Understanding the GH4 Audio Buzz Issue

[jcs]

I analyzed the GH4 audio buzz issue yesterday. While I couldn't find any issue with the internal mics, the external circuit is picking up camera electronics, primarily the electronic shutter (buzz varies predominantly with shutter speed). I tried a few mics with TS, TRS, and TRRS plugs. The problem is not with the plug itself- the buzz is the same with them all.

 

If the Panasonic DMW-MS2 with TRRS plug really works without the buzz, then there's something going on with resistance and/or capacitance/grounding (or more). Reviews of the DMW-MS2 appear to show that the mic is both noisy and not very directional. It's a mid-side mic however decoding is only available on the mic (and controlled with camera settings). I use an Audio Technica BP4029 stereo shotgun on the FS700 (mid-side) and it's great being able to control the stereo field in post varying from pure directional shotgun (mono) to a wide stereo sound stage, and everything in between. The stereo is so good that listening with headphones on you'll turn around sometimes thinking the sound is live- it can be that 3D (similar to binaural).

 

The current workaround is going back to the standard DSLR method: setting in-camera gain to the lowest and using a preamp. For the GH4 that means setting mic gain to -12dB. At -12dB, the buzz is still present, however when testing with a Sennheiser G3 wireless system starting at -30dB receiver output and going up from there, the buzz seems to take a sharp reduction at around around -18 to -12dB G3 receiver output. Perhaps the GH4 has some form of AGC going on? In any case, at around -12dB (or higher) G3 receiver output, the SNR is high enough that examining the waveform in post shows the buzz is inaudible and audio is then usable. When testing with a Rode NTG-2 on battery with a TRS connector, the noise floor (white noise) isn't that bad on the GH4 (compared to the 5D3 for example). However the motor-boat buzz renders the audio unusable.

 

The buzz is a triangle wave, about 2x the amplitude of the noise floor. More info and waveforms + spectral plots here: http://www.dvxuser.com/V6/showthread.php?324401-External-Microphone-Solutions-for-the-GH4-(avoiding-the-buzz)

 

A low-cost, reasonably small and lightweight solution is an iRig Pre, for about $33. Along with a splitter cable (or with the soldering mod) it is small enough to attach to the back of a shotgun mic with Velcro or a rubber band. With a 9v battery, it supplies phantom power and sufficient gain for clean, high-quality audio.

[Michael1]

Good review.  I have been wondering about this.  I think I heard Panasonic has been claiming that the issue is only with TRS plugs.  Your tests prove otherwise.

 

I surprised Panasonic blew this, on an otherwise stellar design.  I can't imagine Panasonic can fix this with firmware.  It seems they didn't isolate the audio circuits sufficiently.  Therefore, I am holding off purchasing the GH4.

 

Michael

[jonpais]

with the GH3, I've been shooting more and more with off-camera microphone on boom pole and synching in post, so this shouldn't present a huge problem. if anything, it'll just encourage me to stop using on-camera microphones altogether. but all is not lost. Dave Dugdale is supposed to be receiving some different plugs or something from Rode in the very near future to see if they help at all - providing it's just a grounding problem. but jcs's post suggests it isn't. hmmm... I just picked up my GH4, so haven't had a chance to do any testing yet.