Yeah. On 11/17/06, Christian Klippel <[EMAIL PROTECTED]> wrote:
hi steffen,Am Freitag, 17. November 2006 17:26 schrieb Steffen: > On 16/11/2006, at 23.53, Chuckk Hubbard wrote: > > My plan was to send PWM from my serial port, then use a low pass > > filter to obtain DC to > > control a Moog synthesizer's pitch. > > I don't quite understand how a PWM'ed signal put through a low pass > filter can give a voltage controlling signal. Care to explain? I'm > not even sure how to make the PWM signal at all - will it only be > using one pin? I might be daft. > imagine you have a always repeating loop from 0 to 255. now compare the wanted pwm value with the counter. if the counter is smaller, you turn on the pin, when it is bigger, you turn it of. that way you have 256 step cycle of always the same length, while the output is on (+5 volts), lets say, for 128 steps (50%), ond off (0 volts) for the remaining 128. in fact, it is a pulse with a fixed frequency but variable pulse width. if you feed that through a r/c lowpass, the resulting output is an average of 2.5 volts (50% of 5 volts). > I thought the easiest way to obtain such a signal from a serial port, > was to make a D/A converter fx with a transistor construction > (couldn't just find a diagram on the internet), such that each bit > sent to the port would translate to a DC signal. I know the precision > of the values might be sloppy and that there will only be 2^{# of > data pins on the port} different control values to mess with - but > still? > its not done with transistors, but resistors. there are serveral methods, but the most common used i think are binary-weighted resistor dacs, and r-2r dacs. the first one uses one resistor per dataline, increasing by powers of two (like, 500 ohms, 1kohm, 2kohm, 4kohm,....). the second one uses only two different resistor values, but twice as many resistors in total. the problem is the accuracy. a binary weighted dac you can build yourself only with very few bits. since the values rise in powers of two, the tolerance of the highest-valued resistor may be already more than the total value of the smalles resistor. a r-2r dac is a bit better, but still for many bits, it is too inaccurate when you build it with standard components. a small overview of how dac's are constructed (figure 1 and 4 are the the ones i talked about): http://www.analog.com/en/content/0,2886,761%255F795%255F91287,00.html note that while dac chips actually use these techniques, you will fail to build such a dac yourself with good enough accuracy. in the chips, they use highly precise, and often laser-trimmed, resistors (usually on-die). > I read about it in Delton T. Horns Music Synthesizers: A Manual of > Design and Construction, if i recall correct, where the author > suggested to use a S/H in connection with such a converter to save > CPU power (when one wanted to keep a given CV for a period of time > (faster then the port)). > this is only usefull if you want to digitize a changing signal. you use the s&h circuit (usually just a capacitor and a switch) to "freeze" the current value, so it stays stable while you digitze it. to digitize a signal, you basically need a dac, s&h and comparator. you load the s&h, set a value at the dac, and compare the two voltages with the comparator. you change the dac value so long until you have a match between the two voltages. then you discharge the s&h, and go ahead with the next sample. > > Christian Klippel, however, has kindly offered to create a USB->CV > > adaptor for me, something others have suggested but which has > > proven too confusing for me. > > That's very nice. I'd really love to make such a lil' donkey myself. > once it is built and working, i will put it online. greets, chris > _______________________________________________ > PD-list@iem.at mailing list > UNSUBSCRIBE and account-management -> > http://lists.puredata.info/listinfo/pd-list _______________________________________________ PD-list@iem.at mailing list UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list
-- "Far and away the best prize that life has to offer is the chance to work hard at work worth doing." -Theodore Roosevelt _______________________________________________ PD-list@iem.at mailing list UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list
