Sensing CCTV baseband video?
austin_48111
Posts: 7
I have project in which I'm attempting to use CCTV video over a pair of baluns (balanced/unbalanced converters) to convert from coax to unshielded twisted pair into a BASIC Stamp. This is an analog signal theoretically at 1Vp-p. I'm receiving utterly inconsistent results with this, likely because of it's nature as an analog signal. Though it is 1vP-P I sense almost zero voltage over a multimeter, in the range of .374v at most. All I see on the BS2 is floating results (high, high, high, low, low, low, high, high, high, etc). If you have any experience working with baseband video (RGBHV, component, non-broadband composite) in any fashion you would likely have some experience that applies here.
Not knowing what I'm doing and putting a full wave bridge rectifier in between gets me 5.74v but again I acheive utterly inconsistent results. I thought that may convert it all to a rounded off DC voltage but clearly I'm incorrect. I have no oscilliscope to determine what the root cause of this issue is.
1. Do I need to amplify the signal?
2. Do I need to convert it from analog to digital signaling?
3. Would I be better off using a third party IC designed to sense or convert this? Any clue where I could find one?
4. Would I be better off avoiding a balun and staying with CCTV over coax?
I'm about the greenest rookie on this forum so please keep it slightly above laymans terms.
Not knowing what I'm doing and putting a full wave bridge rectifier in between gets me 5.74v but again I acheive utterly inconsistent results. I thought that may convert it all to a rounded off DC voltage but clearly I'm incorrect. I have no oscilliscope to determine what the root cause of this issue is.
1. Do I need to amplify the signal?
2. Do I need to convert it from analog to digital signaling?
3. Would I be better off using a third party IC designed to sense or convert this? Any clue where I could find one?
4. Would I be better off avoiding a balun and staying with CCTV over coax?
I'm about the greenest rookie on this forum so please keep it slightly above laymans terms.
Comments
Remember that a Stamp is too slow to do anything with a video signal. A multimeter is much slower. The video signal does jump all over the place normally and you're sampling it (with the Stamp and multimeter) effectively at random.
I assume you want to do some kind of analysis on the video image. If so, the Stamps are way too slow and have way too little memory. A Propeller could handle it using an external analog to digital converter, but it doesn't have enough memory to do anything useful with the information. Sampling a video signal at low resolution (like 160 x 120 in B&W) would work.
Regarding whether to use baluns or not, they do offer the advantage of using CAT5 cable, which is cheaper than coax over long runs. However, over very long runs, there can be some signal degradation, which can be ameliorated with adaptive equalizers.
-Phil
FYI most meters are horrid at anything over 100Hz .
use a scope . or a RF volt meter ( harder to find )
It sounds like we will need to convert back to unbalanced. I definitely need UTP but think the LM1881 fits the bill perfectly. It's not so much the distances involved that will be the primary concern but rather the wiring methods we will need to resort to in this particular environment. Being a locomotive items have to be coupled between cars and coupling a 25 pair cat3, cat5, cat6 or (should we be crazy enough to pay for it) cat6a cable is a lot easier than having a dozen coax to figure out how to couple. We're not homebrewing the UTP baluns but are using products like NVT or Nitek that we've used in the past with good results, and we tend to do a lot of CCTV and IP surveillance. They have passive and active units and can get up to around 7000'-8000' with active units on both ends with a decent cable. I'm going to assume that, besides just amplification, the "active" units make use of the adaptive equalizers you mention.
-Phil
Honestly, It's a commuter rail project in a state where there's nearly no commuter rail to speak of so I'm not certain if by the time it's fully active there will be even that many cars to worry about. I think the local populace may have difficulty switching from their automobiles to a more economical but limited transportation system.