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This is a brief description of a project in progress to play back samples in real time from an SD card to an I2S based DAC on the Midibox. It will play a number of samples simultaneously, mixing the output to the DAC, the number of samples depends upon the performance of the SD card.
You will need:
I have currently only tested this on the LPC1769 based MIOS32 Midibox. It should work in theory if compiled for the STM32, but I don't have one to try!
The system currently looks for a file called 'bank.1' on the SD card root. This is a text file, but must have Unix style line feeds (not DOS style) - just use your friendly programmer's file editor to create one (or make it on a Mac ). It will read up to 64 lines from this file, each line mapping a MIDI note, and the format is e.g:
0x39 c_a.raw 0x3a c_bf.raw
where the first number is the MIDI note number in Hex, and after the space is the filename of the sample to play (up to 8.3 characters).
It looks for the samples in the root of the SD card.
The sample format is fixed as: 44.1kHz 16 bit RAW little endian. Quite a mouthful. Basically use Audacity, Soundforge etc to convert whatever samples you have into this format.
Currently you have to provide a note mapping for each note you want to sound, and usually therefore you will need to generate a sample of the right pitch for each note. This is a lot of samples for a full key range, but hey hopefully you only have to do this once!