Signal attenuation when splitting an output

D

Daniel Postilnik

Junior
Jan 14, 2011
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San Francisco, CA
In one of the install I systems I work with (it's a fitness studio), the vendor has installed an xlr output in the wall that is a direct split from the wireless mic receiver for that room, located in a rack in a different room. The wiring is such that a cable from the RF mic receiver in the rack goes to a phoenix connector input on a DSP a couple of rack slots below it, and the split is achieved by running a line from that same phoenix connector all the way out to the xlr output on the wall. So the phoenix connecter has one cable in and one cable out carrying the same signal.

When we tested this system by running a cable from the wall output into a camera, I detected what sounded like a 6dB drop in signal in the level of mic being piped to the room. The drop occurred as soon as we plugged the cable into the camera.

Is there a principle by which splitting a signal cable drops the signal level by a specific quantity?

Is there any kind of in-line pre-amp that could be used on the split line to offset the signal level drop?
 
Re: Signal attenuation when splitting an output

Daniel Postilnik said:
In one of the install I systems I work with (it's a fitness studio), the vendor has installed an xlr output in the wall that is a direct split from the wireless mic receiver for that room, located in a rack in a different room. The wiring is such that a cable from the RF mic receiver in the rack goes to a phoenix connector input on a DSP a couple of rack slots below it, and the split is achieved by running a line from that same phoenix connector all the way out to the xlr output on the wall. So the phoenix connecter has one cable in and one cable out carrying the same signal.

When we tested this system by running a cable from the wall output into a camera, I detected what sounded like a 6dB drop in signal in the level of mic being piped to the room. The drop occurred as soon as we plugged the cable into the camera.

Is there a principle by which splitting a signal cable drops the signal level by a specific quantity?

Is there any kind of in-line pre-amp that could be used on the split line to offset the signal level drop?
Click to expand...

Plugging in the camera likely unbalanced the signal. It probably took the cold and shorted it to ground. Get a transformer splitter or transformer isolate the camera and it will not have this problem.
 
Re: Signal attenuation when splitting an output

Silas Pradetto said:
Plugging in the camera likely unbalanced the signal. It probably took the cold and shorted it to ground. Get a transformer splitter or transformer isolate the camera and it will not have this problem.
Click to expand...
I will try to make that happen, though I'm surprised that the xlr inputs on a high end input module for a DSLR camera don't have their wiring straight.
 
Re: Signal attenuation when splitting an output

Silas Pradetto said:
Plugging in the camera likely unbalanced the signal. It probably took the cold and shorted it to ground. Get a transformer splitter or transformer isolate the camera and it will not have this problem.
Click to expand...

The other reason to isolate the split is to prevent any phantom power provided by the camera from making its way into the RF receiver. Note, though, that transformer isolation will not prevent level changes due to excessive loading of the line.
 
Re: Signal attenuation when splitting an output

6dB does sound like a balanced signal being dragged unbalanced.

If the inputs are high impedance, there should be negligible loss due to a split.
 
Re: Signal attenuation when splitting an output

Lyle Williams said:
6dB does sound like a balanced signal being dragged unbalanced.

If the inputs are high impedance, there should be negligible loss due to a split.
Click to expand...
The losses in a "split" are dependent on several factors.

1: The number of splits and the impedance of the input circuit. A mic does not have the same low impedance as a line level signal and typically mic inputs are not as high of an impedance as those for a line level signal.

2: The length of the cable going to each split. It all adds up.

So that being said- you can "typically" split a line level signal in to many more inputs than a mic level.

And of course once you throw in an unbalanced input-it all gets gets screwed up---------------
 
Re: Signal attenuation when splitting an output

Daniel Postilnik said:
When we tested this system by running a cable from the wall output into a camera, I detected what sounded like a 6dB drop in signal in the level of mic being piped to the room. The drop occurred as soon as we plugged the cable into the camera.

Is there a principle by which splitting a signal cable drops the signal level by a specific quantity?
Click to expand...
One general mechanism, and one more obscure.

General mechanism first: Outputs have output impedance, and inputs have input impedance. When plugged together you get a simple voltage divider formed by the two impedances. Modern audio interfaces use "bridging" terminations (not same as bridged power amp) where input impedance is 10x or higher than the output impedance for modest (1dB or less) insertion loss.

The obscure mechanism that coincidentally results in exactly 6dB loss, is when a balanced output with two hot signal legs (on XLR pins 2 and 3) interface with an input that shorts one of the two legs to ground, or ignores one leg. This is not very common.
Is there any kind of in-line pre-amp that could be used on the split line to offset the signal level drop?
Click to expand...

Yes, but you should not need one...

JR
 
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