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

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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 Elektor Electronics 3/97 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 21 NC (SHIELD) VIN5 9 20 VIN4 VCC 10 19 NC (SHIELD) 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 Elektor Electronics 3/97 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
Page 1 and 2: The audio mixer board presented in
Page 3: 3 5 Figure 3. Block diagram of the

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

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