Helix MP3 Encoder

The Helix MP3 Encoder was open-sourced by RealNetworks ca. 2005 via the (long-defunct) Helix community project. It originated from the Xing MP3 encoder, which was purchased by RealNetworks.

A current version ("hmp3"), with contributions from HydrogenAudio members, is available as source code over at https://github.com/maikmerten/hmp3. This Wiki page discusses that version.

Features

Supports MPEG-1 and MPEG-2 modes
- 48 kHz, 44.1 kHz, 32 kHz (MPEG-1)
- 24 kHz, 22.05 kHz, 16 kHz (MPEG-2)
LAME headers for gapless playback
CBR and VBR encoding

Listening tests

The Helix MP3 encoder participated in several listening tests and demonstrated to be amongst the highest-quality encoders for MP3 available.

Encoder switches

hmp3 is a command-line operated application. The most basic invocation to generate a MP3 file from WAV:

 hmp3 input.wav output.mp3

This creates a ~128 kbps VBR file for 44.1 kHz stereo input.

Encoder switches
Switch	Function	Example
-B	Set per-channel bitrate. Selects CBR encoding.	-B64 for a 128 kbps stereo CBR file
-F	Frequency cutoff for the encoder lowpass filter. To actually encode anything beyond 16 kHz, also specify the -HF switch.	-F19000 for a 19 kHz lowpass
-HF	Controls encoding of high frequency content (> 16 kHz). Disabled by default. Valid values are 0 (disabled), 1 (partial, only "mode-1 granules"), 2 (full, "all granules"). Note that high-frequency content will only be encoded if the psychoacoustic model deems encoding high frequencies as beneficial for the given bitrate/quality settings.	-HF2 for unrestricted high-frequency encoding
-SBT	Threshold for short-block decisions. Lower values mean more short-block usage. Default is 700.	-SBT500 for more short-blocks (more responsive to transients, might increase bitrate in VBR)
-V	Quality setting for VBR encoding. Ranges from 0 to 150. Default is 50.	-V115 for a ~180-200 kbps stereo VBR file