Fri Mar 29, 2013 6:42 pm
Peter,
DC offsets aren't something that you really "hear".
But, what they do is screw up your metering in production. Since its pushing the AC signal off center, it limits the achievable signal to noise level that you would otherwise be able to get. So, what you would "hear" is poorer SNR.
The are a pain in post production also. You've got to first remove them before you do any normalization, if you are going to be lining up your clips with other audio sources and want to maintain consistency in signal levels.
The DC offsets in the camera at LINE are less than they are at MIC.
But, because of the analog gain structure of the camera, a typical LINE level signal from an ENG mixer will require high gain in the camera at LINE. So, even though the DC offset is smaller, you're taking a hit in SNR because you'll have to use high gain in the camera.
This is kind of the folly when calibrating cameras to the needs of an external mixer. I prefer to find the sweet spot of the camera in terms of SNR (0dB digital gain which is around 20-30% in the BMCC), then calibrate the external meter to the needs of the camera.
In my tests, even with larger DC offsets at MIC level, I can achieve better SNR coming in at MIC level with the camera gain at 20%, versus going in at LINE level with an external ENG mixer tone (where I can't calibrate the meters) where I'm forced to use the camera at high gain. Your clip sounded pretty good for SNR. For some reason, I found that I had to push the gain higher than the 80% you used to calibrate the camera to the LINE tone of the MixPre-D. So, there might be some difference here that I don't have an explanation for.
If the camera had meters, then metering at the SNR sweet spot of the camera is a no-brainer.
Using an external monitor with meters allows you to do the same (the Frank approach) with the camera in the SNR sweet spot.
Using an external meter that has the handles for calibration (like the Riggy-Assist) is another approach to this. The downside is that you're metering BEFORE the camera (like with the ENG mixer). So, if you kick a cable out, or the camera stupidly switches from MIC to LINE, you won't get a visual indication.
I'm going to be releasing another solution to this issue at NAB ...
But hopefully, the camera will get a firmware update fixing its issues, like meters, DC offsets, and the MIC-to-LINE-switch-surprise ...
Robert