What’s “dithering”? And why is it used on CDs?
In: Technology
TLDR dithering is a tiny, quiet amount of noise that’s added to the audio file so when it’s converted to a CD-quality audio file, the file converts more cleanly.
This is a tricky one for me to explain to a 5YO, but I’ll do my best to get as close to a 5YO-level explanation that I can. First, try to imagine the typical graph of a sound wave, the big wavy line that goes above *and* below the x axis. This is the image I’ll be working from.
Let’s first talk about capturing audio on a computer. Computers work with binary code – all information stored is either a 1 or a 0. This makes it hard to record a constantly moving, constantly changing source like a sound file, a signal that has infinite points over time. So, the best way to record audio on a computer is to take “snapshots” of the audio file. As the sound comes in to the computer, we take a picture of where we are above or below the x axis.
This leads us to two terms:
Sample rate – how many times per second we take a picture
Bit depth – how many 1s and 0s we have at our disposal to say how far above or below the x-axis we are. The higher the bit depth, the louder the sound we can record.
Most audio these days are recorded at a sample rate of 48000Hz, which means we’re taking 48,000 pictures a second, and it’s recorded at 24bit, which means we have 24 1s and 0s as our loudness resolution.
CDs are written at 441000Hz and 16 bit, so we lose quality when we go down. When we trim 8bits from every snapshot, we run the risk of introducing noise into the audio file. We cut those 8 bits from the bottom of the audio wave, we might have some low level noise that exists below the threshold of 16bit and is then registered as silence, but sometimes that noise will peak back up into the 16bit spectrum and suddenly you have noise. So in the conversion from 24 to 16 bit, we can have instances where we have digital silence and then little spurts of low level noise. Not ideal
This is where dither comes in. It’s super quiet noise that is just loud enough to still be audible in the 16bit world, but so quiet that virtually no one can hear it or notice it. We put dither on an audio wave before we convert it to CD quality – we just print it on to the audio file.
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