Convert Dolby 5.1 Surround M2TS to WebM Minimal Audio Degradation

Title: Converting Dolby 5.1 Surround M2TS to WebM Minimal Audio Degradation

Introduction

This article will guide you through the process of converting a Dolby 5.1 surround M2TS (H.264-AC3) video to WebM (VP9-Opus) format while minimizing audio degradation. The conversion process involves a two-step process, which we will discuss in detail.

Step 1: Extract AC3 audio from M2TS

First, we will extract the AC3 audio from the M2TS video using FFmpeg. Here's the command:

ffmpeg -i input.m2ts -vn -acodec copy -map 0:a:0 output.ac3

In this command:

• ffmpeg is the tool we're using for the conversion.
• -i input.m2ts specifies the input file.
• -vn tells FFmpeg to ignore the video stream.
• -acodec copy copies the audio stream without re-encoding it.
• -map 0:a:0 selects the first audio stream.
• output.ac3 is the output file for the extracted audio.

Step 2: Encode M2TS to WebM and remux with Opus audio

Next, we will encode the M2TS video to WebM format and remux the extracted AC3 audio with Opus audio. Here's the command:

ffmpeg -i input.m2ts -c:v libvpx-vp9 -crf 23 -c:a libopus -b:a 128k output.webm

In this command:

• ffmpeg is the tool we're using for the conversion.
• -i input.m2ts specifies the input file.
• -c:v libvpx-vp9 specifies the video codec (VP9).
• -crf 23 sets the Constant Rate Factor (CRF) for VP9. A lower value results in a higher quality video but larger file size.
• -c:a libopus specifies the audio codec (Opus).
• -b:a 128k sets the bitrate for Opus audio.
• output.webm is the output file for the converted video.

Minimal Audio Degradation

To minimize audio degradation when converting from AC3 to Opus, you can use a tool like sox to remap the AC3 audio levels to Opus levels. Here's an example command:

sox input.ac3 output.wav gain 1.25 remap output.opus

In this command:

• sox is the tool we're using for audio processing.
• input.ac3 is the input file for the AC3 audio.
• output.wav is a temporary WAV file used for remapping.
• gain 1.25 increases the gain of the audio by 25%.
• remap output.opus remaps the audio levels to Opus levels.
• output.opus is the output file for the remapped audio.

Summary

• Convert Dolby 5.1 surround M2TS to WebM (VP9-Opus) using FFmpeg and (optionally) sox.
• Extract AC3 audio from M2TS using ffmpeg.
• Encode M2TS to WebM and remux with Opus audio using ffmpeg.
• Optionally, use sox to remap AC3 audio levels to Opus levels.

References

• FFmpeg Documentation
• SoX Manual