Question

我有以下方法将IMediaSample中的PCM数据收集到FFT的浮点数中：

    public int PCMDataCB(IntPtr Buffer, int Length, ref TDSStream Stream, out float[] singleChannel)
    {
        int numSamples = Length / (Stream.Bits / 8);
        int samplesPerChannel = numSamples / Stream.Channels;

        float[] samples = new float[numSamples];

        if (Stream.Bits == 32 && Stream.Float) {
            // this seems to work for 32 bit floating point
            byte[] buffer32f = new byte[numSamples * 4];
            Marshal.Copy(Buffer, buffer32f, 0, numSamples);

            for (int j = 0; j < buffer32f.Length; j+=4)
            {
                samples[j / 4] = System.BitConverter.ToSingle(new byte[] { buffer32f[j + 0], buffer32f[j + 1], buffer32f[j + 2], buffer32f[j + 3]}, 0);
            }
        }
        else if (Stream.Bits == 24)
        {
            // I need this code
        }

        // compress result into one mono channel
        float[] result = new float[samplesPerChannel];

        for (int i = 0; i < numSamples; i += Stream.Channels)
        {
            float tmp = 0;

            for (int j = 0; j < Stream.Channels; j++)
                tmp += samples[i + j] / Stream.Channels;

            result[i / Stream.Channels] = tmp;
        }

        // mono output to be used for visualizations
        singleChannel = result;

        return 0;
    }

似乎可以用于32b浮点数，因为我在频谱分析仪中得到了合理的数据（尽管它似乎过度（或压缩？）到较低的频率）。

我似乎也设法使其适用于8,16和32非浮点数，但是当位数为24时我只能读取垃圾。

如何使用24位PCM进入缓冲区？

缓冲区来自IMediaSample。

我想知道的另一件事是，我使用的方法是通过求和除以通道数来将所有通道添加到一个...

Answer 1

我明白了：

byte[] buffer24 = new byte[numSamples * 3];
Marshal.Copy(Buffer, buffer24, 0, numSamples * 3);
var window = (float)(255 << 16 | 255 << 8 | 255);

for (int j = 0; j < buffer24.Length; j+=3)
{
    samples[j / 3] = (buffer24[j] << 16 | buffer24[j + 1] << 8 | buffer24[j + 2]) / window;
}

从三个字节创建一个整数，然后通过除以三个字节的最大值将其缩放到1 / -1范围内。

Answer 2

你试过吗

byte[] buffer24f = new byte[numSamples * 3];
Marshal.Copy(Buffer, buffer24f, 0, numSamples);

for (int j = 0; j < buffer24f.Length; j+=3)
{
    samples[j / 3] = System.BitConverter.ToSingle(
            new byte[] { 
                0, 
                buffer24f[j + 0], 
                buffer24f[j + 1], 
                buffer24f[j + 2]
            }, 0);
}

如何读取IMediaSample 24位PCM数据

2 个答案: