dsp: initial commit

wip/dsp.md

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+# digital signal processing 
+
+A boring lecture about signals and systems, and its digital implementation in DSP.  
+Borderline 'song' less 'poetic'.  
+
+The actual material (text that is read) is in Japanese.  
+
+## Outline
+### Signals and Systems
+- Linear time-invariant systems (and nonlinearity)
+- Signal transformations
+- Impulse signal
+- ...
+
+### Digital Signal Systems
+- The impossibility of real continuous signals in digital systems
+- Discrete time
+- ...