On Fri, 27 Mar 1998 08:33:44 +0700 Ari Wahyudi writes: >Sorry, it's realy off topic.. >Please help me, I realy need to know this.. > - why video signal from a camera does not contain DC component.. > is there a simple DC restorator circuit? The simplest DC restorer is just a diode to clamp the sync tips to a certain level. The video signal, coupled by a capacitor, is applied to a fairly large resistor and diode in parallel, say to ground if you want the sync level set at (1 diode drop below) ground. The resistor tends to drag the video level negative, ensuring the diode will conduct on each sync pulse and keep the video from going too negative. The 'black' and 'white' DC levels are then approximately the same regardless of picture content. A more advanced DC restorer uses an analog switch to clamp the reference blanking level that follows the sync pulse to the proper voltage. The analog switch is driven by a pulse derived from the sync/timing circuit. > - I have build a video acquicition system, the resulting image was >not > so satisfied me.. there're some pixel shift to the right or left.. > I use 8 MHz frequency for sampling.. is that normal? > how's this can be happen..? how to eliminate that? > The sampling frequency needs to be an exact multiple of the horizontal rate. If it is not, the position of the samples will drift left or right on the picture as you describe. Usually this is done by dividing the sampling frequency down to the nominal horizontal rate and comparing it to the horizontal sync pulses. The output of the phase comparator controls the sampling frequency clock in standard PLL fashion. Other outputs from the divider should be used to control the sample timing. This way the PLL will cancel much of the uncertainty involved in synchronizing the process to every sync pulse, especially if the sync pulses are noisy. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]