ashleysmithd wrote:I had another quick question on flash ADC's, the diagram below shows that the output from the comparators will equal 0 when the input voltage is lower than the reference voltage. However, would that not mean there is always a clear divide between the output, meaning it's impossible to get 256 values? As per the diagram, there can only be 00000000, 11111111, or a divided mixture 00001111, 00111111 or 00000011 etc.
Apologies I'm fairly new to this, just trying to get my head around the basics.
ashleysmithd wrote:Wow thanks for the informative reply Jeff, that certainly clears a lot of questions up.
You mentioned sampling at 4x the colour sub-carrier frequency, is this rule typical in most ADC's? From what I've learnt PAL is normally sampled at 13.5MHz in accordance with the Nyquist-Shannon theorem (double and then add a bit), 4x the sub-carrier would make it 17.73MHz which I guess would produce an even more accurate sample and reduce noise.
Would you say that more composite PAL/NTSC to SDI ADC's are Flash ADC based or Successive Approximation? It actually make's more sense to use Successive Approximation ADC's as they sample in the range of hundreds of MHz, minus the 4095 comparators you would need for a 12-bit Flash ADC. I'm just trying to work what area to focus more on in my presentation.
Again, many thanks for the reply it was extremely helpful.
ashleysmithd wrote:Brilliant, thanks Jeff. I've gone with successive approximation, but have explained why within the presentation.
One component I have to try and squeeze into my (8-minute..) presentation is the the testing of the hardware's efficiency. I've done some looking around and found a few bits and pieces including an FFT spectral analysis and histogram statistical analysis, but is there any particular hardware dedicated to testing the efficiency of video ADC's that you know of? Or any particular methods you would recommend talking about? It would be good to do a small case study and explain some of the theory behind it.
PID_Stop wrote:...the second revision ACR-25B featured a digital time base corrector, and our station had one of these. As I recall, the TBC's ADC occupied four or five plug-in circuit boards, each something like 8x10 inches, each crammed full of 14- and 16-pin DIP ICs and a handful of 8-pin metal can ICs. One of our engineers had an unfortunate slip with a scope probe one day, and managed to short the +12 volt supply to the +5 bus -- wiping out dozens of ICs. We spent at least a week trying to repair the damage before finally giving up and ordering replacement boards from Ampex.
w9wi wrote:Hang on, you worked at WISC-TV back in the mid-1980s?
Exactly the same thing happened to us. We had two ACR-25s; one's TBC wasn't behaving properly, so one of our techs went probing in both of them comparing waveforms.
His probe slipped, doing exactly what you describe above.
Unfortunately, he was probing the good TBC at the time. Obviously it only remained "good" for a few more milliseconds...
PID_Stop wrote:That's why I hang on to ancient manuals for long-gone equipment: they often explain things that apply to far newer devices.
PID_Stop wrote:Nowadays the equipment is so reliable that when it does fail, the operators have no idea what to do to keep on the air.
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