我正在开发一个播放音频的 Android 应用程序。为了最大限度地减少延迟,我通过 JNI 使用 C++ 来使用 C++ 库双簧管来播放应用程序。
目前,在播放之前,应用程序必须解码给定的文件(例如 mp3),然后播放解码的原始音频流。如果文件较大,这会导致播放开始之前需要等待一段时间。 所以我想预先进行解码,保存它,当请求播放时,只需播放保存文件中的解码数据。 我几乎不知道如何在 C++ 中进行正确的文件 i/o,并且很难理解它。有可能我的问题可以通过正确的库来解决,我不确定。
所以目前我正在保存我的文件,如下所示:
bool Converter::doConversion(const std::string& fullPath, const std::string& name) {
// here I'm setting up the extractor and necessary inputs. Omitted since not relevant
// this is where the decoder is called to decode a file to raw audio
constexpr int kMaxCompressionRatio{12};
const long maximumDataSizeInBytes = kMaxCompressionRatio * (size) * sizeof(int16_t);
auto decodedData = new uint8_t[maximumDataSizeInBytes];
int64_t bytesDecoded = NDKExtractor::decode(*extractor, decodedData);
auto numSamples = bytesDecoded / sizeof(int16_t);
auto outputBuffer = std::make_unique<float[]>(numSamples);
// This block is necessary to get the correct format for oboe.
// The NDK decoder can only decode to int16, we need to convert to floats
oboe::convertPcm16ToFloat(
reinterpret_cast<int16_t *>(decodedData),
outputBuffer.get(),
bytesDecoded / sizeof(int16_t));
// This is how I currently save my outputBuffer to a file. This produces a file on the disc.
std::string outputSuffix = ".pcm";
std::string outputName = std::string(mFolder) + name + outputSuffix;
std::ofstream outfile(outputName.c_str(), std::ios::out | std::ios::binary);
outfile.write(reinterpret_cast<const char *>(&outputBuffer), sizeof outputBuffer);
return true;
}
所以我相信我将 float 数组,将其转换为 char 数组并保存。我不确定这是否正确,但这是我对它的最好理解。 反正之后有一个文件。 编辑:正如我在分析保存的文件时发现的那样,我只存储了 8 个字节。
现在如何再次加载该文件并恢复我的outputBuffer的内容?
目前我有这一点,显然不完整:
StorageDataSource *StorageDataSource::openPCM(const char *fileName, AudioProperties targetProperties) {
long bufferSize;
char * buffer;
std::ifstream stream(fileName, std::ios::in | std::ios::binary);
stream.seekg (0, std::ios::beg);
bufferSize = stream.tellg();
buffer = new char [bufferSize];
stream.read(buffer, bufferSize);
stream.close();
如果这是正确的,我需要做什么才能将数据恢复为原始类型?如果我做错了,它如何以正确的方式工作?
感谢@Michael 的评论,我知道了如何做到这一点。
这就是我现在保存数据的方式:
bool Converter::doConversion(const std::string& fullPath, const std::string& name) {
// here I'm setting up the extractor and necessary inputs. Omitted since not relevant
// this is where the decoder is called to decode a file to raw audio
constexpr int kMaxCompressionRatio{12};
const long maximumDataSizeInBytes = kMaxCompressionRatio * (size) * sizeof(int16_t);
auto decodedData = new uint8_t[maximumDataSizeInBytes];
int64_t bytesDecoded = NDKExtractor::decode(*extractor, decodedData);
auto numSamples = bytesDecoded / sizeof(int16_t);
// converting to float has moved to the reading function, so now i save decodedData directly.
std::string outputSuffix = ".pcm";
std::string outputName = std::string(mFolder) + name + outputSuffix;
std::ofstream outfile(outputName.c_str(), std::ios::out | std::ios::binary);
outfile.write((char*)decodedData, numSamples * sizeof (int16_t));
return true;
}
这就是我再次读取存储的文件的方式:
long bufferSize;
char * inputBuffer;
std::ifstream stream;
stream.open(fileName, std::ifstream::in | std::ifstream::binary);
if (!stream.is_open()) {
// handle error
}
stream.seekg (0, std::ios::end); // seek to the end
bufferSize = stream.tellg(); // get size info, will be 0 without seeking to the end
stream.seekg (0, std::ios::beg); // seek to beginning
inputBuffer = new char [bufferSize];
stream.read(inputBuffer, bufferSize); // the actual reading into the buffer. would be null without seeking back to the beginning
stream.close();
// done reading the file.
auto numSamples = bufferSize / sizeof(int16_t); // calculate my number of samples, so the audio is correctly interpreted
auto outputBuffer = std::make_unique<float[]>(numSamples);
// the decoding bit now happens after the file is open. This avoids confusion
// The NDK decoder can only decode to int16, we need to convert to floats
oboe::convertPcm16ToFloat(
reinterpret_cast<int16_t *>(inputBuffer),
outputBuffer.get(),
bufferSize / sizeof(int16_t));
// here I continue working with my outputBuffer
我没有或没有得到的重要信息/理解 C++ 是
a) 指针的大小与其数据的大小不同 指向 和 b) 寻找流的工作原理。我需要把 在我在缓冲区中找到任何数据之前,针回到开始处。
另一种选择是使用 libsndfile。我维护着这个被遗忘的分支,它使用 Oboe 库实现了这个库。例如,我在麦克风回调中使用它,如下所示:
class MAudioRecorderCallback : public DefaultDataCallback {
oboe::DataCallbackResult onAudioReady(oboe::AudioStream *oboeStream,
void *audioData, int32_t numFrames) {
auto result =
DefaultDataCallback::onAudioReady(oboeStream, audioData, numFrames);
auto *inputInts = static_cast<const int16_t *>(audioData);
...
mSoundRecording->write(inputInts, numFrames * oboeStream->getChannelCount());
...
return result;
}
AndroidAudioDeviceManager *m_aadm;
SoundRecording* mSoundRecording = nullptr;
public:
MAudioRecorderCallback(AndroidAudioDeviceManager *pManager)
: m_aadm(pManager) {
mSoundRecording = &m_aadm->mSoundRecording;
}
};
文件将开始写入一个简单的:
mSoundRecording.initiateWritingToFile(path.c_str(), mOutputChannelCount, mSampleRate);
SoundRecording 类的完整实现是这里。