What is APE?

How APE Works

APE, the format produced by Monkey's Audio, is a lossless audio codec, which means it compresses a recording so that decoding it reproduces the original samples bit for bit with nothing discarded. Unlike a lossy codec that throws away information judged inaudible, a lossless codec only removes statistical redundancy, so the reconstructed file is mathematically identical to the source. The tradeoff is that lossless compression cannot reach the tiny sizes of lossy formats, since it can never simply delete detail.

Monkey's Audio achieves its compression in two broad stages. First it applies prediction: an adaptive predictor estimates each upcoming sample from the samples before it, and the codec stores only the small difference between the prediction and the true value. Because music is highly correlated from one instant to the next, these residual differences are far smaller and more compressible than the raw samples, which is the heart of how any lossless audio codec earns its savings.

The residuals are then passed through an entropy coder that packs frequently occurring small values into fewer bits. Monkey's Audio is known for using unusually strong, computationally intensive prediction, and it offers several compression levels such as fast, normal, high, and extra high. Higher levels apply heavier prediction to squeeze out a little more size, at the cost of substantially more processing work during both encoding and decoding.

That design choice makes APE notably asymmetric and CPU-heavy. Decoding is not a simple lookup; the decoder must run the same expensive predictive model that the encoder used, so playback demands more processing than lighter lossless formats. In exchange, APE historically achieved slightly higher compression ratios than more mainstream lossless codecs, which was its main selling point for archivists who wanted the smallest possible perfect copies.

History and Standardization

Monkey's Audio was created by Matthew T. Ashland and released as a Windows application, giving lossless audio enthusiasts a way to archive CDs and other sources without any quality loss. It arrived during the era when lossless audio was becoming practical on personal computers, and it built a devoted following among users who prioritized the smallest achievable file size for a perfect copy.

Unlike codecs developed through formal standards bodies, APE grew from a single reference implementation rather than an open, multi-vendor specification. Its licensing and source situation was historically more restrictive and less open than fully free alternatives, and its tooling was strongly Windows-centric. That origin shaped both its strengths, a tightly optimized encoder, and its weaknesses, thinner support outside the Windows world.

Over time the practical center of gravity for lossless audio shifted toward more open and broadly supported formats, and APE became something of a niche choice favored in specific archiving communities. It remains in use and is understood by many audio tools, but it never achieved the ubiquitous hardware and cross-platform support that its main rival enjoys.

APE Compared to FLAC and Other Lossless Codecs

The natural comparison is with FLAC, the dominant open lossless format. On raw compression, APE has historically edged out FLAC by a small margin, producing marginally smaller files at its higher settings. FLAC's advantages, however, are decisive for most users: it is a free and open format, it encodes and decodes far faster with much lighter CPU demands, and it enjoys extremely broad support across software players, operating systems, and dedicated hardware devices.

Against Apple's ALAC, APE again competes on being lossless, but ALAC benefits from deep integration in the Apple ecosystem, and against WavPack, another flexible open lossless codec, APE trades some versatility for its compression edge. In every one of these comparisons the theme is the same: rival codecs give up a sliver of compression in return for openness, speed, and portability, and most listeners find that trade worthwhile.

The honest tradeoff with APE is that its small size advantage is real but slight, while its costs, heavy CPU use, asymmetric and slow decoding, weaker cross-platform and hardware support, and a less open licensing history, are significant for everyday use. It makes the most sense for a Windows-based archivist who values the absolute smallest lossless files and does not need to play those files on a wide range of devices. For general listening, syncing, and portability, the more open and better-supported lossless formats are usually the more practical choice.