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How to adjust audio levels with Blackmagic cameras

Allan Tépper
How to adjust audio levels with Blackmagic cameras 6

 


As several people begin receiving Blackmagic’s Pocket Cinema cameras, many begin to ask me how to deal with their audio with them. They want to know how to connect their mics, how to monitor, and how to adjust levels. This has been further complicated by the fact that surprisingly, Blackmagic has still not added any audio metering to their cameras via a firmware update.

The Blackmagic Camera’s audio settings screen offers manual adjustment of audio recording levels, but currently, no metering, or even a visual indication of audio clipping.

At least at first glance, this means that the user of a Blackmagic camera who chooses to record audio internally (as opposed to dual system, where audio is recorded independently) must consider using some external device with audio metering. That could mean either:

  1. Using an external preamp/mixer with a built-in audio meter (for several hundred dollars more than one without them) and then still needing to determine the exact input setting on the camera to calibrate the system.
  2. Using a video monitor to connect to the Blackmagic Camera’s digital video output (HDMI or SDI, model dependent) with a built-in audio meter.

For those who already own one of those two devices, using it might be fine. However, for those who don’t —especially for those who are purchasing the Blackmagic Pocket Cinema Camera for field interviews— having to spend several hundred dollars for one, or over US$1k for the other just to measure audio levels might seem unjustifiable, especially when even many consumer video cameras include audio meters, AGC, and/or a limiter. (Then again, no consumer camcorder offers video recording to RAW or to ProRes422.) That’s why I asked Blackmagic’s public relations to see whether the company would commit to adding either audio meters, AGC, and/or a limiter via a firmware update. As expected, the response was that they would not commit to adding new features until they are available. So I insisted and asked whether they had any alternative way to set levels without having to spend hundreds of dollars extra for one solution, or over a thousand for another.

An external preamp with VU meters can cost hundreds of dollars more than one without. (Photo courtesy of JuicedLink.)

The surprise response from Blackmagic

The response from Blackmagic public relations department was both surprising and even potentially relieving, although it wasn’t actually a direct response from them. Instead, it was a quote from one of their unnamed happy users of the Blackmagic Pocket Cinema Camera, who suggested solving the issue with a good pair of isolating, over-the-ear headphones to adjust levels. Once I read that, this is what I wrote back to Blackmagic public relations:

This brings up a question that Blackmagic should be able to answer (since it doesn't require committing to potential new features that may or may not be added in the future):

  1. Does the audio signal being monitored via headphones ocurr after the A-to-D (Analog-to-Digital) conversion, and therefore back D-to-A? If so, it would certainly help the operator determine whether clipping is occurring, which is good.
  2. If that is the case, what is the delay (i.e. in nanoseconds, microseconds, or fractions of seconds) between the input and the monitored output?

If the output is not after the A-to-D and D-to-A, then it wouldn't help. Please let me know!

I knew that the public relations department would have to ask the product manager, and that it might take a few days to get a reply. When a few days went by without an answer, I followed up and got the following response:

“…the delay is approximately 3ms. But there could be different scaling going from A-D and then back to D-A in some circumstances.”

I was surprised not to receive a separate answer to my first and second question. Since they answered the second question but not the first (and given the suggestion of the unnamed happy user of a Blackmagic Pocket Cinema Camera), it may be safe to assume that the answer to the first question is: “yes”. If so, then before your recording begins, you may be able to ask your interviewee to speak continually, increase the input level (either in the camera’s menu, or on your external preamp/mixer) until you hear clipping (i.e. intermittent silence) in your isolating headphones (i.e. Sony’s MDR–7506 or Audio Technica’s BPHS1, with built-in microphone), and then back down a safe margin below the clipping threshold.

The Audio Technica BPHS1 can allow the camera operator’s voice to be recorded on one track, while s/he monitors both tracks, one in each ear. The interviewee’s mic would be connected to the opposite track of the camera.

I will confirm this whenever I get the opportunity to verify it myself, or whenever someone who has had the camera confirms it to me.

 

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