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the dude
Joined: 11 Sep 2013
Posts: 179
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| Posted: Tue Feb 23, 2016 11:28 pm Post subject: |
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The article is named "bandwidth (signal processing)" because the term bandwidth is also used for digital data processing and communication, and there it means a transfer rate or capacity of a communication chanel. There is no article "bandwidth (video systems)". This for sure is covered by signal processing.
In digital signal processing the term is "sample rate". And this is at least 2 times the frequency. A sample rate is NOT expressed in Mhz, the equivalent of this is MS/s, Megasamples per second. Computer displays use the term "pixel clock" instead of sample rate and this is expressed in Mhz, but that is a exception that was made because people are more familiar with frequencies and it makes it more intuitive to calculate timings. It is absolutly false to use this together with analoge bandwidth.
Regardless of what you refer to, the whole world is using the terms in this way. And by using the unit Mhz for a analog circuit you say that you refer to a frequency. Especially if you use the term "bandwidth".
This is simply false advertising. And for sure you would not rate your projector this way if the pixelclock would be half the analog bandwidth.
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mp20748
Joined: 12 Sep 2006
Posts: 5689
Location: Maryland
TV/Projector: 9500LC Ultra / Super 02 and 03 VIM
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| Posted: Tue Feb 23, 2016 11:44 pm Post subject: |
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| the dude wrote: | The article is named "bandwidth (signal processing)" because the term bandwidth is also used for digital data processing and communication, and there it means a transfer rate or capacity of a communication chanel. There is no article "bandwidth (video systems)". This for sure is covered by signal processing.
In digital signal processing the term is "sample rate". And this is at least 2 times the frequency. A sample rate is NOT expressed in Mhz, the equivalent of this is MS/s, Megasamples per second. Computer displays use the term "pixel clock" instead of sample rate and this is expressed in Mhz, but that is a exception that was made because people are more familiar with frequencies and it makes it more intuitive to calculate timings. It is absolutly false to use this together with analoge bandwidth.
Regardless of what you refer to, the whole world is using the terms in this way. And by using the unit Mhz for a analog circuit you say that you refer to a frequency. Especially if you use the term "bandwidth".
This is simply false advertising. And for sure you would not rate your projector this way if the pixelclock would be half the analog bandwidth. |
So you disagree with what's in the manuals and what the manufacturers of the pattern generators and processors use as a reference or rule... there should be no such thing as "Analog Pixel Rate"
And what about the calculator, is it wrong to use "bandwidth" as well?
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El Duderino
Joined: 23 Jan 2011
Posts: 4653
Location: Portland, OR
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| Posted: Wed Feb 24, 2016 5:55 am Post subject: |
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| mp20748 wrote: | | So when I put in 1920X1440 75HZ the calculated results are: 311mhz[/b] |
And just above the result part of the calculator, it says that the required analog video bandwidth is the -3db point.
FWIW, I agree that when discussing analog signal bandwidth, it is defined as the -3db point of the analog signal chain. Terms like an 'Analog Pixel' or 'Analog Pixel Clock are an oxymoron. If it's digital BW, it should be expressed in Mb/sec, MB/sec or MegaPixel/sec not Mhz.
To clarify, I haven’t a clue if your analog mods enable >300Mhz BW or not, but I do agree with others here that if you’re claiming they do, then I think it’s reasonable to expect a oscilloscope measurement of an analog signal (sine, square, triangle, trapezoidal, ... ) showing that relative to a lower frequency, the >300Mhz signal amplitude is </= -3db down from it.
Last edited by El Duderino on Wed Feb 24, 2016 7:29 am; edited 1 time in total
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redfox001
Joined: 16 Mar 2009
Posts: 2257
Location: The Netherlands
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| Posted: Wed Feb 24, 2016 7:29 am Post subject: |
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| the dude wrote: | | Maybe a pin diode is not well suited. They have ultra low capacitance, but high resistance on high frequencies, wich maybe include transients. Not sure, normally they are used for RF signal switches. |
LIMITERS
PIN diodes are sometimes used as input protection devices for high frequency test probes. If the input signal is within range, the PIN diode has little impact as a small capacitance. If the signal is large, then the PIN diode starts to conduct and becomes a resistor that shunts most of the signal to ground.
https://en.m.wikipedia.org/wiki/PIN_diode
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redfox001
Joined: 16 Mar 2009
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Location: The Netherlands
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| Posted: Wed Feb 24, 2016 7:54 am Post subject: |
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| the dude wrote: | | Maybe a pin diode is not well suited. They have ultra low capacitance, but high resistance on high frequencies, wich maybe include transients. Not sure, normally they are used for RF signal switches. |
I think I read it is to slow on transients?
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redfox001
Joined: 16 Mar 2009
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Location: The Netherlands
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| Posted: Wed Feb 24, 2016 8:44 am Post subject: |
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| the dude wrote: | Something on the diodes:
It seems i found the only datasheet that says they are 1.5 pf 
Its form PanJit, Taiwan. All other (checked NXP, Infinion, Fairchild, Diodes, Diotec, Transys, Zetex, Siemens) rate them 5 pf at 1 Mhz. |
There is a picture of capacitance against reverse voltage at 10V it is below 1pf. 0.9pf seems a realistic vallue for the bas21.
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redfox001
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mp20748
Joined: 12 Sep 2006
Posts: 5689
Location: Maryland
TV/Projector: 9500LC Ultra / Super 02 and 03 VIM
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| Posted: Wed Feb 24, 2016 9:24 am Post subject: |
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| El Duderino wrote: | | mp20748 wrote: | | So when I put in 1920X1440 75HZ the calculated results are: 311mhz[/b] |
And just above the result part of the calculator, it says that the required analog video bandwidth is the -3db point.
FWIW, I agree that when discussing analog signal bandwidth, it is defined as the -3db point of the analog signal chain. Terms like an 'Analog Pixel' or 'Analog Pixel Clock are an oxymoron. If it's digital BW, it should be expressed in Mb/sec, MB/sec or MegaPixel/sec not Mhz.
To clarify, I haven’t a clue if your analog mods enable >300Mhz BW or not, but I do agree with others here that if you’re claiming they do, then I think it’s reasonable to expect a oscilloscope measurement of an analog signal (sine, square, triangle, trapezoidal, ... ) showing that relative to a lower frequency, the >300Mhz signal amplitude is </= -3db down from it. |
I'm very aware of the -3db video bandwidth. and for the record only a few days ago I posted elsewhere on this forum about a near flat 1920X1080P bandwidth for the mods I have now. We understand well the -3b RULE and that goes without saying. What I'm asking is what is the RULE in the literature, why do the processor manufacturers and Display System maker NOT use the -3DB RULE
And I also agree about the proper measuring of the video chain, where I also posted some days ago that the best way to determine bandwidth IN A VIDEO CHAIN is to use a scope and square wave. But a video chain is different from the final RULE of what is on the screen...
If my video chain is -3db 200mhz, what is the DISPLAYED bandwidth?
And the above the displayed bandwidth, it states what the -3db should be, but why is the "bandwidth" rating of the bandwidth calculator and that of what is on the generators and all the other gear a oxymoron or totally irrelevant?
My point here is RULE. Are we talking about two different things here...
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redfox001
Joined: 16 Mar 2009
Posts: 2257
Location: The Netherlands
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| Posted: Wed Feb 24, 2016 10:27 am Post subject: |
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This one is used on the new marquee neckboards. Two of these shouls do 200V rated 1.5pf but perhaps lower at reverse voltages?
https://www.fairchildsemi.com/datasheets/MM/MMBD7000.pdf
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mp20748
Joined: 12 Sep 2006
Posts: 5689
Location: Maryland
TV/Projector: 9500LC Ultra / Super 02 and 03 VIM
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| Posted: Wed Feb 24, 2016 11:00 am Post subject: |
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They are the standard ones that were almost used by everyone back then. There is also better ones I had samples of some years back that had much lower capacitance ratings. And as mentioned with the newer VDC neck boards, these diodes are not doing what they were put there for. They are totally useless on the newer VDC neck boards.
With the samples I got some years ago, I forgot who I got them from, but was told they were much faster than the BD7000's and provided better clamping/protection, but none should be thought of to work 100%. Not sure why some think these diodes are perfect protect devices. That is why its ALWAYS best to talk to the manufacturer with questions when you request samples. Intersil used to always have an engineer contact you when you requested samples from them. That's when you really get to learn a lot about better using their device.
I have the better solution for the VDC boards, that also allows very low capacitance.
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redfox001
Joined: 16 Mar 2009
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Location: The Netherlands
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| Posted: Wed Feb 24, 2016 1:54 pm Post subject: |
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However I measure the video signal going into the driver there is a roll off (turnes out to be the measuring cable)
But I compared the use of coax from moome to switcher with the standard white little cable. Well I do not know what I see but it looks a little cleaner with coax.
Coax
 Untitled by Radio Head, on Flickr
 Untitled by Radio Head, on Flickr
white cable
 Untitled by Radio Head, on Flickr
 Untitled by Radio Head, on Flickr
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Last edited by redfox001 on Wed Feb 24, 2016 4:55 pm; edited 2 times in total
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redfox001
Joined: 16 Mar 2009
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| Posted: Wed Feb 24, 2016 2:01 pm Post subject: |
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Coax from moome to switcher. Measuring directly on the video 50 Ohm terminated.
 Untitled by Radio Head, on Flickr
 Untitled by Radio Head, on Flickr
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mp20748
Joined: 12 Sep 2006
Posts: 5689
Location: Maryland
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| Posted: Wed Feb 24, 2016 2:13 pm Post subject: |
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| redfox001 wrote: | However I measure the video signal going into the driver there is a roll off.
But I compared the use of coax from moome to switcher with the standard white little cable. Well I do not know what I see but it looks a little cleaner with coax.
Coax
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Absolutely... first time knowing of this, but for sure the COAX should have been a part of that upgrade (Moome).
Video signal paths are very finicky. Coax, especially when going more than 4 inches should ALWAYs be used in these circuits, especially for the quality that we're looking for and anything above 720P
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redfox001
Joined: 16 Mar 2009
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Location: The Netherlands
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| Posted: Wed Feb 24, 2016 2:19 pm Post subject: |
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It is something I also see on the screen but did not know what exactly changed. I am going to solder the coax to the moome on the moome side.
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redfox001
Joined: 16 Mar 2009
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| Posted: Wed Feb 24, 2016 4:52 pm Post subject: |
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Ok I have been doing a number of precise measurements on the driver. I found out that it is really in the length of the wire that is used to measure. So I kept them short as possible and used the same length at the beginning and the end of the driver. I do measure a drop and there is peaking comming from the Moome as was established before.
 Untitled by Radio Head, on Flickr
Beginning driver
 Untitled by Radio Head, on Flickr
End driver
 Untitled by Radio Head, on Flickr
I think the drop can be explained by the AD835.
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the dude
Joined: 11 Sep 2013
Posts: 179
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| Posted: Wed Feb 24, 2016 5:22 pm Post subject: |
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This time Mike is absolutly right 
I thought about changing the video cables annd connectors also. On the mainboard there are coax cables connected to the board connectors, but those connectors are not good for video i think. Also the connection between the switcher and the driver should use better connector. Also the neckboard. Some mini coax connectors of matched impedance should be better there. None of the stock connectors have well defined impedance. Just something more on the list...
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redfox001
Joined: 16 Mar 2009
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Location: The Netherlands
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| Posted: Wed Feb 24, 2016 5:49 pm Post subject: |
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Yes have to look into it but on the switcher connector print there are also limitting diodes. With a good connection they probable can be removed too. Bav99 1.5pF
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redfox001
Joined: 16 Mar 2009
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| Posted: Wed Feb 24, 2016 6:01 pm Post subject: |
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Wow that 1n4148 is 4 pF it is at the end of the driver and beginning and on neckboard.
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Last edited by redfox001 on Wed Feb 24, 2016 6:04 pm; edited 1 time in total
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the dude
Joined: 11 Sep 2013
Posts: 179
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| Posted: Wed Feb 24, 2016 6:01 pm Post subject: |
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Early 1209s don't have them. I think all clamp diodes except those on the neckboards can be removed. I did that for the switcher.
All diodes on the inputs aren't required imho. We use internal video moome card or the HD fury, there aren't long cable runs. The diodes on the imputs provide some protection against different ground potential that can be caused by shorts in electric installations or EMP inducted in long cables during thunder storms. Today we don't need that. If a lightning strikes ocours that can kill the input with those short cables used today, it can kill anything else.
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the dude
Joined: 11 Sep 2013
Posts: 179
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| Posted: Wed Feb 24, 2016 6:04 pm Post subject: |
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Remember that 4 pf is not too bad for this. This is a low impedance circuit with low voltage. The neckboard video amp output is high voltage, so the same capacity will require much more energy to be charged to that voltage.
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