Lab 7 - Brief LAME assembler usage

The LAME is an MP3 encoder, despite the originally assigned recursive acronym standing for 'LAME Ain't an MP3 Encoder'. It claims to be an educational too for learning about MP3 encoding and it has a goal of improving the quality and performance of it. Interestingly, LAME is distributed by the developers only as source code with the purpose of being used merely as an educational tool, which in turn avoids patent infringement. Still, the developers advise acquiring a license before deploying projects implementing any part of compiled LAME code.

MP3

MP3 (aka MPEG-1, MPEG-2 Audio Layer III) is a digital audio format that reduces the size of original audio files by using lossy audio compression algorithms. The format takes advantage of the frequency range in the limited human sound perception (psychoacoustic model) and frequency resolution, i.e noticing changes in frequency. The compression discards or reduces those inaudible components.

The MPEG-1 standard provides example psychoactive models among other specifications, but doesn't provide precise specifications for the encoder, leaving developers to create their own algorithms to remove undesired information from a sound file. This led to the creation of many MP3 encoders, each generating outputs of different quality. For example, some encoders were better at dealing with low bit-rate files, while others (e.g LAME) were better at higher bit-rates. Eventually, LAME evolved and became widely used.

Encoding

Encoding MP3 requires dealing a lot with frequency manipulation involving many samples, and ideally being able to process those samples as fast as possible. On software, this can be done by implementing algorithms like fast Fourier Transform which converts a signal represented in the time domain to the frequency domain, allowing a much more convenient way to analyse manipulate data. Once there is a concrete basis theory that can be used with discrete data, the next level of optimization is to explore the hardware features available to reduce the time taken to process it. LAME is able to make use of hardware features to optimize encoding.

Assembly Usage

LAME extensively uses assembly. It specifically targets x86_64 architecture and can make use of features such as MMX, SSE, SSE2 and '3DNow!'. Although there's very little inline assembly, there are various nasm files that state functions which are called in C code.

The assembly code is used for checking cpu features, performing fast Fourier transforms using different features (depending on what's available) and other discrete calculus operations.

I couldn't compile the source to Aarch64.

Conclusion

LAME has been in development for a long time and it's widely used in many OS's. Their website says: "LAME compiles on Windows, DOS, GNU/Linux, MacOS X, *BSD, Solaris, HP-UX, Tru64 Unix, AIX, Irix, NeXTstep, SCO Unix, UnixWare, Ultrix, Plan 9, OpenVMS, MacOS Classic, BeOS, QNX, RiscOS, AmigaOS, OS/2, SkyOS, FreeMiNT(Atari) and probably a few more". It is really a project that has been optimized and ported a lot, which is a benefit for the user, but it immensely increases the complexity when compiling the code. Furthermore, the assembly code limits the usage of the program's optimizations on different platforms, in which equivalent features may be available.