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A Musical Beginning!

This is my first blog!

I’ll be writing about the hobby project I did this Saturday and Sunday. Friday evening i had a thought of getting a bit musical. I don’t have any instrument with me. So decided on reviving one of my old project titled ‘project mozart’, which was originally written in C language for 8051 kind of microcontrollers. The 8051 I used for that couldn’t support anything more than 2MIPS. At that power only continuous notes, like that from a violin continuously bowed, could be produced. This ability alone didn’t satisfy me. So i decided to implement it in my newly bought Atmega32 controller, supporting upto 16MIPS.

I had architecture of the circuit, part in my mind, and part in the C file. The circuit is rather simple with a microcontroller, a DAC0808 digital to analog convertor, and a 741 op amp configured as a current to voltage convertor. Breadboarded it quickly. Then ported the C file to the new hardware.

My expectations out of the device were as follows. I wanted a device like a piano/synthesiser, that has 32keys (White + black). There are 12 keys per octave on a standard piano/keyboard. This touches 3 octaves, with 2 in full and one partial. For my design it stretched form 175.28Hz to 1108.7kHz. (F2 to C5). Another aim was to have a full polyphony. You can press any number of keys, you should get mixture of all that notes. In addition to this, another objective — to have the Attack-Decay-Sustain-Release sort of sound (sound similar to that of a bell, which is not continuous like a violin). The sample rate chosen was 2793.6kHz for legacy reason and because, anything more is not computationally possible on that 8 bit chip.

The working of the software is as follows (Sorry i can’t make it any simpler). The software would scan the keyboard and identify the keys pressed at approx. 10 times per second. For the keys pressed the timer ISR, would If a new key is pressed, corresponding sinusoidal signal will be played. The sinusoidal signal is made know to the program in the form of a table of size 256bytes, which together represented the values of a single cycle of a sinusoid. If I send each of the table values to the DAC in each sample interval again and again, cycling through the wavetable, I will get a sine wave of 2793.6/256 = 10.955Hz. If I send every 16th entry of the table to the DAC, i will get, 10.955*16=175.28Hz sine wave, which is almost F2. Similarly you can calculate which sample to be played with the help of a bit music theory, which you can learn from wikipedia.

By the method presented so far, we could only get continuous note like that of a violin/saxophone. I need notes like that of a piano/bell/guitar. For this I will generate an envelope based on the duration for which each key is pressed, and will multiply that envelope with the continuous signal produced. This way, I could do the Attack-Sustain-Release phases. The decay phase is yet to be implemented.

At this poor bit depth(8bit) and poor sample rate too, the sound looks pretty good. I know of some possible additions, like addition of wave tables for more real life instruments, a full attack-decay-sustain-release cycle for each instrument, better sampling rate, better bit depth for the sound etc…. Your suggestions and links to your project pages are welcome.

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