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Author Topic: RF signal converted to composite video ? Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-15-2008 08:24 AM                        Presently we are distributing attractions to 3 CRT monitors (32in TV sets to be exact) in our grand lobby via RF on standard 75 ohm coax cable. Yeah, yeah I know...it's down and dirty but it works and it was cheap. The run from the source to the farthest monitor is about 75 feet. The run to the other monitors is only about 30 ft. The 30ft runs, the monitors looks fine. The longer run seems to have some very slight herringbone, probably the coax or even the set picking up channel 2 (the RF is on chan 3). At first I thought, just up convert to a UHF channel to eliminate the interference, but then I thought, why not just go composite video. Question is, can a standard 1v p-p composite video signal run on coax, #1 and #2, can it sustain a distance of 75 ft without loosing so much high frequency so as to make it worse than the slight herringbone? (which, btw, no one, my boss included, seems to be able to see, even when I point it out, which I find unfathomable since it drives me to ). Luckily there is audio distribution and speakers happen to be in the ceiling right above the monitors, so I can still get audio to play when I loose it in the sets themselves if I go to composite instead of RF.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-15-2008 10:53 AM                     Frank, With RG-6 cable, you can go 1000-feet on that composite and only loose something like 3dB (NTSC type signals). Then again, even a GOOD mini-coax should be able to get you 75-feet without dropping 3dB on the signal. I'm wondering if part of your problem isn't in the means of distribution. Areyou merely using a splitter...which is going to introduce anywhere from 3 to 12 dB of loss into the signal. Use a good DA for the composite and preferably one with gain and peaking controls so you can get it to look good. Note, it is really necessary to have a suitable test generator and O'scope to set those controls as most people just set things to look brighter (even trained people have a hard time getting it right). The trick you have in your system is that you have two short runs with one long run...you will need to split the difference if you get a DA with said peaking and gain or put a separate amplifier on the long run. Extron has essentially what you need...called the MDA 4V EQ. It has individually tunable outputs so each run gets the gain/eq needed (note they also have a basic 1 in 3-out DA for composite too (MDA 3V) but it only saves like $28.00). Steve [ 04-15-2008, 07:49 PM: Message edited by: Steve Guttag ]  |  IP: Logged Mark Ogden Jedi Master Film Handler Posts: 943 From: Little Falls, N.J. Registered: Jun 99 posted 04-15-2008 12:11 PM                     Also, Frank, It's good practice to place a 75-ohm termination over the empty outputs of whatever splitter you're using. I've seen just the kind of problem you're describing (herringbone) from situations where an unused output was left open. Give that a shot first before any extra expenditure.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 04-15-2008 01:08 PM                     (I'm dangerously out of my depth here) If he's only got 3 endpoints, I would like to hope he could get away with just passive components. Composite should definitely look better than RF-modulated, right? It should be easy enough to try. Another question is whether your splitters are optimally arranged--if you're using passive splitters, are they 50/50 splitters or 90/10 splitters, or...? (By the way, is there a good "Guide to debugging video problems with an oscilloscope" book/website/etc. out there? There've been a few times recently when it might've been handy, like that time I was staring at HD-SDI on an oscilloscope, scratching my head. That was fun. Turned out to be 24PsF versus 29.97 or somesuch...) --jhawk  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-15-2008 04:22 PM                     If one is using a passive splitter...you definately have to terminate all outputs...that is how the whole thing works. However, look on any splitter, you are going to drop at least 3dB of singal on the best output and often they are worse than that. If the signal was at a good level when it went into the splitter, the splitter will make it marginal. Often a signal needs to be over-driven before the splitter to ensure there is enough level after being split. Steve  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 04-15-2008 06:26 PM                     Go to Radio Shack, and get a 10db RF amp. If you are coming out of a VHS machine, or a small modulator, the RF out is only meant to go a few feet into one TV. Also remember that energy cannot be created or destroyed, only changed in form. If you put energy into a two way splitter, each output will have only HALF the energy (minus losses) that was put into the splitter. If you use composite video, you MUST use a DA, unless your monitors have looping inputs with 75 ohm terminators that can be turned off. If you "Y" the inputs of two monitors together, each one will only get half the signal level, and each will look rather "dark". If you "Y" three monitors together, you may not have enough sync level to lock the picture at all. A 75 foot long cable is nothing for composite video. Look for a video DA that does not even have a gain control, and gives you "unity gain". That way you don't have to use a scope to set the gain. You could go several hunderd feet with RG-59U type cable without any noticeable change in picture quality on a monitor, with composite video. John, what type of scope were you looking at HD-SDI on? HD-SDI runs at 1.5Gb/s, most scopes will not come even close to desplaying that, and there is no sync, just ones and zeros (highs and lows). Could it be that you were looking at component analog HD.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-15-2008 07:48 PM                     quote: Bruce Hansen Look for a video DA that does not even have a gain control, and gives you "unity gain". That way you don't have to use a scope to set the gain. You could go several hunderd feet with RG-59U type cable without any noticeable change in picture quality on a monitor, with composite video. Not so fast there. Depending on the cable, you will most certainly see visible degredation on composite with mere RG-59 running even 100-feet. Is it acceptable? I've seen people run XGA 100-feet on mini coax and not see a problem (until I point out the smearing). However, you are probably correct that with 75-feet being the long run, everything will look just fine. For something like Composite NTSC, I'll admit the signal is not that challenging but you will most certainly see a HF rolloff as the cable gets longer. As someone that has a test generator and O'Scope, I find it just too easy to set up video signals right. An easy signal to get ones hands on is normally NTSC Multiburst. It is a well-defined singal and it is easy to calibrate most any O'scope to. Just adjust your scope to it looking right out of the generator (or DVD player or whatever you have that has the pattern on it)...then go look at the same signal on the other end of the cable...adjust level and peaking to match and whola...a near exact copy at both ends. Multiburst will also show the HF roll off in the cable pretty well. It feels like calibrating an analog A-chain for slit loss. Just make sure you are loading the signal with 75-ohms and only 75-ohms when comparing the signal at each end. Steve  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-15-2008 08:04 PM                     quote: Steve Guttag With RG-6 cable, you can go 1000-feet on that composite and only loose something like 3dB (NTSC type signals). Actually 1000 feet is way too long without some hf roll off and loss of detail in the image which is what is present at the hf end of the spectrum. The longest we were allowed to go in Broadcast Television was 250 feet before it hit a video distribution amp and we were using some stuff that was doubnle shielded and much higher quality than every day RG-6. Of course this was back when we had professionals actually working to keep the signal to a high standard... that professionalism is totally gone in Brodcast today. The turds running today's automated TV ststions can't even set bars and tone! Mark  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-16-2008 08:26 AM                     Mark, I was not claiming that there wouldn't be some HF roll-off...In fact, I have always claimed there was. The capacitance of the cable will determine just how much there is. You can pump a 640x480 (31.5KHz) progressive signal through a modern RG-6 cable like Belden 1694A for 580-feet with only a 3dB fall off. I extrapolated what it would be like for an NTSC signal running at about 1/2 that frequency. Note too, that I advocated using a DA with gain and peeking to compensate for ANY cable roll off (level or HF) even on the shorter runs because it really is too easy to get it right. However, you must admit, with a 75-foot maximum run...with good low-capacitance cable, presuming the source of the signal is at spec, an NTSC composite signal is going to look okay. A lot of ifs in there but it isnt too unreasonable. And with a suitable gain/peeking amp, it should look spot on. Steve  |  IP: Logged Mark Ogden Jedi Master Film Handler Posts: 943 From: Little Falls, N.J. Registered: Jun 99 posted 04-16-2008 03:44 PM                     Only 75 - 250 feet of double-shielded before a VDA? Wow, that's a lot of processing. Our studio building in NYC is an entire city block long, there are routinely video runs of 500 - 1000 feet with no equalization 'till just before the master routing switcher input, and generally everything looks fine. Sometimes, though, it's the distribution amps themselves that cause a problem in the older analog studios; as they get up there in years, their frequency response falls off at different rates, so you sometimes get variations in signal quality from similar sources depending on their path to the main production switcher. Back in my NASCAR tour days, I used to take composite video feeds out of the infield at places like Daytona and Talledaga on three thousand feet of 8281, and with just one Telemet Clamper and a frame-sync made a very broadcast-able picture. Was it absolutely dead-nuts flat out to 4.2 MHz? Not quite. Could anyone tell? Nope.  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 04-16-2008 05:53 PM                     I just made a 500 foot run of 1505A (the newer RG-59 cable), I put a scope on it, and I was surprised at how little roll off there was. BTW, most consumer type TV/monitors have an automatic gain system for the croma. It adjusts the color burst to be the correct level; so a little croma roll off will not be noticeable.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-16-2008 06:32 PM                     1505A is good stuff...for NTSC...all things being equal...at 500-feet you are probably down about 3.5dB. It is probably visible if you A/B it though unlikely to get complaints. Mark O's 8281 is a little better at the lower frequencies (3dB down) like NTSC but would do worse for HD's bandwidth where its high cap/ft would have it decline ever faster as the frequency went up. 1694A is my cable of choice for large coax...good for most everything, including HDSDI...which fits in nicely for DCinema too. Steve  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-16-2008 07:03 PM                     We definately were not using 8281 back in the late 70's in the 2 different places I worked. One station I worked at... CH. 38 was located in the old RCA sound studios in Chicago... a kinda neat place even without the TV end of things. We inherited that place and all of that was wired by the Catholic Church for it's own TV system which was not broadcast over the air... only by cable... we inherited it when the Catholic Church decided it could not fund it any longer. Except for the camera cables that place was mostly wired in and between the three studios with plain old RG-59... Now thats a cable that has lots of roll off at the high end and it very definately required video DA's every 250 feet or so. We rebuilt master control and re-wired it all with double shielded stuff... can't remember what cable though... that was a long time ago. Nothing in Master Control was more than about 100 feet in length all terminating at a central Video DA/Patch point. When we did remotes all the long runs to the truck or recorder got checked and equalized... It never took alot of eq but the Assistant Chief was a stickler for quality. The one thing we had going for us in the studios was the fact that the TK45 Studio cameras large cables all came back to massive camera cable patch bay in master control where the CCU's were also at. That was pretty good cable but massive and heavy. Our remote cameras were TK760's... just glorified TK76 ENG cameras. I still like the look of the TK45's to this day. Mark  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 04-16-2008 10:56 PM                     Bruce asks "John, what type of scope were you looking at HD-SDI on? HD-SDI runs at 1.5Gb/s, most scopes will not come even close to desplaying that, and there is no sync, just ones and zeros (highs and lows). Could it be that you were looking at component analog HD." My memory is sketchy. I was definitely not using a snazzy scope, just using the 2445 that was at arm's length. I was looking at bars, I don't know if that means bits were blurred together and I didn't notice/care. I certainly did look at component HD off an Aja converter, but I'm pretty sure I also looked at SDI. Could be I'm misremembering though... --jhawk  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-17-2008 12:12 AM                        Thanks guys....all good info. Thanks. Our GM told me today that he didn't want me wasting any more time(I really hate it when they call doing your job "wasting time!")on it; he says it looks just fine and he keeps backing up until he can't see any details at all and says, "see....it's as good as anything I see on TV at home....stop obsessing over it or you'll loose more hair." So until I can get at this after hours or when he goes on vacation, I'll have to stay with RF. I'll try the easy stuff first and definately I know there is an unterminated split the 4way RF splitter that I can fix right off. Then still working with the RF, I can try putting an RF amp with separate outputs for each monitor. If that doesn't solve the problem, then it's out with RF and I'll start going the composite route.  |  IP: Logged

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