我一直疑问为什么有些视频解码时显示格式是:H264，大部分又是：AVC1
我在搜索编程资料时在微软的msdn上发现的：
原文:http://msdn.microsoft.com/en-us/library/dd757808v=vs.85).aspx
FOURCC:AVC1 描写叙述:H.264 bitstream without start codes.
FOURCC:H264 描写叙述:H.264 bitstream with start codes.
H.264 Bitstream with Start Codes
H.264 bitstreams that are transmitted over the air, or contained in MPEG-2 program or transport streams, or recorded on HD-DVD, are formatted as described in Annex B of ITU-T Rec. H.264. According to this specification, the
bitstream consists of a sequence of network abstraction layer units NALUs), each of which is prefixed with a start code equal to 0x000001 or 0x00000001.
这段话的大致意思是：带有開始码的H.264视频通常是用于无线发射、有线广播或者HD-DVD中的。这些数据流的開始都有一个開始码：0x000001 或者 0x00000001.
H.264 Bitstream Without Start Codes
The MP4 container format stores H.264 data without start codes. Instead, each NALU is prefixed by a length field, which gives the length of the NALU in bytes. The size of the length field can vary, but is typically 1, 2, or
4 bytes.
这段话的大致意思是：没有開始码的H.264视频主要是存储在MP4格式的文件里的。它的数据流的開始是1、2或者4个字节表示长度数据。
原文中的”NALU”简单说是H.264格式中的最主要的单元，是一个数据包。
H.264 Video Types
The following media subtypes are defined for H.264 video.
| Subtype | FOURCC | Description |
|---|---|---|
| MEDIASUBTYPE_AVC1 | ‘AVC1’ | H.264 bitstream without start codes. |
| MEDIASUBTYPE_H264 | ‘H264’ | H.264 bitstream with start codes. |
| MEDIASUBTYPE_h264 | ‘h264’ | Equivalent to MEDIASUBTYPE_H264, with a different FOURCC. |
| MEDIASUBTYPE_X264 | ‘X264’ | Equivalent to MEDIASUBTYPE_H264, with a different FOURCC. |
| MEDIASUBTYPE_x264 | ‘x264’ | Equivalent to MEDIASUBTYPE_H264, with a different FOURCC. |
The main difference between these media types is the presence of startcodes in the bitstream. If the subtype is MEDIASUBTYPE_AVC1, thebitstream does not contain start codes.
H.264 Bitstream Without Start Codes
The MP4 container format stores H.264 data without start codes.Instead, each NALU is prefixed by a length field, which gives thelength of the NALU in bytes. The size of the length field can vary, butis typically 1, 2, or 4 bytes.
When start codes are not present in the bitstream, the following media type is used.
| Major type | MEDIATYPE_Video |
|---|---|
| Subtype | MEDIASUBTYPE_AVC1 |
| Format type | FORMAT_MPEG2Video |
The format block is an MPEG2VIDEOINFO structure. This structure should be filled in as follows:
- hdr: A VIDEOINFOHEADER2 structure that describes the bitstream. No color table is present
after theBITMAPINFOHEADER portion of the structure, and biClrUsed must be zero. - dwStartTimeCode: Not used. Set to zero.
- cbSequenceHeader: The length of the dwSequenceHeader array in bytes.
- dwProfile: Specifies the H.264 profile.
- dwLevel: Specifies the H.264 level.
- dwFlags: The number of bytes used for the length field that appears before each NALU. The length field indicates the size of the following NALU in bytes. For example, if dwFlags is 4, each NALU is preceded
by a 4-byte length field. The valid values are 1, 2, and 4. - dwSequenceHeader: A byte array that may contain sequence parameter set SPS) and picture parameter set PPS) NALUs.
The MP4 container might contain sequence parameter sets SPS) orpicture parameter sets PPS) as special NAL units in file headers or ina separate stream distinct from the video stream). When the format isestablished, the media type can specify SPS and PPS
NAL units in the dwSequenceHeader array. IfcbSequenceHeader is greater than zero, dwSequenceHeaderis the start of a byte array containing SPS and PPS NALUs, delimited by2-byte length fields, all in network
byte order big-endian). It ispossible to have both SPS and PPS, only one of these types, or none.The actual type of each NALU can be determined by examining thenal_unit_type field of the NALU itself.
When this media type is used, each media sample starts at thebeginning of a NALU, and NAL units do not span samples. This enablesthe decoder to recover from data corruption or dropped samples.
H.264的视频类型
下面媒体亚型定义的H.264视频。
亚型fourcc值描写叙述
MEDIASUBTYPE_AVC1“AVC1”H.264比特流没有启动代码。
MEDIASUBTYPE_H264“H264”開始编码的H.264比特流。
MEDIASUBTYPE_h264’H264’的相当于到MEDIASUBTYPE_H264，与不同的fourcc值。
“X264的’MEDIASUBTYPE_X264的相当于到MEDIASUBTYPE_H264，与不同的fourcc值。
“X264”MEDIASUBTYPE_x264相当于到MEDIASUBTYPE_H264，用不同的fourcc值。
这些媒体类型之间的主要差别是存在的比特流中的startcodes。假设亚型是MEDIASUBTYPE_AVC1，thebitstream不包括启动代码。
H.264的码流没有启动代码
MP4容器格式存储H.264没有開始codes.Instead的数据，每一个的NALU是由前缀长度字段，从而使字节的NALU
thelength。长度字段的大小能够有所不同，butis通常为1，2，或4个字节。
当启动代码是在比特流中不存在，以下的介质类型。
主要类型MEDIATYPE_Video的
亚型MEDIASUBTYPE_AVC1
格式类型FORMAT_MPEG2Video
格式块是MPEG2VIDEOINFO结构。这样的结构应填写例如以下：
HDR：一个VIDEOINFOHEADER2的结构描写叙述的比特流。没有颜色表是存在的BITMAPINFOHEADER结构部分后，biClrUsed必须是零。
dwStartTimeCode：未使用。设置为零。
cbSequenceHeader的在字节dwSequenceHeader数组的长度。
dwProfile：指定H.264的个人资料。
dwLevel：指定H.264水平。
的dwFlags：用于长度字段出如今每次的NALU的字节数。长度字段表示在字节下面的NALU的大小。比如，假设dwFlags是4个，每一个的NALU之前由4个字节的长度字段。有效的值是1，2和4。
dwSequenceHeader：一个字节数组可能包括序列參数集（SPS）和图像參数集（PPS）NALUs。
MP4容器可能含有序列參数集（SPS）orpicture參数集（PPS）的文件头或INA单独的流（从视频流中不同）的特殊NAL单元。当isestablished的格式，媒体类型能够指定在dwSequenceHeader阵列SPS和PPS
NAL单元。假设cbSequenceHeader大于零，dwSequenceHeaderis含SPS和PPS的NALUs，分隔BY2字节长度字段，全部在网络字节顺序（大端）字节数组開始。它ispossible
SPS和PPS，仅仅有这些类型之中的一个，或能够通过审查的NALU本身thenal_unit_type领域确定每一个的NALU none.The实际类型。
当使用这样的媒体类型，每一个媒体样本的NALU的thebeginning開始，NAL单元不跨越样本。这enablesthe解码器来恢复数据损坏或丢弃的样品。