Reducing Latency for NextGen Live Streams

This topic explains how you can reduce latency for your NextGen Live streams by configuring segment duration and understanding the tradeoffs involved.

Introduction

Latency is the time lag between when something is recorded in real life and when it is seen in a video player. There is, of course, some real time required to transmit a signal from the source through the internet to a viewer's player. Latency is aggravated by the fact that online video is transmitted in discrete chunks, called segments.

Latency can be reduced by shortening the length of the video segments, and the following sections will detail how you can do this in NextGen Live.

Understanding the Tradeoffs

Be aware that there is a tradeoff when reducing segment duration: to ensure uninterrupted playback, the player downloads a few video segments ahead of the current playback position (this is called buffering), so that they are ready when needed.

If you shorten the length of segments, keep the following in mind:

  • The player will be pre-fetching less video data, and chances of playback pausing while the player buffers additional segments will be increased
  • The customer's CDN will see increased request traffic
  • Apple's recommendation for standard live stream HLS is a 6-second segment duration. Shortening the segments may make the stream less stable in sub-optimal network conditions
  • Lower segment durations may impact stream stability for viewers with poor network connections

NextGen Live Approach

In NextGen Live, latency reduction is achieved by configuring the segment_duration_seconds parameter within the manifest configuration when creating a live channel via the Live API or by creating a custom ingest profile.

Key Differences from Legacy Live

  • Manifest-level configuration: Segment duration is set once in the manifest block, not per rendition
  • CMAF packaging: NextGen Live uses CMAF as the default packaging format for improved efficiency
  • Simplified configuration: Single parameter controls segment duration for all renditions
  • Better performance: CMAF packaging provides better cache hit ratios with reduced latency

Creating a Custom Ingest Profile

To create NextGen Live channels with reduced latency, you need to create a custom ingest profile that includes the manifest configuration with reduced segment duration. This profile can then be used when creating live events or channels.

Custom Profile Configuration

Create a custom ingest profile with the manifest block configured for reduced latency:

{
  "model_version": 2,
  "name": "Low Latency NextGen Live Profile",
  "description": "Custom profile for reduced latency NextGen Live streams with 2-second segments",
  "account_id": "your_account_id",
  "renditions": [
    {
      "media_type": "video",
      "label": "hls360p",
      "video_codec": "h264",
      "video_bitrate": 780,
      "decoder_bitrate_cap": 936,
      "decoder_buffer_size": 1170,
      "frame_rate": 30,
      "keyframe_rate": 0.5,
      "width": 640,
      "height": 360,
      "video_bframes": 3,
      "sample_aspect_ratio": "1:1",
      "deinterlace": "detect",
      "video_codec_profile": "main",
      "video_codec_level": "3",
      "video_reference_frames": 4,
      "allowed_frame_rates": [
        {
          "as_float": 23.97,
          "as_fraction": "24000/1001"
        },
        {
          "as_float": 24,
          "as_fraction": "24/1"
        },
        {
          "as_float": 25,
          "as_fraction": "25/1"
        },
        {
          "as_float": 29.97,
          "as_fraction": "30000/1001"
        },
        {
          "as_float": 30,
          "as_fraction": "30/1"
        }
      ]
    },
    {
      "media_type": "video",
      "label": "hls540p",
      "video_codec": "h264",
      "video_bitrate": 1500,
      "decoder_bitrate_cap": 1800,
      "decoder_buffer_size": 2250,
      "frame_rate": 30,
      "keyframe_rate": 0.5,
      "width": 960,
      "height": 540,
      "video_bframes": 3,
      "sample_aspect_ratio": "1:1",
      "deinterlace": "detect",
      "video_codec_profile": "main",
      "video_codec_level": "3.2",
      "video_reference_frames": 4,
      "allowed_frame_rates": [
        {
          "as_float": 23.97,
          "as_fraction": "24000/1001"
        },
        {
          "as_float": 24,
          "as_fraction": "24/1"
        },
        {
          "as_float": 25,
          "as_fraction": "25/1"
        },
        {
          "as_float": 29.97,
          "as_fraction": "30000/1001"
        },
        {
          "as_float": 30,
          "as_fraction": "30/1"
        }
      ]
    },
    {
      "media_type": "video",
      "label": "hls720p",
      "video_codec": "h264",
      "video_bitrate": 2400,
      "decoder_bitrate_cap": 2880,
      "decoder_buffer_size": 3600,
      "frame_rate": 30,
      "keyframe_rate": 0.5,
      "width": 1280,
      "height": 720,
      "video_bframes": 3,
      "sample_aspect_ratio": "1:1",
      "deinterlace": "detect",
      "video_codec_profile": "high",
      "video_codec_level": "4",
      "video_reference_frames": 4,
      "allowed_frame_rates": [
        {
          "as_float": 23.97,
          "as_fraction": "24000/1001"
        },
        {
          "as_float": 24,
          "as_fraction": "24/1"
        },
        {
          "as_float": 25,
          "as_fraction": "25/1"
        },
        {
          "as_float": 29.97,
          "as_fraction": "30000/1001"
        },
        {
          "as_float": 30,
          "as_fraction": "30/1"
        }
      ]
    },
    {
      "media_type": "video",
      "label": "hls1080p",
      "video_codec": "h264",
      "video_bitrate": 4500,
      "decoder_bitrate_cap": 5400,
      "decoder_buffer_size": 6750,
      "frame_rate": 30,
      "keyframe_rate": 0.5,
      "width": 1920,
      "height": 1080,
      "video_bframes": 3,
      "sample_aspect_ratio": "1:1",
      "deinterlace": "detect",
      "video_codec_profile": "high",
      "video_codec_level": "4.2",
      "video_reference_frames": 4,
      "allowed_frame_rates": [
        {
          "as_float": 23.97,
          "as_fraction": "24000/1001"
        },
        {
          "as_float": 24,
          "as_fraction": "24/1"
        },
        {
          "as_float": 25,
          "as_fraction": "25/1"
        },
        {
          "as_float": 29.97,
          "as_fraction": "30000/1001"
        },
        {
          "as_float": 30,
          "as_fraction": "30/1"
        }
      ]
    }
  ],
  "packages": [],
  "manifest": {
    "name": "playlist",
    "segment_duration_seconds": 2,
    "hls": {}
  },
  "profile_type": "live"
}

Key Configuration Parameters

Key Profile Parameters
Parameter Description Notes
model_version Profile model version Use 2 for NextGen Live profiles
keyframe_rate Keyframes per second (fractional value) 0.5 = 1 keyframe every 2 seconds
segment_duration_seconds Length of each video segment in seconds 2 for reduced latency, 6 for standard
profile_type Type of profile Must be "live" for live streaming profiles

Creating the Custom Profile

Follow these steps to create a custom ingest profile with reduced latency:

  1. Navigate to the Admin module in Studio
  2. In the left navigation, click Ingest Profiles
  3. Select one of the existing NextGen Live ingest profiles or create a new one
  4. Click Duplicate to create a new profile based on the selected one
  5. In the new profile, change the name to "Low Latency NextGen Live Profile" (or another meaningful name)
  6. Update the manifest block with segment_duration_seconds: 2 as shown in the example above
  7. Click Save to create the profile

Using the Custom Profile

Once you've created the custom ingest profile, you can use it when creating live events or channels, similar to how you would use any other ingest profile.

In Studio

  1. Open the Live module in Studio
  2. Click Create Channel button
  3. In the channel creation form, select your custom "Low Latency NextGen Live Profile" from the Renditions Profile dropdown
    4. Select Low Latency Profile
    Select Low Latency Profile from Renditions Profile dropdown
  4. Complete the rest of the channel configuration as normal
  5. The reduced latency settings will be automatically applied based on your custom profile

Using the Live API

If you want reduced latency, and you use the Live API to create your live jobs, just create the channel as you normally do, and set segment_duration_seconds inside the manifest block to 2:

{
  "manifest": {
    "name": "playlist",
    "segment_duration_seconds": 2,
    "hls": {}
  }
}

Encoder Considerations

When using reduced segment duration, your encoder settings become more critical for maintaining stream stability.

Recommended Encoder Settings

  • Keyframe interval: Set to 60 frames (2 seconds at 30fps) to align with segment boundaries
  • Bitrate mode: Use Constant Bitrate (CBR) for more predictable segment sizes
  • Connection stability: Use a reliable network connection with sufficient bandwidth headroom