Helix MP3 Encoder: Difference between revisions
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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. | 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. | ||
{| class="wikitable" | |||
|+ Bit allocators | |||
|- | |||
! 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). | |||
== External links == | == External links == | ||
Revision as of 12:44, 25 March 2024
| 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).