Audio Impedance

I've recently purchased a BM ATEM 2M/E and a 1M/E. I'm trying to find the analog audio input and output impedance. Can anyone tell me what the actual impedance figure is? Thanks.

I can't find specifications for impedance anywhere in Blackmagic's literature.

Most professional equipment nowadays uses active inputs -- operational amplifiers -- instead of coupling transformers, and the typical case for a line-level input is very high input impedance, on the order of 10,000 ohms or more. Output stages, whether running directly from an op amp or from a bipolar transistor pair, have the output impedance defined by a series resistor that serves mainly to protect the output stage against short circuits. It's pretty common to find output impedance anywhere from 60 to 600 ohms.

In earlier broadcast days, it was general practice to match the source and load impedance; this is a throwback to early telephone standards where the objective is not to achieve flat frequency response and low distortion, but maximum power transmission over a long cable. But since about the mid-1980s, it has been considered better practice in the professional audio world to use low source impedance and high load impedance: among other things, this can drive a longer cable with less attenuation at higher frequencies; for another, the reduced loading on the output stage tends to reduce distortion and heating.

While I cannot absolutely claim that the ATEMs conform to the low output impedance / high input impedance model, all of their other equipment that we use does.

If the specific impedance values are iimportant, there are fairly simple ways to measure them with reasonable accuracy.

To find input impedance, start with a constant-level source of tone -- around 400Hz or 1Khz is reasonable. Connect the high and low sides of the tone feed through a pair of matching value resistors to the low and high sides of the ATEM input. Using an AC voltmeter, measure the direct output of the tone generator; then measure the voltage on the input of the ATEM. You can derive the impedance mathematically, but the easier method is to try different values of resistors until the voltage at the ATEM is half of the voltage coming out of the tone generator. The ATEM's input impedance will be equal to the sum of the two resistors.

Finding output impedance is similar. Run tone through the ATEM, and measure the voltage between pins 2 and 3 on the output connector with nothing else connected. Then, attach a test resistor between pins 2 and 3, and measure the voltage again. Try different resistor values until the "loaded" voltage drops to half of the "unloaded" voltage. Once you get to this point, you have found the ATEM's output impedance: it's the same as the loading resistor.

To be insanely technical, impedance also involves frequency... but a mid-band frequency like what I suggested earlier will provide typical results across the useful audio spectrum. (If you're bored some day, try the same sort of test driving a speaker -- you'll find that reactive loads have quite variable impedance!)

Regards,

-- Jeff

Hi Jeff,

Thank you very much for your comments - it's clear you know what you're talking about.

I was thinking I would do some tests to try and determine the impedance as you suggested, however it seems very strange that BM wouldn't publish this very common specification.

Thanks again.

Thank you very much for your comments - it's clear you know what you're talking about.

My pleasure! There are times when it comes in handy to have been doing radio/TV engineering for 40+ years: I've figured out ways to cheat!



-- Jeff