|I won't go into a great amount of detail about ACID's specifications here as there is a fair amount of information on it already on Sonic Foundry's page, but I will mention some of the (IMO) cooler features of ACID that make it a worthwhile tool for looping.
ACID is an audio/loop based sequencer which allows you to work with audio clips in much the same way you would work with MIDI tracks in a sequencer. The user interface is first rate, and I was able to sit down and start working with it immediately and intuitively. The majority of the main functions (clip browser, track view and clip properties) are all immediately available in the main window for quick access.
Some of the features which make ACID particularly suited for composing with audio clips are (in no particular order):
-Realtime time compression/expansion of audio clips. The time stretching algorithm employed in ACID works quite well and sounds good within it's limits. Almost all time compression/expansion algorithms suffer from aliasing when working at extreme time expansion/compression settings, so this is nothing shocking. It does quite well at realtime processing of audio without noticeable latency though.
-Realtime pitch shifting of audio clips. This is a really nice feature when working with audio. The pitch shifting algorithm sounds quite good and functions as one would expect. You can get some interesting sounds out of it when shifting by an octave or more. Shifting tuned sounds (especially sustained sounds) by more than a few notes up or down sometimes causes unwanted aliasing and distortion in the sound.
-ACID calculates (in most cases) the BPM of a loaded audio clip automatically. I say in most cases because it does not do well with clips which do not contain a lot of rhythmic attacks and peaks in the audio. This is to be expected. The "hit points" within an audio file may be easily adjusted for a better fit to the rhythm of the track as well as the attack transient detection and stretching algorithm. Unfortunately, there are only two stretching algorithms included. One for "general" sounds (e.g. drum loops) and one for sustaining sounds. In some cases, there is noticeable "throbbing" in sustained sounds such as strings, etc. but you can generally adjust the "stretch markers" to remove most of this effect (not always though). It would be nice to have more algorithms better suited to different types of material.
-Ability to draw volume, panning and effects curves curves directly on the audio. -Ability to add ActiveX effects plugins
-Multichannel output (in the "pro" version)
-Synchs to/transmits MIDI clock
-Ability to export tracks or clips as .WAV files w/ the stretching and pitch shifting applied (I often use it for this feature alone and export the audio to other programs for further processing).
-You can set the key for a piece and ACID automatically transposes audio clips that are defined as "pitched" w/ a root note. Clips can be saved w/ the additional ACID header information, so you only have to define this once. You don't need to redefine all your clip parameters for every piece.
-You can define clips as "loops", "one shot sounds" or as hard disk audio (played back linearly)
All that said, ACID is VERY much geared towards the production of dance music (as it's name would imply). However, it can certainly be subverted for use in more esoteric pursuits. The time structure is limited to #/4 time bases, but you can just as easily ignore the time signatures. ACID is as equally useful for doing ambient electronic and even orchestral type music as it is at doing dance music. If you are trying to do odd time signatures though, ACID might not be a good choice. ACID is also a great tool for building up very dense, textured compositions. I generally use this tool for laying out rhythm tracks, but I have also used it on numerous occasions to compose entire pieces. I don't know what the general genre makeup of the Looper's Delight list is, but the included sample disc consists almost entirely of dance oriented samples. They are fun to play with, but not very useful (unless of course you write dance music ;) ). I simply discarded these in favor of my own material. Your mileage may vary depending on what you want to do with ACID.
The playback performance of ACID is also quite good. I have not run into any situation (even with 50-60 tracks on a P200 MMX) where realtime performance or playback as suffered to any degree. (Note: this is not 50-60 simultaneous tracks, I usually have 12-16 simultaneous tracks going at once.) ACID also does recording, but I have not had any experience with it, since I generally just import loops and audio from other applications. Of all my audio applications, this one seems to be the most solid and rarely (if ever) crashes and has no noticably glaring bugs. All in all, rock solid I would say. Again, your milage may vary based on your system configuration.
Overall, I was very impressed with ACID. It has become a valuable tool in my audio and composition arsenal. My only complaints are that it has an inflexible (4/4 oriented) time structure and that it does not have a wider variety of time stretching algorithms.
Richard For Cerebellum/A Most Happy Sound