g1501002@bohr.gbar.dtu.dk (Mekanik of Fysisk Modellering) writes:
>If you know just ANY use of the TEST bit in $d404 (sid+4) please let me know. I'm coding a music tool, and if the TEST bit can be used for any reasonalbe purposes I'd really like to know.
The test bit can be used to reset the waveform generator. If you mix different waveforms such as triangle, square and noise, the waveform can lock up. Using this bit, you can reset the waveform to work again. (When you mix waveforms, the output is a logical AND of the different values)
Another use is when you use the noise waveform to generate random numbers by reading the waveform output of voice 3. By setting this bit, you reset the noise-generator, and thus you can reproduce the random numbers by chosing exact frequencies and synchronizing the reading.
Generally, the test-bit can be used to control the waveform very exactly.
I have studied the behaviour of the SID by mapping the output of the waveform of voice 3 to a sprite's-xcoordinate, effectively doing an oscilloscope. Of course you need to strecth the sprite. Basically this piece of code does the trick:
loop lda $d41b ;Or is it $d41c?
clc
adc #24 ;To enter the screen
sta $d000
rol $d010 ;And set high-bit
lda $d012 ;Read raster-value
sta $d001 ;to stretch the sprite
jmp loop
You would want to use an interrupt routine to play the music, and at the same time, it's a good idea to open the bottom border to have a larger area of view.
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SID Chip ID Number?
From : Ralph Strauser
Nathan Strong wrote to All...
NS> What's the ID number (one that would be on the actual IC) of the C64 SID sound chip?
Well I have sitting in front of me the following info. The sid for the older 64s is a 6581 and requires a 12v supply voltage the sid in the 64c with the narrow board is the 8520 and requires a 9v supply and the sid in the 128 is the 6582. My info may be slightly off but this should give you all three possible chip numbers.
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Craig Chamberlain, etc.
From : David Veatch
To : Jack Vanderwhite
Hi, Jack:
JV> Compute's First book of Commodore 64. Compute's second boook of Commodore 64
DV> Is one of these two books the one that has the type-in listing for Craig Chamberlain's SID composer program? If so, I might be interested.
JV> No, you are thinking of Compute's "All About The Commodore 64, Volumes one and two". The Sid Composer program is in Volume Two.
OK, thanks for that info. I'll know what to look for from now on!
JV> Interesting facts regarding these two volumes:
1. While Craig Chamberlain is the listed author of the two volumes there is someone elses name listed as author of the SID composer program.
2. I believe there is another book formerly published by Compute and authored by Craig Chamberlain which deals only with music. In this book there is a Sid Composer program, I believe labeled "Advanced", that is attributed to Craig as author. Rights to this book have been purchased by John Brown of PARSEC and is being republished in an edited form.
3. Recently someone contacted General Media, parent company of COMPUTE before it was sold. They were told that rights to the "All About The Commodore 64" Volumes and the material in them has reverted to the original authors.
I sorta feel embarrased that I don't already have a copy of the book!
Craig Chamberlain lives here in the Detroit suburbs (Birmingham) and is/was a member of our Commodore users group. In fact, when I was a sysop of the club's bbs, Craig unexpectedly sent me a disk full of SID music - right after the book came out - and he asked me to upload some of the songs to the bulletin board (which I did).
I've wanted to check out the program that composes the songs for quite some time now, but never got around to obtaining a copy. I've amassed about 20-30 double-sided disks full of the SID music over the years. It would be nice to be able to see how they were written, etc.
Thanks for telling me which book it is to look for. At least that's one step in the right direction! :-)
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C64 sound filter
From : alan.jones@qcs.org
Someone asked about the C64 sound filter and its calibration.
The filter uses two external matched capacitors and since the actual capacitance varies with each unit you will have to calibrate your filter to use it properly. The Transactor 6-4 contains an article on this subject. The Commodore docs lead you to believe that their is a fixed affine relationship used to program the filter. Unfortunatly, the values they suggest for setting the filter is just a starting point. It might be possible to switch capacitors or add trimmers to adjust you particular unit to best agree with the Commodore values. In the old days you could buy a C64 from a local discount store and keep returning it untill you found one you liked.
I don't care much for music and sound on my C64/128 computers. However, I am a controls engineer and I can figure out filtering and signal processing. I have never found suitable engineering type data on the C64 programmable filters. Many years ago I wanted to use the C64 as a cheap sinusiodal signal generator. I wrote a simple program in COMAL to read the game paddles, print the vioce frequency on the screen and send the sound to my stereo system. I chose a wave form with most of the signal in the primary frequency and filtered the higher harmonics. I set the filter for maximum peaking. To get higher frequencies I syncronized two voices with a wave forme to maximize the second harminc frequency and tried to filter out the other frequencies. To calibrate the filter input values I set a filter frequency value and swept the SID frequency. The signal was fed into atape recorder with analoge VU meters (convieniently calibrated in Db)
I recorded and plotted signal strength vs. frequency to locate the frequency peak. I repeated this for different filter values and eventually I had a suitable calibration of my filter. Note that I calibrated the filter settings for the peak or corner frequency and for other applications you might want to use the -3Db point instead. My filter just would not work at the higher frequencies that I wanted.
