µP-controlled mixer board No more noisy controls – ever ... - WebHTB

The audio mixer 

board presented in 

this article cannot be 

compared with any 

mixer board previously 

published in 

this magazine. In a 

way, it is the prelude 

to a new era in audio 

engineering. All controls 

are operated by 

integrated digitally 

controlled amplifiers 

(DCAs). The position 

of the slide potentiometers 

is converted 

by a microprocessor 

into a control signal 

for the DCAs. It is 

even possible to do 

away with the potentiometers 

and microprocessor 

and leave 

the control of the 

mixer board to a PC, 

but this will be the 

subject of a future 

article. 

Design by T. Giesberts 

36 

µP-controlled 

mixer board 

No more noisy controls – ever! 

Parameters 

� 8 controlled audio inputs 

� all input signals can be directed to the left-hand or right-hand output 

� maximum attenuation 63 dB in 1 dB steps 

� mute function 

� automatic muting at power up 

� buffered line inputs and outputs; external amplifiers not required 

� signal-to-noise ratio 82 dB with 10 dB headroom 

� distorsion (THD+N) 0.007% 

� standard 3-wire serial interface 

� number of channels may be expanded 

� wide range of applications: multimedia systems; PC sound cards; 

studio mixers; musical instruments, and many more 

Wherever there is a battle between 

digital and analogue technologies, the 

outcome is almost certain to be in 

favour of the digital. Analogue will live 

on, but only in some specialist uses. So, 

we have digital recording, digital control, 

digital television, digital mobile 

telephones, and more. Nevertheless, it 

may still come as a surprise to some to 

find a design for a digital mixer board, 

which many consider a traditional 

stronghold of analogue audio engineering. 

True, you will still find a number 

of slide potentiometers, but these 

operate with direct voltages only, not 

audio signals. The circuit consists of 

just two ICs, one 

of which is entirely digital and the 

other is a hybrid: part digital, part analogue. 

There are no transistors or 

op amps – nevertheless, it works like a 

dream! 

INTRODUCTION 

The circuit is based on an 8×2 digitallycontrolled 

audio mixer Type SSM2163 

from Analog Devices. Each of the eight 

inputs can be mixed under digital control 

to a stereo output A simplified 

block diagram of the device is shown 

in Figure 1. Each input channel can be 

attenuated by up to 63 dB in 1 dB 

intervals, and also fully muted. Furthermore, 

any input can be assigned to 

Elektor Electronics 3/97

either or both outputs. A standard 

3-wire serial interface is used as well as 

a Data Out terminal to facilitate daisychaining 

of multiple mixer ICs. 

The control signal for the mixer 

board is provided by a Type ST62T25 

microcontroller. This devices rapidly 

scans the potential at the wipers of the 

potentiometers and converts this information 

into an 8-bit code. The controller 

also monitors the position of 

two switches at each input so that the 

operator can 

arrange for the 

relevant input signal 

to be applied 

to the left-hand or 

right-hand channel 

or both. This 

information is 

contained in the 

control code. 

THE 

SSM2163 

The SSM2163 

consists of an analogue 

(signal processing) 

section 

and a digital (control) 

section.The 

channel attenuation 

level and 

mixer functions 

are controlled by 

digital registers, 

which are loaded 

via a serial interface. 

A hardware 

mute input is 

included to asynchronously 

force all inputs into the 

muted state. 

Analogue section 

The analogue signal path is shown in 

Figure 2. Each input has a nominal 

impedance of 10 kΩ. Each input therefore 

appears as a digitally programmable 

10 kΩ potentiometer. The 

SSM2163 input impedance remains 

constant as the attenuation level 

changes. So, the sources that drive the 

device do not have to drive complex 

and variable impedances. 

The attenuated input is applied to 

the left-hand and right-hand channel 

inputs of the mixer. Each mixer channel 

consists of an analogue switch and 

a buffer amplifier. If the channel is 

selected (via the appropriate bit in the 

mixer control register), the analogue 

switch is turned on. The buffer amplifier 

is included after the analogue 

switch so that the gain of each channel 

will not be affected by the potentiometer 

setting or by the on-resistance 

[RDS(ON)] of the switch. 

Each mixer channel that is ON is 

then summed into its respective (lefthand 

or right-hand) mixing summing 

Elektor Electronics 3/97 

amplifier. (If both the mixer channels 

are ON, the attenuated analogue input 

will be applied to both the Left and 

Right summing amplifiers). The 

buffered output of the summing 

amplifier will supply a current of 

±500 µA to an external load. 

Digital interface 

The digital interface consists of two 

banks of eight data registers with a serial 

interface (Figure 3). One register 

bank holds the left/right mixer control 

bits, while the other register bank 

holds the 6-bit attenuator value. 

To access the SSM2163, the controller 

writes a value to the serial shift 

register which selects the appropriate 

input channel register for subsequent 

attenuator-load operations. There are 

two ways: if bit 7 (MSB) is 1, the 

SSM2163 interprets the byte as an 

address; if bit 7 is 0, the SSM2163 interprets 

the byte as a data byte. Normally, 

the address byte is sent first. This indicates 

in which of the eight input chan- 

nels the attenuation is 

to be altered and 

whether the signals are 

applied to the left-hand 

or right-hand channel 

or to both. Next, a data 

byte is sent that determines 

the degree of attenuation of the 

selected channel. Normally, this is followed 

by an address byte and a data 

byte for another channel. It is also possible, 

however, to write a sequence of 

data bytes. The selected 

channel remains the 

same, of course, but its 

attenuation changes 

with every data byte. 

This arrangement 

enables fading in and 

out without the necessity 

of writing the 

address every time. If, 

for instance, during the 

2 

1 

Figure 1. Simplified 

block diagram of digitally 

controlled audio 

mixer Type SSM2163. 

Figure 2. The analogue 

signal path of 

the SSM2163. It contains 

the variable 

attenuator and the 

switches that determine 

whether the 

input signal is applied 

to the left-hand or 

right-hand channel. 

fading out of a channel 

a series of values is sent 

to the same address, a 

single write operation 

to the address register 

suffices. 

When the mute input is made high, 

all channels are muted: this action 

does not affect the set attenuation values, 

which are retained. Switch-on 

37 

noises are obviated by 

forcing all inputs into 

the muted state at that 

instant. 

Serial data control 

inputs 

The SSM2163 provides 

a simple 3- or 4-wire 

serial interface. Data is 

input on the DATA IN

3 

5 

Figure 3. Block diagram 

of the digital 

serial data interface of 

the SSM2163. 

pin, while CLK is the serial clock. Data 

can be shifted into the SSM2163 at 

clock rates up to 1 MHz. 

The shift register clock, CLK, is 

enabled when the WRITE input is low. 

The WRITE pin can therefore be used 

as a chip select input. However, the 

shift register contents are not transferred 

to the register banks until the 

leading edge of LOAD. In most cases, 

WRITE and LOAD will be tied 

together, forming a traditional 3-wire 

serial interface. The process is clarified 

by the three-wire mode timing diagram 

in Figure 4. 

To enable a data transfer, the 

WRITE and LOAD inputs are driven 

low. The 8-bit serial data, formatted 

MSB first, is input on the DATA IN 

input and clocked into the shift register 

on the leading edge of CLK. The 

data is latched on the leading edge of 

WRITE and LOAD. If the data is an 

address, the mixer control is updated. 

If the data is an attenuator value, the 

leading edge of WRITE and LOAD 

will update the appropriate attenuator 

SELECTION 

INPUT CHANNEL 1 








1 

1 

1 

1 

1 

1 

1 

1 

ADDRESS MODE 

X 

X 

X 

X 

X 

X 

X 

X 

ADDRESS 

X 

X 

X 

X 

X 

X 

X 

X 

0 

0 

0 

0 

1 

1 

1 

1 

0 

0 

1 

1 

0 

0 

1 

1 

0 

1 

0 

1 

0 

1 

0 

1 

0 

0 

0 

0 

0 

0 

0 

0 

X 

X 

X 

X 

X 

X 

X 

X 

value. 

MUTE input 

The MUTE pin, which is active high, 

provides a hardware input to force all 

the channels asynchronously into the 

muted state at any time. Most µC pins 

are in high impedance state or configured 

as inputs at power-up, so the 

SSM2163 will automatically be muted 

at switch-on. The mute function does 

not affect the attenuator values stored 

in the attenuator control registers. 

Serial data input format 

As mentioned previously, data is written 

to the SSM2163 in two 8-bit bytes. 

The serial data format is shown in Figure 

5. The first byte sent contains the 

channel address and the Left/Right 

4 

CLK 

output mixer control 

bits. The address byte is 

identified by the MSB 

being high. 

The second byte contains the data, 

that is, the attenuator value. The six 

LSBs of this byte set the attenuation 

level from 0 dB to –63 dB. The MSB of 

the data byte must be a logic zero. 

The standard format for data sent 

to the SSM2163 is an address byte followed 

by a data (attenuator level) byte. 

In some cases, however, only one byte 

needs to be sent. For instance, attenuation 

levels are not affected by the 

MUTE input. To turn a muted channel 

on, simply send an address byte with 

the left-to-right mixer bit set. The 

addressed channel will immediately be 

enabled, using the previously set 

D7 

attenuation level. Furthermore, 

once a channel 

is addressed, the 

1 

0 

1 

DATA 

0 

1 

WRITE & LOAD 

0 

MSB LSB MSB LSB 

OUTPUT SELECT 

1 = SELECTED, 0 = NOT SELECTED 

INPUT SELECT 

X = "DON'T CARE", SHADED AREA = DATA 

L 

E 

F 

T 

R 

I 

G 

H 

T 

DATA MODE 

DATA 

0 

0 

0 

. 

. 

. 

. 

. 

1 

1 

1 

attenuation level can be varied by 

sending additional data bytes. For 

instance, fading a channel can be 

accomplished by simply increasing the 

data value sent to the SSM2163. 

CIRCUIT DESCRIPTION 

All connection between the inputs at 

the left and the outputs at the right of 

the circuit diagram in Figure 6 are 

interconnected by screened cable. The 

screen of this cable is earthed. 

Diode D 2 is arranged to make it 

light when data are being sent to IC 2 . 

Controller IC 1 provides the requisite 

control signals, for which purpose I/O 

pins PA 0 , PA 1 , and PA 2 are the DATA, 

CLK and WRITE-LOAD outputs 

respectively. The 16 I/O pins at the left 

of the IC function as analogue inputs. 

D5 D4 D3 D2 D1 D0 

Figure 4. Timing diagram 

of the three-wire 

mode of operation. 

Figure 5. Clarification 

of how two 8-bit bytes 

are constructed. 

0 

0 

0 

. 

. 

. 

. 

. 

1 

1 

1 

970037 - 15 

These pins are sampled 

in turn; the input data 

so found is converted 

into a control signal for IC 2 . 

It will be seen that the inputs into 

IC 1 are all direct voltages. The attenuation 

level as well as the left/right 

information for each channel is set 

with the aid of potential dividers. The 

wipers of the slide potentiometers 

with which the attenuation of the 

input signals is set are linked to ‘P1’, 

‘P2’, and so on, respectively. The other 

terminals of the potentiometers are 

connected to +5 V and 0 respectively. 

When the potentiometers are almost 

closed, the voltage is low and the 

attenuation great. When they are gradually 

opened, the potential at their 

wiper rises and the attenuation is 

reduced. 

A potential divider 

consisting of three 

DATA ATTENUATION 

38 Elektor Electronics 3/97 

0 

0 

0 

. 

. 

. 

. 

. 

1 

1 

1 

0 

0 

0 

. 

. 

. 

. 

. 

1 

1 

1 

0 

0 

1 

. 

. 

. 

. 

. 

0 

1 

1 

0 

1 

0 

. 

. 

. 

. 

. 

1 

0 

1 

0dB 

-1dB 

-2dB 

. 

. 

. 

. 

. 

-61dB 

-62dB 

-63dB 

970037 - 16

2 

L 

R 

LEVEL 

P2 

S3 

S4 

P4 

S7 

S8 

resistors is provided for 

the left/right information 

for each channel. 

The individual resistors 

are switched with the 

aid of two switches. For 

R5 

10k 

5V 


47k 

82k 

47k 

R13 

10k 

5V 

47k 

82k 

47k 

5V +5V 5V 

P5 

S9 

S10 

R17 

10k 

5V 

47k 

82k 

47k 

R6 

R7 

R8 

C2 

330n 

C4 

R14 330n 

R15 

R16 

C5 

R18 330n 

R19 

R20 

C7 

R26 330n 

P1 

S1 

S2 

P7 

R25 

10k 

P8 

S13 

S14 

5V 

47k 

82k 

47k 

R27 

R28 

P3 

S5 

S6 

P6 

S11 

S12 

S15 

S16 

R1 

10k 

5V 

47k 

82k 

47k 

R9 

10k 

5V 

47k 

82k 

47k 

R21 

10k 

5V 

47k 

82k 

47k 

R29 

10k 

5V 

47k 

82k 

47k 

R2 

R3 

R4 

C1 

330n 

C3 

R10 330n 

R11 

R12 

C6 

R22 330n 

R23 

R24 

C8 

R30 330n 

R31 

R32 

Figure 6. Circuit diagram 

of the mixer board. The 

dashed lines near Channel 

2 indicate how the 

potentiometer and 

switches for the left-hand 

and right-hand channels 

must be connected. 

C16 

100n 

C9 

22p 

C17 

10µ 

63V 

1 

2 

3 

4 

5 

6 

7 

8 

3 X1 

8MHz 

C10 

22p 

S17 

MUTE 

instance, in the 

case of channel 2, it 

is easily calculated 

that when both 

switches are open, 

the voltage at the 

10k 

R36 

23 

PA4 

TIMER 

2 

6 

22 

PC7 

PA5 

NMI 

5 

7 

PC6 

21 

PA6 

PA0 

27 

8 

PC5 IC1 PA1 

26 

20 

PA7 

PA2 

25 

9 

PC4 

PA3 

24 

19 

PB0 ST62T25 

12 

PB7 

18 

PB1 

13 

PB6 

17 

PB2 

RESET 

11 

14 

PB5 

16 

PB3 

15 

PB4 

OSC 

IN OUT 

VPP 10 

DGND 1 

VSS 

2 

1 

4 

5V 

28 

D2 

DATA 

5V 

10k 

680Ω 

R33 

C11 

22µ 

40V 

R34 

100n 

28 SYSTEM MUTE 

27 DATA IN 

DATA OUT 3 

26 CLK 

VDD 

4 

25 WRITE 

VIN1 

5 

NC (SHIELD) 6 

SSM2163 

24 LD 

23 NC (SHIELD) 

VIN3 

7 

NC (SHIELD) 8 

TOP VIEW 

22 VIN2 


VIN5 

9 

20 VIN4 

VCC 

10 


VIN7 

11 

18 VIN6 

VEE 

12 

17 AGND 

ACOM 13 

16 VIN8 

VOUTL 

14 15 VOUTR 

C14 

junction of R 7 and R 8 is about 1.3 V. 

The input signal is then not transferred 

to the output of the mixer 

board. When S 3 is closed and S 4 is 

open, the potential at junction R 7 -R 8 is 

about 1.8 V and the input signal is 

39 

100n 

C15 

C12 

10µ 

63V 

28 VDD VCC 

SYSTEM MUTE 

5 

VIN1 

ACOM 

13 

22 

VIN2 IC2 

7 

VIN3 VOUTL 

14 

20 

9 

VIN4 

VIN5 

DATA OUT 

3 

18 

11 

16 

VIN6 VOUTR 

VIN7 

SSM2163 

VIN8 

SHIELD 

15 

6 

27 

26 

25 

24 

DATA IN 

CLK 

WRITE 

LD 

SHIELD 

SHIELD 

SHIELD 

SHIELD 

8 

19 

21 

23 

C13 

10µ 63V 

4 

10 

1 

DGND AGND 

17 

VSS VEE 

5V 

5V 

JP1 

2 

D 

C 

W 

820Ω 

12 

R35 

D1 

5V 

5V 

+5V 

POWER 

0 

– 5V 

970037 - 11 

5V 

5V 

L 

R 

Anzeige

microcontroller control 

The microcontroller used in the mixer board is a Type ST62T25 from SGS- 

Thomson. This device has 20 I/O lines which can be switched at will when 

the program is running to form an input or output in various configurations. 

Sixteen of them may be linked to the internal analogue-to-digital converter 

(ADC) to form the lines via which the positions of the slide potentiometers and 

the switches are written. 

The positions of the slide potentiometers and the switches are continuously 

sampled by the software, which also processes the results of the analogueto-digital 

conversions and stores them in the internal RAM. When even one of 

these settings differs from the previous one, the microcontroller passes all 

data to the mixer IC. 

The allocation of the port lines to potentiometers and switches looks rather 

untidy on the circuit diagram; this results from the requirement of making the 

layout of the printed-circuit board as straightforward as possible. Note that 

PA 0 –PA 3 cannot be linked to the ADC. 

n 

initialization 

scanning 

potentiometers 

settings 

altered 

? 

y 

send to 

mixer board 

970037 - 17 

applied to the left-hand channel. 

When S 4 is closed and S 3 open, the 

potential at junction R 7 -R 8 is about 2.5 

V and the input signal is applied to the 

right-hand channel. When both 

switches are closed, the potential 

across R 8 is 5 V and the input signal is 

applied to both outputs (mono). 

To carry out these tasks, a special 

program is loaded in the ST62T25. The 

manner in which this device converts 

the direct voltages at the inputs into a 


Analogue-to-digital 

The simplest way of operating the ADC of 

the ST62 controllers is starting the conversion 

and waiting in a loop until the EOC 

(end-of-conversion) bit goes high, whereupon 

the normal process of the program 

can be continued. From a software point 

of view, this may be all right, but it appears 

that owing to internal and external noise it 

is impossible to obtain an accuracy of even 

6 bits. The use of a WAIT command, which 

disables a large part of the processor, 

brings about a real improvement. The ADC 

generates an interrupt when the EOC goes 

high and this re-enables the processor. 

The SSM2123 uses only six bits to set the 

attenuators and there is therefore nothing 

against ignoring the two LSBs of the ADC. 

Yet, this is not sufficient to achieve a stable 

setting of the mixer IC over the whole 

control range of the potentiometers: in 

some situations D 2 flashes to indicate that new data are being transferred. 

This is, of course, self-evident, because even if the two LSBs are ignored they 

continue to exert some influence. If bit 1 is high, a variation of just one LSB 

is sufficient to affect higher, relevant bits. 

There are ways and means of solving this. For instance, it would be possible 

to sample a potentiometer a couple of times in succession and average the 

results. In the present design, a simple means is used: the entire 8-bit ADC value 

is retained, while the next sample is accepted only if its value differs by more 

than two LSBs from the previous one. This matter is less problematical when 

the state of the switches is written: in that case only four results are possible: 

some noise and a slight accuracy in the potential divider do not matter much 

Digital to the mixer IC 

Sending data to the SSM2163 is fairly straightforward. Three lines from port 

A of the ST6225 carry the CLK, WRITE and data signals. Unfortunately, with 

this configuration of the port, this cannot be effected with direct setting and 

resetting of the port bits. Because of space considerations, this cannot be further 

explained here. 

control signal for IC 2 is described in 

the box on p. xx. 

Circuit IC 1 is supplied by an asymmetrical 

voltage of 5 V, and IC 2 by a 

symmetrical line of ±5 V. Diode D 1 

functions as on/off indicator. Jump lead 

JP 1 enables the screen of the cables to 

be linked to the negative supply line. 

This may be useful if hum occurs, 

unlikely though that may be. [970037] 

(to be continued) 

In passing … 

Sitting in a small restaurant the 

other day, I was intrigued to overhear 

a conversation between a small 

group of people at the next table 

about the relative merits or otherwise 

of digital cameras (apparently 

one of them had been given one as 

a Christmas present). It was argued 

that pictures from a digital camera 

are nowhere near as good as those 

from a ‘real’ camera. In a sense, this 

is true, of course, because a goodquality 

colour transparency requires 

about 80 million pixels (the tiny 

individual spots that all pictures are 

made up of), whereas a digital camera 

manages not much more than a 

few hundred thousand pixels (since 

its CCD – charge-coupled device – 

contains only that number of 

diodes). A simple 8-bit image allows 

each pixel to have 256 possible 

shades of grey or basic colours. 

With 24-bit colour (which most 

graphics programs use) each pixel 

can have more than 15 million different 

colour possibilities. Now, the 

memory in a digital camera just cannot 

cope with this: even a dozen 24bit 

colour snapshots, each made up 

of only 200,000 pixels, will occupy 

7.2 MB of memory (may be compressed 

by, for instance, JPEG to 

about 2 MB). A picture of the quality 

needed by a magazine such as 

ours would take up 60-80 MB. However, 

in publishing, a dark-room is a 

thing of the past. Colour-negative 

and transparency film is now normally 

machine-processed and 

scanned directly into digital files. 

The pictures are then edited on a 

computer screen and inserted 

directly into the electronic page 

make-up. Consequently, many professional 

photographers are already 

using digital cameras for situations 

where speed is important. 

It is fairly certain that in the not too 

distant future there will be more digital 

cameras around than optical 

ones, in spite of the former’s current 

imperfections. After all, the average 

person will only want to take snapshots, 

not become a photographic 

artist. If you are not convinced of 

this, look at the number of camcorders 

around which operate on 

the same principle as the digital still 

camera. 

41 

LS/975024

µP-controlled mixer board No more noisy controls – ever ... - WebHTB

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