当输入pcm样本数不等于1024时，如何使用ffmpeg-API将重放的pcm音频编码到AAC。
当输入pcm样本数不等于1024时，如何使用ffmpeg-API将重放的pcm音频编码到AAC。
提问于 2013-06-03 19:58:10
我正在努力捕捉和流音频到RTMP服务器在一个时刻。我在MacOS下工作(在Xcode中)，所以为了捕获音频示例-缓冲区，我使用AVFoundation。但是对于编码和流，我需要使用ffmpeg-API和libfaac编码器。因此输出格式必须是AAC (用于支持iOS设备上的流播放)。
我也面临着这样的问题:音频采集设备(在我的例子中，罗技相机)给我512个LPCM采样缓冲器，我可以选择16000,24000,36000或48000 Hz的输入采样率。当我将这512个样本提供给AAC编码器(配置为适当的采样率)时，我会听到一个慢而急促的音频(似乎在每帧之后都会保持沉默)。
我发现(也许我错了)，libfaac编码器只接受1024个样本的音频帧。在编码前，当我将输入采样设置为24000，重采样输入样本缓冲器设置为48000时，我获得了1024个重放样本。在将这1024个样本编码到AAC后，我在输出时听到了适当的声音。但是我的网络摄像头为任何输入采样产生512个样本，当输出采样率必须是48000赫兹时。因此，我需要在任何情况下进行重采样，并且在重采样之后，我将不会在缓冲区中获得确切的1024个样本。
有办法在ffmpeg-API功能中解决这个问题吗？
如果能提供任何帮助，我将不胜感激。
PS:我想我可以积累重放缓冲区，直到样本计数变成1024，然后对其进行编码，但是这是流，因此产生时间戳和其他输入设备会有麻烦，这样的解决方案是不合适的。
当前的问题来自于所讨论的问题：如何用从AVFrame ( CMSampleBufferRef，AVFoundation)获得的数据填充音频AVFoundation？
这里有一个带有音频编解码器的代码(还有视频流，但是视频工作很好)：
/*global variables*/
static AVFrame *aframe;
static AVFrame *frame;
AVOutputFormat *fmt;
AVFormatContext *oc;
AVStream *audio_st, *video_st;
Init ()
{
AVCodec *audio_codec, *video_codec;
int ret;
avcodec_register_all();
av_register_all();
avformat_network_init();
avformat_alloc_output_context2(&oc, NULL, "flv", filename);
fmt = oc->oformat;
oc->oformat->video_codec = AV_CODEC_ID_H264;
oc->oformat->audio_codec = AV_CODEC_ID_AAC;
video_st = NULL;
audio_st = NULL;
if (fmt->video_codec != AV_CODEC_ID_NONE)
{ //… /*init video codec*/}
if (fmt->audio_codec != AV_CODEC_ID_NONE) {
audio_codec= avcodec_find_encoder(fmt->audio_codec);
if (!(audio_codec)) {
fprintf(stderr, "Could not find encoder for '%s'\n",
avcodec_get_name(fmt->audio_codec));
exit(1);
}
audio_st= avformat_new_stream(oc, audio_codec);
if (!audio_st) {
fprintf(stderr, "Could not allocate stream\n");
exit(1);
}
audio_st->id = oc->nb_streams-1;
//AAC:
audio_st->codec->sample_fmt = AV_SAMPLE_FMT_S16;
audio_st->codec->bit_rate = 32000;
audio_st->codec->sample_rate = 48000;
audio_st->codec->profile=FF_PROFILE_AAC_LOW;
audio_st->time_base = (AVRational){1, audio_st->codec->sample_rate };
audio_st->codec->channels = 1;
audio_st->codec->channel_layout = AV_CH_LAYOUT_MONO;
if (oc->oformat->flags & AVFMT_GLOBALHEADER)
audio_st->codec->flags |= CODEC_FLAG_GLOBAL_HEADER;
}
if (video_st)
{
// …
/*prepare video*/
}
if (audio_st)
{
aframe = avcodec_alloc_frame();
if (!aframe) {
fprintf(stderr, "Could not allocate audio frame\n");
exit(1);
}
AVCodecContext *c;
int ret;
c = audio_st->codec;
ret = avcodec_open2(c, audio_codec, 0);
if (ret < 0) {
fprintf(stderr, "Could not open audio codec: %s\n", av_err2str(ret));
exit(1);
}
//…
}以及重采样和编码音频：
if (mType == kCMMediaType_Audio)
{
CMSampleTimingInfo timing_info;
CMSampleBufferGetSampleTimingInfo(sampleBuffer, 0, &timing_info);
double pts=0;
double dts=0;
AVCodecContext *c;
AVPacket pkt = { 0 }; // data and size must be 0;
int got_packet, ret;
av_init_packet(&pkt);
c = audio_st->codec;
CMItemCount numSamples = CMSampleBufferGetNumSamples(sampleBuffer);
NSUInteger channelIndex = 0;
CMBlockBufferRef audioBlockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer);
size_t audioBlockBufferOffset = (channelIndex * numSamples * sizeof(SInt16));
size_t lengthAtOffset = 0;
size_t totalLength = 0;
SInt16 *samples = NULL;
CMBlockBufferGetDataPointer(audioBlockBuffer, audioBlockBufferOffset, &lengthAtOffset, &totalLength, (char **)(&samples));
const AudioStreamBasicDescription *audioDescription = CMAudioFormatDescriptionGetStreamBasicDescription(CMSampleBufferGetFormatDescription(sampleBuffer));
SwrContext *swr = swr_alloc();
int in_smprt = (int)audioDescription->mSampleRate;
av_opt_set_int(swr, "in_channel_layout", AV_CH_LAYOUT_MONO, 0);
av_opt_set_int(swr, "out_channel_layout", audio_st->codec->channel_layout, 0);
av_opt_set_int(swr, "in_channel_count", audioDescription->mChannelsPerFrame, 0);
av_opt_set_int(swr, "out_channel_count", audio_st->codec->channels, 0);
av_opt_set_int(swr, "out_channel_layout", audio_st->codec->channel_layout, 0);
av_opt_set_int(swr, "in_sample_rate", audioDescription->mSampleRate,0);
av_opt_set_int(swr, "out_sample_rate", audio_st->codec->sample_rate,0);
av_opt_set_sample_fmt(swr, "in_sample_fmt", AV_SAMPLE_FMT_S16, 0);
av_opt_set_sample_fmt(swr, "out_sample_fmt", audio_st->codec->sample_fmt, 0);
swr_init(swr);
uint8_t **input = NULL;
int src_linesize;
int in_samples = (int)numSamples;
ret = av_samples_alloc_array_and_samples(&input, &src_linesize, audioDescription->mChannelsPerFrame,
in_samples, AV_SAMPLE_FMT_S16P, 0);
*input=(uint8_t*)samples;
uint8_t *output=NULL;
int out_samples = av_rescale_rnd(swr_get_delay(swr, in_smprt) +in_samples, (int)audio_st->codec->sample_rate, in_smprt, AV_ROUND_UP);
av_samples_alloc(&output, NULL, audio_st->codec->channels, out_samples, audio_st->codec->sample_fmt, 0);
in_samples = (int)numSamples;
out_samples = swr_convert(swr, &output, out_samples, (const uint8_t **)input, in_samples);
aframe->nb_samples =(int) out_samples;
ret = avcodec_fill_audio_frame(aframe, audio_st->codec->channels, audio_st->codec->sample_fmt,
(uint8_t *)output,
(int) out_samples *
av_get_bytes_per_sample(audio_st->codec->sample_fmt) *
audio_st->codec->channels, 1);
aframe->channel_layout = audio_st->codec->channel_layout;
aframe->channels=audio_st->codec->channels;
aframe->sample_rate= audio_st->codec->sample_rate;
if (timing_info.presentationTimeStamp.timescale!=0)
pts=(double) timing_info.presentationTimeStamp.value/timing_info.presentationTimeStamp.timescale;
aframe->pts=pts*audio_st->time_base.den;
aframe->pts = av_rescale_q(aframe->pts, audio_st->time_base, audio_st->codec->time_base);
ret = avcodec_encode_audio2(c, &pkt, aframe, &got_packet);
if (ret < 0) {
fprintf(stderr, "Error encoding audio frame: %s\n", av_err2str(ret));
exit(1);
}
swr_free(&swr);
if (got_packet)
{
pkt.stream_index = audio_st->index;
pkt.pts = av_rescale_q(pkt.pts, audio_st->codec->time_base, audio_st->time_base);
pkt.dts = av_rescale_q(pkt.dts, audio_st->codec->time_base, audio_st->time_base);
// Write the compressed frame to the media file.
ret = av_interleaved_write_frame(oc, &pkt);
if (ret != 0) {
fprintf(stderr, "Error while writing audio frame: %s\n",
av_err2str(ret));
exit(1);
}
}Stack Overflow用户
回答已采纳
发布于 2016-09-26 01:39:07
我也遇到了类似的问题。我将PCM数据包编码为AAC，而PCM数据包的长度有时小于1024。
如果我编码的数据包小于1024，音频将是缓慢的。另一方面，如果我扔掉它，音频会变得更快。从我的观察来看，swr_convert函数没有任何自动缓冲。
最后，我使用了一个缓冲区方案，该方案将数据包填充到1024缓冲区中，并且每次缓冲区满时都会对缓冲区进行编码和清理。
填充缓冲区的功能如下：
// put frame data into buffer of fixed size
bool ffmpegHelper::putAudioBuffer(const AVFrame *pAvFrameIn, AVFrame **pAvFrameBuffer, AVCodecContext *dec_ctx, int frame_size, int &k0) {
// prepare pFrameAudio
if (!(*pAvFrameBuffer)) {
if (!(*pAvFrameBuffer = av_frame_alloc())) {
av_log(NULL, AV_LOG_ERROR, "Alloc frame failed\n");
return false;
} else {
(*pAvFrameBuffer)->format = dec_ctx->sample_fmt;
(*pAvFrameBuffer)->channels = dec_ctx->channels;
(*pAvFrameBuffer)->sample_rate = dec_ctx->sample_rate;
(*pAvFrameBuffer)->nb_samples = frame_size;
int ret = av_frame_get_buffer(*pAvFrameBuffer, 0);
if (ret < 0) {
char err[500];
av_log(NULL, AV_LOG_ERROR, "get audio buffer failed: %s\n",
av_make_error_string(err, AV_ERROR_MAX_STRING_SIZE, ret));
return false;
}
(*pAvFrameBuffer)->nb_samples = 0;
(*pAvFrameBuffer)->pts = pAvFrameIn->pts;
}
}
// copy input data to buffer
int n_channels = pAvFrameIn->channels;
int new_samples = min(pAvFrameIn->nb_samples - k0, frame_size - (*pAvFrameBuffer)->nb_samples);
int k1 = (*pAvFrameBuffer)->nb_samples;
if (pAvFrameIn->format == AV_SAMPLE_FMT_S16) {
int16_t *d_in = (int16_t *)pAvFrameIn->data[0];
d_in += n_channels * k0;
int16_t *d_out = (int16_t *)(*pAvFrameBuffer)->data[0];
d_out += n_channels * k1;
for (int i = 0; i < new_samples; ++i) {
for (int j = 0; j < pAvFrameIn->channels; ++j) {
*d_out++ = *d_in++;
}
}
} else {
printf("not handled format for audio buffer\n");
return false;
}
(*pAvFrameBuffer)->nb_samples += new_samples;
k0 += new_samples;
return true;
}填充缓冲区和编码的循环如下：
// transcoding needed
int got_frame;
AVMediaType stream_type;
// decode the packet (do it your self)
decodePacket(packet, dec_ctx, &pAvFrame_, got_frame);
if (enc_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
ret = 0;
// break audio packet down to buffer
if (enc_ctx->frame_size > 0) {
int k = 0;
while (k < pAvFrame_->nb_samples) {
if (!putAudioBuffer(pAvFrame_, &pFrameAudio_, dec_ctx, enc_ctx->frame_size, k))
return false;
if (pFrameAudio_->nb_samples == enc_ctx->frame_size) {
// the buffer is full, encode it (do it yourself)
ret = encodeFrame(pFrameAudio_, stream_index, got_frame, false);
if (ret < 0)
return false;
pFrameAudio_->pts += enc_ctx->frame_size;
pFrameAudio_->nb_samples = 0;
}
}
} else {
ret = encodeFrame(pAvFrame_, stream_index, got_frame, false);
}
} else {
// encode packet directly
ret = encodeFrame(pAvFrame_, stream_index, got_frame, false);
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/16904841
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