怎样做京东网站,网站找图片做海报侵权,建设网站的工具,蒙牛网站建设报价情况音视频入门基础#xff1a;WAV专题系列文章#xff1a;
音视频入门基础#xff1a;WAV专题#xff08;1#xff09;——使用FFmpeg命令生成WAV音频文件
音视频入门基础#xff1a;WAV专题#xff08;2#xff09;——WAV格式简介
音视频入门基础#xff1a;WAV专题…
音视频入门基础WAV专题系列文章
音视频入门基础WAV专题1——使用FFmpeg命令生成WAV音频文件
音视频入门基础WAV专题2——WAV格式简介
音视频入门基础WAV专题3——FFmpeg源码中判断某文件是否为WAV音频文件的实现
音视频入门基础WAV专题4——FFmpeg源码中获取WAV文件音频压缩编码格式、采样频率、声道数量、采样位数、码率的实现
音视频入门基础WAV专题5——FFmpeg源码中解码WAV Header的实现 一、引言
执行FFmpeg命令
./ffmpeg -i XXX.wav
FFmpeg内部会调用wav_probe函数检测该文件是否为WAV格式的音频文件具体可以参考《音视频入门基础WAV专题3——FFmpeg源码中判断某文件是否为WAV音频文件的实现》。然后如果检测出该文件为WAV格式的音频文件会调用wav_read_header函数解码WAV Header。 二、wav_read_header函数的定义
wav_read_header函数定义在FFmpeg源码本文演示用的FFmpeg源码版本为5.0.3的源文件libavformat/wavdec.c中
/* wav input */
static int wav_read_header(AVFormatContext *s)
{int64_t size, av_uninit(data_size);int64_t sample_count 0;int rf64 0, bw64 0;uint32_t tag;AVIOContext *pb s-pb;AVStream *st NULL;WAVDemuxContext *wav s-priv_data;int ret, got_fmt 0, got_xma2 0;int64_t next_tag_ofs, data_ofs -1;wav-unaligned avio_tell(s-pb) 1;wav-smv_data_ofs -1;/* read chunk ID */tag avio_rl32(pb);switch (tag) {case MKTAG(R, I, F, F):break;case MKTAG(R, I, F, X):wav-rifx 1;break;case MKTAG(R, F, 6, 4):rf64 1;break;case MKTAG(B, W, 6, 4):bw64 1;break;default:av_log(s, AV_LOG_ERROR, invalid start code %s in RIFF header\n,av_fourcc2str(tag));return AVERROR_INVALIDDATA;}/* read chunk size */avio_rl32(pb);/* read format */if (avio_rl32(pb) ! MKTAG(W, A, V, E)) {av_log(s, AV_LOG_ERROR, invalid format in RIFF header\n);return AVERROR_INVALIDDATA;}if (rf64 || bw64) {if (avio_rl32(pb) ! MKTAG(d, s, 6, 4))return AVERROR_INVALIDDATA;size avio_rl32(pb);if (size 24)return AVERROR_INVALIDDATA;avio_rl64(pb); /* RIFF size */data_size avio_rl64(pb);sample_count avio_rl64(pb);if (data_size 0 || sample_count 0) {av_log(s, AV_LOG_ERROR, negative data_size and/or sample_count in ds64: data_size %PRId64, sample_count %PRId64\n,data_size, sample_count);return AVERROR_INVALIDDATA;}avio_skip(pb, size - 24); /* skip rest of ds64 chunk */}/* Create the audio stream now so that its index is always zero */st avformat_new_stream(s, NULL);if (!st)return AVERROR(ENOMEM);for (;;) {AVStream *vst;size next_tag(pb, tag, wav-rifx);next_tag_ofs avio_tell(pb) size;if (avio_feof(pb))break;switch (tag) {case MKTAG(f, m, t, ):/* only parse the first fmt tag found */if (!got_xma2 !got_fmt (ret wav_parse_fmt_tag(s, size, st)) 0) {return ret;} else if (got_fmt)av_log(s, AV_LOG_WARNING, found more than one fmt tag\n);got_fmt 1;break;case MKTAG(X, M, A, 2):/* only parse the first XMA2 tag found */if (!got_fmt !got_xma2 (ret wav_parse_xma2_tag(s, size, st)) 0) {return ret;} else if (got_xma2)av_log(s, AV_LOG_WARNING, found more than one XMA2 tag\n);got_xma2 1;break;case MKTAG(d, a, t, a):if (!(pb-seekable AVIO_SEEKABLE_NORMAL) !got_fmt !got_xma2) {av_log(s, AV_LOG_ERROR,found no fmt tag before the data tag\n);return AVERROR_INVALIDDATA;}if (rf64 || bw64) {next_tag_ofs wav-data_end avio_tell(pb) data_size;} else if (size ! 0xFFFFFFFF) {data_size size;next_tag_ofs wav-data_end size ? next_tag_ofs : INT64_MAX;} else {av_log(s, AV_LOG_WARNING, Ignoring maximum wav data size, file may be invalid\n);data_size 0;next_tag_ofs wav-data_end INT64_MAX;}data_ofs avio_tell(pb);/* dont look for footer metadata if we cant seek or if we dont* know where the data tag ends*/if (!(pb-seekable AVIO_SEEKABLE_NORMAL) || (!(rf64 !bw64) !size))goto break_loop;break;case MKTAG(f, a, c, t):if (!sample_count)sample_count (!wav-rifx ? avio_rl32(pb) : avio_rb32(pb));break;case MKTAG(b, e, x, t):if ((ret wav_parse_bext_tag(s, size)) 0)return ret;break;case MKTAG(S,M,V,0):if (!got_fmt) {av_log(s, AV_LOG_ERROR, found no fmt tag before the SMV0 tag\n);return AVERROR_INVALIDDATA;}// SMV file, a wav file with video appended.if (size ! MKTAG(0,2,0,0)) {av_log(s, AV_LOG_ERROR, Unknown SMV version found\n);goto break_loop;}av_log(s, AV_LOG_DEBUG, Found SMV data\n);wav-smv_given_first 0;vst avformat_new_stream(s, NULL);if (!vst)return AVERROR(ENOMEM);wav-vst vst;avio_r8(pb);vst-id 1;vst-codecpar-codec_type AVMEDIA_TYPE_VIDEO;vst-codecpar-codec_id AV_CODEC_ID_SMVJPEG;vst-codecpar-width avio_rl24(pb);vst-codecpar-height avio_rl24(pb);if ((ret ff_alloc_extradata(vst-codecpar, 4)) 0) {av_log(s, AV_LOG_ERROR, Could not allocate extradata.\n);return ret;}size avio_rl24(pb);wav-smv_data_ofs avio_tell(pb) (size - 5) * 3;avio_rl24(pb);wav-smv_block_size avio_rl24(pb);if (!wav-smv_block_size)return AVERROR_INVALIDDATA;avpriv_set_pts_info(vst, 32, 1, avio_rl24(pb));vst-duration avio_rl24(pb);avio_rl24(pb);avio_rl24(pb);wav-smv_frames_per_jpeg avio_rl24(pb);if (wav-smv_frames_per_jpeg 65536) {av_log(s, AV_LOG_ERROR, too many frames per jpeg\n);return AVERROR_INVALIDDATA;}AV_WL32(vst-codecpar-extradata, wav-smv_frames_per_jpeg);goto break_loop;case MKTAG(L, I, S, T):case MKTAG(l, i, s, t):if (size 4) {av_log(s, AV_LOG_ERROR, too short LIST tag\n);return AVERROR_INVALIDDATA;}switch (avio_rl32(pb)) {case MKTAG(I, N, F, O):ff_read_riff_info(s, size - 4);break;case MKTAG(a, d, t, l):if (s-nb_chapters 0) {while (avio_tell(pb) next_tag_ofs !avio_feof(pb)) {char cue_label[512];unsigned id, sub_size;if (avio_rl32(pb) ! MKTAG(l, a, b, l))break;sub_size avio_rl32(pb);if (sub_size 5)break;id avio_rl32(pb);avio_get_str(pb, sub_size - 4, cue_label, sizeof(cue_label));avio_skip(pb, avio_tell(pb) 1);for (int i 0; i s-nb_chapters; i) {if (s-chapters[i]-id id) {av_dict_set(s-chapters[i]-metadata, title, cue_label, 0);break;}}}}break;}break;case MKTAG(I, D, 3, ):case MKTAG(i, d, 3, ): {ID3v2ExtraMeta *id3v2_extra_meta;ff_id3v2_read_dict(pb, ffformatcontext(s)-id3v2_meta, ID3v2_DEFAULT_MAGIC, id3v2_extra_meta);if (id3v2_extra_meta) {ff_id3v2_parse_apic(s, id3v2_extra_meta);ff_id3v2_parse_chapters(s, id3v2_extra_meta);ff_id3v2_parse_priv(s, id3v2_extra_meta);}ff_id3v2_free_extra_meta(id3v2_extra_meta);}break;case MKTAG(c, u, e, ):if (size 4 got_fmt st-codecpar-sample_rate 0) {AVRational tb {1, st-codecpar-sample_rate};unsigned nb_cues avio_rl32(pb);if (size nb_cues * 24LL 4LL) {for (int i 0; i nb_cues; i) {unsigned offset, id avio_rl32(pb);if (avio_feof(pb))return AVERROR_INVALIDDATA;avio_skip(pb, 16);offset avio_rl32(pb);if (!avpriv_new_chapter(s, id, tb, offset, AV_NOPTS_VALUE, NULL))return AVERROR(ENOMEM);}}}break;}/* seek to next tag unless we know that well run into EOF */if ((avio_size(pb) 0 next_tag_ofs avio_size(pb)) ||wav_seek_tag(wav, pb, next_tag_ofs, SEEK_SET) 0) {break;}}break_loop:if (!got_fmt !got_xma2) {av_log(s, AV_LOG_ERROR, no fmt or XMA2 tag found\n);return AVERROR_INVALIDDATA;}if (data_ofs 0) {av_log(s, AV_LOG_ERROR, no data tag found\n);return AVERROR_INVALIDDATA;}avio_seek(pb, data_ofs, SEEK_SET);if (data_size (INT64_MAX3)) {av_log(s, AV_LOG_WARNING, Data size %PRId64 is too large\n, data_size);data_size 0;}if ( st-codecpar-bit_rate 0 data_size 0 st-codecpar-sample_rate 0 sample_count 0 st-codecpar-channels 1 sample_count % st-codecpar-channels 0) {if (fabs(8.0 * data_size * st-codecpar-channels * st-codecpar-sample_rate /sample_count /st-codecpar-bit_rate - 1.0) 0.3)sample_count / st-codecpar-channels;}if ( data_size 0 sample_count st-codecpar-channels (data_size 3) / sample_count / st-codecpar-channels st-codecpar-bits_per_coded_sample 1) {av_log(s, AV_LOG_WARNING, ignoring wrong sample_count %PRId64\n, sample_count);sample_count 0;}/* G.729 hack (for Ticket4577)* FIXME: Come up with cleaner, more general solution */if (st-codecpar-codec_id AV_CODEC_ID_G729 sample_count (data_size 3) sample_count) {av_log(s, AV_LOG_WARNING, ignoring wrong sample_count %PRId64\n, sample_count);sample_count 0;}if (!sample_count || av_get_exact_bits_per_sample(st-codecpar-codec_id) 0)if ( st-codecpar-channels data_size av_get_bits_per_sample(st-codecpar-codec_id) wav-data_end avio_size(pb))sample_count (data_size 3)/(st-codecpar-channels * (uint64_t)av_get_bits_per_sample(st-codecpar-codec_id));if (sample_count)st-duration sample_count;if (st-codecpar-codec_id AV_CODEC_ID_PCM_S32LE st-codecpar-block_align st-codecpar-channels * 4 st-codecpar-bits_per_coded_sample 32 st-codecpar-extradata_size 2 AV_RL16(st-codecpar-extradata) 1) {st-codecpar-codec_id AV_CODEC_ID_PCM_F16LE;st-codecpar-bits_per_coded_sample 16;} else if (st-codecpar-codec_id AV_CODEC_ID_PCM_S24LE st-codecpar-block_align st-codecpar-channels * 4 st-codecpar-bits_per_coded_sample 24) {st-codecpar-codec_id AV_CODEC_ID_PCM_F24LE;} else if (st-codecpar-codec_id AV_CODEC_ID_XMA1 ||st-codecpar-codec_id AV_CODEC_ID_XMA2) {st-codecpar-block_align 2048;} else if (st-codecpar-codec_id AV_CODEC_ID_ADPCM_MS st-codecpar-channels 2 st-codecpar-block_align INT_MAX / st-codecpar-channels) {st-codecpar-block_align * st-codecpar-channels;}ff_metadata_conv_ctx(s, NULL, wav_metadata_conv);ff_metadata_conv_ctx(s, NULL, ff_riff_info_conv);set_spdif(s, wav);return 0;
} 形参s既是输入型参数也是输出型参数指向AVFormatContext类型的变量。s-pb包含整个WAV Header的二进制数据。执行wav_read_header函数后(WAVDemuxContext *)(s-priv_data)-vst-codecpar中的成员变量会被赋值为WAV Header中Format chunk中的信息包括音频压缩编码格式、采样频率、声道数量、采样位数、码率。 返回值返回0表示解码WAV Header成功。返回一个负数表示解码WAV Header失败。 三、wav_read_header函数的内部实现分析
一读取WAV Header中的“区块编号”
WAV Header中的第一个区块为“区块编号”通过下面语句读取“区块编号”的内容关于avio_rXXX系列函数的用法可以参考《FFmpeg源码avio_r8、avio_rl16、avio_rl24、avio_rl32、avio_rl64函数分析》 /* read chunk ID */tag avio_rl32(pb); 有的同学看到这里可能会有疑问根据文章《音视频入门基础WAV专题2——WAV格式简介》的描述“区块编号”不是大端字节序的吗而avio_rl32函数是按照小端模式读取四个字节数据。所以为什么是用avio_rl32函数而不是avio_rb32函数读取呢avio_rb32函数才是按照大端模式读取四个字节数据的吧 原因是这样的在语句tag avio_rl32(pb)之后又通过MKTAG函数关于MKTAG和MKBETAG宏定义的用法可以参考《FFmpeg源码MKTAG和MKBETAG宏定义分析》将字符比如R, I, F, F转换为整形按小端模式存贮让其跟tag进行比较。所以这就是所谓的“负负得正”“用avio_rl32函数读取然后跟MKTAG转换出来的整数进行比较”这个跟“用avio_rb32函数读取然后用MKBETAG转换出来的整数进行比较”效果是一样的 switch (tag) {case MKTAG(R, I, F, F):break;case MKTAG(R, I, F, X):wav-rifx 1;break;case MKTAG(R, F, 6, 4):rf64 1;break;case MKTAG(B, W, 6, 4):bw64 1;break;default:av_log(s, AV_LOG_ERROR, invalid start code %s in RIFF header\n,av_fourcc2str(tag));return AVERROR_INVALIDDATA;} 所以下面代码块 /* read chunk ID */tag avio_rl32(pb);switch (tag) {case MKTAG(R, I, F, F):break;case MKTAG(R, I, F, X):wav-rifx 1;break;case MKTAG(R, F, 6, 4):rf64 1;break;case MKTAG(B, W, 6, 4):bw64 1;break;default:av_log(s, AV_LOG_ERROR, invalid start code %s in RIFF header\n,av_fourcc2str(tag));return AVERROR_INVALIDDATA;}
等价于 /* read chunk ID */tag avio_rb32(pb);switch (tag) {case MKBETAG(R, I, F, F):break;case MKBETAG(R, I, F, X):wav-rifx 1;break;case MKBETAG(R, F, 6, 4):rf64 1;break;case MKBETAG(B, W, 6, 4):bw64 1;break;default:av_log(s, AV_LOG_ERROR, invalid start code %s in RIFF header\n,av_fourcc2str(tag));return AVERROR_INVALIDDATA;} 读取到“区块编号”后如果值等于“RIFF”表示该文件遵守RIFF格式的规则按默认处理如果值等于“RIFX”让wav-rifx赋值为1表示该文件遵守RIFX格式的规则如果值等于“RF64”让变量rf64赋值为1表示这是WAVE 64位扩展格式中的一种WAV RF64如果值等于“BW64”让变量bw64赋值为1表示这是WAVE 64位扩展格式中的一种WAV BW64如果值不为上述打印日志invalid start code XXX in RIFF header并返回AVERROR_INVALIDDATA表示WAV Header中的数据不合法。 二读取WAV Header中的“总区块大小”
WAV Header中的第二个区块为“总区块大小”通过下面语句读取“总区块大小”。由于该值可以由WAV Header中的其它值推导出来所以对于FFmpeg来讲它没有意义故FFmpeg没有将其存贮到内部的成员变量中 /* read chunk size */avio_rl32(pb); 三读取WAV Header中的“档案格式”
WAV Header中的第三个区块为“档案格式”。通过下面语句读取该值并进行比较如果值不为“WAVE”日志打印invalid format in RIFF header表示WAV Header中的数据不合法 /* read format */if (avio_rl32(pb) ! MKTAG(W, A, V, E)) {av_log(s, AV_LOG_ERROR, invalid format in RIFF header\n);return AVERROR_INVALIDDATA;}四WAVE 64位扩展格式处理
FFmpeg源码内部将WAV和WAVE 64位扩展格式都放到一起处理下面代码处理文件格式为WAVE 64位时的情况
if (rf64 || bw64) {if (avio_rl32(pb) ! MKTAG(d, s, 6, 4))return AVERROR_INVALIDDATA;size avio_rl32(pb);if (size 24)return AVERROR_INVALIDDATA;avio_rl64(pb); /* RIFF size */data_size avio_rl64(pb);sample_count avio_rl64(pb);if (data_size 0 || sample_count 0) {av_log(s, AV_LOG_ERROR, negative data_size and/or sample_count in ds64: data_size %PRId64, sample_count %PRId64\n,data_size, sample_count);return AVERROR_INVALIDDATA;}avio_skip(pb, size - 24); /* skip rest of ds64 chunk */} 五读取WAV Header中的子区块
WAV Header中包含“Format chunk”、“Data chunk”这种必须存在的子区块也包含Fact chunk、Cue points chunk、Playlist chunk、Associated data list chunk等可选区块。通过下面语句中的for循环和switch case语句来判断是哪种子区块然后循环处理 for (;;) {AVStream *vst;size next_tag(pb, tag, wav-rifx);next_tag_ofs avio_tell(pb) size;if (avio_feof(pb))break;switch (tag) {case MKTAG(f, m, t, )://...break;case MKTAG(X, M, A, 2)://...break;case MKTAG(d, a, t, a)://...break;case MKTAG(f, a, c, t)://...break;case MKTAG(b, e, x, t)://...break;case MKTAG(S,M,V,0)://...goto break_loop;case MKTAG(L, I, S, T):case MKTAG(l, i, s, t)://...break;case MKTAG(I, D, 3, ):case MKTAG(i, d, 3, ): {//...break;case MKTAG(c, u, e, )://...break;}}
} 六读取WAV Header中“Format chunk”子区块的信息
下面以读取“Format chunk”子区块中的信息为例子进行讲解如何解码子区块。ff_get_wav_header函数中通过wav_parse_fmt_tag函数解析“Format chunk”子区块
int ff_get_wav_header(AVFormatContext *s, AVIOContext *pb,AVCodecParameters *par, int size, int big_endian)
{
//...for (;;) {switch (tag) {case MKTAG(f, m, t, ):/* only parse the first fmt tag found */if (!got_xma2 !got_fmt (ret wav_parse_fmt_tag(s, size, st)) 0) {return ret;} else if (got_fmt)av_log(s, AV_LOG_WARNING, found more than one fmt tag\n);got_fmt 1;break;}
//...}
} 而wav_parse_fmt_tag函数内部又调用了ff_get_wav_header函数来获取获取音频压缩编码格式、采样频率、声道数量、采样位数、码率信息具体可以参考《音视频入门基础WAV专题4——FFmpeg源码中获取WAV文件音频压缩编码格式、采样频率、声道数量、采样位数、码率的实现》
static int wav_parse_fmt_tag(AVFormatContext *s, int64_t size, AVStream *st)
{AVIOContext *pb s-pb;WAVDemuxContext *wav s-priv_data;int ret;/* parse fmt header */ret ff_get_wav_header(s, pb, st-codecpar, size, wav-rifx);if (ret 0)return ret;handle_stream_probing(st);ffstream(st)-need_parsing AVSTREAM_PARSE_FULL_RAW;avpriv_set_pts_info(st, 64, 1, st-codecpar-sample_rate);return 0;
}
