Adaptive Bitrate Streaming - Ant Media Server

Overview

Adaptive Bitrate (ABR) streaming dynamically adjusts video quality based on network conditions and device capabilities. Ant Media Server provides comprehensive ABR support through real-time transcoding and intelligent bitrate switching.

How ABR Works

When ABR is enabled, Ant Media Server:

Receives the source stream at original quality
Transcodes to multiple quality levels (renditions)
Packages all renditions for delivery
Adapts playback quality based on client bandwidth

Encoder Settings

ABR is configured through the EncoderSettings class, which defines each quality profile:

EncoderSettings.java:8-24

public class EncoderSettings implements Serializable{
    
    private  int height;
    private  int videoBitrate;
    private  int audioBitrate;
    private boolean  forceEncode = true;
    
    public static final String RESOLUTION_HEIGHT = "height";
    public static final String VIDEO_BITRATE = "videoBitrate";
    public static final String AUDIO_BITRATE = "audioBitrate";
    public static final String FORCE_ENCODE = "forceEncode";
}

Key Parameters

height: Resolution height in pixels (e.g., 480, 720, 1080)
videoBitrate: Video bitrate in bits per second
audioBitrate: Audio bitrate in bits per second
forceEncode: Whether to encode even if source is lower resolution

Configuration Examples

Via Application Settings

Configure ABR profiles in your application settings:

encoderSettingsString=[
  {"height":1080,"videoBitrate":2500000,"audioBitrate":128000},
  {"height":720,"videoBitrate":1500000,"audioBitrate":96000},
  {"height":480,"videoBitrate":800000,"audioBitrate":64000},
  {"height":360,"videoBitrate":500000,"audioBitrate":64000}
]

AppSettings.java:865

@Value("${encoderSettingsString:${"+SETTINGS_ENCODER_SETTINGS_STRING+":}}")
private String encoderSettingsString = "";

If encoderSettingsString is empty, Ant Media Server operates in SFU (Selective Forwarding Unit) mode for WebRTC, forwarding streams without transcoding.

Via REST API

Set encoder settings dynamically:

curl -X PUT "https://your-server:5443/AppName/rest/v2/broadcasts/{streamId}/encoder-settings" \
  -H "Content-Type: application/json" \
  -d '[
    {"height":720,"videoBitrate":1500000,"audioBitrate":96000},
    {"height":480,"videoBitrate":800000,"audioBitrate":64000}
  ]'

Encoder Configuration

Encoder Selection

Ant Media Server automatically selects the best encoder:

AppSettings.java:1377

@Value( "${encoderName:${" + SETTINGS_ENCODING_ENCODER_NAME +":}}")
private String encoderName = "";

Hardware Encoders (GPU):

h264_nvenc - NVIDIA GPU
h264_qsv - Intel Quick Sync
h264_videotoolbox - Apple VideoToolbox
h264_vaapi - Linux VA-API

Software Encoders (CPU):

libx264 - H.264 software encoder
libopenh264 - Cisco OpenH264
libvpx - VP8/VP9 encoder

Leave encoderName empty to auto-detect. The server tries GPU encoders first, falling back to CPU encoders.

Encoder Parameters

Configure encoder-specific parameters:

AppSettings.java:1399-1400

@Value("${encoderParameters:{}}")
private Map<String, Map<String,String>> encoderParameters = new HashMap<>();

Example configuration:

{
  "libx264": {
    "preset": "veryfast",
    "profile": "main",
    "tune": "zerolatency"
  },
  "h264_nvenc": {
    "preset": "ll",
    "profile": "main",
    "rc": "cbr"
  },
  "libvpx": {
    "deadline": "realtime",
    "cpu-used": "5"
  }
}

Encoder Settings

encoderThreadCount=0
encoderThreadType=0
gopSize=60
constantRateFactor=23
aacEncodingEnabled=true

GOP Size

AppSettings.java

@Value( "${gopSize:${" + SETTINGS_GOP_SIZE +":60}}")
private String gopSize = "60";

GOP (Group of Pictures) size affects:

Seeking accuracy: Smaller GOP = more precise seeking
Encoding efficiency: Larger GOP = better compression
Latency: Smaller GOP = lower latency

For low-latency streaming, use GOP size of 2-3 seconds (60-90 frames at 30fps).

Quality Profiles

Recommended Profiles

Full HD Multi-bitrate

[
  {"height": 1080, "videoBitrate": 4500000, "audioBitrate": 128000},
  {"height": 720, "videoBitrate": 2500000, "audioBitrate": 128000},
  {"height": 480, "videoBitrate": 1200000, "audioBitrate": 96000},
  {"height": 360, "videoBitrate": 800000, "audioBitrate": 96000},
  {"height": 240, "videoBitrate": 500000, "audioBitrate": 64000}
]

HD Mobile-Optimized

[
  {"height": 720, "videoBitrate": 1500000, "audioBitrate": 96000},
  {"height": 480, "videoBitrate": 800000, "audioBitrate": 64000},
  {"height": 360, "videoBitrate": 500000, "audioBitrate": 64000}
]

Low-Bandwidth

[
  {"height": 480, "videoBitrate": 600000, "audioBitrate": 64000},
  {"height": 360, "videoBitrate": 400000, "audioBitrate": 48000},
  {"height": 240, "videoBitrate": 250000, "audioBitrate": 48000}
]

Advanced Features

Force Encoding

The forceEncode parameter controls whether to transcode when the source resolution is lower than the target:

EncoderSettings.java:15-19

private boolean  forceEncode = true;

true: Always encode to target resolution (may upscale)
false: Skip encoding if source is lower resolution

Original Stream in ABR

Include the original quality stream alongside transcoded versions:

AppSettings.java:1011

@Value ( "${useOriginalWebRTCEnabled:${" + SETTINGS_USE_ORIGINAL_WEBRTC_ENABLED +":false}}" )
private boolean useOriginalWebRTCEnabled=false;

useOriginalWebRTCEnabled=true
addOriginalMuxerIntoHlsPlaylist=true

Hardware Scaling

Use GPU for scaling in addition to encoding:

hwScalingEnabled=true

This offloads scaling to the GPU, reducing CPU usage.

Stats-Based ABR

Ant Media Server includes intelligent bitrate switching based on real-time statistics:

AppSettings.java:733-749

@Value( "${statsBasedABREnabled:${" + SETTINGS_STATS_BASED_ABR_ALGORITHM_ENABLED +":false}}")
private boolean statsBasedABRAlgorithmEnabled;

@Value( "${abrDownScalePacketLostRatio:${" + SETTINGS_ABR_DOWN_SCALE_PACKET_LOST_RATIO +":0.1}}")
private double abrDownScalePacketLostRatio = 0.1;

@Value( "${abrUpScalePacketLostRatio:${" + SETTINGS_ABR_UP_SCALE_PACKET_LOST_RATIO +":0.05}}")
private double abrUpScalePacketLostRatio = 0.05;

@Value( "${abrUpScaleRTTMs:${" + SETTINGS_ABR_UP_SCALE_RTT_MS +":50}}")
private int abrUpScaleRTTMs = 50;

@Value( "${abrUpScaleJitterMs:${" + SETTINGS_ABR_UP_SCALE_JITTER_MS +":5}}")
private int abrUpScaleJitterMs = 5;

Configuration

statsBasedABREnabled=true
abrDownScalePacketLostRatio=0.1
abrUpScalePacketLostRatio=0.05
abrUpScaleRTTMs=50
abrUpScaleJitterMs=5

How it works:

Monitor packet loss, RTT, and jitter
Switch to lower quality when packet loss > 10%
Switch to higher quality when packet loss < 5% and RTT < 50ms

Performance Considerations

CPU Usage

Each additional quality profile requires encoding resources:

1080p transcoding: ~100-150% CPU per stream (software)
720p transcoding: ~50-75% CPU per stream (software)
480p transcoding: ~25-35% CPU per stream (software)

For high concurrency, use GPU encoding or limit ABR profiles to 2-3 renditions.

GPU Encoding

GPU encoding provides:

10-20x faster than CPU encoding
Lower CPU usage for higher concurrency
Slightly lower quality at same bitrate

Encoder Selection Preference

encoderSelectionPreference=gpu_and_cpu

Options:

gpu_and_cpu: Try GPU first, fallback to CPU
only_gpu: Only use GPU (fail if unavailable)

Monitoring ABR

Check encoding status via REST API:

curl "https://your-server:5443/AppName/rest/v2/broadcasts/{streamId}"

Response includes:

{
  "streamId": "stream1",
  "status": "broadcasting",
  "speed": 1.0,
  "encoderBlockedStreams": [],
  "subFolder": "streams"
}

Troubleshooting

Encoding Not Starting

Check encoder availability:
ffmpeg -encoders | grep h264
Verify encoder settings are valid JSON
Check CPU/GPU resources

Quality Issues

Increase bitrate for target resolution
Adjust GOP size
Try different encoder preset
Enable hardware encoding

High CPU Usage

Reduce number of ABR profiles
Enable GPU encoding
Increase encoder preset speed (e.g., ultrafast)
Use lower resolutions

Next Steps

Streaming Protocols

Learn about protocol options for ABR delivery

Architecture

Understand how ABR fits into the system

​Overview

​How ABR Works

​Encoder Settings

​Key Parameters

​Configuration Examples

​Via Application Settings

​Via REST API

​Encoder Configuration

​Encoder Selection

​Encoder Parameters

​Encoder Settings

​GOP Size

​Quality Profiles

​Recommended Profiles

​Full HD Multi-bitrate

​HD Mobile-Optimized

​Low-Bandwidth

​Advanced Features

​Force Encoding

​Original Stream in ABR

​Hardware Scaling

​Stats-Based ABR

​Configuration

​Performance Considerations

​CPU Usage

​GPU Encoding

​Encoder Selection Preference

​Monitoring ABR

​Troubleshooting

​Encoding Not Starting

​Quality Issues

​High CPU Usage

​Next Steps

Streaming Protocols

Architecture

Overview

How ABR Works

Encoder Settings

Key Parameters

Configuration Examples

Via Application Settings

Via REST API

Encoder Configuration

Encoder Selection

Encoder Parameters

Encoder Settings

GOP Size

Quality Profiles

Recommended Profiles

Full HD Multi-bitrate

HD Mobile-Optimized

Low-Bandwidth

Advanced Features

Force Encoding

Original Stream in ABR

Hardware Scaling

Stats-Based ABR

Configuration

Performance Considerations

CPU Usage

GPU Encoding

Encoder Selection Preference

Monitoring ABR

Troubleshooting

Encoding Not Starting

Quality Issues

High CPU Usage

Next Steps