Alpha/Linear Keying

[TheReverendDave]

So...

Our church is making plans for multi-cam and Image Magnification for our worship service (music and pastor). We currently use Propresenter4, with lyrics and backgrounds. We are planning to continue using Propresenter4, but obviously we are going to need to "key" the output. So I have 2 questions.

1) Is it necessary to use the Alpha/Linear type keying? I understand how it would be excellent and the best choice, however I don't really know the quality of the standard keying (ie. green background behind lyrics). Is it "bad"? From doing keying in FCP, I would think dissolves would be inherently impossible, correct?

2) We are looking at the new AW-HS50 from Panasonic. It does have Chroma/Linear key. But the inputs are limited, which could be fine because we were planning 4 cameras with HD-SDI in, and the Presenter display in DVI. My question is this, the only way to output alpha/linear key is to send 2 HD-SDI signals, one of key and one of fill? I'm asking cause if this is the case, the AW-HS50 wouldn't work for us because it doesn't have enough inputs.

Thanks for anyone who spends sometime answering these questions!

[petereit]

We use an older Panasonic AV-HS300G switcher which only has a luma key. We set up our lyrics with white lettering on a black background and key it over live video with only the DVI signal from the computer. Fades between slides in ProPresenter4 work fine.

Where I wish we had a chroma keyer is for nursery alerts. I would LOVE to be able to display a white number in a red square at the bottom left, keyed over the live image, but with only a luma key it would display as white text on a gray square. We kind of get around that by not keying at all -- rather using a diagonal wipe from the lower left and using the T-bar to manually ease it in and reveal the nursery alert.

[tdangelo]

Originally Posted by TheReverendDave

1) Is it necessary to use the Alpha/Linear type keying?

Alpha key is typically a hard clip based on luminance alone. Background is completely replaced by fill channel information.
Linear key is also based on luminance but is sensitive to the amount of luminance present on the key channel. This it is possible to make translucent keys or gradations.

Originally Posted by TheReverendDave

however I don't really know the quality of the standard keying (ie. green background behind lyrics).

What you are calling “standard” keying is a chroma key. There is really nothing standard about a chroma key … it just uses different information to create the key. In the case of a Chromakey, the key is cut using color information. For example, everything green is replaced, everything not green is kept.
My mother in-law was fond of saying that “normal” is only a setting on a washing machine. I think the same is true with “standard.”

Originally Posted by TheReverendDave

Is it "bad"?

Nothing bad about it. Just more opportunity to have a screwed up key. Because the key is color based you need to be very careful to avoid that color in the key channel where you may not intend to cut a key. Additionally, many switchers will cut a cleaner (less noisey) key using luminance (especially non-linear luminance) as it is more basic.

Originally Posted by TheReverendDave

My question is this, the only way to output alpha/linear key is to send 2 HD-SDI signals, one of key and one of fill?

In general terms (not specific to the AW-HS50) no, that is not necessarily correct. IF you want to fill with something other than an internal matte generator in the switcher, or a signal already residing at a switcher input, then YES you would need to send two signals. If you are happy filling with a switcher’s internal matte generator, then NO, one signal will do.

Originally Posted by TheReverendDave

I'm asking cause if this is the case, the AW-HS50 wouldn't work for us because it doesn't have enough inputs.

Yes, it looks that way.

-Tom D'Angelo
NYC

[zactommo]

Tom did a wonderful job of explaining the different types of keying!

What type of computer are you currently using for ProPresenter? Have you considered that to do alpha keying with ProPresenter, your going to need a mac pro with a appropriate video output card (Decklink HD extreme seems to be the card of choice) So if ProPresenter is currently running on a iMac, then there is going to be a significant hardware cost for the upgrade of the computer.

Furthermore, ProPresenter is designed to send the alpha keying over two SDI outputs on the decklink card (or whatever card you use) so the DVI input is not a whole lot of use to you.

My gut feeling is that the HS50 probably isn't the best switcher for your needs. Considering this is your first move into imag, having your switcher at full capacity before you start seems slightly short sighted. Room to grow is always a good thing! You wouldn't by a sound desk with only enough channels for what you are currently doing right? But I do understand that there are probably budget constraints involved (like there always is when working with churches)

My best advice would be to test the keying abilities of the switcher before you buy. Just because a switcher says it can do luma keying, or chorma or alpha, doesn't mean it does it well. Case and point. My work upgraded their old and dated ampex vista switcher, start of last year, to a new Ross Synergy S100, which is a digital SDI system. The ampex did a great job of doing single channel luma keying with our current GFX system, but the new Ross switcher does an awful job of luma keying. The ross switcher does alpha keying amazingly, but we only have the hardware for alpha keying GFX; not enough money for the software yet. So we're stuck with a crappy looking luma key, because appropriate research wasn't done (switcher was purchased before I started working there ) So, check before you buy!

[timg]

Hello Petereit, Hello Reverend Dave -
I have some experience with using a special switcher in location work, and at the school where I teach. It has 4 inputs, chroma and luma key, and its pretty cheap. Since you are using the AV-HS300g and it has both SD and HD inputs, I might suggest an upstream chroma key with this box, then feed that input into your main switcher.

The switcher I use in multiple venues is the modified Pro Fifty version of the Panasonic MX-50. 4 input switcher, chroma and luma key onboard. It is full resolution SD and takes composite or y/c ( s-video) in and out. The retired aerospace engineer, Bob Rall, does the work.

New they were $3500. His modified unit I bought 3 of for $2000 each for use at my school a couple years ago. Now they're around $1,000. Today I saw he has one on Ebay and it is only at $400.

The flexibility of having a chroma key and a luma key adds so much to a live presentation, I registered with ChurchMedia.net to give you this info.

Hope it helps, at least it was well intended. As that this is my first time on the Church Media website I cannot post web links, but I'd suggest you search MX50 from Panasonic on Ebay, this item number is 300428481576

If you search Real Time Engineering, you'll find Mr. Rall for this type of specialty luma key and chroma key work.

All the best to you gentlemen in your important work.

Tim Greenwood
Television Department Head
Academy of Radio and Television
Huntington Beach, California