RTcmix for Linux: Part 1

In the first part of this three-part series on real-time audio synthesis, we take you through the history and basis of RTcmix.

RTcmix is a software package that performs audio synthesis and effects processing. Basically, it's a real-time version of the Cmix language. Standard control over program execution is achieved via a C-style scripting language called MINC (MINC Is Not C). It has been open source long before the phrase was coined. That fact has been key to its evolution.


There have been literally dozens of RTcmix hackers over the past 20 years. I've done my best to give some credit as it relates to this article. You can also check the AUTHORS file included in the recent distribution (see “Getting and Installing” below).

Cmix was derived from the MIX program, a 20-track mixer written by Paul Lansky (at Princeton University) in 1978, which ran on VMS in FORTRAN on IBM mainframes. Synthesis abilities were added, and Paul ported Cmix to run on a PDP11/34 in 1983-84 under BSD2.9 UNIX. In 1985, he moved it over to Ultrix on a DEC MicroVAX. He and Lars Graf added the MINC parser in 1987, with help from Brad Garton and Dave Madole on various parts. Cmix lived happily under the NeXt (and other UNIX systems) for several years with its user base primarily in academic institutions. It has been and continues to be used both for instruction, computer music composition and research development.

Doug Scott and Paul Lansky ported an initial real-time version to SGI in 1995. While sometime in 1993, as a student and research assistant at Columbia University's Computer Music Center (CMC), I took a first stab at a Linux port which had to use Sox to unswap big-endian files. Boy was that fun. In late 1995, Brad Garton (CMC director) and I created a real-time version with a scheduler. RTcmix was born, originally running under IRIX. Porting to Linux basically entailed swapping out the IRIX audio API with OSS (Open Sound System). The initial IRIX version was presented at the International Computer Music Conference (ICMC) '97. Shortly thereafter, Luke DuBois (also at the CMC) added an API to make writing TCP socket data easier. Doug Scott set up a mechanism to dynamically load instruments at run-time in 1997. John Gibson (at UVA) cleaned up the business of reading and writing different audio file formats in 1999 with an interface to Bill Schottstaedt's (at Stanford University) sndlib. John and I recently added two new abilities: routing audio via an internal “bus” mechanism and multichannel audio support. This work was presented this past year at the Society for Electoacoustic Music in the U.S. (SEAMUS) Y2K conference in Denton, Texas. Work also continues on controlling synthesis parameters in real time, creating various interfaces, and more robust socket support.

Setting up CVS (Concurrent Versions System) to help manage code has facilitated recent collaborations and development. The main core of RTcmix is released under the GPL (GNU Public License), with some different licenses for various components (e.g., instruments and effects) depending on their origin.

Getting and Installing RTcmix

In order to make use of RTcmix's real-time audio abilities, you need to install and set up the OSS audio drivers for Linux (see Resources). Support for ALSA (Advanced Linux Sound Architecture) remains untested but is planned. If you want to use RTcmix as a real-time audio processor, you need to make sure your audio card supports full-duplex operation. The Linux version of RTcmix can be downloaded via ftp from:


The latest version as of this article is 3.0. The complete package exists in separate packages. RTcmix-3.0.0.tar.gz: the core engine and scheduler. insts.jg-3.0.0.tar.gz: instruments written by John Gibson. insts.mch-3.0.0.tar.gz: multichannel instruments. insts.std-3.0.0.tar.gz: standard suite of instruments. [At publication time, current versions of all of these packages is 3.0.2—ED]

I should probably comment on some terminology here. An RTcmix “instrument” can be either an effects processor (e.g., DELAY) or a synthesizer (e.g., FM synthesis).

Instrument packages should be unpacked into the same directory as the main code. So if you unpack the main code (RTcmix) to /usr/local/src, you'll end up with /usr/local/src/RTcmix-3.0.0. Untar instrument packages into that directory. In the case of insts.std.tgz, you'd end up with /usr/local/src/RTcmix-3.0.0/insts.std-3.0.0. You can unpack the main code into any directory you like (e.g., your home directory or anyplace else).

In order to keep instruments and RTcmix versions consistent, we've added version tags to their directories. To make compilation simpler, you should symlink these to default names. For example, insts.std.tgz will untar into insts.std-3.0.0. You should symlink this as:

ln -s insts.std-3.0.0 insts.std

Once you've gotten everything unpacked and linked, you can set things up for compilation. There's no ./configure utility here. You have to edit makefile.conf in the RTcmix root directory. Only a few lines need changing. Reading from the top of the file:

# Change stuff in here as needed

# The directory containing the RTcmix directory
TOPDIR = /usr/local/src

# The name of the RTcmix directory
CMIXDIR = $(TOPDIR)/RTcmix-3.0.0

# The dir that will contain links to the
# instrument dynamic shared objects

# add new instruments to this list (e.g.,
# insts.std, insts.jg)
INST_DIRS = insts.base #insts.std insts.jg insts.mch
The only things you really need to potentially change are TOPDIR, depending on where you've unpacked RTcmix, and INST_DIRS, depending on which instrument packages you've downloaded.

Once you've made the appropriate changes, you can do a make && make install from the top-level RTcmix directory. This will compile everything and put a CMIX executable in TOPDIR/bin and shared libraries in TOPDIR/shlib. Make sure the bin directory is part of your PATH.