阅读chrome的插件radio_receiver

... line 280
  /**
   * Sends a USB control message to write to the device.
   * @param {number} value The value field of the control message.
   * @param {number} index The index field of the control message.
   * @param {ArrayBuffer} buffer The buffer to write to the device.
   * @param {Function} kont The continuation for this function.
   */
  function writeCtrlMsg(value, index, buffer, kont) {
    var ti = {
      'requestType': 'vendor',
      'recipient': 'device',
      'direction': 'out',
      'request': 0,
      'value': value,
      'index': index,
      'data': buffer
    };
    chrome.usb.controlTransfer(conn, ti, function(event) {
...

  /**
   * Demodulates the signal.
   * @param {Float32Array} samplesI The I components of the samples.
   * @param {Float32Array} samplesQ The Q components of the samples.
   * @param {boolean} inStereo Whether to try decoding the stereo signal.
   * @return {{left:ArrayBuffer,right:ArrayBuffer,stereo:boolean,carrier:boolean}}
   *     The demodulated audio signal.
   */
  function demodulate(samplesI, samplesQ, inStereo) {
    var demodulated = demodulator.demodulateTuned(samplesI, samplesQ);  //这应该是FM解调
    var leftAudio = monoSampler.downsample(demodulated);                //左声道(L+R)先只做一个抽取
    var rightAudio = new Float32Array(leftAudio);                       //右声道是左声道的copy
    var stereoOut = false;

    if (inStereo) {                                                     //如果设置了要解调立体声
      var stereo = stereoSeparator.separate(demodulated);               //立体声解调
      if (stereo.found) {
        stereoOut = true;
        var diffAudio = stereoSampler.downsample(stereo.diff);          //抽取L-R
        for (var i = 0; i < diffAudio.length; ++i) {
          rightAudio[i] -= diffAudio[i];                                //做L与R分离
          leftAudio[i] += diffAudio[i];
        }
      }
    }

    leftDeemph.inPlace(leftAudio);                                      //去预加重
    rightDeemph.inPlace(rightAudio);
    return {left: leftAudio.buffer,
            right: rightAudio.buffer,
            stereo: stereoOut,
            signalLevel: demodulator.getRelSignalPower() };
  }

      var real = lI * I[i] + lQ * Q[i];
      var imag = lI * Q[i] - I[i] * lQ;

      var sgn = 1;                  //sqn初始化为0~180
      if (imag < 0) {               //这时角度>180
        sgn *= -1;                  //用sqn记住这是个负角度
        imag *= -1;                 //转为180度以内的, 下面的算法应不能算超过180度
      }
      var ang = 0;
      var div;
      if (real == imag) {           //此时为45度, 省一次除法-_-
        div = 1;
      } else if (real > imag) {     //注意imag肯定是>0的, 那么是小于45度的情况
        div = imag / real;
      } else {                      //imag>real,45度至179度.
        ang = -Math.PI / 2;         //在45度~90度的范围也需要+90度?
        div = real / imag;          //这是这种算法在后边象限的普遍选择
        sgn *= -1;                  //已晕
      }

... line 404
  var iavg = new ExpAverage(9999);
  var qavg = new ExpAverage(9999);
  var cavg = new ExpAverage(49999, true);
...

... line 492
function ExpAverage(weight, opt_std) {
  var avg = 0;
  var std = 0;

  /**
   * Adds a value to the moving average.
   * @param {number} value The value to add.
   * @return {number} The moving average.
   */
  function add(value) {
    avg = (weight * avg + value) / (weight + 1);
    if (opt_std) {
      std = (weight * std + (value - avg) * (value - avg)) / (weight + 1);
    }
    return avg;
  }
...

... line 408
  for (var i = 0; i < 8001; ++i) {
    var freq = (pilotFreq + i / 100 - 40) * 2 * Math.PI / sampleRate;
    SIN[i] = Math.sin(freq);    //pilotFreq=19000
    COS[i] = Math.cos(freq);    //sampleRate=336000
  }
...

import numpy as np
from pylab import plot,subplot

i=np.arange(0,8000)
freq=(19000+i/100-40)*2*np.pi/336000
sin=np.sin(freq)
cos=np.cos(freq)

subplot(2,1,1)
plot(sin)
subplot(2,1,2)
plot(cos)

... line 422
      out[i] *= sin * cos * 2;
...

      var corr;                                         //调整用参数
      if (hdev > 0) {
        corr = Math.max(-4, Math.min(4, vdev / hdev));
      } else {
        corr = vdev == 0 ? 0 : (vdev > 0 ? 4 : -4);     //vdev=0
      }
      var idx = Math.round((corr + 4) * 1000);

      var hdev = iavg.add(out[i] * sin);
      var vdev = qavg.add(out[i] * cos);

      var idx = Math.round((corr + 4) * 1000);
      var newSin = sin * COS[idx] + cos * SIN[idx];
      cos = cos * COS[idx] - sin * SIN[idx];
      sin = newSin;

... line 409
    var freq = (pilotFreq + i / 100 - 40) * 2 * Math.PI / sampleRate;
...

...
      var hdev = iavg.add(out[i] * sin);
      var vdev = qavg.add(out[i] * cos);
...
      var corr;                                         //调整用参数
      if (hdev > 0) {
        corr = Math.max(-4, Math.min(4, vdev / hdev));  //>0, 意味着
      } else {
        corr = vdev == 0 ? 0 : (vdev > 0 ? 4 : -4);    
      }
      var idx = Math.round((corr + 4) * 1000);
...