Format Conversion

FlowPick can merge fragments and convert to standard formats while downloading streaming media. This document explains supported conversion paths, underlying technical principles, output format selection, and performance characteristics.

Prerequisite Concepts: Container vs Codec

Before understanding format conversion, need to distinguish two easily confused concepts:

FlowPick's format conversion only changes the container, not the codec (i.e., "remuxing"). This means:

• Video quality and audio quality remain completely unchanged
• Conversion is very fast (no re-encoding involved)
• Cannot change codec formats (e.g., H.265 → H.264)

Conversion Path Overview

FlowPick supports following conversion paths, covering both HLS and DASH mainstream streaming protocols:

"Lossless" means no re-encoding is performed; only container format is changed. Video and audio encoded data remain unchanged.

Technical Principles of Conversion

FlowPick uses two conversion engines, automatically selecting optimal solution based on scenario:

TS Fragments → MP4 (FFmpeg WASM Remuxing)

HLS streams typically use MPEG-TS container. When converting to MP4, FlowPick uses FFmpeg WASM to perform following operations:

Input: Multiple .ts fragment files

Processing Flow:

1. Write all TS fragments into FFmpeg virtual file system (WASM memory)
2. Generate concat file list (filelist.txt)
3. Execute remuxing command:

ffmpeg -f concat -safe 0 -i filelist.txt \
  -c copy \
  -movflags +faststart \
  -bsf:a aac_adtstoasc \
  output.mp4

Key Parameter Explanation:

Performance Characteristics:

• No encoding/decoding involved, low CPU usage
• Main time cost is in file I/O (WASM virtual file system read/write)
• Processing speed approximately 50-100 MB/s (depends on CPU and fragment count)

TS Fragments → MP4 (TSToMP4Muxer Streaming Remuxing)

For large file scenarios, FlowPick prioritizes using proprietary TSToMP4Muxer for streaming remuxing, avoiding FFmpeg WASM memory limitations:

Processing Flow:

1. Parse TS packets one by one (188 bytes/packet), extract PES data
2. Convert to MP4 format media data in real-time
3. Stream write to disk via Streams API (FSA) or memory (Blob)

Advantages:

• Pure JavaScript implementation, no need to load FFmpeg WASM (saves ~30MB memory)
• Streaming processing, memory usage independent of file size
• Supports File System Access API for direct disk writing

Disadvantages:

• Only supports single TS→MP4 conversion path
• Does not support separate audio-video merging

TS Fragments → TS (Direct Concatenation)

This is the fastest "conversion" method — actually performs no conversion at all, only binary concatenation of TS fragments in order.

Processing Flow:

1. Download all TS fragments to memory
2. Concatenate ArrayBuffers sequentially into single Uint8Array
3. Write directly to file

Advantages:

• Zero CPU overhead, speed limited only by disk write speed
• Completely faithful, does not modify any bytes
• Suitable for subsequent processing with professional tools

Disadvantages:

• File size may be slightly larger than MP4 (TS container overhead ~5-15%)
• Some players have poorer support for TS files than MP4

DASH Fragments → MP4

DASH stream fragments are usually MP4 or FMP4 format. Conversion process:

Standard MP4 fragments:

ffmpeg -f concat -safe 0 -i filelist.txt \
  -c copy \
  -movflags +faststart \
  output.mp4

FMP4 fragments (requires init segment):

ffmpeg -i init.mp4 -i video.mp4 -i audio.mp4 \
  -c:v copy -c:a copy \
  -movflags +faststart \
  output.mp4

Audio-Video Separate Merge

DASH streams' video and audio are usually in different AdaptationSets. Merging process:

ffmpeg -f concat -safe 0 -i video_list.txt \
  -f concat -safe 0 -i audio_list.txt \
  -c:v copy -c:a copy \
  -movflags +faststart \
  -bsf:a aac_adtstoasc \
  output.mp4

Output Format Selection Guide

MP4 (Recommended)

Use Cases:

• Need to play on various players and devices
• Need to import into video editing software (Premiere, DaVinci Resolve, etc.)
• Need to upload to video platforms (YouTube, Bilibili, etc.)
• Need to ensure maximum compatibility

Technical Details:

• Container: MPEG-4 Part 14
• MIME Type: video/mp4
• Supports H.264/AVC, H.265/HEVC, AAC, MP3 etc. codecs
• faststart flag ensures network progressive playback

TS

Use Cases:

• Pursue fastest download speed (skip remuxing step)
• Will further process with professional tools (e.g., FFmpeg CLI) later
• Source stream itself is TS format and doesn't need conversion
• Temporary storage, may delete later

Technical Details:

• Container: MPEG Transport Stream
• MIME Type: video/mp2t
• Each fragment independently playable
• File size typically 5-15% larger than MP4

Format Selection Decision Tree

Need max compatibility? ──Yes──→ Choose MP4
      │
      No
      │
      ▼
Pursue fastest speed? ──Yes──→ Choose TS
      │
      No
      │
      ▼
Will process with FFmpeg later? ──Yes──→ Choose TS
      │
      No
      │
      ▼
    Choose MP4 (Recommended)

Performance Comparison

Following is actual test data under typical conditions (Chrome 126, Intel i7-13700, 16GB RAM):

Above data are estimates; actual performance depends on CPU, memory, and browser state. Streaming Muxer shows more obvious advantages when processing large files.

Factors Affecting Conversion Speed

Limitations & Notes

Codec Format Limitations

FlowPick's conversion only changes container format, does not perform re-encoding. This means:

Memory Limitations

FFmpeg WASM runs in browser, subject to following limitations:

Browser Compatibility

Command Line Alternatives

If you're familiar with command line, you can use native FFmpeg for better performance:

# HLS → MP4
ffmpeg -i "https://example.com/playlist.m3u8" -c copy -bsf:a aac_adtstoasc output.mp4

# DASH → MP4
ffmpeg -i "https://example.com/manifest.mpd" -c copy output.mp4

# TS fragment concatenation
cat segment_*.ts > output.ts

# TS → MP4 remuxing
ffmpeg -i output.ts -c copy -bsf:a aac_adtstoasc output.mp4

Advantages of native FFmpeg:

FAQ

Why is converted MP4 smaller than TS?

TS container has header overhead per 188-byte packet, while MP4 container's index structure is more compact. After remuxing, file size typically reduces 5-15%, but audio-video data remains completely unchanged.

Does conversion lose quality?

No. FlowPick uses -c copy (stream copy) mode, only changing container format, no re-encoding. Every byte of video and audio is preserved as-is.

Why can't some videos play after conversion?

Possible reasons:

• Source video uses codec unsupported by your player (e.g., H.265)
• Missing fragments during conversion caused incomplete file
• Issue with source stream itself

Related Documentation

• Video Sniffing — Learn about detection principles for HLS and DASH streams
• Download Engine — Deep dive into streaming processing and memory management
• Configuration Reference — Download settings like output format
• Usage Guide — Step-by-step guide for format conversion operations
• Browser Compatibility — Browser support status for WASM and FSA
• Common Issues Troubleshooting — Diagnosis and resolution of conversion-related issues
• Known Issues — Known limitations of format conversion