On Sunday 26 July 2015 11:13:52 Fons Adriaensen wrote: > On Sun, Jul 26, 2015 at 08:26:25AM -0400, Gene Heskett wrote > > > The jitter is, for some folks minor enough they do not notice it. > > It is however, there. Those of us doing CNC machinery control are > > very aware of it, primarily because when moving steppers at a decent > > rpm, excessive jitter costs us considerable in motion speed. > > Stepper torque about 3 or 4 hundred rpm depends hugely on having the > > step signals at a steady rate. > > USB by itelf is not to blame here, the way it is used to deliver > these stepper signals is. After all USB can transmit 64 streams of > 48 kHz audio samples (e.g. RME MADI-USB) reliably. This requires > some HW buffering of course, but so what ? There is no buffering in a cnc setup. The computer expects to get uptodate data from the machine which I have done at rates to 4,000 times a second, but normal is 1,000/second. Ditto sending data back to the machine, that data cannot lollygag around in a buffer someplace for 20 milliseconds while USB is searching for a suitable sock to put it in. Even 2 milliseconds will likely put the resultant part out of tolerance bad enough its a mantle decorator. > > If you try to exactly control the timing of pulses by controlling > the time they are actually sent by the SW then of course this will > fail. You need an approach similar to how audio is transmitted over > USB. > > It would be quite easy to use a standard audio interface to do this. > Take the analog output, apply a simple first order lowpass at 3 kHz > or so (to have a controlled FR, independent of the DA converter's > filter), then convert to square wave with a classical Schmitt trigger > circuit with minimal hysteresis (a small fraction of the analog signal > range). > > This way you can easily generate pulses up to 10 kHz or more and > control each edge with microsecond accuracy (provided the edges > are at least one sample period apart). All that remains is to > generate the required audio signal. Not trivial, but not rocket > science either. > > Ciao, Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
