相比h264，压缩同样的视频获得同样的质量的情况下，h265可以做到压缩后的大小为前者的一半，但压缩时间复杂度增加。h264编码单元为宏块(MB)，最大划分为16x16，而h265编码单元为编码树单元(CTU)，最大划分为64x64。h264的NALU类型占用5位，理论上最多可以支持32种，但实际上有一些是reserved和unspecified的；h265的NALU类型占用6位，理论上最多可以支持32种，但实际上也有一些是reserved和unspecified的。

char const* nal_unit_type_description_h264[32] = {
  "Unspecified", //0
  "Coded slice of a non-IDR picture", //1
  "Coded slice data partition A", //2
  "Coded slice data partition B", //3
  "Coded slice data partition C", //4
  "Coded slice of an IDR picture", //5
  "Supplemental enhancement information (SEI)", //6
  "Sequence parameter set", //7
  "Picture parameter set", //8
  "Access unit delimiter", //9
  "End of sequence", //10
  "End of stream", //11
  "Filler data", //12
  "Sequence parameter set extension", //13
  "Prefix NAL unit", //14
  "Subset sequence parameter set", //15
  "Reserved", //16
  "Reserved", //17
  "Reserved", //18
  "Coded slice of an auxiliary coded picture without partitioning", //19
  "Coded slice extension", //20
  "Reserved", //21
  "Reserved", //22
  "Reserved", //23
  "Unspecified", //24
  "Unspecified", //25
  "Unspecified", //26
  "Unspecified", //27
  "Unspecified", //28
  "Unspecified", //29
  "Unspecified", //30
  "Unspecified" //31
};
char const* nal_unit_type_description_h265[64] = {
  "Coded slice segment of a non-TSA, non-STSA trailing picture", //0
  "Coded slice segment of a non-TSA, non-STSA trailing picture", //1
  "Coded slice segment of a TSA picture", //2
  "Coded slice segment of a TSA picture", //3
  "Coded slice segment of a STSA picture", //4
  "Coded slice segment of a STSA picture", //5
  "Coded slice segment of a RADL picture", //6
  "Coded slice segment of a RADL picture", //7
  "Coded slice segment of a RASL picture", //8
  "Coded slice segment of a RASL picture", //9
  "Reserved", //10
  "Reserved", //11
  "Reserved", //12
  "Reserved", //13
  "Reserved", //14
  "Reserved", //15
  "Coded slice segment of a BLA picture", //16
  "Coded slice segment of a BLA picture", //17
  "Coded slice segment of a BLA picture", //18
  "Coded slice segment of an IDR picture", //19
  "Coded slice segment of an IDR picture", //20
  "Coded slice segment of a CRA picture", //21
  "Reserved", //22
  "Reserved", //23
  "Reserved", //24
  "Reserved", //25
  "Reserved", //26
  "Reserved", //27
  "Reserved", //28
  "Reserved", //29
  "Reserved", //30
  "Reserved", //31
  "Video parameter set", //32
  "Sequence parameter set", //33
  "Picture parameter set", //34
  "Access unit delimiter", //35
  "End of sequence", //36
  "End of bitstream", //37
  "Filler data", //38
  "Supplemental enhancement information (SEI)", //39
  "Supplemental enhancement information (SEI)", //40
  "Reserved", //41
  "Reserved", //42
  "Reserved", //43
  "Reserved", //44
  "Reserved", //45
  "Reserved", //46
  "Reserved", //47
  "Unspecified", //48
  "Unspecified", //49
  "Unspecified", //50
  "Unspecified", //51
  "Unspecified", //52
  "Unspecified", //53
  "Unspecified", //54
  "Unspecified", //55
  "Unspecified", //56
  "Unspecified", //57
  "Unspecified", //58
  "Unspecified", //59
  "Unspecified", //60
  "Unspecified", //61
  "Unspecified", //62
  "Unspecified", //63
};

h265的参数集比h264的多了一个vps（video parameter set）码流先从vps，sps，pps开始。

h264的NALU头只占一个字节，而h265得NALU头占用两个字节。

h264的NALU头组成：

F(forbidden_zero_bit) ：1 bit

nal_ref_idc ：2 bits

nal_unit_type ：5 bits

h265的NALU头组成：

F(forbidden_zero_bit) ：1 bit

Type(nal_unit_type) ：6 bits

LayerId(nuh_layer_id) ：6 bits

TID(nuh_temporal_id_plus1) ：3 bits

下面以一段h265的码流来分析一下：

 NALU头“40 01”对应类型为32(VPS)

 NALU头“42 01”对应类型为33(SPS)

NALU头“44 01”表示类型为33(PPS)

NALU头“4E 01”表示类型为39(SEI)

h265编码出来的NALU码流的分隔，和h264一样，也是使用00 00 00 01 start code，而且防竞争机制（emulation）也一样。防竞争机制是防止NALU码流中出现00 00 01这种短start code，对NALU分隔造成干扰，在其中插入一个03，即变为00 00 03 01。通常编码器出来的码流都是做过防竞争处理的，在传输或保存NALU时无论是使用start code前缀还是长度前缀，都不需要做去03处理，解码器解码的时候会做这一步操作，当然如果要分析NALU码流，就需要先做去03处理。

具体实现可参考live555源码中的\live\liveMedia\H264or5VideoStreamFramer.cpp文件。