Transcoding

OvenMediaEngine has a built-in live transcoder. The live transcoder can decode the incoming live source and re-encode it with the set codec or adjust the quality to encode at multiple bitrates.

Supported Video and Audio Codecs

OvenMediaEngine currently supports the following codecs:

Video Decoding	Audio Decoding	Video Encoding	Audio Encoding
H.264 (Baseline)	AAC	H.264 (Baseline)	AAC
		VP8	Opus

Encodes and Streams

You can re-encode the incoming video source through the <Encode> setting and make the encoded video and audio into a new stream. The newly created stream can be used according to the streaming URL format.

<Application>
   <Encodes>
      <Encode>
         <Name>HD</Name>
         <Video> ... </Video>
         <Audio> ... </Audio>
      </Encode>
      <Encode>
         <Name>FHD</Name>
         ...
      </Encode>
   </Encodes>
   <Streams>
      <Stream>
         <Name>${OriginStreamName}_o</Name>
         <Profiles>
            <Profile>HD</Profile>
         </Profiles>
      </Stream>
   </Streams>
</Application>

By the above setting, Video and Audio encoded with <Encode><Name> of FHD and HD are newly created and included in <Stream><Name> named ${OriginStreamName}_o. In other words, if a stream inputted as app/stream is transcoded, <Stream><Name> named stream_o is created.

Also, the newly created stream is used in the following URL format:

• WebRTC ws://192.168.0.1:3333/app/stream_o

• HLS http://192.168.0.1:8080/app/stream_o/playlist.m3u8

• MPEG-DASH http://192.168.0.1:8080/app/stream_o/manifest.mpd

• Low-Latency MPEG-DASH http://192.168.0.1:8080/app/stream_o/manifest_ll.mpd

Profile

Video

You can set the video profile as below:

<Video>
    <Codec>vp8</Codec>
    <Width>1280</Width>
    <Height>720</Height>
    <Bitrate>2000000</Bitrate>
    <Framerate>30.0</Framerate>
</Video>

The meaning of each property is as follows:

Property	Description
Codec	Specifies the vp8 or h.264 codec to use
Width	Width of resolution
Height	Height of resolution
Bitrate	Bit per second
Framerate	Frames per second

Audio

You can set the audio profile as below:

<Audio>
    <Codec>opus</Codec>
    <Bitrate>128000</Bitrate>
    <Samplerate>48000</Samplerate>
    <Channel>2</Channel>
</Audio>

The meaning of each property is as follows:

Property	Description
Codec	Specifies the opus or aac codec to use
Bitrates	Bits per second
Samplerate	Samples per second
Channel	The number of audio channels

Bypass transcoding

You can configure Video and Audio to bypass transcoding as follows:

<Video>
    <Bypass>true</Bypass>
</Video>
<Audio>
    <Bypass>true</Bypass>
</Audio>

You need to consider codec compatibility with some browsers. For example, chrome only supports OPUS codec for audio to play WebRTC stream. If you set to bypass incoming audio, it can't play on chrome.

WebRTC doesn't support AAC, so if video bypasses transcoding, audio must be encoded in OPUS.

<Video>
    <Bypass>true</Bypass>
</Video>
<Audio>
    <Codec>opus</Codec>
    <Bitrate>128000</Bitrate>
    <Samplerate>48000</Samplerate>
    <Channel>2</Channel>
</Audio>

Supported codecs by streaming protocol

Even if you set up multiple codecs, there is a codec that matches each streaming protocol supported by OME, so it can automatically select and stream codecs that match the protocol. However, if you don't set a codec that matches the streaming protocol you want to use, it won't be streamed.

The following is a list of codecs that match each streaming protocol:

Protocol	Supported Codec
WebRTC	VP8, H.264, Opus
HLS	H.264, AAC
Low-Latency MPEG-Dash	H.264, AAC

Therefore, you set it up as shown in the table. If you want to stream using HLS or MPEG-DASH, you need to set up H.264 and AAC, and if you want to stream using WebRTC, you need to set up Opus.

Also, if you are going to use WebRTC on all platforms, you need to configure both VP8 and H.264. This is because different codecs are supported for each browser, for example, VP8 only, H264 only, or both.

However, don't worry. If you set the codecs correctly, OME automatically sends the stream of codecs requested by the browser.

Currently, OME doesn't support adaptive streaming on HLS, MPEG-DASH. However, it will be updated soon.