Blv

Introduction

The BLV file format is a proprietary video container format developed by BigBlueButton, an open‑source web conferencing system. BLV files store synchronized video, audio, and interaction data from recorded sessions, enabling replay of online meetings, classes, and webinars. The format is specifically tailored to support the high‑throughput demands of live streaming and real‑time collaboration, with a focus on low‑latency playback and efficient storage. While the BLV format is primarily associated with BigBlueButton installations, its architecture has influenced other e‑learning platforms that require robust recording and playback capabilities.

BLV files typically accompany a separate metadata file, often with a .blv.json extension, that contains session information such as participant list, timestamps, and transcript data. Together, these files form a complete record of an interactive session that can be played back on the BigBlueButton web interface or converted into standard formats like MP4 for broader distribution.

The format's design reflects a compromise between compression efficiency, compatibility with various browsers, and the need to preserve rich interaction data (e.g., screen sharing, whiteboard strokes). As a result, BLV recordings contain both compressed media streams and a custom data channel that records user actions.

History and Development

BigBlueButton was first released in 2010 by the University of Melbourne, with the aim of providing a scalable, low‑cost solution for online teaching. The need for a lightweight, reliable recording format emerged early in the project's development. Early versions of the system used simple MP4 files for video recording, but this approach proved inadequate for capturing the full range of interaction events, especially when screen sharing and whiteboard annotations were involved.

In response, the BigBlueButton team introduced the BLV format in 2013, alongside version 0.9 of the recording server. The initial design leveraged a custom container that could embed multiple data streams, including video, audio, and interaction logs. The format was intentionally kept open for future extensions, with a focus on ensuring that recorded sessions could be replayed accurately on a wide range of devices.

Over subsequent releases, the BLV specification evolved to incorporate improved compression algorithms, support for higher frame rates, and additional metadata fields. By version 2.0, the format had incorporated the ability to interleave multiple video tracks - useful for capturing both the main lecture video and the participant video feeds simultaneously. In addition, support for adaptive bitrate streaming was added, allowing the format to adapt to varying network conditions during playback.

The open‑source community contributed to the refinement of BLV through bug reports, code contributions, and documentation updates. These contributions have helped maintain the format's relevance in educational contexts and enabled third‑party developers to build playback tools that can handle BLV recordings outside the BigBlueButton ecosystem.

Technical Specification

File Structure

BLV files are structured as a binary container consisting of a series of blocks, each prefixed with a header that specifies the block type, size, and a timestamp. The header format is a fixed 12‑byte sequence: 4 bytes for a block identifier, 4 bytes for the block length, and 4 bytes for the timestamp in milliseconds. Following the header, the block contains raw data that corresponds to the block type.

Common block types include VIDE for video frames, AUDI for audio frames, INTER for interaction events, and META for session metadata. The ordering of blocks is strictly chronological; blocks with earlier timestamps appear before later ones, which simplifies real‑time playback and allows the player to seek efficiently.

The file may optionally contain a footer that summarizes key metrics such as total duration, number of blocks, and a checksum for integrity verification. Some implementations write the checksum at the end of the file, while others embed it in a separate metadata file.

Video Encoding

Video data in BLV files is typically encoded using the H.264/AVC codec, chosen for its balance between compression efficiency and compatibility with modern browsers and mobile devices. The video stream is stored in a series of I‑frames and P‑frames, with the GOP (Group of Pictures) size set to 60 frames for typical 30‑fps recordings. This GOP configuration enables efficient random access and supports playback from arbitrary points within the recording.

To accommodate high‑definition sessions, later versions of the format support H.265/HEVC encoding, offering roughly 30% better compression at the same quality level. However, H.265 support remains optional; players that do not support the codec will fall back to H.264 if both are present in the same BLV file.

Audio Encoding

Audio streams in BLV recordings are encoded with the Advanced Audio Coding (AAC) format at 44.1 kHz sampling rate and 128 kbps bitrate. The choice of AAC ensures low latency and high fidelity while remaining widely supported. For recordings that include background music or narration, the format supports dual‑audio channels, allowing separate tracks for user commentary and background sound.

In some educational use cases, especially where bandwidth is limited, the audio stream may be down‑sampled to 22.05 kHz with a bitrate of 64 kbps. This optional setting is configurable through the BigBlueButton recording configuration file.

Metadata

Metadata is stored in a separate JSON file that accompanies the BLV recording. This file contains essential information such as session title, start and end times, participant identifiers, and a chronological log of interaction events (e.g., chat messages, slide changes). The JSON format is deliberately lightweight to avoid introducing dependencies on specialized parsers.

The metadata also includes optional fields that describe the recording environment, such as the BigBlueButton version, server host, and recording plugin configuration. These fields aid in troubleshooting and in ensuring compatibility when re‑playing recordings on newer versions of the software.

Interaction Data

Interaction blocks (INTER) encode events that occur during a session. Each interaction block contains a type field (e.g., CHAT, SHARE, ANNOTATE), a payload with event‑specific data, and a timestamp. The payload is serialized as a binary blob; its internal format is documented in the BigBlueButton interaction specification.

For example, a chat message event includes the sender ID, message text, and the time the message was sent. An annotation event includes coordinates, stroke color, and shape type. The interaction data is vital for reproducing the exact sequence of events during playback, especially when the recording is used for compliance or quality‑assurance reviews.

Applications and Use Cases

Online Education Platforms

Universities and colleges use BigBlueButton to host live lectures and seminars. The BLV recording format ensures that each session can be archived for later review, providing students with access to full recordings that include video, audio, and interactive annotations. Because BLV files are tightly integrated with the BigBlueButton server, the transition from live session to archived content is seamless.

In many institutions, recorded lectures are subsequently converted into MP4 or HTML5‑friendly formats to enable playback on learning management systems (LMS) such as Moodle, Canvas, or Blackboard. The conversion process preserves the original timing and interaction metadata, ensuring that students can view the lecture in the same order as the live event.

Corporate Training

Corporate training programs frequently employ webinars and workshops that require recorded sessions for internal compliance audits. The BLV format’s ability to capture detailed interaction logs - including screen sharing and whiteboard annotations - makes it an attractive option for training managers who need to verify that specific instructional content was delivered.

Large enterprises also use BLV recordings to support asynchronous learning. Employees can review recorded webinars at their convenience, with playback features that include search functionality and interactive transcript synchronization.

Archiving Lectures

Long‑term archiving of academic content necessitates formats that preserve both media quality and interaction fidelity. BLV’s custom container allows for efficient storage of multi‑track recordings, reducing the overall file size without sacrificing quality. Archiving teams can apply compression tools to BLV files and store them on institutional repositories.

Because BLV includes explicit timestamps for each interaction event, archival systems can provide time‑based retrieval capabilities. Researchers can query the archive to locate all instances where a particular concept was discussed or where a specific participant was active.

Compatibility and Tools

Playback Software

The BigBlueButton web interface includes a built‑in BLV player that streams recordings directly to users' browsers. The player is written in JavaScript and leverages the HTML5 video element for media playback, while rendering interaction events through a canvas overlay. The player supports full screen mode, quality selection, and keyboard shortcuts for navigation.

Other browsers can play BLV recordings using third‑party plugins. VLC Media Player supports H.264 and AAC streams found within BLV files, though the interaction overlay is not rendered. Users can view the raw media but must refer to the JSON metadata for interaction events.

Conversion Utilities

FFmpeg is widely used to convert BLV files into standard video formats. The command line syntax typically involves specifying input and output streams, along with codec options. A common conversion workflow extracts the video and audio streams, then writes them to an MP4 container:

ffmpeg -i input.blv -map 0:vid -map 0:aud -c copy output.mp4

For users who require the interaction data, the JSON metadata file can be parsed by custom scripts to generate captions or highlight specific segments. Python scripts using the json module and ffmpeg-python can automate this process.

Editing Software

Non‑linear editing (NLE) suites such as Adobe Premiere Pro or DaVinci Resolve can import BLV files indirectly by first converting them to MP4. Once imported, editors can synchronize the video with external audio or subtitle tracks. Some editors support the import of JSON metadata to overlay annotations, but this requires manual mapping between event timestamps and clip timelines.

Open‑source editors, including Shotcut and OpenShot, can handle BLV files after conversion. They provide basic editing tools, such as trimming, cutting, and adding titles, which can be useful when producing short instructional clips from a full lecture recording.

Advantages and Limitations

Pros

Efficient storage of multi‑track media and interaction data.
Built‑in compatibility with the BigBlueButton ecosystem.
Support for high‑definition video and adaptive bitrate streaming.
Robust timestamping that enables accurate playback and retrieval.
Open specification that allows third‑party developers to create tools.

Cons

Limited support outside the BigBlueButton platform; other players often ignore interaction data.
Dependency on specific codecs; older devices may not support H.265 or high‑bitrate streams.
Complexity of the container format can hinder direct editing without conversion.
No standardized licensing; distribution of third‑party playback tools may be restricted by BigBlueButton’s terms.

Security and Privacy Considerations

BLV recordings may contain personally identifiable information (PII) such as participant names, usernames, and video feeds. Organizations must ensure that recordings are stored securely, using encryption at rest and in transit. BigBlueButton can be configured to encrypt recordings using AES‑256 before writing them to disk.

Access controls are also critical. Role‑based permissions should restrict playback to authorized users. In many institutional deployments, the BigBlueButton server integrates with LDAP or SAML authentication to enforce these controls.

Additionally, privacy policies should address the retention of interaction data. Depending on jurisdiction, organizations may be required to delete chat logs or other logs after a certain period. The JSON metadata file can be processed to purge or anonymize sensitive fields.

Future Outlook

The BLV format is likely to remain closely tied to the BigBlueButton ecosystem, but its open specification provides room for evolution. Potential future directions include:

Support for newer video codecs such as AV1, which offer improved compression.
Enhanced encryption schemes that embed metadata encryption directly within the container.
Improved interoperability with Learning Record Stores (LRS) by adding xAPI statements to the interaction data.
Integration with WebRTC data channels to capture real‑time interactions without relying on post‑session processing.

As the demand for high‑quality, low‑latency recordings grows, especially in remote work and e‑learning contexts, the BLV format may serve as a reference for new container designs that balance media quality with interaction fidelity.

Table of Contents

Blv

Introduction

History and Development