I really do like to test myself :/
My plan (coding wise) for this weekend was to get stuck into some effects in the playback of the tracker.Â I added a simple jump and also sample looping quite easily (although they have both proven how code and fix is a terrible development methodology 🙂 a re-write is on the cards.. but still a prototype 🙂 ).Â For my next feat I though adding volume adjust in would be easy enough, plus the module I am working with features a gradual volume increase over a looped sample at the start of the tune, so a good test.
The volume setting for modules is a range, 0-64, 0 being off and 64 being full volume, 0-100%.Â My initial thoughts were a lookup table, so for any sample value I would have a table of all 64 volumes, (I’d ommit 0% and 100% for obvious reasons), alas this would end up with a table of around 16KB, which whilst not a huge amount of RAM, is more than double the cache RAM of the RISC CPU, which as this is where I want this code ro reside and I don’t want it faffing about on the main bus any more than it needs too, pretty much rules that option out.Â I thought on it a bit more, and played with some excel spreadsheets (spent most of my coding time playing with numbers in excel and drawing line graphs etc!).Â I could half the table if I only considered samples of 0-127 and then used this for negative values also, I could possibly shrink it further if I only consider 33-63 as the values of interest as I could use a right shift to give me 50%.. these solutions all seemed workable, but that lookup table was still around 1KB at the most reduced size I could think of, more than I was hoping it could be.
So machine off and some PS3 time, films and Dead Space 2 (both relaxing and stimulating 🙂 ).Â I then had an idea, which on paper looks like it will work perfectly, and given the range 0-64 makes perfect sense!Â I am probably coming at this backwards but figuring this stuff out for myself is the one thing I do this kind of thing for, its where the buzz starts, ending with the implementation of working code.Â Quite simply using arithmetic right shifts on the sample, I can generate 6 volume levels easily (7 if you count full volume), 50%, 25%, 12.5%, 6.25%, 3.125% and 1.5625%Â now 6 doesn’t sound at all like 64 levels of volume adjust… however, if you combine those 6 in the appropriate combinations you can produce any of the valid volume levels in the range of 0-64!Â But how to determine the combination? simple, the binary of the volume level value indicates which of those volumes need to be added together to produce the desired final volume!
So 61 would be 95.3125% in binary 61 is 111101 so we add together 1.5625+12.5+25+50 = 95.3125Â Â HURRAH! 😀
Of course this will require the original sample is processed a maximum of 6 times (with the output of each pass being added together), but this is not a huge amount of processing, even for multiple channels.Â I may yet find a neater approach, but this one feels right to me at the moment.Â Now I just have to wait until after work tomorrow before I can implement it! curses! 🙂