A typical chipmod sample
consists of a single looped waveform
which allows fitting of many different "instruments
" in a tiny space. Volume envelopes
are created by the tracker software's
own commands, and the modern PC
trackers feature dedicated envelope parameters
for each instrument.
While it is also possible to create drum sounds with the simple waveforms, a more common technique is using normal samples which are as short in duration and low in bytes as possible.
Because the amount of channels is kept to the bare minimum, C64-style fast arpeggios are used to form chords. This enables you to create a 3-note chord in just one track, and bring tears to every nostalgia freak's eyes. :)
Back in 1992-1995 when I was in the Amiga demo scene, I used to love chiptunes. While I did compose mods with big samples as well, chipmods were the thing for me. I used to finish at least one tune each day, resulting in a huge amount of small modules, some good and some bad. The high point of my (not-so-impressive) career was when a well known group Scoopex released one of my chips.
What is the appeal of chipmods from the composer's point of view? For me, it's simple. With very minimal sounds you only have your real mod-creating talent to rely on. You can't cheat using sampled multichannel loops and similar tricks, forcing you to concentrate on the melody and composition itself. Not that good music always needs a melody, but in the scene back then it was usually considered pretty important.