Why is the audio out of sync after converting?

Common causes: (1) variable framerate source rendered as constant (use -vsync vfr to preserve VFR); (2) different audio sample rates not resampled (add -ar 48000 ); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run ffprobe on both source and output to compare timing.

Why is the audio out of sync after converting?

Common causes: (1) variable framerate source rendered as constant (use -vsync vfr to preserve VFR); (2) different audio sample rates not resampled (add -ar 48000 ); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run ffprobe on both source and output to compare timing.

Guide

FLV Files: Flash Video Format History, Codecs, and Conversion Guide

PC By Pablo Cirre • Updated May 7, 2026

FLV Files: Flash Video Format — History, Conversion, and Rescue

FLV (Flash Video) was the dominant online video format from approximately 2005 to 2015. Built on Adobe Flash Player, FLV powered YouTube's early years, Vimeo, Hulu, Dailymotion, and thousands of other video-sharing platforms. The death of Flash — officially retired by Adobe on December 31, 2020, and blocked by all major browsers — left enormous archives of FLV content that can no longer play in modern browsers. Understanding FLV's structure and knowing how to convert it is essential for digital preservation, media archival, and recovering old video content.

Why FLV Was Dominant

When FLV emerged around 2003, it solved a genuine problem: there was no universal web video format. QuickTime required an Apple plugin, Windows Media Video required Microsoft's plugin, and RealVideo required RealPlayer. Flash Player was already installed on 95%+ of desktop web browsers as the dominant plugin for games and animations. Adding video support to Flash made FLV the de facto web video standard with no additional installation required.

YouTube launched in 2005 using FLV exclusively. When Google acquired YouTube in 2006, the library was already millions of FLV files. The format held for several more years until HTML5's <video> element and H.264/WebM support in browsers made Flash unnecessary — a transition that took until around 2010-2015.

FLV Technical Structure

FLV is a container format. The .flv extension contains multiplexed audio and video streams:

File header (9 bytes):

Signature: FLV (3 bytes)
Version: 01 (1 byte)
Flags: bit field indicating audio/video presence
Data offset: 00 00 00 09 (points to first tag)

Tag structure: FLV consists of sequential tags, each with:

Tag type (1 byte): 8=audio, 9=video, 18=script/metadata
Data size (3 bytes)
Timestamp (3 bytes) + timestamp extension (1 byte) = effective 32-bit ms timestamp
Stream ID (3 bytes, always 0)
Tag data

Preceding each tag is a 4-byte previous tag size field (for backward seeking).

Video Codecs in FLV

Sorenson H.263 (codec ID 2) — the original FLV codec, used by early YouTube. Low quality by modern standards.
Screen Video (codec ID 3/5) — screen capture codec, lossless, high compression for desktop captures
On2 VP6 (codec ID 4/5) — better quality than Sorenson, supported alpha channel. Used by YouTube 2007–2010.
H.264/AVC (codec ID 7) — the highest quality option; added when Flash Player 9 Update 3 shipped in 2007. Most FLV files from 2008+ use H.264.

Audio Codecs in FLV

ADPCM — 8 or 16-bit differential PCM
MP3 — the most common early FLV audio codec
Linear PCM — uncompressed PCM audio
Nellymoser — low-bitrate speech codec for Flash phone calls and voice chat
AAC (codec ID 10) — added alongside H.264; high quality, used in later FLV files
Speex (codec ID 11) — voice codec for real-time streaming

Converting FLV Files

Checking FLV Contents First

Before converting, identify what codecs are inside:

ffprobe input.flv

Look for Video: h264, flv1 (Sorenson), or vp6f (VP6), and Audio: mp3 or aac.

FLV to MP4 (Most Common Conversion)

If the FLV contains H.264 video and AAC/MP3 audio, it can often be re-wrapped without transcoding:

# Stream copy (fast, no quality loss — only works if codecs are compatible)
ffmpeg -i input.flv -c copy output.mp4

# Full transcode (needed for Sorenson H.263 or VP6 video)
ffmpeg -i input.flv -c:v libx264 -crf 22 -c:a aac -b:a 192k output.mp4

FLV to WebM

ffmpeg -i input.flv -c:v libvpx-vp9 -b:v 0 -crf 30 -c:a libopus -b:a 128k output.webm

FLV to GIF (for short clips)

ffmpeg -i input.flv -vf "fps=12,scale=640:-1" -t 10 output.gif

Batch FLV Conversion

# Linux/macOS — convert all FLV files to MP4
for f in *.flv; do
  ffmpeg -i "$f" -c:v libx264 -crf 22 -c:a aac "${f%.flv}.mp4"
done

# Windows Command Prompt
for %f in (*.flv) do ffmpeg -i "%f" -c:v libx264 -crf 22 -c:a aac "%~nf.mp4"

Preserving Metadata

FLV files may contain a metadata onMetaData script tag with duration, width, height, frame rate, and creation time. Extract metadata:

ffprobe -v quiet -print_format json -show_format -show_streams input.flv

To preserve metadata in the MP4 output:

ffmpeg -i input.flv -c:v libx264 -c:a aac -movflags +faststart output.mp4

The -movflags +faststart flag moves the MP4 moov atom to the beginning of the file for web streaming (progressive download).

FLV from the Web: Downloading Old Flash Video

Many old FLV files exist only in web archives. Tools for recovering them:

yt-dlp (successor to youtube-dl): Downloads FLV content from archived URLs and YouTube's legacy format:

yt-dlp --format flv "https://www.youtube.com/watch?v=OLD_VIDEO_ID"
# Or download best available format
yt-dlp "URL"

Wayback Machine: The Internet Archive (web.archive.org) has archived Flash content including FLV files. Use their Wayback CDX API to locate cached versions.

Browser cache recovery: FLV files downloaded by Flash Player were sometimes cached in the browser's temporary directory. On Windows XP/Vista/7, Flash cached to %APPDATA%\Macromedia\Flash Player\#SharedObjects\ and %LOCALAPPDATA%\Microsoft\Windows\Temporary Internet Files\.

FLV Parsing Quirks

Sorenson H.263 in FLV (codec ID 2) is a modified variant of H.263 — not exactly standard H.263, with some proprietary extensions. Most decoders handle it, but some may have issues with non-standard resolution constraints.
VP6 alpha (codec ID 5): VP6-F is used for video without alpha; VP6-A is used for video with alpha. The alpha channel was used for compositing in Flash animations.
Timestamp wraparound: 32-bit millisecond timestamps support up to ~49.7 days of continuous video. Long recordings may exhibit timestamp wraparound bugs in some players.
Seekability: FLV files without a keyframe index in the metadata tag are not seekable efficiently. Many FLV rippers and downloaders produce files without proper keyframe indexes.

FLV in 2025

No modern browser plays FLV natively. Flash Player is dead. FLV files require either:

A standalone player (VLC, MPC-HC) with FLV support
Conversion to MP4, WebM, or another modern format

For digital archivists, FLV conversion is triage work: converting millions of files before they become completely inaccessible as software support for the old codecs erodes. FFmpeg's FLV support, including Sorenson and VP6, is likely to remain robust for many years, making it the key tool for FLV preservation.

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Frequently Asked Questions

Yes — several media players support FLV independently of Flash Player. VLC Media Player (free, all platforms) plays FLV files including Sorenson H.263, VP6, and H.264 variants. Media Player Classic (MPC-HC) on Windows also handles FLV natively. MPlayer and mpv on Linux and macOS support FLV. The web browser cannot play FLV without Flash (which is dead), but standalone players work fine. For long-term accessibility, convert FLV to MP4 with H.264, which all modern devices and browsers play natively.

Yes — several media players support FLV independently of Flash Player. VLC Media Player (free, all plataformas) plays FLV arquivos including Sorenson H.263, VP6, e H.264 variants. Media Player Classic (MPC-HC) no Windows also handles FLV natively. MPlayer e mpv no Linux e macOS support FLV. The web browser cannot reproduzir FLV sem Flash (which is dead), mas standalone players funcionar fine. para long-term accessibility, converter FLV to MP4 com H.264, which all dispositivos modernoos e browsers reproduzir natively.

Yes — several media players support FLV independently von Flash Player. VLC Media Player (free, all Plattformen) plays FLV Dateien including Sorenson H.263, VP6, und H.264 variants. Media Player Classic (MPC-HC) auf Windows also handles FLV natively. MPlayer und mpv auf Linux und macOS support FLV. The web browser cannot abspielen FLV ohne Flash (which is dead), aber standalone players work fine. für long-term accessibility, umwandeln FLV to MP4 mit H.264, which all moderne Geräte und browsers abspielen natively.

Yes — several media players support FLV independently de Flash Player. VLC Media Player (free, all plataformas) plays FLV archivos including Sorenson H.263, VP6, y H.264 variants. Media Player Classic (MPC-HC) en Windows also handles FLV natively. MPlayer y mpv en Linux y macOS support FLV. The web browser cannot reproducir FLV sin Flash (which is dead), pero standalone players work fine. para long-term accessibility, convertir FLV to MP4 con H.264, which all dispositivos modernoos y browsers reproducir natively.

AV1 is the most efficient (royalty-free, ~30% smaller than H.265) but encoding is slow. H.265 (HEVC) saves ~30–50% over H.264 and is supported by every modern phone and desktop. H.264 remains the safest baseline for legacy compatibility. Rule of thumb: archives → AV1, daily use → H.265, broadest reach → H.264.

The original FLV was recorded at low resolution or low bitrate, and conversion cannot add quality that was not in the source. Early YouTube FLV files (2005–2008) used Sorenson H.263 at 320×240 or 480×360 pixels at 150–500 kbps — upscaling these to 1080p will look blurry. If you are using ffmpeg -crf 22 and the output still looks blurry, the source quality is the limiting factor. Check the original resolution with `ffprobe input.flv` and do not upscale beyond the native resolution.

The original FLV was recorded at baixa resolução ou baixa bitrate, e conversion cannot add quality that was not no source. Early YouTube FLV arquivos (2005–2008) used Sorenson H.263 at 320×240 ou 480×360 pixels at 150–500 kbps — upscaling these to 1080p will look blurry. If you are usando ffmpeg -crf 22 e the output still looks blurry, the source quality is the limiting factor. Check the original resolução com `ffprobe input.flv` e do not upscale beyond the native resolução.

The original FLV was recorded at niedrig Auflösung oder niedrig Bitrate, und conversion cannot add quality that was not im source. Early YouTube FLV Dateien (2005–2008) used Sorenson H.263 at 320×240 oder 480×360 pixels at 150–500 kbps — upscaling these to 1080p will look blurry. If you are using ffmpeg -crf 22 und the output still looks blurry, the source quality is the limiting factor. Check the original Auflösung mit `ffprobe input.flv` und do not upscale beyond the native Auflösung.

The original FLV was recorded at baja resolución o baja bitrate, y conversion cannot add quality that was not en el source. Early YouTube FLV archivos (2005–2008) used Sorenson H.263 at 320×240 o 480×360 pixels at 150–500 kbps — upscaling these to 1080p will look blurry. If you are using ffmpeg -crf 22 y the output still looks blurry, the source quality is the limiting factor. Check the original resolución con `ffprobe input.flv` y do not upscale beyond the native resolución.

CRF (Constant Rate Factor) is the best default for offline files: ffmpeg picks the bitrate frame-by-frame to maintain perceived quality. Two-pass is only better when you must hit an exact final size (DVD targets). Constant bitrate is for streaming with a fixed channel. For "smallest at quality X" always use CRF.

Yes. Use FFmpeg to extract the audio stream directly: `ffmpeg -i input.flv -vn -c:a copy output.mp3` (if the audio is already MP3). If the audio is AAC: `ffmpeg -i input.flv -vn -c:a copy output.aac`. If you want to convert the audio to a different format: `ffmpeg -i input.flv -vn -c:a aac -b:a 192k output.m4a`. The `-vn` flag disables video processing so only the audio track is extracted.

Yes. usar FFmpeg to extract the audio stream directly: `ffmpeg -i input.flv -vn -c:a copy output.mp3` (if the audio is already MP3). If the audio is AAC: `ffmpeg -i input.flv -vn -c:a copy output.aac`. If you querer converter the audio para um different formato: `ffmpeg -i input.flv -vn -c:a aac -b:a 192k output.m4a`. The `-vn` flag disables video processing so only the audio track is extracted.

Yes. verwenden FFmpeg to extract the audio stream directly: `ffmpeg -i input.flv -vn -c:a copy output.mp3` (if the audio is already MP3). If the audio is AAC: `ffmpeg -i input.flv -vn -c:a copy output.aac`. If you want to umwandeln the audio zu einem different Format: `ffmpeg -i input.flv -vn -c:a aac -b:a 192k output.m4a`. The `-vn` flag disables video processing so only the audio track is extracted.

Yes. usar FFmpeg to extract the audio stream directly: `ffmpeg -i input.flv -vn -c:a copy output.mp3` (if the audio is already MP3). If the audio is AAC: `ffmpeg -i input.flv -vn -c:a copy output.aac`. If you want to convertir the audio a un different formato: `ffmpeg -i input.flv -vn -c:a aac -b:a 192k output.m4a`. The `-vn` flag disables video processing so only the audio track is extracted.

Common causes: (1) variable framerate source rendered as constant (use <code>-vsync vfr</code> to preserve VFR); (2) different audio sample rates not resampled (add <code>-ar 48000</code>); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run <code>ffprobe</code> on both source and output to compare timing.

FLV is a video container format that stores video and audio streams. SWF (Shockwave Flash) is a Flash application format that can contain animations, interactive elements, games, and FLV video embedded as content. A typical Flash website consisted of an SWF file (the application shell) that loaded FLV files for video playback. SWF files cannot be converted to video directly unless you record the screen while they play. FLV files can be directly converted to MP4 using FFmpeg.

FLV is a video container formato that stores video e audio streams. SWF (Shockwave Flash) is a Flash aplicativo formato that can contain animations, interactive elements, games, e FLV video embedded as content. A typical Flash website consisted of an SWF arquivo (the aplicativo shell) that loaded FLV arquivos para video playback. SWF arquivos cannot be convertido to video directly unless you record the screen while they play. FLV arquivos can be directly convertido to MP4 usando FFmpeg.

FLV is a video Container Format that stores video und audio streams. SWF (Shockwave Flash) is a Flash Anwendung Format that can contain animations, interactive elements, games, und FLV video embedded as content. A typical Flash website consisted von an SWF Datei (the Anwendung shell) that loaded FLV Dateien für video playback. SWF Dateien cannot be umgewandelt to video directly unless you record the screen while they play. FLV Dateien can be directly umgewandelt to MP4 using FFmpeg.

FLV is a video contenedor formato that stores video y audio streams. SWF (Shockwave Flash) is a Flash aplicación formato that can contain animations, interactive elements, games, y FLV video embedded as content. A typical Flash website consisted de an SWF archivo (the aplicación shell) that loaded FLV archivos para video playback. SWF archivos cannot be convertido to video directly unless you record the screen while they play. FLV archivos can be directly convertido to MP4 using FFmpeg.

Yes if you only change the container: <code>ffmpeg -i in.mkv -c copy out.mp4</code>. This remuxes the stream without re-encoding, takes seconds even for hours of footage. Limitations: codec must be supported by the target container (e.g. you cannot put H.264 in WebM, only VP8/VP9/AV1). To shrink size you must re-encode.