Bit depth and sampling rates

Bit depth and sampling rates
Bit depth and sampling rate are the two things that determine the fidelity of a digital audio file. You may have seen pairs of numbers rattled off in equipment brochures such as 16/44.1, or 24/96. The first number generally refers to the bit depth, the second to the sample rate.

In the previous section, we talked about taking measurements of an analog input at specific intervals. This is the sample rate. As the sample rate increases, the digital representation more closely resembles the analog original. You must choose a sample rate that is high enough to give you the fidelity you require.

Bit depth is the other variable in the fidelity equation. Bit depth refers to the number of bits used to represent the measurement of the analog voltage. For example, you could represent the distance to the nearest street corner as a block, 50 yards, 143 feet, or 1,717 inches. As you increase the accuracy of your measurement, you need more digits to represent the number. The same is true for our digital samples of our audio input.

In the binary world, each bit you add doubles the range of numbers you can represent. Looked at in another way, each bit you shave off your bit depth cuts your accuracy in half. If we use eight bits, we can have up to 256 different values. With 16 bits, we get more than 65,000 possible values, and 24 bits provides more than 16 million values. So how much accuracy do we need?

Choosing your digitization settings
Ideally, you want to record using the highest sample rate possible, and use the largest bit depth you can. The problem is that using high sample rates and large bit depths creates larger files. Increasing the bit depth from 16 to 24 bits increases the file size by 50 percent, and increasing the sample rate from 44.1KHz to 96KHz more than doubles the file size. Even though storage is relatively cheap and getting cheaper all the time, there are some practical limits to how much fidelity you really need, especially given that podcasts are encoded using lossy codecs such as MP3 that compromise the fidelity. A better master provides a higher podcast quality, so how much fidelity do you really need?

Compact discs use 16 bits and a 44.1 KHz sample rate. Most people consider this sufficient to capture the full range of audio that the human ear is capable of hearing (though many audio engineers would disagree). This is probably what you should use for your master recordings. Although many recording devices and audio editing platforms now offer higher sample rates and bit depths, it's debatable whether the extra quality justifies the increase in storage requirements. You'll just end up using lots more storage space for your archived masters.

If you're producing your podcasts to an extremely high standard, consider using 24/96 (24 bits/96KHz sample rate). This "future-proofs" your masters and will impress all your audio engineering buddies. For most folks, however, the standard 16/44.1 setting will more than suffice.

Tip If you're producing a video podcast, Digital Video (DV) audio is sampled at 48KHz, or sometimes even at 32KHz. These sampling rates were chosen when digital video standards were developed. They're different from the 44.1KHz audio sampling rate, because the sampling rate for audio predates them and was chosen due to technological restrictions at the time. This is fine; the key is not to resample the audio to 44.1KHz (or any other sampling rate). Re-sampling introduces artifacts and degrades the quality of the original audio.