使用 soxr 和 libsndfile 重新采样 PCM 文件数据崩溃

Question

我正在构建一个应用程序，它需要将任何不是 44100Hz 的输入 PCM 音频文件重新采样到 44.1（或至少尽最大努力这样做）。

为了处理重采样，我使用了soxr。 soxr 没有依赖关系并且是轻量级的，这在这种情况下是理想的，但它不提供本机文件 I/O。我对 C 中的 IO 流的经验非常有限，所以我碰壁了。该应用程序是模块化设计的，因此我需要重新采样过程来创建一个输出文件，然后可以将其传递给其他处理器，而不是简单地直接处理输出流。

为了创建该输出文件，我正在尝试获取soxr 重采样过程生成的数据，并将其传递给libsndfile，它应该能够将音频写入文件。

下面是对我所在位置的极其详细的解释，尽管我不知道它为什么会崩溃。我怀疑这与缓冲区的分配和使用方式有关。 （注意：输入文件是在此代码之前使用 sndfile 读取的）

（这是整个事情的single gist）

基本的重采样器选项

// Use "high quality" resampling
unsigned int q_recipe = SOXR_HQ;

// No 
unsigned long q_flags = 0;

// Create the q_spec
soxr_quality_spec_t q_spec = soxr_quality_spec(q_recipe, q_flags);

将 sndfile 格式映射为 soxr 格式

soxr_datatype_t itype;

// Get the SFINFO format
int iformat = self.inputFileInfo.format;

// Set the soxr itype to the corresponding format
if ((iformat & SF_FORMAT_FLOAT) == SF_FORMAT_FLOAT) {
  itype = SOXR_FLOAT32_S;
} else if ((iformat & SF_FORMAT_DOUBLE) == SF_FORMAT_DOUBLE) {
  itype = SOXR_FLOAT64_S;
} else if ((iformat & SF_FORMAT_PCM_32) == SF_FORMAT_PCM_32) {
  itype = SOXR_INT32_S;
} else {
  itype = SOXR_INT16_S;
}

设置 soxr IO 规范

// Always want the output to match the input
soxr_datatype_t otype = itype;

soxr_io_spec_t io_spec = soxr_io_spec(itype, otype);

线程

// A single thread is fine
soxr_runtime_spec_t runtime_spec = soxr_runtime_spec(1);

构造重采样器

soxr_error_t error;

// Input rate can be read from the SFINFO    
double const irate = self.inputFileInfo.samplerate;

// Output rate is defined elsewhere, but this generally = 44100
double const orate = self.task.resampler.immutableConfiguration.targetSampleRate;

// Channel count also comes from SFINFO
unsigned chans = self.inputFileInfo.channels;

// Put it all together
soxr_t soxr = soxr_create(irate, orate, chans, &error, &io_spec, &q_spec, &runtime_spec);

读取、重采样和写入

我对以下任何代码都不是很有信心，但我已经三次检查了数学，一切似乎都符合库 API 的预期。

// Frames in sndfile are called Samples in soxr

// One frame is 1 item per channel
// ie frame_items = 1 item * channels
size_t const iframeitems = (1 * chans);

// item size is the data type size of the input type
//
size_t iitemsize;

if ((iformat & SF_FORMAT_FLOAT) == SF_FORMAT_FLOAT) {
  iitemsize = sizeof(Float32);
} else if ((iformat & SF_FORMAT_DOUBLE) == SF_FORMAT_DOUBLE) {
  iitemsize = sizeof(Float64);
} else if ((iformat & SF_FORMAT_PCM_32) == SF_FORMAT_PCM_32) {
  iitemsize = sizeof(int32_t);
} else {
  iitemsize = sizeof(int16_t);
}

// frame size is item size * items per frame (channels)
// eg for 2 channel 16 bit, frame size = 2 * 2
size_t const iframesize = (iframeitems * iitemsize);

// Number of frames to read (arbitrary)
sf_count_t const ireqframes = 1024;

// Size of the buffer is number of frames * size per frame
size_t const ibufsize = iframesize * ireqframes;

void *ibuf = malloc(ibufsize);

// Output
//////////////////////////////

// These match the input
size_t const oframeitems = iframeitems;
size_t const oitemsize = iitemsize;

// frame size is item size * items per frame (channels)
size_t const oframesize = (oframeitems * oitemsize);

// Number of frames expected after resampling
// eg
// orate = 44100
// irate = 48000
// ireqframe = 1024
// expect fewer frames (downsample)
// (44100 / 4800) * 1024 = 940.8
// Add 0.5 to deal with rounding?
sf_count_t const oexpframes = (ireqframes * (orate / irate)) + 0.5;

// Size of the buffer is number of frames * size per frame
size_t const obufsize = oframesize * oexpframes;

void *obuf = malloc(obufsize);

// Go
//////////////////////////////

size_t total_resample_output_frame_count = 0;
size_t need_input = 1;
sf_count_t num_frames_written = 0;

do {

  sf_count_t num_frames_read = 0;
  size_t actual_resample_output_samples = 0;

  // Read the input file based on its type
  // num_frames_read should be 1024
  if (otype == SOXR_INT16_S || otype == SOXR_INT32_S) {
    num_frames_read = sf_readf_int(self.inputFile, ibuf, ireqframes);
  } else if (otype == SOXR_FLOAT32_S) {
    num_frames_read = sf_readf_float(self.inputFile, ibuf, ireqframes);
  } else {
    num_frames_read = sf_readf_double(self.inputFile, ibuf, ireqframes);
  }

  // If there were no frames left to read we're done
  if (num_frames_read == 0) {
    // passing NULL input buffer to soxr_process indicates End-of-input
    ibuf = NULL;
    need_input = 0;
  }

  // Run the resampling on frames read from the input file
  error = soxr_process(soxr, ibuf, num_frames_read, NULL, obuf, oexpframes, &actual_resample_output_samples);

  total_resample_output_frame_count += actual_resample_output_samples;

  // Write the resulting data to output file
  // num_frames_written should = actual_resample_output_samples
  if (otype == SOXR_INT16_S || otype == SOXR_INT32_S) {
    num_frames_written = sf_writef_int(self.outputFile, obuf, actual_resample_output_samples);
  } else if (otype == SOXR_FLOAT32_S) {
    num_frames_written = sf_writef_float(self.outputFile, obuf, actual_resample_output_samples);
  } else {
    num_frames_written = sf_writef_double(self.outputFile, obuf, actual_resample_output_samples);
  }

} while (!error && need_input);

soxr_delete(soxr);
free(obuf), free(ibuf);

这会在soxr_process 上给出 EXC_BAD_ACCESS。在这一点上，我不知道还有什么可以尝试的。

Answer 1

像SOXR_INT32_S 这样的数据类型中的_S 表示您正在使用拆分通道，从示例4-split-channels.c 看来，在这种情况下您需要传递一个指针数组，每个通道一个指针。

但是，在上面的代码中，您只传递了一个分配的内存块，所以我猜您期望的是交错的通道数据。也许您可以尝试将_S 更改为_I。