Before we jump into the wide world of audio gear, it might be helpful to quickly go over how audio is actually recorded. This will help you understand what will work best for you and your setup. If you’ve jumped ahead and want to navigate to the previous lesson, you can click here.
Microphones: Capturing a Wave
Audio recording starts with a microphone, which is a transducer: it takes one type of energy and transforms it into another type of energy.
So what types of energy are we talking about? On the acoustic side, we have sound, vibrations or pressured waves that travel through the air. The microphone’s job is to take that and transform it into an electrical signal.
For the purposes of on-camera sound, I’m going to say that there are only two types of microphones to consider: dynamic and condenser. Both types essentially do the same thing: they take acoustic energy and transform it into electrical energy. This electrical signal travels down a cable into something called a preamp.
Preamps: Amplifying the Signal
The preamp’s job is to take the electrical signal and amplify it because when it comes out of the microphone, what is generated by the microphone’s capsule is tiny. This makes the signal pretty much useless, and it needs to be amplified before the next stage of the process. The next stage is the conversion from analog information, the electrical waveform, into digital bits, ones and zeros.
When the signal gets to that point of analog-to-digital conversion, sometimes you’ll have some options based on your camera’s system and the audio chips that are inside it as to how the signal is converted into digital bits. There are two things you’ll have to understand about that: bit depth and sample rate.
Sample Rate
First, let’s talk about sample rate. Sample rate is the number of times per second the audio is sampled. Without getting into a huge technical discussion, I’m just going to give you the answer here: 48 kilohertz. That’s the answer to the question, “What sample rate do you choose?”
Why 48 kHz? The number one reason is that it sounds awesome! Are there higher-resolution audio formats available out there? Yup, there’s 96 kHz or 192 kHz. Most people cannot hear the difference, especially considering that your video will be consumed primarily on a mobile device. Someone who has fully decked-out audio gear and a treated room will probably watch your video, but overall, it’s a safe bet that it will be consumed on mobile. When you are out in the world, there is noise, and where there is noise, it becomes impossible for even a well-trained ear to tell the difference between 48 kHz and a higher sample rate. So in this case more is not better—it’s just more memory.
Audio actually doesn’t take up that much memory in general, so why not record at the highest rate? The answer is that you just don’t need to. Your time would be better used trying to get a better signal-to-noise ratio, working on your mic placement techniques, working on your audio processing, and working on your audio processing in post-production. Don’t worry about sample rate—just stick to 48 kHz. It’s what I use, and it’s what a lot of people use. It sounds fantastic, and you can get wonderful-sounding recordings by sticking with it.
Bit Depth
It’s time to get into bit depth. Now don’t be confused—this is different from bit rate, which is the number of bits per second. Bit depth is the total number of bits of information that each one of the samples can hold. This directly corresponds to the resolution of each sample. Now, resolution in this case refers to amplitude resolution, which is basically your signal-to-noise ratio: how much signal you can record over the noise. 16-bit audio has a maximum theoretical signal-to-noise ratio of 96 dB, whereas 24-bit audio is 144 dB. Those are theoretical maximums because there’s noise that’s generated at several points in the audio recording process, so you can’t actually get to those.
You may be thinking that 144 dB is much louder than 96 dB, and if we were talking about sound-to-pressure ratio out in the real world, you would be very right—144 dB would melt your face. We’re not talking about loudness levels here, though—what we’re talking about is the amount of range you get over the noise, and that, unlike sample rates, is something that you can hear. So if you have the option to record in 24-bit, go that route.
Audio Codec
The last thing to talk about is the recording codec, or the audio codec. So you’ve converted your audio from analog to digital, you’ve selected the preferred parameters (48 kHz, 24-bit), and now we need to save it. How are we going to save it, and what format do we choose?
Some cameras don’t have an option when it comes to how the audio is encoded. If your camera records to something like AVCHD, H.264, or H.265, it’s going to be AAC audio. Is that terrible? No, but it is a lossy codec, meaning it takes information you could actually hear and throws it out the window in order to make the file size smaller. But as we mentioned before, audio doesn’t take up that much space, so there’s no need to do that. It’s a bad route to go down because it creates a painful experience when it comes to editing the audio in post-production.
So if you have the option to choose between a lossy codec like AAC and a lossless codec like WAV or AIFF, choose the lossless audio. You may have to look up what your camera calls each codec if you have options: it may be PCM, LPCM, WAV, AIFF, etc. If it says it’s a lossy codec on the wiki, choose the other one. If you don’t have a choice, don’t worry about it, but when it comes to getting a new camera, make sure you have the option. If you can, you want to choose uncompressed audio—this is going to make post-processing so much easier.
Wrapping It Up!
With that, you now know the basics of audio recording. I did talk about some more technical things, but don’t let that confuse you. The two main takeaways that you need from this are the general signal flow that happens (the microphone, cable, preamp, and analog-to-digital conversion) and how to choose a sample rate (48 kHz), bit rate (24-bit), and codec (uncompressed).
So now that you have an understanding of the audio recording process, watch out for the next tutorial in this series, in which you’re going to learn about mic placement and signal-to-noise ratio.
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