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L i ght up your music with the . . . LE D MUSICOL OUR C HRISTMAS IS NOT far off so if you don’t have a light show up and running already you’d better get started! Our new LED Musicolour makes it easier than ever. It drives up to 16 sets of LEDs directly. These can be strips, strings, single LEDs or a set – whatever, as long as they run off 12-24V DC. You can even build multiple LED Musicolours and run them in parallel, to control 32 or even 64 sets of LEDs. You can drive the LED Musicolour 30  Silicon Chip using any line source such as CD or MP3 player, or you plug in an SD card which has been loaded with WAV files. In the latter case, it can be a selfcontained sound and light controller with no need for any extra hardware apart from a power supply. The unit supports high-capacity SDHC cards so you can load it up with lots of music (organised in folders) and use a universal infrared remote control to skip through them. If you build more than one, you can use one as the “master” to play the audio and feed it to the others for a synchronised light show, as well as to an amplifier so you can hear the music at the same time. The LED Musicolour uses a 40MHz, 16-bit digital signal controller which is actually a specialised DSP (digital signal processor) microcontroller. It is powerful enough to do real-time frequency analysis using a Discrete Fourier Transform. The unit also incorporates a Wolfson WM8759 audio DAC for good-quality line level sound siliconchip.com.au Pt.1: By NICHOLAS VINEN Now you can have a kaleidoscope of colour which continually changes in time to music. This consists of 16 strings of LEDs which are individually controlled by 16 frequency bands. Louder signals in each of those bands means that the respective LED string will be brighter. Use it for a Christmas light show, a disco or just for fun when playing music. output. It all fits into a tiny plastic case which seems quite innocuous considering all the fancy processing it is performing. Echoes of the past This unit is intended as an easierto-use version of the DSP Musicolour which was published in SILICON CHIP from June-August 2008. It was also somewhat inspired by the Digital Lighting Controller featured in the October-December 2010 issues. siliconchip.com.au The Digital Lighting Controller controls the brightness of up to 32 mains-powered lights or LED strips, in time to music. But its light sequences are pre-arranged, ie, you program a specific sequence to go along with each sound file. That is a somewhat laborious process but it gives you full control over the light show. It has no option to feed in external audio and its output sound quality is a bit so-so. Also, the Digital Lighting Controller required you to build a master unit and between one and four slave units, with one slave controlling eight lights. The new LED Musicolour, on the other hand, is fully self-contained and can control 16 LED strips per unit. And as explained earlier, you can chain multiple LED Musicolour units together if you need to control more LEDs. So to sum up, the LED Musicolour is more flexible (having an audio input) and is easier to build and set up but doesn’t give you quite as much control as the Digital Lighting Controller from October 2012  31 8x DUAL N-ch MOSFETs – 12/24V LED STRIP + +12/24V Q1a 16-BIT SERIAL PORT EXPANDER (IC5, IC6) – 12/24V LED STRIP + Q1b (UP TO 16 LED STRIPS) – ADC INPUTS SPI1 12/24V LED STRIP + Q8b LINE OUTPUT LINE INPUT BUFFER (IC4) LPF 34kHz CON11  INFRARED RECEIVER (IRD1) DIGITAL SIGNAL CONTROLLER (MICRO + DSP) (IC1) 24-bit 192kHz STEREO DAC (IC2) 2 DCI (I S) PWM (MCLK) CON12 SPI2 POWER/ACK LED (LED1) SD CARD (CON13) CLOCK 4 (IC3) BCLK Fig.1: block diagram for the LED Musicolour. At its heart is IC1, a dsPIC33 Digital Signal Controller/Microcontroller. IC5 and IC6 are used to drive the Mosfets which control the brightness of up to 16 LED strips. Audio can either be fed in to CON11 or played back from an SD card in CON13, via stereo DAC IC2, to CON12. 2010. Having said that, the LED Musicolour’s light shows are quite impressive and it is much simpler to build. How it works Take a look now at the block diagram of Fig.1. At its heart is IC1, a dsPIC33 digital signal controller (DSC). As stated above, this is a 40MHz 16-bit microcontroller and it features a fixedpoint digital signal processing (DSP) unit and extra features to enhance its performance, such as multi-ported random access memory (RAM). The brightness of the 16 strips of LEDs is controlled by switching 16 Mosfets at 200Hz, using pulse width modulation (PWM). IC1 only has 28 pins and that isn’t enough to drive the Mosfets directly and leave some over for other purposes. So the Mosfet gates are driven from the outputs of two 8-bit serial-to-parallel latch ICs (IC5 & IC6) which act as a port expander. IC1 updates their output state using one of its internal Serial Peripheral Interface units, SPI1. When a given output from IC5 or IC6 is high, this turns on the corresponding Mosfet, which sinks current from the negative supply line of the corresponding LED strip. The positive supply lines are permanently tied to the 12-24V supply. By controlling the proportion of the time that the Mosfet 32  Silicon Chip is on, we control the average current through the LED strip and therefore its brightness. Audio can be fed into the unit via 3.5mm phono socket CON11. The audio signals (left & right) are then AC-coupled to IC4 which buffers them and applies a DC offset (~1.65V). This offset is necessary since the circuitry runs off a single DC supply rail. Before being fed to IC1, the signals go through a low-pass filter with a corner frequency of 34kHz. This removes high-frequency signals which could cause aliasing when IC1 digitises the audio at a sampling rate of 48kHz, using its internal 12-bit analog-to-digital converter. Alternatively, the unit can play back the audio from an SD card at CON13. IC1 reads WAV audio data off the card using its other SPI peripheral, SPI2. It then simultaneously analyses the data to determine the brightness of the LED strips and sends it to IC2, a stereo digital-to-analog converter (DAC), using its data converter interface (DCI) unit. Audio data is transmitted from IC1 to IC2 in I2S format. The DAC also requires a “master clock” which is a multiple of the sample clock – in this case, 192 times. For example, when playing 48kHz audio, the master clock is 9.216MHz. This is generated by IC1 using a PWM output, which outputs a rate proportional to its instruction clock. The ratio we are using is 4:1, which gives an instruction clock of 34-37MHz, depending on the audio sampling rate. This is derived from an 8MHz crystal by changing the multiplication and division factors of IC1’s internal phase-locked loop (PLL). IC3 acts as a clock divider to convert the master clock (192 times sample rate) to the appropriate bit clock rate for the I2S stream. This has a fixed ratio; we are transmitting 48 bits for each sample (24 per channel) which means we need a division ratio of 192 ÷ 48 = 4. This bit clock is fed to both IC1 and IC2. The audio from IC2 is AC-coupled to CON12, a 3.5mm phono socket. If you want to control the LED Musicolour with an infrared remote control, which is handy when playing back WAV files (but not strictly necessary), the commands are received by IRD1 and sent to IC1 which decodes them. The Power/Ack LED (LED1) flashes in response; it is normally lit while the unit is powered, to indicate that it is operating. Circuit description Refer now to Fig.2 for the full circuit details. As shown, IC1 sends serial data to and controls shift registers IC5 siliconchip.com.au LED Musicolour Parts List 1 PCB, code 16110121, 103 x 118mm 1 front panel, code 16110122, 134.5 x 30mm 1 rear panel, code 16110123, 134.5 x 30mm 1 instrument case, 140 x 110 x 35mm (Jaycar HB5970, Altronics H0472) 1-16 12V or 24V LED strips with 2-pin or 4-pin sockets 1 12V or 24V DC power supply sufficient for LED strips 1 SD or SDHC card (optional) 1 universal infrared remote control (optional) (eg, Jaycar AR1726, Altronics A1012) 2 PCB-mount M205 fuse clips (F1) 1 10A M205 fuse (F1) 8 8-pin dual row right-angle pin headers, 2.54mm pitch (may be snapped from larger headers) (CON1-CON8) 1 PCB-mount DC socket (CON9) 1 2-way right-angle pluggable terminal block (CON10) 2 PCB-mount switched 3.5mm phono sockets (CON11, CON12) 1 Oupin SMD SD card socket (or equivalent) (CON13) (Altronics P5720) & IC6 using four data lines: DS (serial data), SRCK (serial clock), LCK (latch control) and MR (master reset). While microcontroller IC1 runs off 3.3V, IC5 and IC6 run off 5V, to drive Mosfets Q1a-Q8b with sufficient voltage to switch them on. Unfortunately, a 74HC595 with a 5V supply will not work reliably with 3.3V input signals (according to the data sheet), so these signals need to be level shifted to 5V. For SRCK, DS and LCK, the corresponding IC1 pins (RB8, RB6 & RB5) are set as open drain outputs. These pins are 5V tolerant and with 1kΩ pull-up resistor to +5V, they can operate to at least 1MHz. For MR, we have used a different arrangement as we want this line to be low by default, keeping the outputs of shift registers IC5 and IC6 off until IC1 brings it high. NPN transistor Q9 acts as an inverter/level shifter; the 100kΩ pull-up resistor between its base and siliconchip.com.au 1 8MHz HC-49 crystal (X1) 1 6073B type TO-220 heatsink (Jaycar HH8502, Altronics H0630) 1 M3 x 6mm machine screw 1 M3 x 10mm machine screw 2 M3 shakeproof washers 2 M3 hex nuts 4 No.4 x 9mm self-tapping screws 1 28-pin narrow IC socket 1 8-pin IC socket (optional) 1 14-pin IC socket (optional) 2 16-pin IC sockets (optional) Semiconductors 1 dsPIC33FJ128GP802-I/SP microcontroller programmed with 1611012A.hex (IC1) 1 WM8759 24-bit 192kHz stereo DAC (IC2) (Element14 1776274) 1 74HC393 dual binary counter (IC3) 1 LM358 dual op amp (IC4) 2 74HC595 serial-to-parallel shift registers (IC5, IC6) 8 Si4944DY dual SMD Mosfets (or equivalent) (Q1-Q8) (Jaycar ZK8821) 1 BC547 NPN transistor (Q9) 1 BC327 PNP transistor (Q10) 1 7805T 5V 1A regulator (REG1) collector holds it on when IC1 is not driving it. IC5 and IC6 drive the Mosfet gates via 100Ω series resistors which form low-pass RC filters in conjunction with the Mosfet gate capacitances (about 1nF each). This prevents oscillation and overshoot when switching on or off, due to copper track inductance. We are using eight dual-Mosfets to keep the cost and size down. Each can switch up to 9.4A at 30V. The LED strings are connected to a series of 4-pin headers. Their outer two pins are connected to the high side of the supply (12-24V) and the inner two to the Mosfet drain. That way, you can plug the LED strip connector in either way around and it will still work. It also gives more contact area to safely allow up to 2A per LED string. Audio inputs Turning now to the audio inputs, 1 LM3940IT-3.3 3.3V LDO regulator (REG2) 1 1N4004 1A diode (D1) 4 BAT85 small signal Schottky diodes (D2-D5) 1 green 3mm LED (LED1) 1 3-pin infrared receiver (IRD1) (Jaycar ZD1952, Altronics Z1611A) Capacitors 1 220µF 25V low-ESR electrolytic 2 220µF 16V electrolytic 5 100µF 25V electrolytic 4 10µF 16V electrolytic 1 10µF 6V SMD ceramic (3216) 13 100nF MKT or MMC 2 10nF MKT or MMC 2 100pF ceramic 2 33pF ceramic Resistors (0.25W, 1%) 2 1MΩ 6 1kΩ 2 120kΩ 2 470Ω 3 100kΩ 1 220Ω 3 47kΩ 19 100Ω 5 10kΩ 1 10Ω 1 4.7kΩ Note: the PCB and front & rear panels are available from the SILICON CHIP PartShop. the left & right channel signals from CON11 first pass through low-pass RC filters consisting of 100Ω resistors and 100pF capacitors. These filters prevent RF (radio frequency) signals from entering the device. There are also 1MΩ bias resistors in case the source’s output is AC-coupled. The value of these resistors can be lowered if you want to feed in the output of an iPod, eg, to 1kΩ each. The signals are then AC-coupled by 100nF capacitors to a resistive divider/ DC bias network. This forms a highpass filter with a -3dB point at 7Hz. It also sets the input impedance of the device to 1MΩ || 220kΩ = 180kΩ. The 100kΩ/120kΩ dividers allow an input signal of up to 2.2V RMS before clipping. The signal fed into IC1 is limited by its supply rails to 3.3V peak-to-peak. This translates to 3.3V ÷ (2√2) = 1.17V RMS. However, many CD/DVD/Blu-ray players, computers October 2012  33 F1 10A 12/24V DC (3A MAX) D1 1N4004 A CON9 K 12/24V DC 10A MAX 2 +12/24VF REG1 7805 +5V OUT IN REG2 LM3940IT-3.3 GND 100F IN GND 100F 1 +3.3V OUT 100F 220 CON10 100nF +5V 2 K D2 100nF 100k 1M 100pF 100nF RB6 2 8 2 470 1 IC4a RchIn RA4 LINE INPUT 6 RB7 1.65V 100F CON11 1M 100pF 6 K D5 D3 A 24 25 5 K 23 IC4: LM358 100nF 100k 7 22 10k 120k 100 +3.3V LchIn IC4b 4 7 470 LCK 12 MR 1k AN4/RB2 RB9 10k 14 IC1 dsPIC33FJ128GP802 10nF 120k SRCK DS RA1 Vcore/Vcap A 3 17 15 RA0 10F D4 A RB8 RB5 20 K Vdd LED1 K 1k 100nF 13 MCLR ON/ACK 3 D2–D5: BAT85 28 AVdd 1 1  100  100F 3 10k A 100 IR RECEIVER IRD1 1k 100nF 26 RB4 AN5/RB3 MCLK 18 LRC 11 SD 21 BCK RB10 RB11 RB1 RB12 RB0 RB13 RA3/OSC2 RB14 RB15 RA2/OSC1 AVss Vss 8 27 10nF 16 5 DACEN 4 SDpwrCon 10 X1 8.0MHz 9 Vss 19 33pF 33pF A GND ANALOG GND +3.3V SD CARD SOCKET WP 8 7 6 5 4 3 2 1 9 47k DATA OUT (from card) CLK Q10 BC327 10 DATA IN (to card) E B 4.7k C 10k CS 10F CD 47k 100nF CON13 SC 2012 LED MUSICOLOUR Fig.2: the LED Musicolour circuit diagram. Audio fed into CON11 is filtered, AC-coupled, attenuated, buffered and filtered again before passing to IC1’s internal ADC. IC1 communicates with IC5 & IC6 via a serial (SPI) bus and these ICs drives Mosfets Q1a-Q8b. Another SPI bus is used to read/write the SD card in CON13 while a similar I2S serial bus is used to send audio data to stereo DAC IC2. IC3 ensures that the DAC’s master clock (MCLK) and serial data bit clock (BCK) are synchronised. and so on will put out 2V RMS or more. So we attenuate the signal by a factor of 120kΩ ÷ (100kΩ + 120kΩ) = 0.55 to allow for this. The bottom end of these resistive dividers is connected to a half-supply point of about 1.65V, derived using two 34  Silicon Chip 10kΩ resistors and filtered by a 100µF capacitor. The signals fed to analog input pins AN4 and AN5 of IC1 thus swing symmetrically around its halfsupply point. Op amps IC4a & IC4b buffer the input signal so that the following RC low-pass filter (470Ω/10nF) does not load up the signal source or previous stages. Schottky diodes D2-D5 protect the inputs of IC4 from going outside its supply rails when the unit is not powered. While not strictly necessary, this makes the circuit more “bulletproof”. siliconchip.com.au +12/24VF +5V CON1a 4 3 1k 12 11 14 13 16 Vdd LCK Q1 SRCK Q2 Q3 DS Q5 OE Q6 MR Q7' Q7 10 Vss C E 15 Q9 BC547 1 G 3 11 14 13 10 LCK G D Q2a +5V S CON7b CON7a CON8b CON8a 4 3 +12/24VF 1 2 LEDs14 Q0 15 TO Q5a, Q5b, Q6a, Q6b 1 2 3 IC6 Q4 4 74HC595 5 Q6 Q7 Q7' S S S S 6 1 7 O3 CP O2 IC3a 8x100 2 O1 MR O0 6 13 5 4 12 3 14 1 2 3 4 O1 O0 8 9 10 100nF 11 1N4004 IRD1 LRCIN DIN 1 2 K 3 A IC2 WM8759 EN CAP HPOUTL DEEMPH DGND AGND 12 10 7 Audio outputs As explained earlier, the master clock for DAC IC2 (ie, from output RB7 5 220F 10F 100nF BC327, BC547 D2 D2 D1 D1 ous sub-harmonics being detected. The filters (mostly) prevent that from happening. LINE OUTPUT 9 Si4944DY G2 G1S2 S1 220F 6 HPOUTR BKIN LED1 K 8 AVdd 11 DVdd FMT 10F 100nF 100nF MCLK 47k siliconchip.com.au IC3b MR O2 +5V 13 The low-pass RC filters following IC4a & IC4b both have a -3dB point of 34kHz and are tuned to the sampling rate of IC1’s ADC (48kHz). If there is much signal above the Nyquist frequency (48kHz / 2 = 24kHz), this can cause aliasing which results in spuri- O3 CP 7 10F K +5V IC3: 74HC393 +3.3V A LOW ESR Q8b G Q1-Q8: Si4944DY DUAL MOSFET Vss 8 BAT85 220F 25V D Q8a 14 9 7 D Q7b G G LEDs15 5 8 D Q7a G 3 LEDs16 6 7 8 1 2 4 LEDs13 5 D Q5 A Q2b G S TO Q3a, Q3b, Q4a & Q4b 9 Q3 MR Q1b G 6 Q2 OE 7 D D LEDs3 5 8 7 Q1 DS 3 LEDs4 6 S 6 SRCK 1 S 100nF 12 4 7 Q1a CON2a 2 LEDs1 5 D 2 8 16 Vdd LEDs2 8 IC5 Q4 4 74HC595 5 10k 100k Q0 6 8x100 CON2b 1 2 +5V 100nF B CON1b 1k 1k 7805, LM3940IT-3.3 B E C CON12 GND IN GND OUT of IC1) is generated by a PWM peripheral which divides IC1’s instruction clock by four. This is fed to IC2 and also to IC3a’s CP-bar clock input (pin 1). Its O1 output is one-quarter of the input frequency and this is the audio data bit clock, BCK. BCK is fed to both October 2012  35 Specifications • • LED voltage: 12-24V DC LED current: up to 10A total (ie, 120-240W maximum) • Number of LED strings: up to 16 per unit • LED control method: PWM, 200Hz, 255 brightness steps • LED connectors: 2-pin or 4-pin male headers, 2.54mm pitch • Audio input: 0.5-2.2V RMS nominal, 180kΩ || 100pF input impedance • Audio output: 1.1V RMS, THD+N 0.004%, signal-to-noise ratio 100dB • Audio file support: 8-48kHz 16-bit mono or stereo WAV files • Maximum directory depth: eight levels • • Maximum files per directory: 100 • • • Dimensions: 140 x 110 x 35mm Control method: universal infrared remote (optional) Supply voltage: 12-24V DC Current drain: ~110mA at 12V IC1 & IC2, which use it to send and receive the audio data respectively. IC3a’s O3 output (1/16th MCLK) is connected to input CP-bar of IC3b, the other half of the dual binary counter. This is for testing purposes; the IC3b outputs give various frequencies related to the sampling rate being used. For example, with a 48kHz sampling rate, pin 9 of IC3 (O2) will measure 72kHz (48kHz x 1.5) and pin 8 (O3) will measure 36kHz (48kHz x 0.75). The I2S data stream from IC1’s DCI peripheral comes from pins RB9 (LRC) and RB4 (SD). LRC is the left/right clock and represents the sampling rate (eg, 48kHz). It is produced by the data framing output of the DCI module. SD is the serial audio data and this is clocked according to the signal received at its RB10 input (pin 21). By default, IC2 is in standby mode as its enable pin (pin 4) is pulled to ground by a 47kΩ resistor. When IC1 is transmitting audio, it brings output RB1 (pin 5) high, pulling IC2’s enable pin high and thus turning on the DAC. IC2 has a pair of bypass capacitors (MKT and electrolytic) between each pair of supply pins, digital (DVDD/ GND) and analog (AVDD/AGND). 36  Silicon Chip Its format input (pin 13) is tied high to 3.3V, setting it to I2S mode. Its DEEMPH input (pin 12) is low since we don’t need digital de-emphasis. A pair of capacitors between its CAP pin (pin 5) and AGND filter its internal half-supply rail. Audio is available at HPOUTL (pin 9) and HPOUTR (pin 6). These signals are AC-coupled using 220µF electrolytic capacitors, as IC2 can drive loads down to 16Ω. The 1kΩ DC bias resistors set the average output level to 0V. SD card interface The SD card interface is quite simple and is a tweaked version of the same interface we have used in the past. Microcontroller input RB11 (pin 22) is connected to the socket’s Card Detect line which is pulled to ground when a card is inserted. IC1’s weak internal pull-up is enabled for RB11, allowing it to sense when this occurs. The SD card is operated in 1-wire mode and IC1’s SPI2 unit is used to send and receive data. This is mapped to pins RB15 (pin 26, card select), RB14 (pin 25, data to card), RB13 (pin 24, serial clock) and RB12 (pin 23, data from card). The card select (CS) and data from card (DATA OUT) lines are pulled up to VDD to prevent any card operations from occurring when IC1 is reset or being programmed. The SD card’s VDD line is not connected directly to 3.3V but rather switched by PNP transistor Q10, which is controlled by output RB0 (pin 4) of the micro. This allows it to turn power on for the card only after it has been inserted. The associated 100nF and 10µF capacitors bypass the SD card’s supply, while a 10Ω series resistor prevents excessive current from being pulled from the 3.3V rail when the SD card is first powered up. A 47kΩ bleeder resistor shunts any leakage from Q10 so that the supply bypass capacitors don’t charge up when it is off. Remaining parts Infrared receiver IRD1 detects IR pulses from the remote and converts them into digital signals which it sends to input RA1 (pin 3) of IC1. IC1 then uses a pin-change interrupt handler routine to decode the Philips RC5coded transmissions. IRD1 is powered from a 5V supply which is filtered using a 100Ω series resistor and 100µF and 100nF capacitors. IC1 has two 100nF supply bypass capacitors, for its 3.3V VDD and AVDD lines, plus a 10µF capacitor on the output of its internal 2.5V regulator at pin 20 (VCAP). A 10µF SMD monolithic ceramic capacitor is the preferred type here, as it has good performance and a long life. A 10µF through-hole tantalum capacitor could also be used and this is catered for in the PCB design. Power indicator/acknowledge LED1 is switched from output RA0 (pin 2) of IC1. It’s fed via a 220Ω series currentlimiting resistor, giving a LED current of around 5mA when it is on, ie, when RA0 is driven low. Power supply The incoming 12-24V DC supply is applied to either DC socket CON9 (up to 3A) or to pluggable terminal block CON10 (up to 10A). The positive line is fed to the 16 LED strips via a 10A fuse, while a 220µF low-ESR electrolytic capacitor prevents the supply voltage from drooping too much when the LED strips are switched on in unison. Current also flows from CON9/ CON10 to 5V regulator REG1 via reverse polarity protection diode D1. The 5V output from REG1 powers infrared receiver IRD1, buffer op amp IC4, serial latches IC5 & IC6 and the analog section of DAC IC2. REG2, 3.3V low-dropout regulator, is also fed from 5V and supplies power to the remaining components: microcontroller IC1, clock divider IC3, the SD card (via transistor Q10), LED1 and the digital section of DAC IC2. It also acts as the half-supply bias generator for the analog inputs (IC4a & IC4b). Software For those interested, the source code will be available for download from the SILICON CHIP website. We won’t go into detail here but will just give some basic information on its operation. IC1 only needs to shift one word of data (ie, 16 bits) to IC5/IC6 in order to update the state of all LED outputs. IC5 and IC6 are cascaded, with IC5’s Q’H output going to IC6’s serial input. This is triggered off one of IC1’s internal timers that is set to a rate of 200Hz. When the timer interrupt occurs, the SPI1 peripheral is used to shift a new word to IC5 & IC6, turning on any LEDs that have a brightness above zero. A second timer is then set to generate an interrupt at the time when the dimsiliconchip.com.au The 16-channel outputs from the unit are connected to the coloured LED strings via 4-way pin headers (two pins for the positive rail and two for the switched negative rail). All parts are on a single PCB, with no external wiring (apart from the LED strings). mest LED strip needs to be turned off for the correct duty cycle; this delay is calculated as 5ms x (duty cycle) ÷ 100. When this second interrupt is triggered, the handler routine turns off that LED strip and any others with an identical brightness, then re-schedules the timer for the next dimmest LED strip and so on. As a result, depending on the exact brightness value for each LED output, up to 200 x 16 = 3200 interrupts per second are needed to control the LEDs. Thus the overhead is relatively low, given that IC1 runs at around 35MHz. The LED brightness is updated for every 1024 audio samples. A 1k Fast Fourier Transform (FFT) is then applied, with a Blackman-Harris window, to convert the time domain data to the frequency domain. The magnitudes of the resulting vectors are calculated, giving the frequency content for each bins and the bins are siliconchip.com.au averaged in bands to give the brightness values for the LED outputs. Audio playback Since IC2 (the stereo DAC) has no volume control, we need the ability to digitally attenuate the audio data sent to it. Thus, we send 24-bit audio data, even though the WAV files only store 16-bit samples. They are converted to 24 bits by multiplying them by the 8-bit volume level. For 48kHz audio, IC1’s internal clock is set to 36.857MHz using its PLL (8MHz x 129 ÷ 28). MCLK is set to this frequency divided by four, ie, 9.214MHz and BCLK = 9.214MHz ÷ 4 = 2.304MHz. This is the rate at which audio data is serially transmitted to DAC IC2. The DCI (data converter interface) is set to I2S mode with 24 bits per sample, giving us a sample clock (LRCK) of 2.304MHz ÷ 24 ÷ 2 = 47.991kHz, which is very close to the target of 48kHz. Getting the DCI to transmit 24-bit data is a little tricky since it works with 16-bit words. We set it to transmit two 12-bit words per sample and the audio data is stored in memory in 32-bit chunks, with eight bits of each unused. The rest of the software is fairly straightforward and involves the reuse of much of our existing dsPIC33 codebase, including the SD card interface, FAT16/32 file system layer, interrupt-based infrared protocol decoding and so on. Besides the LED strip control, the main area of new code for this project is the audio playback layer which has been enhanced to support WAV files residing in multiple folder. It also supports folder hierarchies several levels deep. That’s all we have space for this month. Next month, we will describe the assembly and show you how to SC drive it. October 2012  37