Transcoding

OvenMediaEngine has embedded live transcoder. The live transcoder can decode the incoming live source and encode it to the appropriate codec to be published or can encode it in multi bitrates by adjusting the quality.

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, audio will not play on chrome.

WebRTC does not 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 do not set a codec that matches the streaming protocol you want to use, it will not 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, you do not worry. If you set the codecs correctly, OME automatically sends the stream of codecs requested by the browser.

Currently, OME does not support adaptive streaming on HLS, MPEG-DASH. However, it will be updated soon.