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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Jon Risch, Please Simply Explain Shorting Plugs/Caps posted by Steve Bruzonsky on May 13, 1999 at 07:26:26:
[ Please explain the type of plug, if any, you recommend for unused inputs and outputs: ]In general, You can usually short an analog input, if doing so causes pops or transients when you switch past that input, it means that the shorting plug is upsetting a DC servo circuit, and the shorting plug should be replaced by a terminated load plug instead (approx 470 ohms). Problems with DC servo circuits are rare.
DONT: Short an analog or digital out!
DO terminate an unused digital out (except fot optical) with its characteristic impedance, 75 ohms for SP/DIF
and 110 ohms for AES/EBU.Now for specifics:
[ 1. For unused analog audio inputs. ]Either a short or a 470 ohm termination. See above.
[ 2. For unused analog audio outputs. ]
Nothing.
[ 3. For unused analog AES/EBU outputs. ]
110 ohms. All you need to do to terminate an AES/EBU signal coming out of an XLR connection is to buy the appropriate connector at RS, and solder a 110 ohm resistor across pins 2 and 3. 100 ohms would work OK, as it is mainly there to reduce any tendency for ghost signals to reflect back into the digital oputput and back load the rest of the circuitry, causing jitter.
[ 4. For unused digital coaxial inputs. ]
Terminating a digital input might affect input switching, or some other aspect that is not immediately apparent, so check youe owners manual for details. It shouldn't hurt anything, unless the equipment uses the presence of a plug to autoswitch the input, but they usually sense the presence of a digital data stream, so uless you have a really weird piece of gear, it couldn't hurt anything to try it. 75 ohm resistor.
[ 5. For unused digital coaxial outputs. ]
Assuming SP/DIF, 75 ohms.
[ 6. FOr unused digital AES/EBU inputs. ]
See #4, only use 110 ohms.
[ 7. For unused video composite inputs. ]
Video is less sensitive to signal ghosting at the low levels it would occur, anything below approx 40-44 dB is invisible, but it shouldn't hurt anything to use a 75 ohm termination.
[ 8. For unused video composite outputs. ]
Same as #7, except, if there are multiple outputst that are not separted electrically, loading them all down with 75 ohm terminations will load the output driver. I would be inclined to not terminate unused video, but it can't hurt to experiment and see if it helpd or hurts the video.
[ 9. For unused S-VHS video inputs.
10. For unused S-VHS video outputs. ]
See precious two, same basic idea.
Why do these things?
In the case of the analog in's, any RFI or EMI that might find it's way into the preamp/receiver would effectively be denied access, and shorted out by the shorting plug, and in the case of the digital output, terminating it in it's desired characteristic impedance would assure that there would be no digital signal ghosts or refelctions from the open and unterminated digital output, which could keep the output circuit from causing any troubles upstream.
Digital circuitry is much more finicky and sensitive to rather simple and understandable problems, than many people realize. The old saw about "it is just one's and zero's" implies that it is totally robust and incapable of
being impacted sonically unless you actually start loosing the correct identity of the one's and zero's. This is just not the case.
Jon Risch
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Follow Ups
- Re: Jon Risch, Please Simply Explain Shorting Plugs/Caps - Jon Risch 08:47:52 05/16/99 (0)
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