I am not a professional musician (just played on a music keyboard for ~8 years till I stopped ~10 years ago), but sometimes various ideas flow into my mind or I want to quickly transcribe (at least some parts of) heard music. The problem is with figuring out and writing down the stuff (or staff if you prefer, but literal staff is not needed, anything easily convertible to SMF or music XML would be fine). And doing it efficiently, obviously.
I lack sound recognition skill (cannot immediately reconstruct heard or thought sounds, have to find them out using trial and error method, which sometimes becomes more effective if I am getting into vibe of currently analyzed tune).
I have my old Casio CTK-750, but:
1. It's not next to my PC, so I have to relocate (it would be acceptable if not the other things below).
2. It has memory only for 2 songs.
(There are though many tracks, so I can abuse them going with single idea per track and muting all others during recording/playing. Cumbersome.)
3. After recording I still lack the score and next day I can easily forget how I played it before.
There is a MIDI stuff. Haven't tried it recently, because my current desktop doesn't have gameport or I don't have the cable anymore or both (don't remember now the exact state, sounds ridiculous, I know). But I tried it many years ago on my AWE32 (that was really expensive and huge back then!) and Cakewalk was getting some more notes than I played (some very high and very low tones), so it was practically useless. Never figured out whether it was bad cable (didn't have the second one), bad keyboard (didn't have the second one) or bad gameport (didn't have second sound one). Maybe I should buy MIDI-on-USB, cable, and recheck it nowadays.
Ad rem. I tried some applications (haven't checked them thoroughly, though) like musescore, noteedit, nted, rosegarden, milky tracker, (modplug on Windows also) and possibly some others I don't remember their names now, but apparently no one has sane recording/playing via keyboard feature. The best what you can get are shortcuts for notes, like C for C, D for D, etc. which are possibly ok for slow editing/tuning purposes, but far from being even acceptable for "live performances". Having some music keyboard mode is so obvious that I am amazed not seeing it anywhere.
Well, there is VMPK, but it doesn't have good enough way of mapping tones (tones -> keys instead the other way, which means you cannot have same tones on two different keys) and I failed at seeing/hearing its output in musescore, i.e. did set the connection via qjackctl's connect dialog box, but musescore doesn't have any way to choose the input provided to it in IO tab. Musescore is not the best target actually, as it is ok only for storing notes alone, without timestamp and duration information.
(I have voluntary preemptable Linux kernel right now in my debian squeeze, so it's suboptimal for MIDI stuff and audio in general, but still have to say that latency I get in VMPK is extremely awfully annoying.)
So tell me HN, what are your music prototyping solutions, suggestions, tips, favorite apps, etc.?
My first AskHN thing here, hopefully it's not too long and I'll get some insightful comments.
http://en.wikipedia.org/wiki/Tracker_(music_software)
I have yet to find a faster way from idea to music. It's clunky to get into until you spend some time to learn the idea and build up a collection of music and wrap your brain around the entire concept, but it's a few decades old a very mature concept with tens of thousands of works created with this method.
Here's a site that streams tracked music 24-7 http://www.scenemusic.net/demovibes/
Some of it is quite good (thought the quality obviously ranges quite a bit though both talent and technology).
If you want to see the music with a visual production try http://demoscene.tv/
I've been working with a piece of software called MadTracker 2 for a while, but I'm about to transition to Renoise (MadTracker 2 is long since a dead project) which is available for Windows, OS X and Linux and is about as professional as a piece of software has any right being.
http://www.renoise.com/
In a rush I can crank a reasonable tune out in a few hours. I've heard of at least one major artist cranking out an entire album (vocals and all) in less than a week.
http://hunz.com.au/
Most of the modern trackers will let you render individual tracks to file for final downmixing.
Note: These are most definitely not intended for live performance, though a few folks have used them for that purpose.