FFmpeg–音频解码流程：aac解码pcm
音频解码流程
aac—音频解码器–pcm数据
API
avcodec_find_decoder：根据指定的AVCodecID查找注册的解码器
av_parser_init：初始化AVCodecParserContext
avcodec_alloc_context3：为AVCodecContext分配内存
avcodec_open2：打开解码器
av_parser_parse2：解析获得⼀个Packet
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt); // 将AVPacket压缩数据给解码器
int avcodec_receive_frame ( AVCodecContext * avctx, AVFrame * frame ) // 获取到解码后的AVFrame数据
分析：
首先读文件（原始码流），将原始码流解析为Packet, 通过解码器获得AVFrame ，最后写文件 ，循环获取Packet，写文件
伪代码
// 读文件
data_size = fread(inbuf, 1, AUDIO_INBUF_SIZE, infile); // data_size 20480
while(data_size) {
av_parser_parse2() //解析获得⼀个Packet
decode(); // (压缩AVPacket,解码AVFrame数据，之后写数据)
// data_size 减少
}
void decode() {
ret = avcodec_send_packet； //将AVPacket压缩数据给解码器
while (ret ) {
ret = avcodec_receive_frame; // 获取到解码后的AVFrame数据
if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
return; // fwrite 结束后 ret == AVERROR(EAGAIN,需要新的输⼊, 返回
av_get_bytes_per_sample //获取每个sample中的字节数
fwrite();
}
}
code:
#define AUDIO_INBUF_SIZE 20480
#define AUDIO_REFILL_THRESH 4096
static char err_buf[128] = {0};
static char* av_get_err(int errnum)
{
av_strerror(errnum, err_buf, 128);
return err_buf;
}
static void print_sample_format(const AVFrame *frame)
{
printf("ar-samplerate: %uHzn", frame->sample_rate);
printf("ac-channel: %un", frame->channels);
printf("f-format: %un", frame->format);// 格式需要注意，实际存储到本地文件时已经改成交错模式
}
static void decode(AVCodecContext *dec_ctx, AVPacket *pkt, AVFrame *frame,
FILE *outfile)
{
int i, ch;
int ret, data_size;
/* send the packet with the compressed data to the decoder */
ret = avcodec_send_packet(dec_ctx, pkt);
if(ret == AVERROR(EAGAIN))
{
fprintf(stderr, "Receive_frame and send_packet both returned EAGAIN, which is an API violation.n");
}
else if (ret < 0)
{
fprintf(stderr, "Error submitting the packet to the decoder, err:%s, pkt_size:%dn",
av_get_err(ret), pkt->size)；
return;
}
/* read all the output frames (infile general there may be any number of them */
while (ret >= 0)
{
// 对于frame, avcodec_receive_frame内部每次都先调用
ret = avcodec_receive_frame(dec_ctx, frame);
if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
return;
else if (ret < 0)
{
fprintf(stderr, "Error during decodingn");
exit(1);
}
data_size = av_get_bytes_per_sample(dec_ctx->sample_fmt);
if (data_size < 0)
{
fprintf(stderr, "Failed to calculate data sizen");
exit(1);
}
static int s_print_format = 0;
if(s_print_format == 0)
{
s_print_format = 1;
print_sample_format(frame);
}
for (i = 0; i < frame->nb_samples; i++)
{
for (ch = 0; ch < dec_ctx->channels; ch++) // 交错的方式写入, 大部分float的格式输出
fwrite(frame->data[ch] + data_size*i, 1, data_size, outfile);
}
}
}
int main(int argc, char **argv)
{
const char *outfilename;
const char *filename;
const AVCodec *codec;
AVCodecContext *codec_ctx= NULL;
AVCodecParserContext *parser = NULL;
int len = 0;
int ret = 0;
FILE *infile = NULL;
FILE *outfile = NULL;
uint8_t inbuf[AUDIO_INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
uint8_t *data = NULL;
size_t data_size = 0;
AVPacket *pkt = NULL;
AVFrame *decoded_frame = NULL;
if (argc <= 2)
{
fprintf(stderr, "Usage: %s <input file> <output file>n", argv[0]);
exit(0);
}
filename = argv[1];
outfilename = argv[2];
pkt = av_packet_alloc();
enum AVCodecID audio_codec_id = AV_CODEC_ID_AAC;
if(strstr(filename, "aac") != NULL)
{
audio_codec_id = AV_CODEC_ID_AAC;
}
else if(strstr(filename, "mp3") != NULL)
{
audio_codec_id = AV_CODEC_ID_MP3;
}
else
{
printf("default codec id:%dn", audio_codec_id);
}
// 查找解码器
codec = avcodec_find_decoder(audio_codec_id); // AV_CODEC_ID_AAC
if (!codec) {
fprintf(stderr, "Codec not foundn");
exit(1);
}
// 获取裸流的解析器 AVCodecParserContext(数据) + AVCodecParser(方法)
parser = av_parser_init(codec->id);
if (!parser) {
fprintf(stderr, "Parser not foundn");
exit(1);
}
// 分配codec上下文
codec_ctx = avcodec_alloc_context3(codec);
if (!codec_ctx) {
fprintf(stderr, "Could not allocate audio codec contextn");
exit(1);
}
// 将解码器和解码器上下文进行关联
if (avcodec_open2(codec_ctx, codec, NULL) < 0) {
fprintf(stderr, "Could not open codecn");
exit(1);
}
// 打开输入文件
infile = fopen(filename, "rb");
if (!infile) {
fprintf(stderr, "Could not open %sn", filename);
exit(1);
}
// 打开输出文件
outfile = fopen(outfilename, "wb");
if (!outfile) {
av_free(codec_ctx);
exit(1);
}
// 读取文件进行解码
data = inbuf;
data_size = fread(inbuf, 1, AUDIO_INBUF_SIZE, infile);
while (data_size > 0)
{
if (!decoded_frame)
{
if (!(decoded_frame = av_frame_alloc()))
{
fprintf(stderr, "Could not allocate audio framen");
exit(1);
}
}
ret = av_parser_parse2(parser, codec_ctx, &pkt->data, &pkt->size,
data, data_size,
AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);
if (ret < 0)
{
fprintf(stderr, "Error while parsingn");
exit(1);
}
data += ret; // 跳过已经解析的数据
data_size -= ret; // 对应的缓存大小也做相应减小
if (pkt->size)
decode(codec_ctx, pkt, decoded_frame, outfile);
if (data_size < AUDIO_REFILL_THRESH) // 如果数据少了则再次读取
{
memmove(inbuf, data, data_size); // 把之前剩的数据拷贝到buffer的起始位置
data = inbuf;
// 读取数据 长度: AUDIO_INBUF_SIZE - data_size
len = fread(data + data_size, 1, AUDIO_INBUF_SIZE - data_size, infile);
if (len > 0)
data_size += len;
}
}
/* 冲刷解码器 */
pkt->data = NULL; // 让其进入drain mode
pkt->size = 0;
decode(codec_ctx, pkt, decoded_frame, outfile);
fclose(outfile);
fclose(infile);
avcodec_free_context(&codec_ctx);
av_parser_close(parser);
av_frame_free(&decoded_frame);
av_packet_free(&pkt);
printf("main finish, please enter Enter and exitn");
return 0;
}
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