Question

我有兴趣使用JavaScript WebAudioAPI来检测歌曲节拍，然后在画布中渲染它们。

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

我已经尝试过关注this article，但在我的生活中，不能连接每个函数之间的点来制作功能性程序。

我知道我应该向你展示一些代码，但老实说，我没有任何代码，我的所有尝试都失败了，并且在前面提到的文章中都有相关的代码。

无论如何，我真的很感激一些指导，甚至更好地演示如何用WebAudioAPI实际检测歌曲节拍。

谢谢！

Answer 1

要理解the referenced article by Joe Sullivan的主要内容是即使它提供了大量的源代码，但它远不是最终的完整代码。要达到可行的解决方案，您仍然需要一些编码和调试技能。

此答案从引用的文章中抽取了大部分代码，原始许可适用于适当的情况。

以下是使用上述文章所述功能的简单示例实现。

代码包含为答案编写的准备代码：

通过the FileReader API
使用the AudioContext API

然后，如文章所述：

过滤音频，在此示例中使用low-pass filter
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任意值.98;在分组时，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样本。

请注意，评论很少，以保持示例实现简短，因为：

处理背后的逻辑在参考文章中解释
语法可以在相关方法的API文档中引用

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>

Answer 2

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

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
从阵列缓冲区中修剪10秒的剪辑
向下采样数据
规范化数据
数量分组
从分组计数中推断速度

下面的代码解决了这个问题。

将音频文件加载到阵列缓冲区并运行低通滤波器

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');

你现在有一个低通过滤歌曲的数组缓冲区（原创）

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

从歌曲中剪掉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);

羽绒样本

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秒分组计数缩放至60秒以获得每分钟节拍数

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

如何使用JS WebAudioAPI进行节拍检测？

2 个答案: