"Bleed-through" or "CrossTalk" on Toaster

Product: Video Toaster

Platforms: 2000 3000 3000T 4000 4000T

Problem: Ghosted images of other video appear in Toaster Main Output:

Situations which may cause 'bleed-thru' or 'cross-talk':

*badly shielded I/O video cables

*partially unshielded I/O video cables

*improper termination with either video or RGB buses

*bad RGB cable/port/monitor

*improper/nonexistent termination levels on the source/record decks

*electrical ground loops

*(very rarely) Video Toaster card itself

If the equipment is otherwise ok then a ground loop is the most likely culprit for this. The best way to check this is to reduce the system to the simplest setup. The computer system and a monitor for program out, with the attached video equipment powered down, detached, and unplugged, including the Flyer drives. Boot the system up, run the software, select your framebuffer with the logo on preview, then alternately select inputs 3 and 4 on your program out. If you do see ghosting, isolate the computer itself by using two prong adaptors on the RGB and the Program out monitors, and retest. If you still see the ghosting, then you have a problem either in the system or in the Toaster or Flyer. If the Toaster or the Flyer is the source of the problem, contact Tech Support for repair authorization.

If you see no ghosting, then the Toaster and Flyer are okay. Down the system, add a component, and retest. (Note: this procedure is similar to how you would test for a faulty component which is feeding voltage or signal into its chassis ground - a "ground fault".)

If you don't want to go through the entire painstaking process of putting the system back together a piece at a time to figure out the loop, you could simply set things up so that only the the computer system is grounded, and the attached equipment all goes to a power strip (or several) which are not grounded, plugged in using a two-prong adapter. This prevents the possibility of a loop, and the video and audio interconnections between the equipment cover the issue of grounding on the attached equipment.

The Toaster exhibits only the same level of crosstalk that, by the laws of physics, exists in any circuit board. Any trace or wire produces a minute field, and thus affects the signal in other traces or wires. In normal operations this kind of "crosstalk" is certainly not visible, and is most often undetectable. Ground loops in particular have the capability to amplify such crosstalk, and also to act as an antenna to pick up RF noise in the environment and inject it into the signal path.

Ground loops will thus also have an effect on the video signal quality. The incoming composite video on a Toaster BNC is by necessity referenced to ground when it hits the first amp. If there is noise in the ground, that noise is translated directly into the video signal.

Ground loops are a fact of life for any electronic equipment, particularly noticable in video and audio applications; this is a fact of physics, not an error in our engineering.

By its very nature, setting up any video system creates ground loops. Take the example of a very simple Toaster setup: The computer housing a Toaster, a destination deck, and one monitor for each, one grounded power strip. In examining just the computer and the deck, we have the following connections:

Computer (power supply) to power strip --> this connects the computer's chassis ground into the house ground.

Deck to power strip --> this connects the deck's chassis ground into the house ground.

Toaster to Deck via BNC video cable --> the shield line on the BNC connects the chassis ground of the Computer to the chassis ground of the Deck, and a ground loop is thus created, circuiting around the deck, the computer and the power strip.

As you add more components, each component creates a loop each with every other component connected to the power strip.

A ground loop will act as an antenna, picking up RF, and is susceptible also to induced current from surrounding magnetic fields. Again, this can amplify symptoms such as crosstalk and introduce noise and interference patterns into video equipment. The way cabling is arranged in a suite can increase or decrease the problems, and the condition of the cabling and other ground plane problems may also have some effect.

In point of fact, when you set up a system with only one item plugged into the ground, and the rest of the items plugged in using an adaptor which lifts the ground on the power cord, you are not ungrounding the equipment at all. In both BNC and RCA cords (at least, the kind of RCA cords you should be using), the shield ground interconnects the chassis grounds of equipment they link together, thus all interconnected equipment is grounded, in series, through the one piece of equipment that connects to house ground. All you have really done is eliminate ground loops, not grounding.

By eliminating all those "unintentional antennas", you should get cleaner signals throughout your system.