Steven Spielberg has famously said that “Sound is Half the Picture” - but it’s actually a little more complicated than that. Really bad unlistenable sound will ruin a picture faster than bad lighting or shoddy camera work. And in some cases like documentary or corporate industrial work, good clean sound is actually more important than just about anything else. Fortunately, getting decent sound isn’t that hard if you understand the fundamentals of recording and implement some solid practices.
So for this discussion lets imagine recording sound as a signal chain. At the source a microphone converts sound energy into analog electric signals. This signal is carried down a cable and into a preamp on an audio recorder or camera where it is converted into a digital file. Now let’s dive in and look look each piece of this signal chain starting with the end.
The Audio Recorder
The first thing you need to decide when recording digital audio for video is whether to use the single or double system for recording
In a single system setup, audio is fed directly into the camera and recorded with the image. In a double system or dual system, sound is recorded onto an independent dedicated audio recorder. Sound from the camera is still recorded if it’s available but used as a sync or scratch track.
Let’s compare each setup. With a single system, recording audio with video means there is no need to sync up the footage in post production. This can be a huge time saver especially in tight turnaround situations like the news or documentary. With a camera designed for broadcast that has a robust and professional audio inputs and a preamp, the single system also avoids the cost purchasing additional recorders. Video codecs have predefined settings for audio, most of them recording 16 bit, 48khz uncompressed audio - more on what that means in a second.
If you want hassle-free synched audio without having to mess with audio settings and you have a camera that has good sound inputs -the single system may be a good choice for you.
Why would anyone use the double system? First and foremost is if you don’t have a camera with professional audio inputs like many DSLRs but there are 3rd party preamps on the market that can make any camera into a single system setup for audio.
But in my opinion there is a better reason - The major benefit of the double system is audio quality. Digital audio recorders have some great features that make for better recording. The first is higher sampling rate.
When an analog signal is converted to digital, the smooth analog curves of the wave signal have to be quantized - that is split up into samples and the amplitude measured.
How many times we sample the wave determines how accurately our digital representation matches the original analog waveform. We call this sampling rate and it’s measured in kilohertz - not to be confused with the frequency or pitch of a sound wave. At the low end we have values like 11 kHz - that’s 11,000 times per second we sample the audio. This is used for low quality internet voice transmissions. It doesn’t really sound that good but it makes for small file sizes. 44.1 kHz 44,100 samples per second is CD quality audio. 48 kHz is the standard for digital video. This rate was chosen because it could deliver a 22 kHz frequency response (that’s refering to pitch) and work with 29.97 frames per second NTSC video - as well as 25 frame/s, 30 frame/s and 24 frame/s systems.
But I really like to record audio at 96 kHz. That’s twice the sampling rate of 48 kHz and to me that extra resolution just sounds better. I’m not sure I could pick one or the other in a blind hearing test but there just seems like something translucent about 96 kHz that 48 doesn’t have. Besides just sounding better, having extra resolution makes post processing 96 kHz audio easier as we’ll discuss in the next lesson.
Dedicated audio recorders can go up to 192 kHz - that’s 4 times the standard of 48 kHz, but to my ears that’s sort of overkill.
Besides sampling resolution, dedicated audio recorders can also deliver greater bit depth. Bit depth is how many different values of amplitude each sample can be.
With 16 bit audio - each sample can have one of 65,536 values - that’s 2 to the 16th power. That’s what most professional cameras and codecs record but with a good dedicated audio recorder you can record at 24 bit which gives each sample 16,777,216 possible values - This extra resolution contributes to that translucent quality and ease of processing in post.
Another reason to like recording double system is you are no longer tethered to the camera. This is really useful in situations where the camera needs to be moving like on a steadicam or a dolly where cables can easily snag. This is also a consideration if you’re shooting events and you don’t want to run long cables between a mix board and the camera.
Dedicated audio recorders also have the ability to compress audio wave files and record MP3 files. For recording audio for film this is a no-no. Always record uncompressed -that’s WAV files unless you have a dying need to conserve space on your recording medium - say you need to record 8 hours continuously and you can’t get a bigger card. Compressed audio throws away a lot of useful information that will come in handy in the post processing side and with today’s memory capabilities uncompressed audio files aren’t really that big and problematic to deal with (certainly dwarfed by the size of your video files).
So for higher quality sound and freedom of movement, I’m a big proponent of the double system recording at 96kHz 24bit uncompressed wav files. You will have to sync the audio in post but you can use slates to line up the audio on each shot or use sync programs so long as you record a scratch audio track on your camera. But if speed and ease are your goal, there’s nothing wrong with sacrificing a little bit of quality to record synced sound using the single system.