Hello, When bt878 AGC is enabled (bit 4 of ADC register 0x068), the chip provides its own AGC mechanism. In this situation, if you switch inputs AGC starts again trying to raise the optimal level for this input. This process takes time, and that is why you see "brightness" fluctuations just after switching and during several ms (up to even more than one sec. is my experience). It is possible to disable AGC, but you must then provide an external reference voltage to the bt878. This is all explained in the chip's documentation, explanations on ADC interface register (0x068). Page 5-23 in my docs. Hope this helps, Miguel On 15 Sep 2001 21:53:19 +0400, Dmitry Alexeyev wrote: > Hello. > > Maybe I haven't exactly described the video source. It is really the > problem of Gain Control. > I have _extrenal_ multiplexr ( and chip has it's own too - internal ) > > so I have: > > cameras: > 1: --- | > 2: --- |M ---- [TV input of chip ] --- > .. --- |U > 8: --- |X > So i receive only one frame at the moment. > When chip detects blank frame it increases brightness ( not at the > moment, it lasts some frames ). > So it is really AGC. > > Regards, > Dmitry. > > P> On Thu, 13 Sep 2001, Dmitry Alexeyev wrote: > > > >> Hello. > >> > >> I have BT878 card and external multiplexer with several monochrome cameras > >> connected to it. If one camera is off ( blank screen ), bt878 > >> automaticaly increases brightness, so I can see nothing on frames from > >> other cameras. > >> How can i turn off AGC ? > >> > >> Dmitry > >> > > P> Seems your problem is the multiplexer not the AGC. > P> If properly clamped to black level AGC will probably > P> no longer interfere. > > P> This of course requires minimum expertise to > P> clamp the multiplexer input to black level > P> when camera is off (using a capacitor, resistor > P> or combination, after analysing the input circuit). > > P> However, if you have such ability it must be > P> pretty simple and "safe" once it's just a simple > P> clamping. (clamping activation via camera power) > > P> But with today VLSI "closed source" trend things > P> simple like this may even be impossible. :-( > > P> Open source hardware too will be great.... > P> schematics, information access... or better > P> put the closed hard crap in trash together > P> with the closed source soft too. ;) > > P> No need to mess the AGC that seems to be not > P> the cause but just the symptom of a floating > P> input signal. > > P> Hope this helps. > > P> -- > P> HOM: http://www.momentus.com.br/users/hook/ > P> GPG: http://www.momentus.com.br/users/hook/GPG-PauloCastro.asc > > > > _______________________________________________ > Video4linux-list mailing list > Video4linux-list@xxxxxxxxxx > https://listman.redhat.com/mailman/listinfo/video4linux-list
