Okay, here goes... (deep breath) ...more than
you ever wanted
to know about loop-throughs and degradation!
The supercilious answer:
Yes, there is degradation. It's inevitable every time you add another connector or another inch of cable. Impedance mismatches in the equipment and cable connectors cause reflections of the video signal back down the cable (return loss), and the electrical characteristics of the cable cause loss of high frequency (chroma and fine detail) information (roll-off).
Even a one-foot piece of cable with a connector on each end will show degradation if your instrumentation is sensitive enough. Fortunately, the human eye is VERY forgiving. The real question is, "Is there VISIBLE degradation?" The answer in this case is: "Maybe, maybe not."
The practical answer:
In fact, there is no way your question can be answered from the information given. It is entirely possible that with the right kind of cable, in short enough lengths, equipped with proper connectors that have been properly assembled, and connected to equipment with proper return loss values, you will not see any degradation at all. On the other hand, this sort of loop-through arrangement is a recipe for problems.
The most common is when somebody bumps a termination switch and suddenly your video drops to half-level due to double-termination. The second most common is when a piece of equipment changes its input impedance when it is powered off. All of a sudden you notice your video doesn't look quite as good as you remember it did a little while ago. Then somebody unwittingly turns the offending gear back on, return loss goes back to spec, and the problem disappears. You blink and wonder if you were imagining things...until someone turns the unit off again.
Proper engineering doctrine demands "source distribution amplifiers" (Source DAs) in sufficient quantity so that each piece of equipment that is connected to a given source is being fed directly from a Source DA output, without looping. The only loops permitted are DA-to-DA jumpers when more than one DA is needed to provide enough outputs.
Most of us don't have deep enough pockets to follow that doctrine religiously, though.
The objective engineering answer:
The only way to answer your question is to set up a test. The rigorous way is to use test a signal generator and a scope, and it's really the ONLY way you can positively identify equipment-related degradation. These problems propagate throughout the loop-through chain, including back toward the source, so they show up EVERYWHERE you look in the chain. They include irregular frequency response, reflections, and other distortions caused by impedance/return loss problems.
The subjective engineering answer:
But you CAN test for cable-related degradation such as roll-off and color smearing using one high-quality monitor with A/B inputs and a cable. (Do not use two side-by-side monitors! If you do, you can't be sure that any "degradation" you see isn't just differences in monitor setup.) Follow the steps below, realizing that any visible changes caused by introducing the test monitor and cable invalidate the whole test.
- Step 1: Place the high-quality test monitor as close as possible to the source of the video signal.
- Step 2: Evaluate the video in the existing loop-through chain. Use the highest quality monitor and scopes you currently have in the chain. Remember what it looks like for future comparison. (The higher the quality of the monitoring you can do here, the more reliable this test will be.)
- Step 3: Break the chain at the source and insert the test monitor. Loop through the A input AND the B input. Use the shortest known-good cables you can.
- Step 4: Evaluate the video in the existing loop-through chain again. Use the same monitor and scopes you did in Step 2, and make sure that introducing the test monitor has caused no visible changes.
- Step 5: At the test monitor, compare the A and B inputs. They should be IDENTICAL (same signal, same monitor). If not, your monitor's input circuitry doesn't "qualify," and it won't work for this test.
- Step 6: Now that you've "qualified" your test monitor's inputs, replace the short jumper between the A and B inputs with the shortest possible "test cable" that is long enough to reach from the test monitor to the farthest point in the loop-through chain. (Use high-quality wire such as Belden 8281 or Belden 1505A to minimize loss.) Repeat Steps 4 and 5. If there are still no visible differences, you have "qualified" your test cable and you can continue.
- Step 7: Change the loop-through chain so it only goes through the test monitor's A input. Leave the test cable connected to the B input, and terminate it with an external high-precision commercial terminator. Repeat Step 4. If there is still no visible difference, you are ready to try and make a test.
- Step 8: Go to the end of the loop-through chain. Remove the terminator and replace it with your test cable. Repeat Step 4. If you see a change, it means the test monitor terminator differs from the one you removed. Since you put a high- precision terminator on the test monitor, the one you removed may have problems. Replace it with a high-precision terminator, remove the test monitor from the chain and restore the original connection to the video source, and start again from Step 2. (Sigh! Takes a LOT of work to eliminate all the possible variables, doesn't it!)
- Step 9: If you got past Step 8, you've finally managed to set up a valid test! Go to your test monitor and do an A/B comparison. If you see a difference, you know the following:
- A. It's not the test monitor A/B inputs (Step 5).
- B. It's not the test cable (Step 6).
- C. It's not the B input terminator (Step 8).
Therefore it can only be degradation in the loop-through chain that has been introduced by cables and/or equipment between where it loops out of the A input (at the beginning of the chain) and returns to the B input (at the far end of the chain).
Now that you have a test setup that you can have some confidence in, you can start moving the test cable back along the chain, breaking in at various points and looking for significant changes such as chroma or gain variations caused by an "active loop-through" of the type sometimes found on "pro-sumer" equipment.
A significant improvement caused by breaking in on the "upstream" vs. the "downstream" side of a piece of equipment may indicate that it has problems (especially if it has a supposedly-passive loop-through), and it should be pulled out and tested rigorously with test signals and a scope.
If all you get is a gradual improvement as you work your way back toward the source, it means you're looking at the cumulative attenuation and roll-off caused by all the equipment and cable in the chain, and there's little you can do about it. Move the most critical equipment as near to the source end of the chain as possible to minimize roll-off, and start saving up for DAs.
--Joseph. S. Kidd
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