FLV
是FLASH VIDEO的简称,FLV
流媒体格式是随着Flash MX的推出发展而来的视频格式。由于它形成的文件极小、加载速度极快,使得网络观看视频文件成为可能,它的出现有效地解决了视频文件导入Flash后,使导出的SWF文件体积庞大,不能在网络上很好的使用等问题。RTMP
和HTTP_FLV
内部使用FLV
协议封装H.264
和AAC
音视频包,FLV属于大端字节序
。
FLV格式由FLV Header
+FLV Body
构成:
从上图可知,FLV封装主要由以下几个部分组成:
FLV Body = PreviousTagSize0 + Tag1 + ..+ PreviousTagSize n + Tag n
PreviousTagSize + Tag = FLV Tag header + Data tag
Data tag = Video(video header + h264 data
) / Audio (audio header + aac data
)
为了进一步熟悉FLV协议,我们探讨下在RTMP中对FLV的封装。
RTMP的Video FLV Data (封装H.264)
通常,FLV的Video Tag应该由11字节
的FLV tag header
和Video tag header
构成,即:
FLV tag Header说明:
PreviousTagSize
: 表面之前的 Tag 的长度(tag 头+tag 数据),第一个 tag值是 0
TagType
:音频:8, 视频:9
DataSize
:flv tag 的数据长度,其实如图里 audio tag 头及其数据长度
Timestamp
:时间戳,貌似flv播放需要
TimestampExtended
:当时间Timestamp不能表示的时候,启用本扩展字段
StreamID
:都填 0.
但是,在RTMP协议中,Video Tag不包含FLV tag header
,即只有Video tag header
,并且封装的H.264数据需要区分PPS/SPS NALU
和普通NALU
,具体结构如下:
- PPS/SPS NALU
以MediaCodec采集的SPS NALU
和PPS NALU
为例:
fun getConfigurationFlvVideoTag(ppsByteBuffer: ByteBuffer, spsByteBuffer: ByteBuffer): ByteArray {
/**
* 构造AVCDecoderConfigurationRecord
*/
val configRecordData = generateAVCDecoderConfigurationRecord(ppsByteBuffer, spsByteBuffer)
/**
* ====================================================
* FLV Video tag (5字节) + configRecord
* ====================================================
*/
val flvVideoTagLen = 5
val videoTagLen = flvVideoTagLen + configRecordData.size
val videoTag = ByteArray(videoTagLen)
/**
* FLV Video tag (5字节):
*
* UB[4]FrameType = 指定帧类型,1表示IDR帧,2为非关键帧
* UB[4]CodecID = 指定数据类型,7表示AVC数据
* UB[8]AVCPacketType = 0x00表示AVC Sps/Pps;0x01表示普通AVC
* UB[24]Composition Time = 0x00 0x00 0x00(忽略)
*/
videoTag[0] = 0x27 // FrameType(2) + CodecId(7) = 0010 0111
videoTag[1] = 0x00 // AVCPacketType = 0x00
videoTag[2] = 0x00 // Composition Time = 0x00 0x00 0x00
videoTag[3] = 0x00
videoTag[4] = 0x00
System.arraycopy(configRecordData, 0, videoTag, flvVideoTagLen, configRecordData.size)
return videoTag
}
fun generateAVCDecoderConfigurationRecord(ppsByteBuffer: ByteBuffer, spsByteBuffer: ByteBuffer): ByteArray {
/**
* Rtmp flv不需要nalu起始地址
* 去掉pps nalu和sps nalu的起始地址(4字节,0x00000001)
*/
spsByteBuffer.position(4)
ppsByteBuffer.position(4)
/**
* ====================================================
* 拷贝sps、pps数据到缓存,预留11个字节配置
* sps configuration(8字节) + sps data + pps configuration(3字节) + sps data
* ====================================================
*/
val spsLen = spsByteBuffer.remaining()
val ppsLen = ppsByteBuffer.remaining()
val totalLen = 11 + spsLen + ppsLen
val recordData = ByteArray(totalLen)
spsByteBuffer.get(recordData, 8, spsLen)
ppsByteBuffer.get(recordData, 8 + spsLen + 3, ppsLen)
/**
* 配置sps
* UB[8]configurationVersion = 0x01
* UB[8]AVCProfileIndication = 0x4D
* UB[8]profile_compatibility = 0x40
* UB[8]AVCLevelIndication = 0x15
* UB[8]lengthSizeMinusOne = FF
* UB[8]numOfSequenceParameterSets = 0xE1
* UB[16]sequenceParameterSetLength:sps长度,占2个字节
*/
recordData[0] = 0x01.toByte()
recordData[1] = 0x4D.toByte()
recordData[2] = 0x40.toByte()
recordData[3] = 0x15.toByte()
recordData[4] = 0xFF.toByte()
recordData[5] = 0xE1.toByte()
recordData[6] = (spsLen shr 8 and 0xFF).toByte()
recordData[7] = (spsLen and 0xFF).toByte()
/**
* 配置pps
* UB[8]numOfPictureParameterSets = 0x01
* UB[16]pictureParameterSetLength: pps长度,占2个字节
*/
recordData[8 + spsLen] = 0x01.toByte()
recordData[8 + spsLen + 1] = (spsLen shr 8 and 0xFF).toByte()
recordData[8 + spsLen + 2] = (spsLen and 0xFF).toByte()
return recordData
}
- 普通NALU(IDR/I帧/non-I帧)
以MediaCodec采集的NALU
为例:
fun geFlvVideoTag(buffer: ByteBuffer, bufferInfo: MediaCodec.BufferInfo): ByteArray {
/**
* Rtmp flv不需要nalu起始地址
* 去掉nalu的起始地址(4字节,0x00000001)
*/
buffer.position(bufferInfo.offset + 4)
buffer.limit(bufferInfo.offset + bufferInfo.size)
/**=====================================================
* FLV Video tag (5字节) + nalu长度值(4字节) + nalu data
* =====================================================
*/
val flvVideoTagLen = 5
val naluLen = 4
val naluDataSize = buffer.remaining()
val videoFlvDataLen = flvVideoTagLen + naluLen + naluDataSize
val videoFlvData = ByteArray(videoFlvDataLen)
/**
* FLV Video tag (5字节):
*
* UB[4]FrameType = 1表示IDR帧,2为非关键帧
* UB[4]CodecID = 7表示AVC数据
* UB[8]AVCPacketType = 0x00表示AVC Sps/Pps;0x01表示普通AVC
* UB[24]Composition Time = 0x00 0x00 0x00(忽略)
*/
videoFlvData[0] = 0x17 // FrameType(1) + CodecId(7) = 0001 0111
videoFlvData[1] = 0x00 // AVCPacketType = 0x00
videoFlvData[2] = 0x00 // Composition Time = 0x00 0x00 0x00
videoFlvData[3] = 0x00
videoFlvData[4] = 0x00
/**
* NALU数据长度值(4字节)
*
* UB[32]naluLen = nalu长度
*/
videoFlvData[5] = (naluDataSize shr 24 and 0xFF).toByte()
videoFlvData[6] = (naluDataSize shr 16 and 0xFF).toByte()
videoFlvData[7] = (naluDataSize shr 8 and 0xFF).toByte()
videoFlvData[8] = (naluDataSize and 0xFF).toByte()
/**
* NALU数据
*/
buffer.get(videoFlvData, flvVideoTagLen + naluLen, naluDataSize)
return videoFlvData
}
RTMP的Audio FLV Data
通常,FLV的Audio Tag应该由11字节
的FLV tag header
和Audio tag header
构成,但是,在RTMP协议中,Audio Tag不包含FLV tag header
,即只有Audio tag header
,格式如下:
Flv Audio tag说明(占2
字节):
SoundFormat[4]
:表示音频格式,10 = AAC
SoundRate[2]
:表示音频采样率,3 = 44000Hz
soundSize[1]
:表示采用精度,0=8bit 1=16bit
SoundType[1]
:表示音频类型,1 = aac
AACPacketType[8]
:表示AAC packet类型,0 = aac header; 1 = aac raw(不含ADTS头部)
以MediaCodec采集的AAC
为例:
fun getFlvAudioTag(buffer: ByteBuffer, bufferInfo: MediaCodec.BufferInfo): ByteArray {
buffer.position(bufferInfo.offset)
buffer.limit(bufferInfo.offset + bufferInfo.size)
/**
* =====================================================
* FLV Audio tag (2字节) + aac raw data(没有adts头)
* =====================================================
*/
val audioTagLen = 2
val aacDataSize = buffer.remaining()
val audioTagData = ByteArray(audioTagLen + aacDataSize)
/**
* FLV Audio tag (2字节):
*
* UB[4] 10=AAC
* UB[2] 3=44kHz
* UB[1] 1=16-bit
* UB[1] 0=MonoSound
* UB[8] 0=AAC header, 1=AAC raw
*
* 1010 11 1 0 (0xAE)
*/
audioTagData[0] = 0xAE.toByte()
audioTagData[1] = 0x01.toByte() // 0x00=aac header
/**
* AAC raw data
*/
buffer.get(audioTagData, audioTagLen, aacDataSize)
return audioTagData
}
参考文献
【流媒体协议】图解 FLV 协议 快速入门
音视频基础:FLV封装格式介绍及解析
如何理解 rtmp 通过 flv 格式推送音视频流(h264/aac)