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nuckolls.bob(at)aeroelect Guest
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Posted: Fri Oct 20, 2017 4:06 pm Post subject: Constructing an automatic RCA video camera feed splitte |
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At 12:37 PM 10/20/2017, you wrote:
Quote: | --> AeroElectric-List message posted by: "Eric Page" <edpav8r(at)yahoo.com>
nuckolls.bob(at)aeroelect wrote:
>
Bob, I'm having trouble making sense of this graph. I read the National Instruments page ( http://www.ni.com/white-paper/4750/en/) it comes from, but I'm not sure I've got it straight. See if this sounds right...
The Y axis ("IRE") is a somewhat arbitrary unit. |
Yeah, it's dimensionless . . . like dB in
audio, it describes
a ratio of voltages for video painting a
max white, black, blacker-than black, burst
amplitude and sync amplitude.
Quote: | That is, zero IRE is "usually 0V" but (by implication) it doesn't have to be. The zero-reference and blanking level in the signal is set by the "back porch" following the horizontal sync pulse, so any DC offset within reason is automatically compensated by the nature of the format. |
I think I understand your assertion . . . I've seen
various ways for video to be transported on a DC
stable reference. The time base correctors I used
to build had three points of dc restoration as
the signal progressed through the system. One is
advised to do this so that variations in gross
video energy don't manifest as momentary
blooms or potholes in observed luminance.
You can find video being managed at various
voltage levels, 1 v pk-pk in 75 ohms was adopted
for shipping video around studios and stations
on coax. This is equivalent to the legacy "0
VU" in audio systems being pegged at about 1.25
vrms across 600 ohms.
On further reflection (it's been 30 years or
so) I decided to refresh my recollection about
the 1 v pk-pk number. Found this little diddy
on RS-170, the industry convention for television
signals.
http://tinyurl.com/382a25m
According to this paper, the 1v value was originally
crafted in the black-n-white days where 1v = 100
IRE
RS-170 was the original "black-and-white" television signal definition, per EIA. The original standard defined a 75 ohm system and a 1.4 volt (peak-to-peak, including sync) signal. Signal level specifications form RS-170 were: White = +1.000V; Black = +0.075V: Blank = (0V reference): Sync - -0.400 Volts.
In this world, the conversion would be 10Mv/IRE
Read more at: http://www.epanorama.net/documents/video/rs170.html
So after you took a 'clean' white level of 100
IRE and added 40 IRE of sync, you had a 1.4 volt
pk-pk signal in 75 ohms
When color came along, the various levels of luminance
from black to white COULD be enhanced with some hue and
saturation of COLOR carrier that might add another 40
IRE to the white level. So add sync to that and you're
up to 180 IRE or perhaps 1.8v pk-pk.
It will be interesting to see how the cameras are supplied.
Most monitors are able to accept and accurately display
signals from 0.5 v pk-pk to perhaps 4 volts . . . after
all, it's just a 'volume control'.
The the ratio of elements in the audio stream
remains the same irrespective of actual power
or transmission impedance. IRE is video analog
to audio VU or DB.
Quote: | As long as the signal is pretty close to 1V pk-pk, it doesn't matter if it's centered on 0V -- or, as we're proposing, on 2.5V -- the zero point is wherever the "back porch" sits.
So, if you find a signal from your test camera that's riding somewhere between 0V and 3.3 or 5V, and we power the CD4066 from a single 5V rail, we should be in good shape.
Since the IRE scale spans -40 to +120 in the NTSC format, and an NTSC signal is supposed to be 1V pk-pk (into 75 ohms), then you could say that 1 IRE = 12.5mV.
Is that in the ballpark? |
Yup, the cameras we're looking at are fitted
with coaxial termination that is, no doubt,
intended to work smoothly with the constellation
of video products in a 75 ohm world.
It will be interesting to see what the cameras give us.
Bob . . .
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Eric Page
Joined: 15 Feb 2017 Posts: 243
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Posted: Fri Oct 20, 2017 11:02 pm Post subject: Re: Constructing an automatic RCA video camera feed splitt |
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nuckolls.bob(at)aeroelect wrote: | Yeah, it's dimensionless . . . like dB in audio, it describes a ratio of voltages for video painting a max white, black, blacker-than black, burst amplitude and sync amplitude.
. . . [SNIP] . . . |
Thank you, Bob. I think all of that makes sense.
Eric
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Eric Page
Joined: 15 Feb 2017 Posts: 243
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Posted: Fri Oct 20, 2017 11:03 pm Post subject: Re: Constructing an automatic RCA video camera feed splitt |
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Gents, may I suggest that we abandon the dual-CD4066B approach and substitute a single CD4051B instead? It's an 8:1 multiplexer whose inputs are selected by 3 digital control pins referenced to 0V.
http://www.ti.com/lit/ds/symlink/cd4051b.pdf
This will replace two 14-DIP ICs with a single 16-DIP, save 5 PIC I/O pins, solve any worries about control signals referenced to a negative rail, and eliminate concerns about output bus contention.
We can easily return to an 8-pin PIC:
Vdd
Vss
Control A
Control B
Control C
Left Switch
Right Switch
Program Mode LED -or- Spare
If it turns out the cameras' output is all above 0V as Bob suspects, I can also eliminate the negative rail components. I might be able to get this thing into a D-25 backshell again, even with 8 channels...
Bob, do you have a part number for the D-25 backshell you posted a drawing of recently? It looks bigger than the ones I've been able to find on Digi-Key. Also, the dimensions of the biggest PCB that will fit, assuming it's mounted between rows of solder cups?
Thanks,
Eric
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