Reusing Paddings As Supplements

05/21/2023

Despite having launched a couple of weeks ago, we had nothing newsworthy on GAPCM. Basically, it converts 8-bit PCM from and to a format devised for a 28-year old video game, with the main goal of ripping its soundtrack. PCM is short for pulse-code modulation and is the basis of digital audio.

Today however, we are releasing Revision 3 of the project with non-standard support for 16-bit PCM while retaining 8-bit compatibility. Normally, an 8-bit sample is preceded by a padding byte, an alignment constraint from reading data in 16-bit words into an 8-bit bus. The padding byte is thus ignored during playback on the Ricoh RF5C164. GAPCM can safely use this byte as a supplement to the sample. Decoders that ignore it--including the one in-game--should remain playing the 8-bit sample, while GAPCM can combine both bytes into a 16-bit little-endian sample. This extension is disabled by default and its setting is hard-coded.

Revision 3 also brought two significant changes. Firstly, the encoder's default loop start position (mark) value has been changed to follow the stream length for the shortest possible loop. The previous default of zero can be set with -m 0. Secondly, given a game PCM file, its stream length must be greater than and not equal to its mark; so loops can not be empty, and a stream length of zero is invalid.

In addition to the above, we also made the GAMplay playback helper script, to which you give a game PCM file you want to listen to. It provides a simple interface for near-authentic playback without having to learn much about anything, save for basic command-line skills. Decoding and playback parameters are determined between two modes: single, in which the file is played in one sweep; and loop, in which the file is programmatically looped six times followed by a 10-second fade-out. Ultimately, its behavior is adjustable in script.

This project got us officially started with the C programming language, which turned 50 last year. We wanted to take full advantage of the latest standard revision as well as the GCC 13 compiler. However, Debian testing is currently under a freeze in preparation for the next stable release, so new software releases are deferred until then. We provide binaries for Debian and Windows, but the source code should compile in any modern POSIX-compliant environment. Notably, macOS is POSIX-certified out-of-the-box.

In other news, SheFF--our shell FFmpeg frontend--was updated earlier this month for AV1 support. We decided to go with the SVT-AV1 encoder for now and may switch to rav1e in a future release.