75 Ohm Terminators why?
 

Okay, I have a broadcast monitor that requires 75 Ohm BNC terminators to be put on the monitor outputs.

I didn't realize that I was supposed to do this at first, so I calibrated the monitor without the terminators on. As far as I can tell, the only difference it made was that I had to turn down the brightness a tiny bit. It made no difference in the gamma or color tracking. I used a high quality colorimeter and calibration software to white balance everything, and it tracked *perfectly* without the terminators on it.

My question is, why exactly do I need to use the terminators?

Thanks

Keeps the impedance matched and the levels correct

Just for the sake of doing that, or is there an important reason to "keep the levels correct" at that point in the chain, or is it totally fine to skip the 75 ohm terminators and just lower the brightness?

I'm not seeing any downside to skipping them, and no one has told me that running it like that is going to hurt anything

The monitor input is designed to take 1V pp - If you are running without terminations, it is getting 2V pp. Apparently it has enough headroom to tolerate this and not clip whites or peak chroma carrier on full amplitude yellow and cyan, but you shouldn't count on that.

Okay. Thanks for that. So there is a good reason to run with the terminators in place; mainly to not overly tax the input components.

Yes, it doesn't seem to be an issue without the terminators. I was able to properly calibrate the monitor and tracks beautifully and all colors are about as close to perfect as I have ever seen.

I ordered some more terminators (others are being used elsewhere) and I'll install those and recalibrate when I get them

If you don't terminate, the signal will be reflected from the monitor input back towards the source. If the source has a good, accurate 75 ohm impedance it will absorb that reflection and no harm is done. Otherwise part or all of the signal will be reflected back towards the monitor. Depending on cable length this can cause frequency response errors or possibly ghost images.

In practice, it's often not a problem if a monitor is left unterminated. The frequency response errors and ghost images will often be negligible.Most monitors will tolerate excess video amplitude. There's no question of stressing or harming a monitor with 2Vp-p of video. But just because you can often get away without terminating doesn't make it right.

The fun starts when somebody puts a 50 ohm termination in the box of 75 ohm ones. This can be malicious, by accident or because you're in an environment where both 50 and 75 ohm equipment is being used.

Okay. Thanks for that explanation. That makes a lot more sense now. Yes, the sources I'm using are all 75 ohm so that seems okay. I have the terminators coming in the mail already, so I will "make it proper" when they arrive, but it was just kind of driving me crazy that I was doing it without understanding why.

They original built broadcast equipment without terminators and with 2 jacks so they could daisy chain multiple monitors. If you were to plug a second monitor or a VCR with internal termination into the other port it would also work as good as a terminator.
Later makers got fancy and started making them with switchable internal termination.

Yes, the monitor I'm talking about is an Ikegami from ~2000, and it has the 2 jack design for daisy chaining. They are just empty right now, but I have the terminators coming in the mail. I don't have a VTR with RGB I/O, so there's no way for me to do that. I'm not even sure if there were any VTRs that did have RGB I/O

I make my terminators, 75 ohm resistor inside a crimp type BNC with the center pin soldered to the resistor and all is well.

Just because a video source claims to be 75R, there's no reason to believe it unless it quotes the return loss** figures. For example 40dB return loss means it will reflect 1% of the signal. Return loss varies with frequency, usually worse at higher frequencies. Traditionally, analogue broadcast grade kit aims for a return loss of better than 40dB up to 5MHz for both inputs and outputs. For consumer grade kit it's variable. Outputs can be very poor, sometimes down to 10dB or less. Inputs are usually better.

I have measured the return loss of many video inputs and outputs over the years. The results have been very varied. You can't do this without special equipment (return loss bridge or vector network analyser) which I have, as a design engineer. I have designed a lot of equipment with video inputs and outputs.

I too have made many of my own 75R terminations with a BNC plug and a resistor.

**The radio frequency people use VSWR. It's the same thing expressed in a different way that's more meaningful to them.

This impedance matching issue can harm more with basically-VHF signals from eg. 1600*1000 @ 85Hz or more for analog RGB monitors; this requires a lot of bandwidht, making mismatch readily visible. I have a Dell P991 (Sony chassis) operating at 1600x1200 (at 75Hz), and even a little loss of conduction from VGA connector makes ghosting.

Monitors can sometimes handle the mismatch without issues like streaking, clipping but things like recording equipment need to be terminated as they have a far narrower tolerance before streaking, clipping ect. Digital signalling like SDI & HD-SDI is far less tolerant of the mismatch and must be terminated to work correctly.

SDI and HD-SDI only specify 15dB return loss across the bandwidth but running without a termination will often cause them to fail completely. It's rare to have looping inputs and separate terminations with SDI so the problem doesn't usually arise.

When you come from a traditional professional broadcast environment correct termination isn't even something you think about. You just do it right. I've certainly heard horror stories concerning younger broadcast technicians who have absolutely no idea about how to connect analogue video correctly. One good one was a string of monitors with looping inputs. The tech connected one end of the line to an output of the distribution amplifier. And the other end of the line to another output of the same VDA! Some of the monitors seemed OK, others were mysteriously in monochrome. My friend who was on set told me it took him a while to work out what had happened.

The termination is not super-important when connecting a video source to a single display six or ten feet away, but once you feed a chain of several displays, each 50 feet or more apart, the proper impedance becomes critical.

Think of airport terminals in the days when the arrival and departure displays were all black-and-white. They were all fed with looped composite video in those days. One loose or missing termination (separate piece or built-in switch) would mess up a concourse full of monitors. :eek: