pySSTV/sstv.py

#!/usr/bin/env python

from __future__ import division, with_statement
from math import sin, pi, floor
from random import random
from contextlib import closing
import struct
import wave

FREQ_VIS_BIT1 = 1100
FREQ_SYNC = 1200
FREQ_VIS_BIT0 = 1300
FREQ_BLACK = 1500
FREQ_VIS_START = 1900
FREQ_WHITE = 2300
FREQ_RANGE = FREQ_WHITE - FREQ_BLACK

MSEC_VIS_START = 300
MSEC_VIS_SYNC = 10
MSEC_VIS_BIT = 30


class SSTV(object):

    def __init__(self, image, samples_per_sec, bits):
        self.image = image
        self.samples_per_sec = samples_per_sec
        self.bits = bits
        self.vox_enabled = False

    BITS_TO_STRUCT = {8: 'b', 16: 'h'}

    def write_wav(self, filename):
        """writes the whole image to a Microsoft WAV file"""
        fmt = '<' + self.BITS_TO_STRUCT[self.bits]
        data = ''.join(struct.pack(fmt, b) for b in self.gen_samples())
        with closing(wave.open(filename, 'wb')) as wav:
            wav.setnchannels(1)
            wav.setsampwidth(self.bits // 8)
            wav.setframerate(self.samples_per_sec)
            wav.writeframes(data)

    def gen_samples(self):
        """generates discrete samples from gen_values()

           performs quantization according to
           the bits per sample value given during construction
        """
        max_value = 2 ** self.bits
        alias = 1 / max_value
        amp = max_value / 2
        lowest = -amp
        highest = amp - 1
        for value in self.gen_values():
            sample = int(round(value * amp + alias * (random() - 0.5)))
            yield max(min(highest, sample), lowest)

    def gen_values(self):
        """generates samples between -1 and +1 from gen_freq_bits()

           performs sampling according to
           the samples per second value given during construction
        """
        spms = self.samples_per_sec / 1000
        param = 0
        samples = 0
        for freq, msec in self.gen_freq_bits():
            offset = param
            samples += spms * msec
            tx = floor(samples)
            for sample in xrange(int(tx)):
                t = sample / self.samples_per_sec
                param = t * freq * 2 * pi + offset
                yield sin(param)
            samples -= tx

    def gen_freq_bits(self):
        """generates tuples (freq, msec) that describe a sine wave segment

           frequency "freq" in Hz and duration "msec" in ms
        """
        if self.vox_enabled:
            for freq in (1900, 1500, 1900, 1500, 2300, 1500, 2300, 1500):
                yield freq, 100
        yield FREQ_VIS_START, MSEC_VIS_START
        yield FREQ_SYNC, MSEC_VIS_SYNC
        yield FREQ_VIS_START, MSEC_VIS_START
        yield FREQ_SYNC, MSEC_VIS_BIT  # start bit
        vis = self.VIS_CODE
        num_ones = 0
        for _ in xrange(7):
            bit = vis & 1
            vis >>= 1
            num_ones += bit
            bit_freq = FREQ_VIS_BIT1 if bit == 1 else FREQ_VIS_BIT0
            yield bit_freq, MSEC_VIS_BIT
        parity_freq = FREQ_VIS_BIT1 if num_ones % 2 == 1 else FREQ_VIS_BIT0
        yield parity_freq, MSEC_VIS_BIT
        yield FREQ_SYNC, MSEC_VIS_BIT  # stop bit

    def horizontal_sync(self):
        yield FREQ_SYNC, self.SYNC


def byte_to_freq(value):
    return FREQ_BLACK + FREQ_RANGE * value / 255