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, Audio and Image Codecs

OvenMediaEngine currently supports the following codecs:

Type	Decoder	Encoder
Video	VP8, H.264, H.265	VP8, H.264, H.265(GPU only)
Audio	AAC, OPUS	AAC, OPUS
Image		JPEG, PNG

Type

Decoder

Encoder

Video

VP8, H.264, H.265

VP8, H.264, H.265(GPU only)

Audio

AAC, OPUS

Image

JPEG, PNG

OutputProfiles

OutputProfile

The <OutputProfile> setting allows incoming streams to be re-encoded via the <Encodes> setting to create a new output stream. The name of the new output stream is determined by the rules set in <OutputStreamName>, and the newly created stream can be used according to the streaming URL format.

<OutputProfiles>
    <OutputProfile>
        <Name>bypass_stream</Name>
        <OutputStreamName>${OriginStreamName}_bypass</OutputStreamName>
        <Encodes>
            <Audio>
                <Bypass>true</Bypass>
            </Audio>
            <Video>
                <Bypass>true</Bypass>
            </Video>
            <Audio>
                <Codec>opus</Codec>
                <Bitrate>128000</Bitrate>
                <Samplerate>48000</Samplerate>
                <Channel>2</Channel>
            </Audio>
        </Encodes>
    </OutputProfile>
</OutputProfiles>

According to the above setting, if the incoming stream name is stream, the output stream becomes stream_bypassand the stream URL can be used as follows.

WebRTC ws://192.168.0.1:3333/app/stream_bypass
HLS http://192.168.0.1:8080/app/stream_bypass/playlist.m3u8
MPEG-DASH http://192.168.0.1:8080/app/stream_bypass/manifest.mpd
Low-Latency MPEG-DASH http://192.168.0.1:8080/app/stream_bypass/manifest_ll.mpd

Encodes

Video

You can set the video profile as below:

<Encodes>
    <Video>
        <Codec>vp8</Codec>
        <Width>1280</Width>
        <Height>720</Height>
        <Bitrate>2000000</Bitrate>
        <Framerate>30.0</Framerate>
        <Preset>fast</Preset>
        <ThreadCount>4</ThreadCount>
    </Video>
</Encodes>

The meaning of each property is as follows:

Property

Description

Codec*

Specifies the vp8 or h264 codec to use

Bitrate*

Bit per second

Width

Width of resolution

Height

Height of resolution

Framerate

Frames per second

Preset

Presets of encoding quality and performance

ThreadCount

Number of threads in encoding

Table of presets

A table in which presets provided for each codec library are mapped to OvenMediaEngine's presets. Slow presets are of good quality and use a lot of resources, whereas Fast presets have lower quality and better performance. It can be set according to your own system environment and service purpose.

Presets

openh264

libvpx

h264/265 NVC

h264/265 QSV

slower

Quantizer(10-41)

best

slow

Quantizer(10-41)

best

llhq

medium

Quantizer(10-51)

good

fast

Quantizer(25-51)

realtime

faster

Quantizer(25-51)

*realtime

*llhp

References

https://trac.ffmpeg.org/wiki/Encode/VP8
https://docs.nvidia.com/video-technologies/video-codec-sdk/nvenc-preset-migration-guide/

Audio

You can set the audio profile as below:

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

The meaning of each property is as follows:

Property

Description

Codec*

Specifies the opus or aac codec to use

Bitrate*

Bits per second

Samplerate

Samples per second

Channel

The number of audio channels

It is possible to have an audio only output profile by specifying the Audio profile and omitting a Video one.

Image

You can set the Image profile as below:

<Encodes>
    <Image>
        <Codec>jpeg</Codec>
        <Width>1280</Width>
        <Height>720</Height>
        <Framerate>1</Framerate>
    </Image>
</Encodes>

The meaning of each property is as follows:

Property

Description

Codec

Specifies the jpeg or png codec to use

Width

Width of resolution

Height

Height of resolution

Framerate

Frames per second

The image encoding profile is only used by thumbnail publishers. and, bypass option is not supported.

Bypass without 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.

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

Keep the original with transcoding

If you want to transcode with the same quality as the original. See the sample below for possible parameters that OME supports to keep original. If you remove the Width, Height, Framerate, Samplerate, and Channel parameters. then, It is transcoded with the same options as the original.

<Encodes>
    <Video>
        <Codec>vp8</Codec>
        <Bitrate>2000000</Bitrate>
    </Video>
     <Audio>
        <Codec>opus</Codec>
        <Bitrate>128000</Bitrate>
    </Audio>  
    <Image>
        <Codec>jpeg</Codec>
    </Image>
</Encodes>

Rescaling while keep the aspect ratio

To change the video resolution when transcoding, use the values of width and height in the Video encode option. If you don't know the resolution of the original, it will be difficult to keep the aspect ratio after transcoding. Please use the following methods to solve these problems. For example, if you input only the Width value in the Video encoding option, the Height value is automatically generated according to the ratio of the original video.

<Encodes>
    <Video>
        <Codec>h264</Codec>
        <Bitrate>2000000</Bitrate>
        <Width>1280</Width>
        <!-- Height is automatically calculated as the original video ratio -->
        <Framerate>30.0</Framerate>
    </Video>
    <Video>
        <Codec>h264</Codec>
        <Bitrate>2000000</Bitrate>
        <!-- Width is automatically calculated as the original video ratio -->        
        <Height>720</Height>
        <Framerate>30.0</Framerate>
    </Video>    
</Encodes>

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.

Last updated 2 years ago