When you’re recording digital audio, there are two main settings that you will come across at some point – bit depth and sample rate.
We won’t get into sample rate today. I’ll save that for a future article. To summarize, sample rate measures how many times per second the audio is “sampled,” or measured. Common sample rate values are 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz, and 192 kHz. Is higher better? Hmm…I have my own take on this, but that’s for another day.
For today, just know that if you record at 44.1 kHz, that means the volume of the audio signal gets measured 44,100 times per second. When you put all these tiny measurements together, you get a waveform.
Okay, moving on.
What is bit depth?
Sample rate determines the frequency with which the system measures the volume of the audio. Bit depth determines how many different volume measurements the system has to work with.
In other words, if you think of the audio as being measured by a ruler, the bit depth is how many notches that ruler has. Some rulers only measure in whole inches (low bit rate), while others allow you to measure within one-sixteenth of an inch.
A ruler with more notches allows for a great number of measurement options, and therefore a more accurate measurement.
Comparing Bits to Inches
Okay, so the higher the bit depth the more individual measurements we can achieve. How does that relate specifically to recording?
If you’re recording at 44.1 kHz, then you’re telling your analog-to-digital converter (i.e. your audio interface) to take a volume measurement of the audio once every 1/44,100th of a second. How does it measure the volume? In bits.
1 bit = 6 dB
“dB” stands for decibel. It is logarithmic measurement of volume. If you increase the level of a signal by 6 dB, it will sound twice as loud.
So, for our converter to measure the signal at 2 bit rather than 1 bit, it needs to be twice as loud.
The 16-Bit Ruler
Digital systems don’t have “in between” measurements. Everything is cut and dry in the Land of Didge (shout-out to Slau).
So, the smallest unit of measuring volume in a digital system is 1 bit. There’s no 1.5 bit or 1.42983003 bit.
This means that in a 16-bit system, you have 16 notches on your ruler. 16 potential measurements for your audio. That may not sound like a lot, but keep in mind that these measurements are being taken thousands of times per second.
Alright, we said that 1 bit equals 6 dB of dynamic range (or volume). What is the dynamic range of a 16-bit recording? The answer is 96 dB.
96 dB? That’s great, right? Sure it is…in a perfect world.
The problem you run into is noise. Every audio system out there has some amount of inherent noise.
In other words, no recording system is perfectly quiet. The electrical components generate a low-level noise. Each piece of your system contributes to the noise party. All of this noise adds up, and it’s called the noise floor.
This doesn’t even take into account any room noise that might get picked up by a microphone.
The noise floor essentially “steals” away some of your dynamic range. Let’s say that all the noise added together was 18 dB. That’s 3 bits.
Since this noise occupies the bottom 3 bits of your system, the level of your audio needs to be recorded ABOVE 3 bits (or 18 dB), or it will be lost in the noise. So instead of having 96 dB of dynamic range, you realistically only have 78 dB (or 13 bits).
The gap between your recorded signal and the noise floor is getting smaller. This means that if you don’t record your signal loud enough, you’ll end up hearing this noise in your recordings. On the flip-side, if you record your signal too loud (to stay well above the noise), you’re in danger of clipping.
24-Bit to the Rescue!
Enter 24-bit recording, super-hero cape blowing in the wind.
Now, instead of giving your converter 16 measurement options, you’re giving it 24. And if you kept your calculator our from earlier, then you know that 24 bits x 6 dB = 144 dB dynamic range!
An audiologist will tell you that our ears aren’t even capable of hearing a full 144 dB of dynamic range. However, having this much available dynamic range allows you to create greater separation between the recorded audio signal and the noise floor.
When you add in the 18 dB of noise we have in our make-believe system, and you drop the usable dynamic range down to 126 dB, you still have a TON of breathing room left.
Check out this diagram:
As you can see, the 16-bit system is still fairly close to the noise floor. The 24-bit system, however, towers above the noise floor, making it much less of an issue when recording.
In a 24-bit system, you don’t need to record the levels super-hot, because you’re signal is not nearly as likely to drop down into the noise floor. This leads to better sound quality, less noise, and less stress when recording.
Do you record at 24-bit? Leave a comment.
[Photo by internets_dairy]