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Constructional Project Part 1 by Tim Blythman Many people now have home theatre/ surround sound systems and need to control the volume of six or more audio channels. This Multi-Channel Volume Control has a touchscreen and receives infrared remote signals. It can be expanded up to 20 channels, although six or eight will suit most applications. Multi-Channel Volume Control T he Multi-Channel Volume Control can be built as a stand-alone unit in its own enclosure, or as a modular system that can be incorporated into a multi-channel preamplifier or amplifier. It can control up to 20 channels in synchrony. A microcontroller senses inputs from the touchscreen, rotary encoder or infrared remote control and drives digital potentiometers to control the volume of each channel. One of the reasons behind this project was our publication of the Hummingbird Amplifier module in the December 2022 issue. That small, low-cost 100W amplifier module makes it easy to build an amplifier with four, six or even more channels. You need something like this design to adjust the volume of all those modules together. The alternative would be a fourganged or six-ganged potentiometer, but then you would have poor tracking and messy wiring. Pots can also go scratchy after a while, unlike a digital pot, which generally continues working flawlessly for decades. This design is based on our previous Touchscreen Digital Preamp (September & October 2022 issues), Fig.1: the performance is similar to the Touchscreen Digital Preamp, with THD+N typically less than 0.002% across much of the range. The dashed lines show the degradation with 1μF ceramic coupling caps instead of 2.2μF tantalums. 30 which only sports two channels and is not expandable. Besides this design, another option is Phil Prosser’s “The Digital Potentiometer” (March 2024). While it is a two-­channel design, it does allow you to gang up multiple boards. However, it is a more ‘bare bones’ design than this one, lacking the touchscreen option or any onboard audio sockets. Performance The Multi-Channel Volume Control is based on a Baxandall-style volume control circuit but uses a digital potentiometer rather than the regular Fig.2: a plot of THD+N against input level. The sweet spot for input levels is ~1.5V, but anywhere in the range of 1-2.3V RMS is fine. Above 2.3V RMS starts to cause clipping while lower levels suffer due to the closer noise floor. Practical Electronics | November | 2024 Multi-channel Volume Control, part 1 kind. Handily, this circuit provides a logarithmic response from a linearly changing resistance, making it easy to map the volume settings to levels in decibels. Figs.1-4 show the resulting performance. Fig.1 plots total harmonic distortion plus noise (THD+N) against frequency for an input signal level of 1.6V RMS. As you’d expect, the plots are similar to those for the Touchscreen Digital Preamp. The red plot was taken with a 20Hz22kHz bandwidth, which best represents the normal audible range and thus what you would hear. The cyan/ blue plot was taken with a wider bandwidth, up to 80kHz, which includes the harmonics of higher-­frequency signals. They are not directly audible but could intermodulate to affect audible frequencies. As you can see, THD+N is below 0.001% for up to 2kHz and below 0.002% up to about 7kHz. Fig.2 shows the total harmonic distortion plus noise (THD+N) against signal level for three volume (gain) settings. The distortion is higher at lower signal levels as the fixed noise dominates the smaller signals. The sharp rise around 2.5V RMS is where the Volume Control enters clipping. The sweet spot is with an input signal around 1.5V RMS. Fig.3 shows the crosstalk between channels. The two plots show the extremes that can be expected within a single Volume Module (explained later). The cyan/blue plot shows the Features & Specifications » » » » » » » » » » » » » Can control the volume of four, eight, 12, 16 or 20 channels RCA sockets for inputs and outputs Volume levels/gain settings: mute (-100dB) and -48dB to +16dB 1dB or smaller steps from -30dB to +16dB Digital controls, including touchscreen, rotary encoder and IR remote 2.8in LCD screen shows the volume and mute status Can be programmed to support many NEC-compatible IR codes Each channel can have a preset offset applied Volume and mute settings kept in EEPROM for next power on Optional small OLED status display with rotary encoder THD+N: typically less than 0.002% (see Fig.1) Channel separation: >80dB (see Fig.3) Signal handling: 0.1-2.5V RMS Image source: unsplash.com/photos/HLhmbBw6xpY crosstalk from adjacent channels and is around -80dB or better; these channels share some components. The red plot shows the separation between channels at opposite ends of a module, which is a little better. Fig.4 shows the frequency responses at three gain settings: +5dB (red), 0dB (green) and -5dB (cyan/blue). There is a slight but uniform roll-off at lower frequencies, but the level difference is uniform across the audible band. In other words, the volume adjustment is consistent, as is expected. Modular design While we are boasting up to 20 channels, most constructors will not need that many. So rather than offering a design based around a single PCB, the Volume Control is modular. The Fig.3: channel separation is about 80dB at worst and only for adjacent channels that share an op amp in their audio paths. Practical Electronics | November | 2024 three modules are the Control/Power Supply Module, the four-channel Volume Module and an optional OLED Module. The main Control and Power Supply Module incorporates a microcontroller that converts the user input into the necessary actions to implement the volume control. It also contains all the power supply circuitry needed to drive any other connected modules. The power supply section only needs a 12V AC supply, which can be provided from a small mains transformer or AC plugpack if your system doesn’t already have a suitable source. You probably won’t need a separate transformer if your system has a larger transformer incorporating 12V AC taps. Fig.4: a frequency response plot at three different volume settings (-5dB, 0dB and +5dB). There is only a slight roll-off at the bottom end, and the effect of the volume settings is very uniform across the spectrum. 31 Constructional Project The Control and Power Supply Module has parts on both sides. This side mostly has the power-supply components. Although we haven’t tested it, a pair of ±15V DC rails could be fed to the Control and Power Supply Module instead. The Control/Power Supply module also incorporates an infrared receiver and a 2.8-inch LCD touchscreen. The Module’s PCB is the same size as the LCD panel at 50mm tall, making it a comfortable fit for a 3U rack case, or anything taller. The Volume Module can control the volume of up to four channels and incorporates an AD8403 precision quad digital potentiometer. This Module also has four input and four output RCA sockets, plus other components to buffer and drive the audio signal as it passes through. A third type of module, the OLED Module, is a simple and compact alternative (or supplement) to the touchscreen display. It includes a small OLED screen for those who want something smaller and simpler; its input control is a rotary encoder. The various modules are connected by a ribbon cable punctuated by IDC connectors. This carries all power and control signals between the modules. The simplest configuration is a single Control and Power Supply Module with between one and five Volume Modules, allowing 4-20 channels to have their levels adjusted as they pass through. If the OLED Module is added, only four volume modules can be connected, which limits the number of controlled channels to sixteen. Still, we 32 don’t see that as a major limitation! Due to the various ways this project can be arranged, and the fact that many will be building it as part of a larger system, we will not specify a particular enclosure. You can choose an enclosure based on your requirements that will also fit any amplifier modules, transformers and other necessary bits. We’ll start by describing how each module works. Then, next month, we’ll follow up by outlining the assembly of each type of module, along with instructions on how they are wired together, tested and used. Control and Power Supply Module Fig.5 is the circuit diagram for the Control and Power Supply Module. It receives 12V AC via CON7 or CON8. CON7 is a barrel socket and thus can only accept a single 12V AC input. On the other hand, CON8 has three terminals and is intended to connect to a 24V AC centre-tapped transformer, or two 12V AC phases; however, you could connect a single winding via CON8 if that’s all you had. Either way, the AC supply passes through bridge rectifier BR1 and a pair of 1000µF electrolytic capacitors to provide a nominally ±17V DC supply to be regulated. Feeding in two AC phases via CON8 is preferred as the capacitors only need to hold up through the 10ms of each half-cycle rather than 20ms for a full mains cycle. The board generates five regulat- ed DC rails. REG2 (78L12) and REG4 (79L12) provide +12V and -12V, respectively, to power the analog circuitry. Their outputs are filtered by 100µF capacitors and taken to CON11, which feeds the ribbon cable bus noted earlier, for distribution to the other modules. The +12V rail is further regulated to 5.5V by REG5, an LM317L and its accompanying components. The accompanying resistors and VR2 allow its output voltage to be trimmed to account for resistor tolerances. A 220µF capacitor sits on the output of REG5, and this voltage is also taken to CON11. If the 5.5V rail is adjusted too high, zener diode ZD1 conducts and protects other circuitry downstream. The unregulated positive rail is also dropped via a 22W 5W resistor (to spread dissipation) and reduced to 5V by REG1 (7805). It has a 100µF bypass capacitor on its input and a 220µF filter capacitor on its output. This 5V rail is used on the Control and Power Supply Module to power the LCD touchscreen; the power needed by the LCD backlight, and the resulting higher dissipation, is the main reason why this regulator is a larger TO-220 type, while the others are in smaller TO-92 packages. The 5V rail is reduced to 3.3V by REG3 (MCP1700) to power microcontroller IC9. The 3.3V rail is also available on CON11 to power the microcontroller on the OLED Module. Broadly, the +12V and -12V rails power analog components, while the 5.5V rail primarily powers the digital potentiometers. The 5V and 3.3V rails power the digital components. The separate power domains help to minimise any intrusion of digital signals into the analog, which would affect audio quality. On the Control and Power Supply Module, the remaining circuitry consists mainly of the microcontroller, the LCD touchscreen and their essential ancillaries. IC9 is a 20-pin, 8-bit PIC16F18146 microcontroller. It is powered from the 3.3V rail, so it can easily interface with the LCD controller, which also runs at 3.3V from its own regulator on the LCD’s PCB. A 10kW resistor pulls IC9’s pin 4 MCLR pin to the 3.3V rail, preventing spurious resets, while a 100nF capacitor bypasses its supply to pins 1 and 20. These three pins, plus the Practical Electronics | November | 2024 Multi-channel Volume Control, part 1 Fig.5: the Control and Power Supply Module circuit derives +12V, -12V, +5.5V, +5V and +3.3V rails to power the control circuitry and external modules. The microcontroller handles the LCD touch panel and receives inputs from an infrared receiver. It also sends signals to the two other module types over a 20-way bus via CON11. Practical Electronics | November | 2024 33 Constructional Project PGC and PGD programming pins, go to ICSP header CON10 for in-circuit programming if needed. Another nine of the micro’s digital pins are wired to CON11 to control the other modules. The SCK, MOSI and MISO pins form the SPI serial bus that controls the other modules. The PIC16F18146 can remap its digital pe- 34 ripherals to any digital pin, so the pin allocation was chosen to simplify the PCB routing. Five CS (chip select) lines are also broken out, allowing up to five different slave modules to be independently addressed for the SPI bus. The SHDN (shutdown) line allows the micro to signal to all Volume Modules that their outputs should be muted. The SPI bus pins are also wired to the LCD touchscreen via CON9, along with the other four digital control lines it needs. A pair of Mosfets plus pullup and pulldown resistors power the LED backlight in the LCD touchscreen, controlled by a digital signal (LED_CON) from IC9. Practical Electronics | November | 2024 Multi-channel Volume Control, part 1 Infrared receiver IRRx1 takes its power from the 5V rail, smoothed by the 100W resistor and 1µF capacitor. Its output goes to the last of IC9’s unused pins via a 1kW resistor. These receivers typically have a 30kW internal pullup, so even though it is powered from 5V, it can safely interface to the microcontroller expect- ing 3.3V levels. That’s because the 5V drive is quite weak and easily clamped by the microcontroller’s internal input protection diodes. Volume Module The circuit diagram for a single Volume Module is in Fig.6. CON5 connects to the Control and Power Supply Module’s CON11 via a 20-way ribbon cable. An adjustable padded divider that includes trimpot VR1 is used to derive a 2.75V rail from the 5.5V supply. VR1 trims this voltage, which is bypassed by a 220µF capacitor. Test points are provided to allow easy measurement during setup. The 2.75V rail must be Fig.6: the Volume Module circuit contains four substantially identical volume control stages, each using one channel of the AD8403 precision digital potentiometer. It receives SPI control signals over the 20-way bus via CON5. Practical Electronics | November | 2024 35 Constructional Project Up to five of these Volume Modules can be connected together, allowing the volume of up to 20 channels to be controlled. The modules can be incorporated into a larger system with other parts, like amplifiers. set accurately to maximise the signal swing and ensure symmetrical clipping if the signal level is excessive. The Volume Module has a single AD8403 quad precision digital potentiometer (IC10) powered from the 5.5V rail bypassed by a 100nF capacitor. The potentiometer’s RS (reset) pin is pulled up to the 5.5V rail by 10kW since we do not use this feature (which forces the potentiometers to their midpoints). The remaining digital pins of IC10 (SCK, MOSI, MISO and SHDN) connect to the corresponding lines on the control bus via CON5. The CS pin goes to five-way jumper JP2, so you can choose which of the five CS lines from the microcontroller on the Control Module will control this Volume Module. Each ‘slave’ module in the system is assigned a different CS line. The SHDN pin is also connected to a 47kW pulldown resistor to ground. 36 This ensures that the Volume Control is muted until the microcontroller initialises and decides otherwise. The microcontroller can also drive the pin low to enforce muting, helping to eliminate noises during startup. The remainder of the Volume Module consists of four practically identical analog sections, each using one of the 10kW potentiometers internal to IC10. This combination of four potentiometers (on each Volume Module) and five CS lines gives us the 20-­channel limit of the Multi-Channel Volume Control. Analog circuitry We will describe the operation of the first channel only, as they all work the same. All 16 op amps (in eight IC packages) are high-performance (low noise and distortion) LM833 types powered from the ±12V rails. Each op amp has a 100nF bypass capacitor. A 100kW resistor biases the input from the RCA socket to ground so it doesn’t float if disconnected. The 100W resistors, 470pF capacitor and ferrite bead before the first op amp stage protect the inputs from excessive voltage swing and filter out RF noise and ultrasonic frequencies. The first op amp stage is a unitygain buffer to provide a high input impedance and low source impedance for the subsequent stages. The output signal is AC-coupled and biased to the 2.75V rail. Dual diode D1 clamps the signal if it happens to deviate below the 0V rail or above the 5.5V rail, with the 2.2kW resistor limiting the current that flows in this case. The 0V to 5.5V span has been chosen as the widest that the AD8403 can handle in normal operation. The 2.2kW resistor also forms a divider with the 10kW of the digital potentiometer, meaning that signals up to 2.3V RMS (or 6.5V peak-to-peak) are accepted at the input without clipping via D1. The following two op amp stages implement the Baxandall volume control. The first stage is a unity buffer while also being a type of mixer, while the second stage is an inverting amplifier with a gain of 14.7. One effect of this is that this stage has its output polarity inverted with respect to its input. The potentiometer is connected between the input (‘A’ end) and output (‘B’ end) of these two stages, providing logarithmic gain changes despite the circuit using a linear potentiometer. Another dual diode at the other end of the digital potentiometer’s track to protects it from excessive voltages from the output of the gain stage. When the SHDN signal is asserted (low), the digital potentiometer disconnects the A end of each potentiometer and connects the wiper to the B end of the track; this means that output is fully muted. The potentiometer exhibits a small amount of resistance at the end of each wiper, so even at the extreme ‘low’ setting, a small amount of signal will pass through to the output unless the SHDN signal is used. Another capacitor and resistor bias the signal back to its original ground reference, and the final op amp stage for each channel applies more gain to allow us to get an output swing of up to 2.5V RMS, despite the 5.5V peak-toPractical Electronics | November | 2024 Multi-channel Volume Control, part 1 peak limitation imposed by the digital pot ICs. A 100W resistor between the output and RCA socket isolates the op amp from capacitive loads. There is an option to use a jumper to bypass the last op amp stage to reduce the gain, saving a handful of components in the process. An important feature is that the four digital potentiometers can all be set independently, allowing the channels to have different levels if needed to maintain audio balance. OLED Module The OLED Module provides an optional set of compact, tactile controls. Its circuit is very simple (see Fig.7) as it has been designed so that the PCB also forms a front panel with its components on the back. This Module is intended to be mounted like a bezel over a cutout in an existing panel. It connects to the ribbon cable bus via its CON12, using the same 20-way IDC box header as the other modules. On this Module, only the SPI control lines, the five CS lines, and 3.3V power and ground are connected; its operation is entirely digital and does not require the other supply rails. A 14-pin 8-bit PIC16F15224 microcontroller (IC11) is connected to This optional OLED Module provides a more compact status display and a rotary volume control. If you don’t want to use the touchscreen, you could leave it off and make this as the full interface (and with an IR receiver wired back to the Control Module). the SPI bus as a slave, with its CS line picked from the five on the bus by JP7. JP7 uses solder shorting pads rather than pin headers and a jumper to keep this module compact. A further three pins of IC11 are connected to a rotary encoder, which provides the user input. Two of these pins are for the quadrature encoder and are either pulled to ground by the contacts in the rotary encoder or pulled up by the 10kW resistors. The pushbutton in the rotary encoder is connected to a similar arrangement, with each of these three pins also having a 100nF capacitor to ground to help filter out contact bounce. Two further pins from IC11 are also used to drive the OLED via an I 2C serial interface. This OLED shows the Volume Control’s state as it is updated. The typical arrangement for 8-bit PICs is also present, consisting of a supply bypass capacitor between 3.3V and ground with a pullup to the MCLR pin. These three pins and the PGC and PGD pins used for in-circuit programming are also taken to the incircuit programming header, CON13. Volume control range limitations Using a quad 8-bit digital potentiometer (IC10), rather than a purpose-­ designed volume control IC, helps keep costs down. However, it imposes a limitation on the effective volume control range. A dedicated volume control IC might give a range of 100dB or more, but an 8-bit digital potentiometer only has Fig.7: the OLED Module circuit; it is a simple microcontroller-based board with an OLED and rotary encoder. Since it only requires the 3.3V rail and the SPI bus, only 13 of the 20 pins on CON12 are connected Practical Electronics | November | 2024 37 Constructional Project 256 steps, meaning it has an effective volume control range of about 60dB. That’s enough for most applications. Still, you should check that the highest output level makes sense, or you might end up without good control at lower volume settings. The default configuration has a maximum gain of 16dB, giving a fullscale output of around 2V RMS from an input signal close to 300mV RMS. That’s well below line level, which is typically more like 775mV RMS. In this configuration, the lowest signal output level with a 1V RMS input before muting is 4mV RMS or -48dB. The steps above that are 9mV (-41dB), 15mV (-36.5dB), 21mV (-33.5dB), 27mV (-31.4dB), 33mV (-29.6dB) and steps of about 1dB or less from there up. While -48dB is 64dB below the maximum output level, the steps are pretty large until around -30dB, giving a useful control range of about 46dB. A variation of, say, 10dB between different input signal sources will reduce the effective volume control range to 36dB. That gives a 4000:1 ratio between the highest and lowest power output with decent control; if maximum volume results in 100W from your amplifier, you will have fine control down to just 25mW. That’s certainly good enough, but the more the maximum possible gain is above what you need, the more apparent the steps at lower volume levels will become. The system’s overall gain is set with resistors, so you can easily adjust it at the construction stage. Practically speaking, if your power amplifier will reach full power with less than 2V RMS (as many will, and all your input-signals are at least line-level), we suggest you omit the final 6dB op amp gain stages. That will give you 6dB more room at the lower end of the volume range. Control firmware We’ve chosen an 8-bit microcontroller for the Control Module as the requirements are not too burdensome. Although it is driving an LCD panel, the user interface is not complex, with only a single screen configuration needed (no menus etc). The LCD screen is overlaid by a touch panel, which the micro scans for user input. We know of two otherwise inter38 Parts List – Multi-Channel Volume Control 1 Control and Power Supply Module (see below) 1-5 Volume Modules (each handles four channels; see below) 1 OLED Module (optional; see below) 1m length of 20-way ribbon cable (cut to suit application) 1 universal IR remote control (optional; see text) 1 12V AC single winding or 24V AC centre-tapped transformer and appropriate wiring/fusing RCA cables to interface to existing hardware Other mounting hardware to suit your application Control and Power Supply Module 1 double-sided PCB coded 01111222, 87 × 50mm 1 2.8in LCD touchscreen with ILI9341 controller & SPI interface 1 2.1mm or 2.5mm DC jack socket (CON7; optional) 1 3-way 5mm/5.08mm pitch screw terminal block (CON8; optional) 1 14-way 0.1in female header (CON9; for LCD touchscreen) 1 5-way right-angle pin header (CON10; optional, for ICSP) 1 20-way box header and IDC inline plug (CON11) OR 1 20-way IDC transition header (CON11) 1 500W mini top-adjust trimpot (VR2) 1 3-pin infrared receiver, 38kHz (IRRx1) [eg, TSOP4138, TSOP33438 etc] 9 M3 × 5mm panhead machine screws 4 M3 × 12mm tapped spacers 1 M3 hex nut and washer (for mounting REG1) Semiconductors 1 W04M bridge rectifier (BR1) 1 PIC16F18146-I/SO microcontroller programmed with 0111122B.HEX, wide SOIC-20 (IC9) 1 IRLML2244TRPBF or SSM3J372R 20V 1A+ logic-level P-channel Mosfet, SOT23 (Q1) 1 2N7002 60V 115mA N-channel Mosfet, SOT-23 (Q2) 1 7805 +5V 1A linear regulator, TO-220 (REG1) 1 78L12 +12V 100mA linear regulator, TO-92 (REG2) 1 MCP1700-3.3 3.3V 250mA linear regulator, SOT-23 (REG3) 1 79L12 -12V 100mA linear regulator, TO-92 (REG4) 1 LM317L 100mA adjustable linear regulator, TO-92 (REG5) 1 5.6V 1W zener diode (ZD1) Capacitors 2 1000μF 25V electrolytic 2 220μF 10V electrolytic 4 100μF 16V electrolytic 1 1μF 10V X7R ceramic, SMD M3216/1206 size 2 100nF 50V X7R ceramic, SMD M3216/1206 size Resistors (all SMD M3216/1206 size 1% except as noted) 2 1kW 1 910W 1 560W 2 10kW 1 100W 1 22W 5% 5W axial 1 110W changeable versions of this panel with touch panels rotated by 180° compared to each other. Our workaround is to display the user control buttons in the bottom half of the screen. Touches in the top half of the panel are assumed to correspond to touches on the bottom half of the rotated display, so either should work with no changes. In retrospect, we might have chosen a more powerful (and faster) microcontroller, such as the 16-bit PIC24FJ- 256GA702. At the time of writing, they are not dissimilar in price, although the SSOP version of the PIC24FJ256GA702 is somewhat more tricky to solder than the SOIC part we are using. The lesser flash memory available on the PIC16F18146 meant that we needed to compress the large font that’s necessary to provide a clear and legible display. The PIC16F18146 microcontroller we used for this project can store Practical Electronics | November | 2024 Multi-channel Volume Control, part 1 Volume Module 1 double-sided PCB coded 01111221, 82 × 94mm 2 quad right-angle RCA socket assemblies (CON1, CON2) [Altronics P0214] 1 20-way box header and IDC inline plug (CON5) OR 1 20-way IDC transition header (CON5) 4 SMD ferrite beads, M3216/1206 size (FB1-FB4) [Fair-Rite 2512066017Y1] 1 5×2 pin header and a jumper shunt (JP2) 1 500W mini top-adjust trimpot (VR1) 8 M3 × 6mm panhead machine screws 4 M3 × 12mmm tapped spacers (or other mounting hardware to suit the application) Semiconductors 8 BAT54S dual series schottky diodes, SOT-23 (D1-D8) 8(6) LM833 low-noise dual op amps, SOIC-8 (IC1-IC8) 1 AD8403ARZ10 quad precision digital potentiometer, SOIC-28 (IC10) Capacitors (all SMD M3216/1206 size unless noted) 1 220μF 10V electrolytic 4 22μF 16V electrolytic 4 10μF 16V electrolytic 4 2.2μF 25V SMA size SMD tantalum 🔷 11(9) 100nF 50V X7R 4 470pF 50V C0G/NP0 4 100pF 50V C0G/NP0 Resistors (all SMD M3216/1206 size 1%) 8 100kW 5 47kW 4 22kW 5 10kW 10(2) 1kW 4 680W 12 100W 4 2.2kW n numbers in brackets refer to requirements if the last op amp gain stage is omitted 🔷 not recommended but 22μF 4V+ X5R/X7R ceramics in M3216/1206 can be substituted (see panel on “Lessons learned during development”) OLED Module 1 double-sided PCB coded 01111223, 51 × 76mm 1 0.96in I2C OLED module (MOD1) 1 PIC16F15224-I/SL microcontroller programmed with 0111122C.HEX, SOIC-14 (IC11) 1 pulse-type rotary encoder with 18 tooth spline shaft (RE1) 1 knob to suit RE1 1 20-way SMD box header or 20-way dual-row SMD header (CON12) 🔵 1 20-way IDC inline plug 1 5-way pin header (CON13; optional, for ICSP) 4 M3 screws, washers and nuts to suit mounting requirements 4 100nF 50V X7R M3216/1206 size SMD ceramic capacitors 4 10kW M3216/1206 size SMD 1% resistors several short pieces of solid wire (eg, component lead offcuts) 🔵 can be made by cutting 10 rows from a long DIL SMD header around 16kB of font data in its flash memory, so you can see how important careful managing font data is. We were very close to running out of flash memory before we started looking into compression. Font compression To display text on a graphical screen requires some form of font data to encode the ‘glyphs’ (character representations) to show. The glyphs usualPractical Electronics | November | 2024 ly correspond to a subset of the ASCII character set, perhaps with minor alterations to suit the project, such as including the degrees (°) symbol instead of some other less-used character. The font data format we typically use is widely known. It consists of two header bytes describing the width and height of the font in pixels, followed by another two bytes denoting the first ASCII character code point (eg, 32 for a space is typical) and the Control Module Kit SC6793 (~£31): also comes with 1m of ribbon cable Volume Module Kit SC6794 (~£34): includes all the listed parts OLED Module Kit SC6795 (~£15): includes all the listed parts Each kit includes all the parts listed under each module in the parts list. The only other items needed are a case, power supply and remote control. number of characters within the font. This is followed by bits of pixel data in bitmap form for each character within the font. Since each byte can hold eight pixels, a small 8×8 pixel font of 95 characters (the full ASCII set) takes up 764 bytes. Fig.a (in the panel overleaf) shows how such a font is translated from bitmap data into the corresponding C code. Other languages, such as MMBasic, use a similar format adapted to the syntax of the specific language. A 16×24 pixel font, our typical choice for legible text on a typical 3.5in LCD panel, uses around six times as many bytes or just over 4kB. Larger fonts are possible by upscaling smaller fonts, but the result does not look as good. The flash memory of these 8-bit PICs uses 14-bit words. They are often described as having 28kB of flash memory, but it is arranged as 16k x 14-bit words rather than 28k x 8-bit bytes. The ideal font size for the large dB display in this project is 44×60 pixels; that would take 31,354 bytes to store as a full ASCII font, which obviously wouldn’t fit in the PIC16F18146. We can reduce the space required by only encoding the characters we need. For example, truncating the 12×16 font we use for buttons and smaller text to only include from the space character up to the capital letters brings its size down from 2284 bytes to 1420 bytes. Similarly, we reduced the font used for the dB display to the digits 0-9, a blank space, a negative sign, a decimal point (or full stop) and the ‘d’ and ‘B’ characters. That takes it down to 4954 bytes before we apply RLE compression. 39 Constructional Project RLE compression RLE stands for run length encoding and takes advantage of repetitive values in data; in this case, runs of the same pixel colour. RLE has existed in displays and computing for at least 50 years. RLE is used in the JPEG image standard (although it is only part of the compression used there). It was also used by fax machines (remember them?). For our implementation, rather than storing bitmap data, the data encodes a run of pixels of the same state (on/off). The top bit indicates whether the pixels are on or off (ie, add 128 for on pixels), while the remaining seven lower bits encode the number of consecutive pixels with that property. Fig.b shows a glyph encoded using our RLE strategy. This small font is not a great example for this sort of compression, as the resulting data has ballooned from eight bytes up to 25 bytes. RLE could be seen to encode how often the pixel state changes on each line. So characters such as “1” should encode better than “0” and, indeed, the RLE data only comes to 15 bytes for “1” in this particular font. The space character for this font is encoded as a single byte of value 64, meaning all 64 pixels are off. This variability in glyph size is accounted for by adding a header specifying the number of bytes before each group of RLE data. It’s easy to write code to step through the data jumping forward by the header’s byte count until we reach the glyph we need to display; we then have the count of the number of bytes we need to decode. The RLE-compressed font we ended up with only uses 2013 bytes (including all header data) compared to the 4954 bytes for the uncompressed version. As an extreme example, the space character for this large font (which consists of 2640 black pixels) takes up 330 bytes uncompressed, but only 22 bytes after compression. Another extreme, the ‘0’ character, takes up the same 330 bytes uncompressed but only 196 bytes when compressed, a 40% saving. For the numerals 0-9 and space, the RLE encoding provides a notable saving for fonts as small as 16×24 pixels. Different font subsets, including letters like M and W, will not compress as well as they include shorter runs. Other advantages The microcontroller can copy the RLE-compressed data to the screen faster than bitmap data. There is less data to be read from flash in the RLE case and the decoding is simpler too. The uncompressed bitmap data needs to be decoded one bit at a time. Each individual bit has its value checked, then the appropriate colour pixel is written to the LCD. For the RLE data, a group of consecutive pixels is decoded and can be efficiently sent to the LCD in a tight loop. Similarly, the code to show the RLE-encoded font is smaller 0b01111100 → 0b11000110 0b11000110 0b11000110 0b11000110 0b11000110 0b01111100 0b00000000 0 1  1  1  1  1 0  0   → 1  1 0  0  0 1  1 0   1  1 0  0  0 1  1 0   1  1 0  0  0 1  1 0   1  1 0  0  0 1  1 0   1  1 0  0  0 1  1 0   0 1  1  1  1  1 0  0   0  0  0  0  0  0  0  0 const char TinyFont[764] = { 0x08, 0x08, 0x20, 0x5F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // Space 0x18, 0x3C, 0x3C, 0x18, 0x18, 0x00, 0x18, 0x00, // ! 0x66, 0x66, 0x24, 0x00, 0x00, 0x00, 0x00, 0x00, // " 0x6C, 0x6C, 0xFE, 0x6C, 0xFE, 0x6C, 0x6C, 0x00, // # 0x18, 0x3E, 0x60, 0x3C, 0x06, 0x7C, 0x18, 0x00, // $ 0x00, 0xC6, 0xCC, 0x18, 0x30, 0x66, 0xC6, 0x00, // % 0x38, 0x6C, 0x38, 0x76, 0xDC, 0xCC, 0x76, 0x00, // & ... 0x7c, 0xC6, 0xC6, 0xC6, 0xC6, 0xC6, 0x7C, 0x00, // 0 Fig.a: bitmap font data is encoded with a ‘1’ bit indicating a pixel is set to the foreground colour or a ‘0’ bit if the pixel is the background colour. The last line shows the bitmap data for the number 0. than its bitmap equivalent, about half the size in program flash memory. The bitmap routine also calls another function to perform a multiplication; there would be further savings if the same routine were not needed by other code. Being able to update the display more quickly is a clear upside. We suspect the effect would not be so pronounced with a faster microcontroller, but it would still be present. OLED fonts We have not used RLE fonts in the firmware for the OLED Module. Firstly, these displays are monochrome, so they do not require individual pixel colours to be written. Instead, they expect data to arrive in blocks of eight bits at a time, which maps to eight pixels on the screen. In other words, they natively work with bitmap data. Also, the OLED has a much lower resolution, so it does not need to display very large fonts as often. Given that larger fonts benefit more from RLE, there is less incentive to apply it to the smaller fonts. Bitmaps image compression The Silicon Chip logo shown on the LCD screen is a bitmap and is stored similarly to the bitmap fonts. So, we experimented with a different encoding that stores a run of pixels (from 0 to 15) in each nibble of a byte. The top nibble is assumed to be the background colour, while the lower nibble is the foreground colour. The code to decode this data is similar in speed and program flash memory usage, and we found this algorithm offered about 40% compression on the Silicon Chip logo. So we used this encoding for the logos and icons that are displayed. Conclusion RLE encoding a larger font gives superior image quality than upscaling a smaller font, is faster and can use a similar amount of flash or even less. So it seems like the way to go. The only downside is the extra complexity in the initial encoding. Some font examples can be found at: www.rinkydinkelectronics.com/r_fonts.php Download the FontTweak Font editing program from: www.c-com.com.au/MMedit.htm convert to RLE encoding 0111110011000110110001101100011011000110110001100111110000000000 ↓ 1x0,5x1,2x0,2x1,3x0,2x1,1x0,2x1,3x0,2x1,1x0,2x1,3x0,2x1, 1x0,2x1,3x0,2x1,1x0,2x1,3x0,2x1,2x0,5x1,10x0 ↓ 1,133,2,130,3,130,1,130,3,130,1,130,3,130,1,130,3,130,1,130,3,130,2,133,10 Fig.b: mapping of the pixels to bitmap data is from top left to bottom right in horizontal rows. For an eight-pixel wide font, each row maps to one byte of data. With RLE encoding, each run of same-coloured pixels maps to a byte of data, so one byte can encode up to 127 pixels. It is much more effective for fonts that have more glyphs. 40 Practical Electronics | November | 2024 Multi-channel Volume Control, part 1 The compression technique used is called run-length encoding (RLE). Rather than storing simple bitmaps for the characters to show on the display, RLE stores a run of pixels as its count and on or off state (hence runlength encoding). The resulting font data is less than half its original size. That is helpful when the font uses up nearly a third of the available flash memory. Another advantage is that the encoding is easy to decompress for display, making screen updates faster! We’ve gone into this in greater detail in a panel, for those interested in that sort of thing. Infrared remote control The IR decoding routine is designed to receive signals encoded with the NEC protocol. It uses pulse position encoding with 38kHz modulation; the IR receiver demodulates this carrier, so the microcontroller only needs to decode the pulse encoding. The microcontroller uses a timer/ counter to measure the pulse lengths and thus extract the necessary data. The NEC protocol transmits 32 data bits, including a device byte, its inverse, a command code and the command code’s inverse. The inverses allow the microcontroller to reject corrupted codes. The Control Module can be programmed to accept different device and command codes, so it can be used with fixed or programmable IR remote controls. We’ll get more into those details later. The Control Module communicates over the SPI bus on the ribbon cable. If an OLED module is present, data is sent out from the Control Module to update its display; the Control Module also receives data back if there has been activity on the rotary encoder or its button. The micro on the control module signals the digital potentiometers to set a new volume level whenever the volume is changed. If the mute function is activated, the volume is ramped down as low as possible, after which the SHDN pin is pulled low to mute the audio fully. Unmuting is simply the reverse. The SHDN pin goes high, then the volume level ramps up. The same process occurs when the unit is powered on, avoiding clicks and pops. Otherwise, the main loop updates Practical Electronics | November | 2024 Lessons learned during development We solved two significant but subtle problems in developing this design. The first was that, to save components and simplify setup, we generated the +2.75V rail on the Power Supply & Control board and fed it via the ribbon cable to the Volume Modules. The problem with this was that any tiny amount of noise or ripple picked up in the ribbon cable ends up getting injected into the signals because this is a virtual ground rail. Adding extra capacitance to ground for this rail on each Volume Module didn’t fix it. The only way to get acceptable performance was to move the +2.75V rail generation circuitry onto the Volume Modules. The other problem we ran into was that we accidentally used 1μF X7R multilayer ceramic capacitors to couple the signal to the last op amp stage (eg, from pin 1 of IC2a to pin 3 of IC3a). This type of capacitor simply isn’t very linear and the result was a significant rise in distortion below 200Hz, shown by the dashed portions of the curves in Fig.1. There are two solutions to this. Our preferred solution is to switch to using 2.2μF tantalum capacitors, which luckily are available in the same size (the SMA tantalum case is basically the same dimensions as M3216/1206 ceramic chip capacitors). Being electrolytic capacitors, these are not as linear as say plastic film types, but significantly more linear than X7R ceramics. Unlike ceramic capacitors, tantalum capacitors are polarised. There is 2.75V between these points, so the capacitors are orientated with the positive leads to the op amp pin 1 outputs. As a less-desirable alternative, X7R multi-layer ceramic capacitors can still be used but with a significantly higher value; at least 10μF, and ideally 22μF or more. That pushes the distortion down so instead of starting below 200Hz, it starts below 20Hz, which is in the inaudible part of the frequency range. There usually won’t be much signal below 20Hz; our concern is that, if there is, the resulting distortion harmonics could be in the audible range. Hence our preference for the tantalum capacitors. the display when necessary and reads input from the touch panel. We had provision for an IR receiver on the OLED Module, but since the Control Module is mandatory and already has an IR receiver, we have not fitted it to our prototypes and there is no support for it in the firmware. OLED module firmware The microcontroller on the OLED Module monitors the rotary encoder and button for action, sends and receives data to and from the Control Module and updates the OLED screen as required. It acts as an SPI slave device, meaning it must be ready to respond whenever the Control Module wants to communicate. For simplicity, we designed the communication between these two modules to only use a single byte in each direction. The Master sends out the volume level (in steps of 0.5dB) relative to a value of 128. A value of zero means that the mute is active. Thus, the OLED Module doesn’t need to retain any state data; it simply updates its display whenever data is received. The data that the OLED Module sends back to the Control Module is in a similar format. The OLED module counts the number of rotary encoder steps (forward or backward) that have accumulated and sends that to the Control Module, offset from a value of 128. The offset helps avoid receiving spurious commands if no OLED Module is connected. The data is designed not to use the values of 0 or 255 (00000000 and 11111111 in binary) as might occur if the data line was pulled up or pulled down permanently. A special value of 51 (00110011 in binary) indicates a press of the rotary encoder’s button. This works as a toggle, so the OLED Module does not need to know the current state and simply reports to the Control Module that the mute state needs to change. Next month That’s all we can fit in this month’s article; we will describe the construction and assembly of the Preamp in the following issue. Short-form kits for all three modules are available, so you might like to gather the parts together in preparation. PE 41