DSP Musicolour

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It may be apparent from the description so far that there are no op amp filters, so the frequency bands
for each channel are user-selectable. Furthermore,different arithmetic operations can be applied to the
digitised audio data to amplify or attenuate it.This means that each input channel is effectively post-
amplified in software. Moreover, the various input channels can be selected and mixed in software with
different weighting. In previous designs, the switching was accomplished using a toggle switch.Providing
chaser modes is also much easier with a digital design.Again, it’s all done in the software and no extra
circuitry is involved.

Circuit operation Let’s now look in more detail at the circuit operation of the DSP Musicolour. It’s based
on microcontroller IC1, a dsPIC30F4011 from manufacturer Microchip. This combinesmost of the
common peripherals of a PIC with a high-performance RISC(reduced instruction set
computer)processing core that has instructions optimised for digital signal processing(DSP).The
Microchip dsPIC series of microcontrollers incorporate DSP features like MAC (multiply
accumulate),variable bit shifting, bit-reversed addressing modes, dual data fetching (multiple operands),
high-precision arithmetic operations (including fractional hardware divide and multiply) and multiple 40-
bit accumulators. All DSP instructions are single cycle, meaning the dsPIC series is surprisingly powerful,
with throughputs in the tens of MIPS (millions of instructions per second). In this design, IC1 runs off an
internalsystem clock operating at about 29MHz. Among other things, it’s responsible fordigitising the
audio inputs; processing the audio inputs (including computing the FFT); synchronising with the mains
frequency and implementing real-time phase control; driving the multiplexed LED display and
responding to key presses. The operation of the software is explained in greater detail later in this

Output stages

The output stages of the DSP Musicolourhave been uprated using heavy-dutyBTA41-600B triacs. These
are isolated tab triacs rated at 600V, with continuouscurrent ratings of up to 40A and a surge current
rating of close to 400A.This high surge-current rating isnecessary in order for the triacs towith stand the
large fault currents which can occur if a 150W floodlightblows its filament. This is a particular problem if
the floodlight is facing upwards. In that case, the brokenfilament can flail around and short to the
adjacent filament support. The resulting arc continues until the stemfuse inside the lamp blows.In
addition, the software programmed into the dsPIC has a facility to setthe filament pre heat current of
each output channel. This helps minimisesurge currents to protect the lamps.Note that the four triacs,
which arehoused in TOP3 packages, are mountedin pairs on the main board and eachpair shares a
heatsink. These heatsinksare necessary, as the triacs will get hotduring normal operation, even if each
channel is only rated at 800W.Each triac is triggered by the dsPICvia a MOC3021 optocoupler (OPTO1to
OPTO4), to ensure full mains isolationfrom the low voltage part ofthe circuit. As shown, IC1’s ‘output
compare’ pins (OC1 to OC4) are usedto drive the internal LEDs of the optocouplers,which then switch on
the triacs until the next zero crossing of the mains waveform.

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