Transcoding

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>
    <!-- 
    Common setting for decoders. Decodes is optional.

    <Decodes>
	Number of threads for the decoder.
	<ThreadCount>2</ThreadCount>

	By default, OME decodes all video frames. If OnlyKeyframes is true, only the keyframes will be decoded, massively improving thumbnail performance at the cost of having less control over when exactly they are generated
	<OnlyKeyframes>false</OnlyKeyframes>
    </Decodes>
    -->

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

• LLHLS http://192.168.0.1:8080/app/stream_bypass/llhls.m3u8

• HLS http://192.168.0.1:8080/app/stream_bypass/ts:playlist.m3u8

Encodes

Video

You can set the video profile as below:

<Encodes>
    <Video>
        <Name>h264_hd</Name>
        <Codec>h264</Codec>
        <Width>1280</Width>
        <Height>720</Height>
        <Bitrate>2000000</Bitrate>
        <Framerate>30.0</Framerate>
        <KeyFrameInterval>30</KeyFrameInterval>
        <BFrames>0</BFrames>
<!--
        <Preset>fast</Preset>
        <ThreadCount>4</ThreadCount>
        <Lookahead>5</Lookahead>
        <Modules>x264</Modules>
-->
    </Video>
</Encodes>

Property	Description
Codec*	Type of codec to be encoded See the table below
Bitrate*	Bit per second
Name*	Encode name for Renditions No duplicates allowed
Width	Width of resolution
Height	Height of resolution
Framerate	Frames per second
KeyFrameInterval	Number of frames between two keyframes (0~600) default is framerate (i.e. 1 second)
BFrames	Number of B-frames (0~16) default is 0
Profile	H264 only encoding profile (baseline, main, high)
Preset	Presets of encoding quality and performance See the table below
ThreadCount	Number of threads in encoding
Lookahead	Number of frames to look ahead default is 0 x264 is 0-250 nvenc is 0-31 xma is 0-20
Modules	An encoder library can be specified; otherwise, the default codec See the table below

* required

Supported Video Codecs

Type	Description	Codec	Modules
Video	VP8	vp8	SW: libvpx*
	H.264	h264	SW: openh264*, x264 HW: nv, xma
	H.265 (Hardware Only)	h265	HW: nv, xma

Preset

A table in which presets provided for each codec library are mapped to OvenMediaEngine 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	h264_nvenc	vp8
slower	QP( 10-39)	p7	best
slow	QP (16-45)	p6	best
medium	QP (24-51)	p5	good
fast	QP (32-51)	p4	realtime
faster	QP (40-51)	p3	realtime

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>
        <Name>opus_128</Name>
        <Codec>opus</Codec>
        <Bitrate>128000</Bitrate>
        <Samplerate>48000</Samplerate>
        <Channel>2</Channel>
    </Audio>
</Encodes>

Property	Description
Codec*	Type of codec to be encoded See the table below
Bitrate*	Bits per second
Name*	Encode name for Renditions No duplicates allowed
Samplerate	Samples per second
Channel	The number of audio channels
Modules	An encoder library can be specified; otherwise, the default codec See the table below

* required

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

Supported Audio Codecs

Type	Description	Codec	Modules
Audio	AAC	aac	SW: fdkaac*
	Opus	opus	SW: libopus*

Bypass (Passthrough)

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>

Additional Features

Conditional Encoding

If the codec or quality of the input stream is the same as the profile to be encoded into the output stream. there is no need to perform re-transcoding while unnecessarily consuming a lot of system resources. If the quality of the input track matches all the conditions of BypassIfMatch, it will be Pass-through without encoding

Matching elements in video

Elements	Condition	Description
Codec (Optional)	eq	Compare video codecs
Width (Optional)	eq, lte, gte	Compare horizontal pixel of video resolution
Height (Optional)	eq, lte, gte	Compare vertical pixel of video resolution
SAR (Optional)	eq	Compare ratio of video resolution

* eq: equal to / lte: less than or equal to / gte: greater than or equal to

Matching elements in audio

Elements	Condition	Description
Codec (Optional)	eq	Compare audio codecs
Samplerate (Optional)	eq, lte, gte	Compare sampling rate of audio
Channel (Optional)	eq, lte, gte	Compare number of channels in audio

* eq: equal to / lte: less than or equal to / gte: greater than or equal to

To support WebRTC and LLHLS, AAC and Opus codecs must be supported at the same time. Use the settings below to reduce unnecessary audio encoding.

<Encodes>
	<Video>
                <Bypass>true</Bypass>	
	</Video>
	<Audio>
		<Name>cond_audio_aac</Name>
		<Codec>aac</Codec>
		<Bitrate>128000</Bitrate>
		<Samplerate>48000</Samplerate>
		<Channel>2</Channel>
		<BypassIfMatch>
			<Codec>eq</Codec>
			<Samplerate>lte</Samplerate>
			<Channel>eq</Channel>			
		</BypassIfMatch>
	</Audio>		
	<Audio>
		<Name>cond_audio_opus</Name>
		<Codec>opus</Codec>
		<Bitrate>128000</Bitrate>
		<Samplerate>48000</Samplerate>
		<Channel>2</Channel>
		<BypassIfMatch>
			<Codec>eq</Codec>
			<Samplerate>lte</Samplerate>
			<Channel>eq</Channel>	
		</BypassIfMatch>	
	</Audio>
</Encodes>

If a video track with a lower quality than the encoding option is input, unnecessary upscaling can be prevented. SAR (Storage Aspect Ratio) is the ratio of original pixels. In the example below, even if the width and height of the original video are smaller than or equal to the width and height set in the encoding option, if the ratio is different, it means that encoding is performed without bypassing.

<Encodes>
	<Video>
		<Name>prevent_upscaling_video</Name>
		<Codec>h264</Codec>
		<Bitrate>2048000</Bitrate>
		<Width>1280</Width>
		<Height>720</Height>
		<Framerate>30</Framerate>
		<BypassIfMatch>
			<Codec>eq</Codec>
			<Width>lte</Width>
			<Height>lte</Height>
			<SAR>eq</SAR>
		</BypassIfMatch>
	</Video>
</Encodes>

Keep Original and Change Bitrate

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>  
</Encodes>

Rescaling with 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>

Decoding Thread Configuration

The software decoder uses 2 threads by default. If the CPU speed is too low for decoding, increasing the thread count can improve performance.

<OutputProfiles>
    <!-- 
    Common setting for decoders. Decodes is optional.
    -->
    <Decodes>
	<!-- Number of threads for the decoder.-->
	<ThreadCount>2</ThreadCount>
    </Decodes>

    <OutputProfile>
    ....
    </OutputProfile>
</OutputProfiles>