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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.

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How ABR Works

When ABR is enabled, Ant Media Server:
  1. Receives the source stream at original quality
  2. Transcodes to multiple quality levels (renditions)
  3. Packages all renditions for delivery
  4. Adapts playback quality based on client bandwidth

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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";
}

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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

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Configuration Examples

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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.

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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}
  ]'

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Encoder Configuration

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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.

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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"
  }
}

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Encoder Settings

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

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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).

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Quality Profiles

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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}
]

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HD Mobile-Optimized

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

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Low-Bandwidth

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

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Advanced Features

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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

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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

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Hardware Scaling

Use GPU for scaling in addition to encoding: 
hwScalingEnabled=true
This offloads scaling to the GPU, reducing CPU usage.

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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;

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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

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Performance Considerations

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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.

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GPU Encoding

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

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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)

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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"
}

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Troubleshooting

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Encoding Not Starting

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

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Quality Issues

  1. Increase bitrate for target resolution
  2. Adjust GOP size
  3. Try different encoder preset
  4. Enable hardware encoding

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High CPU Usage

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

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Next Steps

Streaming Protocols

Learn about protocol options for ABR delivery

Architecture

Understand how ABR fits into the system

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