Question

我试图连续播放声音，应该更改smoothley频率。例如，我的AudioTrack当前正在以100 Hz的频率播放，新频率为1000 Hz。然后我不希望频率跳跃，但是要使smoothley适应变化。我目前的代码如下：

int buffSize = AudioTrack.getMinBufferSize(sampleRate,
                                           AudioFormat.CHANNEL_OUT_MONO,
                                           AudioFormat.ENCODING_PCM_16BIT);
AudioTrack audioTrack = new AudioTrack(AudioManager.STREAM_MUSIC, sampleRate,
                                       AudioFormat.CHANNEL_OUT_MONO,
                                       AudioFormat.ENCODING_PCM_16BIT, buffSize,
                                       AudioTrack.MODE_STREAM);
short samples[] = new short[buffSize];
int amp = 10000;
double twopi = 8. * Math.atan(1.);
double phase = 0.0;
audioTrack.play();
while (playing)
{
    double newFrequency = getFrequency();
    for (int i = 0; i < buffSize; i++)
    {
        samples[i] = (short) (amp * Math.sin(phase));
        phase += twopi * newFrequency / sampleRate;
    }
    audioTrack.write(samples, 0, buffSize);
}
audioTrack.stop();
audioTrack.release();

当然，这是在后台运行的。我的sampleRate自动设置为可能的最低值：

private int getMinSampleRate()
{
    for (int i = 1; i < 100000; i++)
        if (AudioTrack.getMinBufferSize(i, AudioFormat.CHANNEL_OUT_MONO,
                                        AudioFormat.ENCODING_PCM_16BIT) > 0)
            return i;

    return -1;
}

Answer 1

您可以使用low-pass filter来平滑过渡。这是另一个example。

double currentFrequency = getFrequency();
double smoothing  = 10;   // calculate based on your sample rate and desired time delta for the change
while (playing)
{
    double newFrequency = getFrequency();
    for (int i = 0; i < buffSize; i++)
    {
        // because you are filling the buffer, deltaTime will be a constant: the time between samples
        currentFrequency += deltaTime * ( newFrequency - currentFrequency ) / smoothing;
        samples[i] = (short) (amp * Math.sin(phase));
        phase += twopi * currentFrequency / sampleRate;
    }
    audioTrack.write(samples, 0, buffSize);
}

保留频率的历史记录，并使用基于增量的增量步骤将其慢慢更改为新值。对于大的增量，值将在开始时更快地变化，然后在结束时更慢。 IIRC，这称为一阶滤波器，因为您无法控制变化的加速度。二阶滤波器可以平滑加速度。

有许多不同的技巧，请查看Wikipedia处的过滤器。

频率变化平稳

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