BMCC audio input sensitivity changed during shoot?

Hello,

This may have nothing to do with the camera (BMCC 2.5k, latest firmware I think), still I can't troubleshoot it. I shot around 50 clips of a few seconds each yesterday. I had a couple of mics set up, connected to a mixing desk that was feeding a laptop via an audio interface, and I also fed an auxiliary send from the mixer to input 1 of the BMCC for sync reference and as an aid to the editor. This signal was line level and mono, and I set the camera to record it in both channels (line level, presumably +4dBu).

I set the levels very carefully in the beginning (I'm an audio guy mainly). Most of the clips turned out ok, with very clean audio. However, some of them were suddenly around 15-20 dB lower, with increased noise, to the point that you could barely hear a thing! The thing is, they weren't even a continuous range of clips (like if something had gone bad with a cable or something), and this never happened during a clip. Just some of them (around 20) were bad like that.

I measured my cables after (the wiring was balanced, but the source aux send from the mixer was not), they were fine.I also had an XLR to TRS adapter to feed the camera, which also measured ok.

Any idea on what might have happened? Is there any sort of overload protection on the BMCC? I did change the gains on the (external) preamps a occasionally, though not more than 3-4 dB in either direction. The audio recorded in the laptop is perfect.

We did clip a couple of times, not on useable audio, just the hand clap we use for syncing afterwards (sometimes accidentally too close to the mics). Could that cause the BMCC to lower its recording level or something? Because if it were an intermittent cable, it would be highly unlikely that it would come in and out like that, always between takes, with no other audio issue at all. I would send a balanced signal, but I didm;t have a balanced output handy at the time, and I didn't really care for the extra headroom or interference, as the camera audio was just a guide for the first edit and to help us sync the proper audio from the sound subsystem. And it did work around 60% of the time...

Any ideas? This was an easy shoot and I can sync manually easily, but next time it won't be so, and I want to figure out what went wrong...

Unfortunately I have no answer to your problem as something like that never occurred on my camera.

But a small piece of advice for audio recording on the BMCC:
I would always recommend to put the internal levels to 50%. Anything above only adds digital gain, which increases the noise floor. I don't know what putting it below 50% exactly does, but it doesn't keep your signal from clipping noises.

Dani, that is strange as the BMCC would use overload protection only in MIC mode. If the input signal gets too hot it will switch to LINE mode.

But as you wrote you were already on LINE mode that should not be the case.

Thanks for the responses guys.

This is very difficult to troubleshoot, since I tried it again and it appeared to be working. Maybe it was a funky cable, who knows... Next time I'll send a stereo signal, so that I can tell if it's in both channels (clearly then it'd be the camera) or just the one (which might suggest a bad cable is more likely).

The levels were at 50%, I didn't change that (although it was incidental that I didn't). Maybe Blackmagic should change the labels to proper units, so it is clear where unity gain is... Instead of 50%, it should read 0dB, with +/- dB marks above and below. And it would be good to know what level input signal it likes, and how much headroom before clipping we can have. Logic suggests a +4dBu reference, but the headroom is important as well for external monitoring. It can't be expected of the camera operator to check the tiny meters on the camera screen.

I should note that sometimes clipping is unavoidable, yet harmless. Sometimes the sound we're going for is soft dialogue from a boom, or even footsteps (either for post-cleanup or as sync references for sound design/foley). Yet the clapperboard will always be at least 20dB above that. I let it clip... who cares? It'll still show as a pulse on the waveform. But I need to know that the converter can handle the clipping without any funky business...

You are definitely an audio guy
... and you are going to enter a very foggy island without a map when dealing with the audio on the BMCC

Yes, it expects studio level (+4dB), balanced. Unfortunately the noise floor is not studio level so you have to record lauder than you normally would if you want to use the camera recorded audio and usually that means that the clapper will clip as the headroom above +4dB is almost nothing in my experience.

Concerning your issue: I have no idea what might have caused this. Your approach looks fine to me. Have you asked the support directly whether there might be some protection going on?

Johannes

I haven't yet. I try not to bother support for things that may be beyond their control (this could still be just an adapter or cable). Next time I shoot something I'll try and clip it intentionally to see if something happens then.