2

Outline

• Digital music• The power of FPGA• The “DigitalSynth” project

– Hardware– Software

• Conclusion• Demo

3

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.

4

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?

5

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.

6

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),

7

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).

8

Block diagram

9

Oscillator unit

• The design uses oscillators to generate two kinds of waves:

– sawtooth

– square

10

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.

11

Amplitude modulation

• The ADSR envelope model is used for modulation.

12

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.

13

Amplitude modulation

14

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.

15

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.

16

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.

17

Polyphony arbitration

18

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.

19

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.

20

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.

21

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.

22

Audio effects unit

23

Audio effects unit

• Audio effects can be represented as a system:

24

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.

25

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.

26

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.

27

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.

28

The software application

29

The software application

30

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.

31