own tweaks and produced a working system I got my PCB guru, Mike Grindle, to design a professional set
of PCBs. The vocoder comprises 14 boards, including an optional
Chips and VCAs
The transconductance VCAs are the biggest source of noise
in the whole system. However, this is masked by the muC *
C * d e p e n d s o n f r e q u e n cy b a n d
3 9 n F
I C 1 * * u se T L 0 8 2 o r si m i l a r
sical sounds generated. Interestingly, it does not detract
C a n ce l l a t i o n o f h a l f - w a ve r e ct i f i e d
from the overall effect, the slight roughness introduced
si g n a l w i t h t h e p o si t i ve h a l f o f t h e
1 %
i n p u t si g n a l o cu
rs h e re .
makes it sound more ‘vocal’ and ‘breathy’, which is a positive effect for this system.
My prototype shown in Fig.16 used the classic CA3080
1 %
1 %
transconductance amplifier, which ceased production in
–
IC 2 b * *
2005. Later vocoders and this design use the LM13600/700
1 N 4 1 4 8
C la m p
+
O u tp u t
d io d e
which is effectively two 3080s with added Darlington
buffers. Linearising diodes are also added for distortion
In p u t
1 N 4 1 4 8
1 %
0 V
–
0 V
+
– V
t
F u l l - w a ve - r e ct i f i e d
o u t p u t ( i n ve r t e d )
+ V
IC 2 a * *
+ V
I
W h e n i n p u t vo l t a g e si t s a t 0 V
th e o p a m p o u tp u t is a t – V B E .
t
0 V
– V
0 V
2 2 n F
Fig.17. A precision full-wave rectifier circuit is used to generate DC
from the output of each speech analysis filter. The circuit depends
on accurate subtraction of a half-wave rectified signal from the
rectifier input. Note that 1% resistors are needed.
t
+
2 2 n F
O u tp u t to
V - t o - I co n ve
rto r
–
t
– V
– V
S m o o t h e d o u t p u t vo
lta g e
Fig.18. The output from the rectifier is smoothed by a low-pass filter.
These filters need different values for each frequency band.
Practical Electronics | December | 2021
–
I n p u t n e g a t i ve g o i n g vo l t a g e
0 V
IC 3 b
+
T R 1
B C 5 5 9 C
t
– V
1 0 0 n F
In p u t
fro m
r e ct i f i e r
1 N 4 1 4 8
N e g a t i ve cu r r e n t t o
t r a n sco
n d u ct a n ce
a m p l i f e r co n t r o l p i n
I
H i g h i m p e d a n ce
O u tp u t
Fig.19. The voltage-to-current converter controls the
transconductance op amp to form a VCA. The VBE of the
transistor is cancelled with negative feedback. This circuit is a
design from Jacob Moskowitz’s article, Current-compensated
op amp improves OTA linearity, Jan 1977. I can’t trace the
original publication, but it is available on ResearchGate.net.
I’ve made minor improvements consisting of a diode to
protect the base-emitter junction from reverse bias and a
capacitor to prevent HF instability has been added.
61
the control current and the Darlington input to obtain both
a high impedance and a high maximum current output over
the full range of control currents. The LM13700 is a later version where better processing gave higher current gain in the
Darlingtons, allowing this link to be dispensed with. The
LM13700 is currently the only transconductance amplifier IC
in production, and I use the surface-mount version in my Dubreq Theremin. The CA3280 is the best choice, but is hard to
find. It also has linearising diodes, but no buffer stages. Using
dual devices, such as the LM13600 means that two vocoder
filter channels have to be placed on one board.
There is also the NE571 telephone compander chip that
includes a couple of full-wave rectifiers, in addition to two
VCAs. The Paia 6710 eight-band vocoder kit uses these. Another odd ball is the CA3060, which has three 3080s on one
chip. All these VCA chips are shown in the IC rogues’ gallery of Fig.20.
Transconductance amplifiers are voltage in / current out
devices. However, here we are using a gain-controlled transconductance amplifier with an additional linear current-driven
gain control pin. There are even more complicated VCAs
available, such as the 2180 from That Corp. These have logarithmic control characteristic, which complicates the circuits
excessively. Also, their Hi-Fi specification is not necessary,
possibly making the vocoder too clean sounding! I suspect the
soft clipping produced by non-linearised transconductance
Fig.20. Suitable transconductance VCA chips for vocoders. Here
we see, the classic CA3080, the LM13600/700, the oddballs
NE571 and CA3060 – plus the most recently developed and
best, the CA3280. Only the LM13700 and ‘That Blackmer Corp’
topology 2180 chips are still in production, although the DIL version
of the LM13700 will soon be discontinued by Texas Instruments
(TI). All are available from the AO shop – see page 59.
cancellation, allowing a higher drive level, and thereby giving
lower noise. This is activated by installing R25 (see Fig.21).
There is no difference in practice between the LM13600 and
LM13700. Originally, the LM13600 provided a link between
T P 1 *
C 2
3 3 n F
T o o th e r
ch a n n e l
1 . 2 5 kH
z
In p u t
sp e e ch
8 6 0 H z
D 1
1 N 4 1 4 8
R 9
–
IC 1 a
T L 0 8 2
R 2
C 5
1 0 n F
–
+
V –
C o n tro l
o u tp u t
IC 1 b
T L 0 8 2
R 8
R 1 2
R 1 1
–
–
+
R 5
D 2
1 N 4 1 4 8
IC 2 b
T L 0 8 2
IC 2 a
T L 0 8 2
+
+
R 1 0
0 V
0 V
C 7
1 0 0 n F
R 1 5
R 1 6
–
IC 3 a
T L 0 8 2
C 8
2 2 n F
C 9
2 2 n F
+
T R 1
B C 5 5 9 C
C 1 1
3 3 n F
In p u t
syn
th
z
0 V
4 (1 3 )
–
+
R 3 6
R 2 4
1 1
2
(1 5 )
1 (1 6 )
3 (1 4 )
C 1 2
3 3 n F
R 2 8
R 3 5
6
R 2 6
7 (1 0 )
R 3 1
M i x b u se
–
R 3 3
IC 5 b
5 5 3 2
s
O u tp u t
r ig h t
T o o th e r
ch a n n e l
R 3 0
8 (9 )
O u tp u t
le ft
R 3 2
+
R 2 9
IC 4 a /(b )* *
L M 1 3 7 0 0
5 (1 2 )
T P 3
8 6 0 H z
–
+
R a il
b u se s
V – 1 5 V
* * I C sh a r e d w i t h
o t h e r ch a n n e l
R 2 3
R 2 2
R 2 7
0 V
0 V
F i l t e r va
C 1 6 +
1 0 µF
2 5 V
C 1 3
3 3 n F
R 2 5
R 2 1
IC 5 a
5 5 3 2
R 2 0
0 V
V +
( o p tio n a l)
C 1 0
3 3 n F
R 1 9
C 1 4
4 7 0 n F
V + 1 5 V
R 3 4
R 1 8
0 V
1 . 2 5 kH
T o IC
p o w e r
p in s
0 V
+
C 1 5 +
1 0 µF
2 5 V
R 1 7
D 3
1 N 4 1 4 8
IC 3 b
T L 0 8 2
–
T o o th e r
ch a n n e l
R 1 4
C 6
3 9 n F
R 1 3
R 6
C 3
3 3 n F
R 4
T P 2 *
V C A b ia s
V R 1 *
R 7
C 4
3 3 n F
R 3
C 1
3 3 n F
R 1
V +
* M o u n t o n to p e d g e
o f b o a r d f o r a ce
s
F ilte r
o u tp u t
l u e s sh
o w n a r e f o r 8 6 0 H z a n d 1 . 2 5 kH
z si n g l e ch
a n n e l o n ly .
V –
Fig.21. Assembling all the building blocks to form a complete vocoder channel. We will build two channels per PCB – see Fig.15.
62
Practical Electronics | December | 2021
D r i ve
b u s e s
S p e e ch
S yn
M i xi n g b u se
th
L (o d d )
R
s
( e ve
n )
20 Hz
50
100
200
500
1k
2k
5k
10k
20k
20 Hz
50
100
200
500
1k
2k
5k
10k
20k
20 Hz
50
100
200
500
1k
2k
5k
10k
20k
0dB
–10
L P
1 3 0 H z
H P
8 kH
1
1 5 0 H z
–20
–30
z
–40
0dB
2
–10
2 1 0 H z
–20
3
3 0 0 H z
4
4 5 0 H z
5
6 5 0 H z
6
9 0 0 H z
–30
–40
0dB
–10
–20
–30
–40
1 . 3 kH
z
8
1 . 8 kH
z
9
2 . 6 kH
z
1 0
3 . 8 kH
z
1 1
5 kH
Fig.23. The total frequency response of the stereo output of
the vocoder is produced using the left channel (top) and right
channel (middle) – here plotted on top of each other (bottom).
The shaded area in the bottom plot highlights the difference
between left and right channels. This produces a unique
musical effect.
7
z
amp. This amplifier
will also drive output transformers.
Summing up
The outputs of the
channels can either
1 2
6 . 8 kH z
all be mixed together
into mono or alterM i x r e si st o r s p o si t i o n
natively a stereo mix
se t o n b o a r d
can be created. In
Tim Orr’s vocoder
Fig.22. I use a special mixing trick in all my vocoders to give
designs, the phase of
a stereo effect. The outputs of the channels are panned
each alternate filter
alternately to the left and right outputs apart from the lower
was inverted bebands, which are mono to ensure an even power distribution
fore being summed
between left and right.
to mono to give a
smoother response. This is because there
amplifiers is actually musically benefiis a phase cancellation where the final
cial. I suggest leaving R25 out in your
12dB/oct filter slopes crossover. In the
first build.
design we are building I don’t do this,
All the circuit elements described in the
which gives a more spacious stereo at
vocoder channel block diagram shown
the expense of slightly duller sound in
in Fig.4 (see last month) are assembled
mono. The low-pass filter, the two lowin the complete circuit diagram for a sinest band-pass filters and the high-pass
gle vocoder band-pass module in Fig.21.
filter are fed into both output channels
to maintain essential mono compatibility
Driving
in the bass. The remaining bandpass filThe total input impedance of all 14 filters are then fed alternately into the left
ters is of the order of 130Ω, which is too
and right channels. A block diagram is
low for a single op amp to drive (one
shown in Fig.22. Thus, the frequency
section of a 5532 can drive 600Ω). This
responses of the left and right channels
means a small power amplifier or mulare opposite, where one goes down the
tiple op amps are required. This can be
other goes up, as shown in Fig.23. This
achieved by adding a current-boosting
gives a lovely stereo spread between the
push-pull emitter follower stage to an op
Practical Electronics | December | 2021
two speakers. This special mixing is accomplished in the driver PCBs.
Next month
Now that we have described the system,
we’ll start constructing th driver amplifier PCB next month. This will be a useful
module in its own right for feeding headphones, spring-lines, transformers and
enabling amplifier bridging. The other
boards will follow later.
Acknowledgements
An excellent reference on filter designs is The Filter Handbook by Stefan
Niewiadomski (a previous contributor to
Practical Electronics). Also, The Active
Filter Cookbook by Don Lancaster. Vocoder principles were taught by senior
lecturer Tim Orr at the London College
of Furniture in 1985. Yes, I did study
electronics at a furniture college! (now
the London Metropolitan University).
He wrote many articles in Electronics
Today International magazine on electronics for music. These culminated in
the ETI vocoder of September and October 1980, marketed as a kit for £175 +
VAT by Powertran Electronics. He later
expanded on the filter design in Bandpass and beyond (ETI, December 1980).
This design was also based on a student
HND project report from around 1982
by JM Hawkes. Where is he now, does
anyone know?
63
