Getting your sound from another source.
Most consumer camcorders dont have the luxury of a line in connection when used in the camera mode. This can be a disadvantage in situations where it is possible to get an audio feed from another point, eg: an audio mixer desk or a public address feed. With a bit of fiddling it is possible to get around this problem.
I had the opportunity to do some work in a hall and the prospect of having to set up mics all over the place made the task daunting. Fortunately there was a mixer desk and stage mics set up and all I had to do was find a way of getting them into my camera. A typical mic input sensitivity of a camera is around 60dbm. The line output of a mixer desk could be around +4dbm so it is not possible to connect the two together without overloading the camera. Also the input impedance of a mic input is typically 10k ohms unbalanced whereas a desk would have an output impedance of 600 ohms, often balanced. The box I made up will make the two compatible.
It is simple and relatively inexpensive, requires a bit of metal bashing, and some basic soldering skills. The box consists of two major areas, the input balun, and the attenuator pad.
The input balun is a high quality audio transformer which isolates the balanced output of the desk from the unbalanced input to the camera. The transformers I used were scavenged from some old broadcast gear but as I have indicated in the parts list, good quality ferrite core transformers are available from Altronics or Jaycar. The ones I used are a 1:1 impedance ratio.
Note: I used these transformers because they were available. You could use a 600/10k matching transformer and alter the design of the pad.
The attenuator pad is designed to (1) Reduce the incoming level from the desk to something compatible with the mic input and (2) Match the low impedance output of the desk with the high impedance input of the camera. The values given here will give you a pad with a loss of about 55db.
Most cameras have some kind of auto level control on mic inputs. Mine has a soft limiting characteristic and I have set the pad attenuation such that the input goes into limiting when the line level is around +4dbm. This equates to 0VU to which most audio people will set their peak levels. The 10k pot in the circuit will enable alignment to be done with an audio source and level meter. When setting up mine, I fed a tone at +4dbm into the line input and hung an audio level meter on the AV output of the camera. I then set the pot so that any level above +4dbm caused no further increase in output from the camera.
You could get away with just these basics but I have fitted a couple of extras to make life easier. The isolating input jack on the pad allows me to feed in unbalanced high level audio and the phase switch will overcome the problem with out of phase cables. There is also a stereo/mono switch to allow me to common both of the inputs. The whole thing fits into a diecast aluminium box (just). Layout is not critical and logic will determine the location of components.
A belt clip would be a handy addition, or something to clip on the leg of the tripod. Belt clips can be sourced in the bargain bin for superseded mobile phone pouches. I bought mine for 93c at the local Tandy store.
A commercial unit similar to this is readily available from camera shops for $100s. One I have seen is very professional with variable attenuators and mic inputs and fits between the camera and the tripod. Total cost of a homebrew unit should be about $73.00 if you buy all the bits.
- 2xTransformer 600ohm/600ohm Altronics M0707 $19.45.
- 2xChassis mount XLR Female Amphenol 3 pin Altronics P0904 $7.00.
- Switching 6mm Socket - Altronics P0071 $2.05
- Min Slide Switch Altronics S2010 $0.50.
- 3.5 mm Socket Altronics P0094 $0.75.
- Trimpot 10k Altronics R2460 $0.75.
- Diecast Box to suit Approx. $10.00.
- Resistors - 2x10kohm, 2x12ohm,2x620ohm. Altronics $0.30 per pack of 10.
- Capacitor 1mfd 63v Altronics R5018 $0.25.
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For a schematic diagram click Line Level Adaptor For Mic. Inputs