Back from Botswana, I want better audio for my video


This blog has been silent the last month because I've been on an amazing trip to Botswana and a few other places. There will be full reports and lots of pictures later, but today's idea comes from experiments in shooting HD video using my Canon 5D Mark II. As many people know, while the 5D is an SLR designed for stills, it also shoots better HD video than all but the most expensive pro video cameras, so I did a bit of experimenting

The internal mic in the camera is not very good, and picks up not just wind but every little noise on the camera, including the noises of the image stabilizer found in many longer lenses. I brought a higher quality mic that mounts on the camera, but it wasn't always mounted because it gets a little in the way of both regular shooting and putting the camera away. When I used it, I got decent audio, but I also got audio of my companion and our guide rustling or shooting stills with their own cameras. To shoot a real video with audio I had to have everybody be silent. This is why much of the sound you see in nature documentaries is actually added later, and very often just created by Foley artists. I also forgot to turn on my external mic, which requires a small amount of power, a few times. That was just me being stupid -- as the small battery lasts for 300 hours I could have just left it on the whole trip. (Another fault I had with the mic, the Sennheiser MKE 400, was that the foam wind sleeve kept coming off, and after a few times I finally lost it.)

When shooting people who are talking, a wireless lav mike is the common choice, though they take time to set up and to get the transmission right. Last year I wrote a number of plans for smart wireless digital microphones and I still am interested in most of those, particularly the idea of using beam forming with multiple microphones to focus on sound at the location and distance the camera is pointing and focused on. Bluetooth remains valuable because it's cheap and useful, and should be in all cameras anyway for a variety of reasons. It means no fiddling with plugs or even the need for plugs.

There's an immediate benefit to having the microphone be some distance from you, even on the hood of your vehicle as you stop to photograph wildlife -- it just won't pick up as much sound from you and your cameras, and the cameras of others. You can't aim such a microphone, though, unless you have a beamforming array. With techniques I am about to describe, you could actually place an array of microphones around the edges of the vehicle and then use beamforming after the fact to isolate your sounds.

With any digital protocol you are going to get a problem with latency and sync. (You will have such a problem on any remote sound. For example, an elephant 100 feet away is going to have 100ms of latency on the sound due to the speed of sound itself.) To help solve this, I propose a protocol where you aim the video camera at the microphone and it is triggered to pulse an LED, noting the time it pulsed the LED into the audio stream. The camera puts it in the video stream. You can then sync the audio and video superbly, and re-sync when you need to. Though frankly clock drift in modern electronics should be quite minor when it comes to audio sync.

While having lots of controls on your camera would be great, it may make more sense to just build standalone microphones that record all the hear to flash. Using the LED pulse, you can sync their audio after the fact. This allows fairly fancy audio to be done in the microphones without requiring anything in the cameras -- not bluetooth or wireless receivers. You would have to set audio levels at the microphones unless you also had transmission. While I am not an expert in beamforming, my understanding is that with high speed sampling of the sound field, it should be possible to combine the microphones after the fact and get better isolation of the desired sounds. The implementation of a compass/gyros in both the camera and the microphone array could help automate this, so that we have an idea where the camera was pointing.

The use of a wireless protocol or independent recording even makes sense when you are putting the mic on the hotshoe of the camera. It saves the trouble of having to plug a physical wire in, and lets you quickly remove the mic and put it somewhere while still using it. (You might find, though, that you are still using it even when you stuffed it in its bag.) The easy solution to that is to always have the camera record multiple audio tracks. The internal microphone track should always be recorded, even if an external mic is active, or at least this should be an option. All available microphones should be recorded, so I can choose what I want or mix after the fact. If I carry a microphone on myself that can be recorded either to emphasize my voice, or to make it easier to remove it later. Pure bluetooth audio is probably not enough, both because of data rates and errors. You would want some compression and error correction for real quality work.

It might also make sense to have a cheap microphone for no other reason than to pick up wind noise, so you can know when to mute it out of other microphones near the cheap one.

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