Readme

README.md

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+# Manchester encoder/decoder
+
+# What
+[Manchester code](https://en.wikipedia.org/wiki/Manchester_code) is an encoding algorithm created originally to store data in the Manchester university's Mark 1 computer drum disk. Later used to store data on audio tape, in ethernet communications (base10), NFC etc.
+
+This is my implementation of the specifications. There are two python scripts, one for encoding and one for decoding. The only audio format supported is WAV. This implementation tollerates a theoretical maximum phase shift (tape speed change) of 50%, and accepts silence and noise (at lower levels than the signal) before start and after end of signal.
+
+# Why
+I needed a simple and interoperable way to store data on my (Pat80)[https://github.com/penguin86/pat80] homebrew computer. Audio is a good idea because:
+- it was used in a lot of home computers in the 80s
+- it is standard: almost any phone or computer has a headphone exit that can be used to load applications on a home computer
+- if one want the original feeling, audio tapes and audio recorders are still available in shops in 2022, while floppy disks are not
+
+I wrote this code to teach myself the Manchester code and to prototype the whole process before trying to implement it with electronics and Z80 assembly. Therefore, the implementation is didactic: it works (very well), but is inefficient, and the code is written to be more understandable than efficient.
+
+# How
+## Encode a file to audio
+Specify the input file, the audio output wav file and the clock.
+The clock is the frequency used to encode the data. Faster clock = more data per second. Slower clock = more robust on bad quality tape (less high frequencies).
+```
+./encode.py input.txt encoded.wav 1000
+```
+Now you can write this file to your tape.
+
+## Decode a wav file to the original file
+Same of encoding, but clock speed is detected from the signal itself (if you play the signal, you can hear a first part used to extimate clock frequency).
+```
+./decode.py encoded.wav output.txt
+```
+
+## Troubleshooting
+Use -v or -d flag to have printed on the terminal all the debug infos.
+Use -h flag for more infos on script usage.
+
+If the decoding script couldn't determine clock speed (or determines a wrong one), fiddle with the volume control on the tape recorder. If this doesn't work, you can adjust the minimum accepted signal volume changing the constant `AUDIO_MIN_VOLUME` in `decode.py` between 0 and 32768.