Outline

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Outline

• Digital music
• The power of FPGA
• The DigitalSynth project
• Hardware
• Software
• Conclusion
• Demo

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Digital music

• Music has a powerful presence in our culture.
• Instruments have evolved from rocks and bones to
  digital devices, becoming more and more
  sophisticated.

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Digital music

• Music has a powerful presence in our culture.
• Instruments have evolved from rocks and bones to
  digital devices, becoming more and more
  sophisticated.
• Digital instruments have been around for a long
  time already.
• High performance high prices?

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The power of FPGA

• Newer families of FPGA are both powerful and
  cheap. But are they suitable for digital
  instruments?
• The main advantage is that the architecture is
  easily reconfigurable.
• FPGAs have started to offer most of what
  expensive chips do
• high speed circuits,
• large memory blocks,
• multipliers, etc.

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The DigitalSynth project

• The design is based on two components
• hardware (the synthesizer with four-note
  polyphony, capable of applying digital audio
  effects (delay, echo, reverb, vibrato, flange),
  capable of playing MIDI files and recording MIDI
  files),

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The DigitalSynth project

• The design is based on two components
• hardware (the synthesizer with four-note
  polyphony, capable of applying digital audio
  effects (delay, echo, reverb, vibrato, flange),
  capable of playing MIDI files and recording MIDI
  files),
• software (the user interface that handles the
  MIDI files, displays their music sheet and
  controls the audio effects).

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Block diagram
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Oscillator unit

• The design uses oscillators to generate two kinds
  of waves
• sawtooth
• square

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Oscillator unit

• There are four oscillators. Each of them
  generates one wave whose type can be selected by
  the user.
• The amplitude of the generated wave is modulated
  ? AM (amplitude modulation) synthesis.

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Amplitude modulation

• The ADSR envelope model is used for modulation.

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Amplitude modulation

• The attack phase starts when a key is
  pressed.The decay phase follows.The sustain
  phase lasts as long as the key is pressed.The
  release phase starts when the key is released.
• Modulation is realized by multiplying the audio
  data by the values of the ADSR envelope.

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Amplitude modulation
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Note selector unit

• There are two sources of control
• the keyboard,
• the software application.
• When the user plays a MIDI file on the FPGA
  board, the software application controls the four
  oscillators.In the rest of the time, the
  keyboard controls them.

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Keyboard unit

• The design uses 17 keys of the PS/2 keyboard.
  Pressing a valid combination of one, two, three
  or four notes will generate an equal number of
  waves.

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Polyphony arbitration

• There are 17 keys and only 4 oscillators, so a
  polyphony arbitration rule is needed
• if there are more than four keys pressed, the
  excess of notes will not be rendered,
• if one of the four pressed keys is released, then
  a new key can be handled.
• By keeping track of the status of the 17 keys,
  the design determines which notes get to be
  rendered by the four oscillators.

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Polyphony arbitration
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USB unit

• The design implements the Digilent Parallel
  Interface Model.
• The software application controls the wave type,
  the audio effects parameters and the oscillators
  during MIDI file playing.

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USB unit

• On the software side, USB communication means
  calling the methods that are defined in the
  dpcutil.dll dynamic library.
• On the hardware side, USB communication needs
  implementing a parallel communication protocol
  that uses an 8-bit data bus and strobes.

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Audio effects unit

• The design allows applying audio effects such as
  delay, echo, reverb, vibrato and flange on the
  sounds generated by the oscillators.
• The design generates audio samples with the
  frequency of 48 kHz.
• These effects use samples generated at various
  moments in the past, and for this purpose they
  use a circular buffer.

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Audio effects unit

• Writing in the circular buffer is done with the
  frequency of 48 kHz.
• Reading is allowed in the rest of the time.
• Writing is done at a certain address which is
  increased for every new sample of data.
• Reading at various offsets in relation to the
  write address leads to accessing older data.

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Audio effects unit
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Audio effects unit

• Audio effects can be represented as a system

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Audio effects unit

• The delay effect requires reading data with a
  constant delay.
• The echo effect is similar to the delay effect,
  but its system has feedback, for more depth.(The
  larger the feedback, the longer the effect is
  perceived.)
• The reverb effect requires reading several data
  with constant delays.

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Audio effects unit

• The vibrato effect requires reading data with a
  modulated delay. The modulation can be sinusoidal
  or triangular. The system outputs only the
  processed data.
• The flange effect is similar to the vibrato
  effect, but its system has feedback for more
  depth, and outputs a mix of the original and the
  processed data.

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SPI unit

• The result of the oscillators is processed by the
  effect units or it can pass unmodified.
• In order to be output by the speaker, data must
  be serially sent to the digital-to-analog
  converter.
• The converter receives the samples and outputs
  corresponding voltages. The speaker transforms
  voltage values into sounds.

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The software application

• DigitalSynth provides a user interface that
  allows
• opening format 1 MIDI files and displaying the
  music sheet,
• playing the MIDI file on the FPGA board,
• recording a new MIDI file from the FPGA board,
• controlling the audio effects.

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The software application
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The software application
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Conclusion

• FPGA architecture is suitable for digital
  instruments (and more).
• DSP functions can be easily implemented.
• The Digilent Nexys board offers the possibility
  to experiment and even innovate.

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