LAME 4.0 cVBRb

This repository contains the LAME 4.0 cVBRb source tree maintained by Cole Williams, based on the LAME MP3 encoder source.

The cVBRb work extends the encoder with constrained-VBR behaviour and bitrate-boost controls while retaining the wider LAME source tree and its existing Windows components.

Project website: https://www.lamemp3.co.uk/

cVBRb

The cVBRb branch adds encoder-side controls including:

• strict VBR minimum bitrate handling (--vbr-min-strict)
• bitrate boost levels (--bitrate-boost)
• cVBR/cVBRb status reporting in the command-line encoder
• corresponding libmp3lame API support
• Media Foundation encoder support for CBR, normal VBR and cVBRb

The implementation is primarily in the main encoder/frontend and libmp3lame code. The Media Foundation Transform under MFT/ also exposes cVBRb-specific configuration.

Windows components

Several Windows codec interfaces are included in the source tree, but they are not all cVBRb-enabled.

Component	Location	cVBRb status
Command-line LAME encoder	frontend/	cVBRb-enabled
libmp3lame	libmp3lame/	cVBRb-enabled
Media Foundation encoder	MFT/	cVBRb-enabled
DirectShow MP3 encoder	dshow/	Not patched to expose/use cVBRb-specific controls
DirectShow MP3 decoder	DShowDecoder/	Decoder only; no cVBRb encoder controls
ACM codec	ACM/	Legacy CBR/ABR compatibility interface; cVBRb intentionally not exposed

The DirectShow and ACM sources are therefore included as separate LAME components and should not be described as cVBRb implementations in this repository. The ACM interface is intentionally kept conservative for compatibility with older Windows applications; cVBRb, strict minimum bitrate and bitrate boost belong to the main encoder/libmp3lame and MFT paths.

Building

The tree contains the original LAME build system together with Windows/Visual Studio project files under vc_solution/. Historical build files that are part of the LAME source distribution are retained, while local development-only build experiments and generated build products are not included.

For the newer Windows components, see:

• MFT/README.md — Media Foundation encoder
• dshow/README.md — DirectShow encoder
• DShowDecoder/README.md — DirectShow decoder
• ACM/README.md — ACM codec
• vc_solution/README.vs2019.txt — existing Visual Studio notes

Build products are intentionally not included in the GitHub source archive.

Repository layout

• frontend/ — LAME command-line frontend
• libmp3lame/ — MP3 encoding library and cVBRb implementation
• include/ — public LAME API headers
• MFT/ — Windows Media Foundation MP3 encoder
• dshow/ — DirectShow MP3 encoder
• DShowDecoder/ — DirectShow MP3 decoder
• ACM/ — Windows ACM codec
• directshow-baseclasses/ — DirectShow BaseClasses used by the filter projects
• vc_solution/ — Visual Studio projects
• mpglib/ — MPEG audio decoding code from the LAME tree
• Dll/ — legacy LAME encoder DLL interface

Licensing

The existing LAME licensing in this source tree remains in place. No new licence terms are introduced by this README.

See:

• COPYING for the GNU Library General Public License version 2 text supplied with the source
• LICENSE for the existing LAME licensing guidance
• individual source files for their respective copyright and licence notices

Third-party code included in the tree may carry its own notices and terms. Those notices remain authoritative for the relevant files.

Credits

LAME is the work of the LAME Project and its contributors. cVBRb modifications in this branch are by Cole Williams.

LAME project: https://lame.sourceforge.io/

cVBRb builds and project information: https://www.lamemp3.co.uk/