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A November 3 report describes adapting the Ultra Low Complexity Codec (ULC) for Nintendo 64 audio. Its author says it can play 44.1 kHz stereo at bitrates near Opus while using about 4% of the console’s CPU time; independent testing and detailed test conditions are not provided in the source.
A developer says the Ultra Low Complexity Codec (ULC) can play 44.1 kHz stereo audio on the Nintendo 64 at bitrates near Opus while using about 4% of the console’s CPU time. The November 3 report describes ULC as a potential alternative to audio formats that either consume substantial cartridge space or are too demanding for continuous playback during gameplay.
The report comes from the developer of Xibalba 64, who says the game currently stores its music using the N64’s real-time ADPCM decoder. That approach works, but uses about 31 MB of a 32 MB ROM for sound and music, despite the game having about 15 minutes of music. The author says this makes ADPCM wasteful when cartridge capacity is available but still limited.
The author says Libdragon, an open-source alternative to Nintendo’s Libultra development library, already includes an RSP-accelerated Opus decoder. The report describes its compression and sound quality as strong, but says it is too slow for use during gameplay. The developer presents ULC as a lighter option, reporting that it achieves the stated audio quality and CPU use on the N64. The source does not give detailed benchmarks, a comparison test setup or independent measurements.
ULC was created by developer Aikku93, who, according to the report, began work on it in 2019 for a Game Boy Advance demo. It uses a transform approach based on the modified discrete cosine transform, but represents audio data with a relatively simple four-bit, nibble-based scheme. The author says this avoids the more involved entropy decoding used by MP3 and makes the format practical on low-power hardware. The report says the codec’s code is permissively licensed.
More Music, Less N64 Processing
If the reported performance holds in other projects, ULC could make recorded music and richer audio more practical on N64 games without dedicating as much storage to uncompressed or lightly compressed sound. It may also offer developers a way to keep audio decoding within a smaller processing budget than the report attributes to Opus and MP3.
The result is relevant beyond one game: retro developers often work within constraints that modern hardware no longer faces, including limited cartridge capacity and the N64’s division of work between its main CPU and Reality Signal Processor (RSP). A codec that balances sound quality, storage and decoding cost could expand what is feasible on the platform. However, the report’s headline performance figures are the author’s claims, not a published independent benchmark, and they do not by themselves establish how the codec will perform across different games or audio mixes.
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N64 Audio’s Cartridge Constraints
The N64’s cartridge format gave developers less room for large music files than the CD-based PlayStation. The report says Nintendo’s ADPCM decoder could decompress audio on the RSP, reducing sample storage by roughly a factor of four, but even compressed, five minutes of 44.1 kHz stereo audio could take about 12 MB. For comparison, the report notes that Super Mario 64 occupied an 8 MB ROM, while the largest N64 games reached 64 MB.
As a result, N64 games commonly generated music by arranging short samples rather than storing full-length recordings. Some later titles received direct access to the RSP; the report cites Conker’s Bad Fur Day as using MP3 decoding for speech samples. The author says MP3 decoding was expensive enough to affect frame rates and was better suited to cutscenes than continuous audio during gameplay.
The report says Libdragon has since made both ADPCM and Opus decoding available on the RSP. ULC’s proposed advantage is not that it uses an entirely new transform, but that its data representation is designed to make decoding less demanding. The underlying codec has been in development since 2019, according to the source.
““44100 Hz stereo with near-Opus bitrates using about 4% of the N64’s CPU time.””
— The author of the November 3 report
Benchmark Conditions Remain Unreported
The report gives headline figures for sample rate, stereo playback, bitrate and CPU use, but does not detail the test setup, the exact bitrate, the audio material used or how CPU time was measured. It also does not include independent testing or comparison data for ULC against Opus, ADPCM or MP3 under the same conditions.
It is not clear from the source whether the N64 implementation is ready for general use, whether the reported performance applies to full-length game audio in varied situations, or whether the developer plans to replace Xibalba 64’s ADPCM tracks with ULC. The report also does not specify when, or in what form, the N64 implementation and encoding tools will be released. Those details matter to developers considering adoption.
Testing Across N64 Projects
The immediate next step for readers and developers is to look for implementation details, reproducible tests and examples of ULC running in complete N64 projects. Publicly available code and encoding tools, if confirmed for the N64 version, would let other developers examine the reported CPU cost, audio quality and storage requirements on their own hardware.
Until those details or independent measurements are available, the 4% CPU-use figure should be treated as a claim from the original report rather than a broadly established benchmark. The source does not announce a release date, a formal test schedule or a confirmed change to Xibalba 64’s audio system.
Key Questions
What is ULC?
ULC is the Ultra Low Complexity Codec, a transform audio codec created by Aikku93. The report describes it as using a modified discrete cosine transform and a simple nibble-based format for representing coefficients.
What performance does the N64 report claim?
The author reports 44.1 kHz stereo audio at bitrates near Opus, using about 4% of the N64’s CPU time. The source does not provide detailed test conditions or independent confirmation.
Why not use ADPCM for N64 music?
ADPCM can be decoded in real time by the N64’s RSP, but the report says it still takes substantial cartridge space. In the author’s Xibalba 64 example, sound and music occupy about 31 MB of a 32 MB ROM.
Does this mean ULC will replace Opus or ADPCM?
No replacement is confirmed. The author presents ULC as a possible alternative and says Opus is too slow for gameplay in the tested setup, but the report does not announce a finalized change to the game or adoption by other developers.
Can developers use the N64 version now?
The report says ULC code is permissively licensed, but it does not clearly state whether the N64 implementation and its encoding tools are available, or provide a release date. Developers should check the project’s published materials for current availability.
Source: phoboslab.org
