Understanding HLG Video Format [A Quick Guide]

"What exactly is the point of HLG and do you grade it?" - Reddit user

HLG (Hybrid Log-Gamma) is an HDR format designed for live broadcasts, but many wonder if it's worth using. Unlike other HDR formats, HLG works without metadata, making it ideal for real-time content while staying backward compatible with SDR displays. In this guide, we’ll break down how HLG video works, its advantages, and applications.

Part 1. What Is HLG Video?

HLG (Hybrid Log-Gamma) is a type of HDR (High Dynamic Range) video format. It was developed by BBC and NHK in 2014 to provide a more compatible HDR format for a wide range of TVs.

Unlike other HDR formats, HLG doesn’t apply metadata. HLG combines a traditional gamma curve with a logarithmic curve to enhance brightness and contrast. This makes it more flexible for live TV, streaming, and even standard displays without needing separate SDR signals.

HLG is unique because it works with both SDR (Standard Dynamic Range) and HDR screens. This means that the same video can be played on older SDR TVs and newer HDR TVs without needing separate versions. Here’s how it compares to other popular HDR formats:

Part 2. Benefits of Using HLG Format

HLG is a simple and flexible HDR format for broadcasting, streaming, and content creation. It is free to use, easy to set up, and works without extra processing. Its biggest advantage is that it works on both HDR and SDR screens, ensuring good picture quality on any display.

Key Benefits of HLG:

• Works on both HDR & SDR displays. No need for extra conversion; HDR screens show full brightness and colors, while SDR screens display it as a standard SDR signal.
• Offers no licensing fees (unlike Dolby Vision), and no extra hardware is needed. Many devices can support it with a simple firmware update.
• No extra bandwidth is needed. Unlike other formats, broadcasters don’t need a separate SDR signal, making it easier to implement.

Where Is HLG Used?

• Ideal for real-time broadcasts like live TV & sports. Since it does not require metadata, it reduces delays and image issues.
• Best for HDR content creation as it captures natural lighting variations for a more realistic image.
• Approved for HDR broadcasting in Japan (ARIB STD-B67) and adopted by global standards like ATSC 3.0, DVB UHD-1 Phase 2, and ITU Rec. 2100 for TV and streaming.

Part 3. How to Fix Corrupted HLG Videos?

HLG videos can sometimes get corrupted due to issues like improper encoding, transmission errors, or compatibility problems with playback devices. Since HLG uses a unique hybrid gamma curve, errors can appear in different ways, such as missing colors, distorted brightness, or playback failures.

Here are common issues:

• Distorted colors happen when the device or software misreads the BT.2020 color space, making the video look dull.
• Playback errors occur if the file is not properly encoded, which causes stuttering or freezing.
• A black screen happens when the media player or display does not support HLG HDR.
• Audio-video sync issues result from encoding errors, which result in delays or mismatched sound.

For minor issues, ensure your system has the latest video codecs and GPU drivers for smooth playback. For severe corruption, professional video repair software like Repairit Video Repair is recommended. It offers:

• Quick repair for minor HLG video glitches, missing frames, or playback errors.
• Advanced repair, which uses a sample HLG video from the same device to reconstruct corrupted HLG files.
• Batch repair for fixing multiple HLG video formats at once for efficiency.

Repairit preserves the HLG video’s original color, brightness, contrast, and metadata while fixing playback issues. Ths tool simplifies HLG video recovery in just a few clicks. Just follow the below instructions:

Step 1: Run Repairit. Click Professional Mode and hit Start. Select Add or drag and drop your corrupted HLG video formats.

Step 2: Click Repair to start fixing your HLG video formats automatically. Wait until the program fixes them. Once done, preview the repaired HLG video formats before saving.

Step 3: Click Save and find a saving location folder. Avoid saving the HLG video formats in the same location where they were lost.

Part 4. How HLG Video Works and How to Use HLG Video?

HLG improves brightness, contrast, and color depth without relying on metadata like traditional HDR formats. It uses a blend of two curves:

1. Gamma Curve (for SDR displays): Maintains image quality on older screens.
2. Logarithmic Curve (for HDR screens): Captures a broader range of brightness for enhanced detail.

This combination allows a single HLG signal to work on both SDR and HDR screens for broadcasting and streaming.

The format uses the BT.2020 color gamut, offering a wider color range than the BT.709 standard used in SDR. It supports 10-bit depth for smoother color transitions with less banding compared to 8-bit SDR. This works on SDR UHDTV (4K TVs) without conversion but appears as SDR with reduced contrast and color range. However, on older SDR displays (HD/SD TVs), it requires tone mapping, color conversion, and bit reduction, which may lead to color clipping, desaturation, or noise.

Various devices and platforms widely support HLG:

• HDMI 2.0b: Enables seamless HLG signal transmission.
• HEVC & VP9: Popular video compression formats for HDR streaming.
• BBC iPlayer: Streams HDR content in HLG.
• DirecTV & Freeview Play: Broadcasts HDR movies, shows, and events.
• YouTube: Supports HLG HDR video uploads and streaming.

Conclusion

HLG is a flexible HDR format that works with both SDR and HDR displays. This is ideal for live broadcasts, streaming, and content creation. Unlike other HDR formats, it doesn’t need metadata, making it easier to use across devices. It supports the BT.2020 color gamut and 10-bit depth for better color and detail. Platforms like YouTube, BBC iPlayer, and DirecTV support HLG, and if issues arise, tools like Repairit can fix corrupted videos.