javascript - 如何使用 JS WebAudioAPI 进行节拍检测？

Question

我有兴趣使用 JavaScriptWebAudioAPI来检测歌曲节拍，然后在画布中呈现它们。

我可以处理画布部分，但我不是一个大音频人，真的不明白如何在 JavaScript 中制作节拍检测器。

我已经尝试过这篇文章，但在我的一生中，无法将每个函数之间的点连接起来以制作一个函数式程序。

我知道我应该给你看一些代码，但老实说我没有，我所有的尝试都失败了，相关代码在前面提到的文章中。

无论如何，我真的很感激一些指导，甚至更好的演示如何使用WebAudioAPI.

谢谢！

score 21 · Accepted Answer

关于Joe Sullivan 所引用的文章，需要理解的主要内容是，尽管它提供了很多源代码，但它远非最终和完整的代码。要获得可行的解决方案，您仍然需要一些编码和调试技能。

这个答案从引用的文章中提取了大部分代码，原始许可适用于适当的地方。

下面是使用上述文章描述的函数的简单示例实现，您仍然需要找出函数解决方案的正确阈值。

该代码由为答案编写的准备代码组成：

通过FileReader API读取本地文件
使用AudioContext API将文件解码为音频数据

然后，如文章中所述：

过滤音频，在本例中使用低通滤波器
使用阈值计算峰值
分组间隔计数，然后是速度计数

对于阈值，我使用了最大值和最小值之间范围的 0.98 的任意值；分组时，我添加了一些额外的检查和任意舍入以避免可能的无限循环并使其成为易于调试的示例。

请注意，很少有评论来保持示例实现的简短，因为：

处理背后的逻辑在参考文章中进行了解释
语法可以参考相关方法的API文档

audio_file.onchange = function() {
  var file = this.files[0];
  var reader = new FileReader();
  var context = new(window.AudioContext || window.webkitAudioContext)();
  reader.onload = function() {
    context.decodeAudioData(reader.result, function(buffer) {
      prepare(buffer);
    });
  };
  reader.readAsArrayBuffer(file);
};

function prepare(buffer) {
  var offlineContext = new OfflineAudioContext(1, buffer.length, buffer.sampleRate);
  var source = offlineContext.createBufferSource();
  source.buffer = buffer;
  var filter = offlineContext.createBiquadFilter();
  filter.type = "lowpass";
  source.connect(filter);
  filter.connect(offlineContext.destination);
  source.start(0);
  offlineContext.startRendering();
  offlineContext.oncomplete = function(e) {
    process(e);
  };
}

function process(e) {
  var filteredBuffer = e.renderedBuffer;
  //If you want to analyze both channels, use the other channel later
  var data = filteredBuffer.getChannelData(0);
  var max = arrayMax(data);
  var min = arrayMin(data);
  var threshold = min + (max - min) * 0.98;
  var peaks = getPeaksAtThreshold(data, threshold);
  var intervalCounts = countIntervalsBetweenNearbyPeaks(peaks);
  var tempoCounts = groupNeighborsByTempo(intervalCounts);
  tempoCounts.sort(function(a, b) {
    return b.count - a.count;
  });
  if (tempoCounts.length) {
    output.innerHTML = tempoCounts[0].tempo;
  }
}

// http://tech.beatport.com/2014/web-audio/beat-detection-using-web-audio/
function getPeaksAtThreshold(data, threshold) {
  var peaksArray = [];
  var length = data.length;
  for (var i = 0; i < length;) {
    if (data[i] > threshold) {
      peaksArray.push(i);
      // Skip forward ~ 1/4s to get past this peak.
      i += 10000;
    }
    i++;
  }
  return peaksArray;
}

function countIntervalsBetweenNearbyPeaks(peaks) {
  var intervalCounts = [];
  peaks.forEach(function(peak, index) {
    for (var i = 0; i < 10; i++) {
      var interval = peaks[index + i] - peak;
      var foundInterval = intervalCounts.some(function(intervalCount) {
        if (intervalCount.interval === interval) return intervalCount.count++;
      });
      //Additional checks to avoid infinite loops in later processing
      if (!isNaN(interval) && interval !== 0 && !foundInterval) {
        intervalCounts.push({
          interval: interval,
          count: 1
        });
      }
    }
  });
  return intervalCounts;
}

function groupNeighborsByTempo(intervalCounts) {
  var tempoCounts = [];
  intervalCounts.forEach(function(intervalCount) {
    //Convert an interval to tempo
    var theoreticalTempo = 60 / (intervalCount.interval / 44100);
    theoreticalTempo = Math.round(theoreticalTempo);
    if (theoreticalTempo === 0) {
      return;
    }
    // Adjust the tempo to fit within the 90-180 BPM range
    while (theoreticalTempo < 90) theoreticalTempo *= 2;
    while (theoreticalTempo > 180) theoreticalTempo /= 2;

    var foundTempo = tempoCounts.some(function(tempoCount) {
      if (tempoCount.tempo === theoreticalTempo) return tempoCount.count += intervalCount.count;
    });
    if (!foundTempo) {
      tempoCounts.push({
        tempo: theoreticalTempo,
        count: intervalCount.count
      });
    }
  });
  return tempoCounts;
}

// http://stackoverflow.com/questions/1669190/javascript-min-max-array-values
function arrayMin(arr) {
  var len = arr.length,
    min = Infinity;
  while (len--) {
    if (arr[len] < min) {
      min = arr[len];
    }
  }
  return min;
}

function arrayMax(arr) {
  var len = arr.length,
    max = -Infinity;
  while (len--) {
    if (arr[len] > max) {
      max = arr[len];
    }
  }
  return max;
}

<input id="audio_file" type="file" accept="audio/*"></input>
<audio id="audio_player"></audio>
<p>
  Most likely tempo: <span id="output"></span>
</p>

score 11 · Accepted Answer

我在这里写了一个教程，展示了如何使用 javascript Web Audio API 来做到这一点。

https://askmacgyver.com/blog/tutorial/how-to-implement-tempo-detection-in-your-application

步骤概要

将音频文件转换为数组缓冲区
通过低通滤波器运行阵列缓冲区
从阵列缓冲区修剪 10 秒剪辑
下采样数据
规范化数据
计算卷分组
从分组计数推断速度

下面的代码完成了繁重的工作。

将音频文件加载到数组缓冲区并通过低通滤波器运行

function createBuffers(url) {

 // Fetch Audio Track via AJAX with URL
 request = new XMLHttpRequest();

 request.open('GET', url, true);
 request.responseType = 'arraybuffer';

 request.onload = function(ajaxResponseBuffer) {

    // Create and Save Original Buffer Audio Context in 'originalBuffer'
    var audioCtx = new AudioContext();
    var songLength = ajaxResponseBuffer.total;

    // Arguments: Channels, Length, Sample Rate
    var offlineCtx = new OfflineAudioContext(1, songLength, 44100);
    source = offlineCtx.createBufferSource();
    var audioData = request.response;
    audioCtx.decodeAudioData(audioData, function(buffer) {

         window.originalBuffer = buffer.getChannelData(0);
         var source = offlineCtx.createBufferSource();
         source.buffer = buffer;

         // Create a Low Pass Filter to Isolate Low End Beat
         var filter = offlineCtx.createBiquadFilter();
         filter.type = "lowpass";
         filter.frequency.value = 140;
         source.connect(filter);
         filter.connect(offlineCtx.destination);

            // Render this low pass filter data to new Audio Context and Save in 'lowPassBuffer'
            offlineCtx.startRendering().then(function(lowPassAudioBuffer) {

             var audioCtx = new(window.AudioContext || window.webkitAudioContext)();
             var song = audioCtx.createBufferSource();
             song.buffer = lowPassAudioBuffer;
             song.connect(audioCtx.destination);

             // Save lowPassBuffer in Global Array
             window.lowPassBuffer = song.buffer.getChannelData(0);
             console.log("Low Pass Buffer Rendered!");
            });

        },
        function(e) {});
 }
 request.send();
}


createBuffers('https://askmacgyver.com/test/Maroon5-Moves-Like-Jagger-128bpm.mp3');

您现在有一个低通滤波歌曲（和原始歌曲）的数组缓冲区

它由许多条目组成，sampleRate（44100 乘以歌曲的秒数）。

window.lowPassBuffer  // Low Pass Array Buffer
window.originalBuffer // Original Non Filtered Array Buffer

从歌曲中修剪 10 秒剪辑

function getClip(length, startTime, data) {

  var clip_length = length * 44100;
  var section = startTime * 44100;
  var newArr = [];

  for (var i = 0; i < clip_length; i++) {
     newArr.push(data[section + i]);
  }

  return newArr;
}

// Overwrite our array buffer to a 10 second clip starting from 00:10s
window.lowPassFilter = getClip(10, 10, lowPassFilter);

向下采样您的剪辑

function getSampleClip(data, samples) {

  var newArray = [];
  var modulus_coefficient = Math.round(data.length / samples);

  for (var i = 0; i < data.length; i++) {
     if (i % modulus_coefficient == 0) {
         newArray.push(data[i]);
     }
  }
  return newArray;
}

// Overwrite our array to down-sampled array.
lowPassBuffer = getSampleClip(lowPassFilter, 300);

规范化您的数据

function normalizeArray(data) {

 var newArray = [];

 for (var i = 0; i < data.length; i++) {
     newArray.push(Math.abs(Math.round((data[i + 1] - data[i]) * 1000)));
 }

 return newArray;
}

// Overwrite our array to the normalized array
lowPassBuffer = normalizeArray(lowPassBuffer);

计算扁线分组

function countFlatLineGroupings(data) {

 var groupings = 0;
 var newArray = normalizeArray(data);

 function getMax(a) {
    var m = -Infinity,
        i = 0,
        n = a.length;

    for (; i != n; ++i) {
        if (a[i] > m) {
            m = a[i];
        }
    }
    return m;
 }

 function getMin(a) {
    var m = Infinity,
        i = 0,
        n = a.length;

    for (; i != n; ++i) {
        if (a[i] < m) {
            m = a[i];
        }
    }
    return m;
 }

 var max = getMax(newArray);
 var min = getMin(newArray);
 var count = 0;
 var threshold = Math.round((max - min) * 0.2);

 for (var i = 0; i < newArray.length; i++) {

   if (newArray[i] > threshold && newArray[i + 1] < threshold && newArray[i + 2] < threshold && newArray[i + 3] < threshold && newArray[i + 6] < threshold) {
        count++;
    }
 }

 return count;
}

// Count the Groupings
countFlatLineGroupings(lowPassBuffer);

将 10 秒分组计数扩展到 60 秒以得出每分钟节拍数

var final_tempo = countFlatLineGroupings(lowPassBuffer);

// final_tempo will be 21
final_tempo = final_tempo * 6;

console.log("Tempo: " + final_tempo);
// final_tempo will be 126

javascript - 如何使用 JS WebAudioAPI 进行节拍检测？

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