This project has compiled a set of specifications over its lifetime which influence the design of its hardware (i.e. the add-on board) and the capabilities of the device and its software overall. The buildup of these "specs" happened fairly naturally, or arbitrarily; however, these need to be fine-tuned for the upcoming hardware iteration since it aims to be a more-or-less "finished" design. As a result, a few of these specifications have changed. I'll take this chance to list out all of the current design specifications, as well as discuss some of the decisions that led up to them.

I'm sharing these now since the schematic for the next add-on board is nearly complete. These are the specifications it follows, and the next project log will show how the implemented circuits meet these requirements.

Sampling rate and buffer size

These two values determine the speed and quantity of incoming data that needs to be processed. Limits for these values depend on the given application; for DSP PAW, educational and audio applications are the primary target.

Educational projects will generally lean towards simplicity, so we would like to avoid super-fast sampling rates and huge sample buffer sizes. On the other end, too low of a sampling rate would cause slow algorithm reactions and make testing a nuisance.

For the lower bound, a fairly arbitrary choice of 8 kHz was made. This allows for working with slower signals as well as some audio since the frequency is a telephony standard. This rate is also easy to create with microcontroller (MCU) clock -- the MCU needs an 8 kHz clock to sample signals at 8 kHz.

The upper bound sampling rate was inspired by audio applications. The two most common audio recording rates are 44.1 kHz and 48 kHz. Generating a 44.1 kHz clock proved to be difficult, especially since the one clock would also have to support the lower frequencies (e.g. 8 kHz). So, design leaned towards 48 kHz, ultimately choosing its double (96 kHz) as the maximum. This gives a maximum Nyquist frequency of 48 kHz, meaning signals up to 48 kHz can be sampled without aliasing/distortion.

The STM32L476 microcontroller can actually support sampling at up to 5 MHz, but for audio and education there is little need to go faster than the chosen limit. Getting anywhere close to the MHz range would both reduce the algorithm execution window to an unusable size, and interfere with the microcontroller’s essential USB communications with the computer.

For buffer size, the configurable range was made to be between 100 and 4,096 samples. The upper limit was partially a result of the microcontroller's constrained memory, though this allows for up to half a second of signal data when sampling at 8 kHz. Smaller buffer sizes are handy for simple algorithms and/or faster algorithm reaction times.

Signal amplitude

The other primary factor of a signal apart from its frequency is its amplitude. Supporting larger amplitudes means flexibility with external signals, though it also requires caution regarding electrical safety of the hardware. The MCU can only handle voltages between 0V and the ADC reference voltage (3.3V by default), so the add-on board needs to scale signals to or from that range.

Previously, an arbitrary decision to support +/- 3.3V was made. This range is wide, and allowed us to rely on the MCU’s default ADC reference. The next design iteration will see a reduction to +/- 2V, for a few reasons: first, this means the addition of an external ADC reference which will eliminate power supply noise from affecting the signals; second, the MCU's use of the 2V reference will lead to better accuracy and precision...

Third, the project's target applications do not need the additional range that was previously allowed. Educational applications will either use the on-board signal generator, which follows the chosen limit, or external hardware which can most often be configured to an acceptable amplitude. For audio, line levels should practically always be within +/- 2V. For the rare case that incoming signals exceed this specification, protection diodes are being added to the add-on board.

Other specifications

The other features of the add-on board are more or less "set in stone," as they have not changed since the initial prototype:

• One signal input port and one signal output port: supporting multiple DSP streams at once would be a daunting task.
• One signal generator output: there is little if any need for multiple generator outputs.
• Two "parameter" potentiometers: more may be added if enough applications warrant it; a software interface for this feature could also be made.
• 0.1"-pitch headers and 3.5mm jacks for the signal I/O: these seem to be the most common and easy-to-use connectors.
• MCU interface through the "ST Morpho" headers: chosen since the project is meant for NUCLEO development boards, though this could change or expand if other boards are to be supported.