Helix MP3 Encoder
| Developer(s) | RealNetworks,
|
| Release information | |
|---|---|
| Stable release | |
| Compatibility | |
| Operating system | Linux, Windows |
| Additional information | |
| Use | Encoder |
| License | RPSL |
| Website | GitHub repo |
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.
- Public MP3 listening test, October 2008
- Personal listening test by Kamedo2, ~224 kbps, May 2013
- Personal listening test by Kamedo2, ~192 kbps, December 2016
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.
| 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.
High frequencies will only be encoded if -V >= 80 or -B >= 96. |
HF2 for unrestricted high-frequency encoding |
| -M | Stereo-mode/Mono selection. 0: stereo, 1: M/S stereo (default), 2: dual channel, 3: mono | -M3 to downmix to mono |
| -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 |
| -X | Control writing of Xing/LAME header information. 0: No headers, 1: only basic Xing information header, 2: Xing header with VBR-TOC and LAME header (gapless information) (default) | -X0 to disable headers (in very rare cases of incompatibility) |
Bit allocators
The Helix MP3 Encoder, apparently for historical reasons, has two distinct bit allocators, which are selected depending on operating modes. Bit Allocator 1 (bitallo1.cpp) appears to be the older one, most likely inherited from early Xing days, while Bit Allocator 3 (bitallo3.cpp) is a newer, overall more-capable mechanism that is utilized by default.
| Feature | Bit Allocator 1 | Bit Allocator 3 |
|---|---|---|
| CBR | supported | supported |
| VBR | not supported | supported |
| >16 kHz encoding | not supported | supported |
| Long/Short block switching | not supported | supported |
| Intensity stereo | supported | not supported |
Bit Allocator 1 thus is mostly interesting for very low-bitrate CBR encodings, where intensity stereo can lead to bitrate savings to spend somewhere else. Example:
hmp3 input.wav output.mp3 -F16000 -B48 -N8
for somewhat bearable low-bitrate stereo-ish MP3 encoding (the -N parameter enables intensity stereo).