The quality of an audio file depends upon the sample rate, bit rate, file format, the method of encoding, and the ability of the encoder to get the important bits right. This post will cover some basic concepts of digital audio and how they affect audio fidelity. I’ll focus on sample rate, bit depth, and bit rate, plus a few related topics.
- Sample Rate Explained
- Bit Depth Explained
- Bit Rate Explained
- What Sample Rate and Bit Depth Should I Use?
The sample rate or sampling frequency refers to the number of samples recorded every second. A sample is the signal amplitude, and it contains the information of the amplitude value of the signal waveform over some time. The sample rate is measured in hertz abbreviated as Hz, or Kilohertz abbreviated KHz, where I kHz is equal to 1000 Hz.
The higher the sample rate, the better a digital signal compares to the original analog audio signal. Also, the higher the sample rate, the higher frequencies a system can record.
It is important to note that you cannot upgrade the sample rate of an audio file once it is recorded. If you record a musical instrument at 44100 samples/second, you can not convert it to 96000 samples/second. If you manage to convert the sample rate to a higher value, the playback audio will still be 44100 samples/second. Because of this, it is important to take note of the sample rate before recording.
For most applications, a sample rate of 44100 Hz or 44.1 kHz is used. This format is used in CDs, Mp3s, or AAC files like the ones sold by the iTunes store. The reason for sampling at 44.1 kHz is because of a theorem called the Nyquist Theorem.
The Nyquist theorem or Nyquist frequency states that to prevent loss of information when sampling a signal digitally, you have to sample at a rate of twice the highest expected signal frequency. So, when sampling at a rate of 44.1 kHz, it allows for accurate reproduction of audio frequencies up to about 22.05 kHz.
Choosing to work with higher sample rates tends to be in multiples of 44.1 kHz or 48 kHz, like 88. kHz, 96 kHz, 176.4 kHz will result in higher Nyquist frequencies. This allows for supersonic frequencies to be recorded and recreated.
When audio is sampled, it is stored in bits. This is where bit depth comes into play. The bit depth determines how much information can be stored. The bit depth shows the number of possible amplitude values in a sample. You can calculate the number of possible amplitude values of a given bit depth. This is done using equation 2 to the power of n (substitute n with the bit depth).
Using the equation;
- A bit depth of 16 will be 2^16 = 65536 possible values
- A bit depth of 24 will be 2^24 = 16777216 possible values
The bit depth also relates to the dynamic range. The more bits you have, the wider the dynamic range. A wider dynamic range means three things;
- Better signal-to-noise ratio
- Better precision when mixing
- Less worrying about headroom because you do not have to run your levels so hot.
In a nutshell, an audio sample file with a higher bit depth will have a higher resolution. Lower bit depth sample audio will produce a lower signal-to-noise ratio but will yield a smaller file size.
Bit Rate or BitRate refers to the number of bits of processed data over a specific time, which usually means kilobits per second, abbreviated Kbps. For example, if a music track is 320 kilobits per second, there are 320 kilobits of data stored in every second of the track.
If a track has a higher bitrate, it will take up more space on your computer. Compressing a track to reduce the bitrate, reduce its size, and the quality of the track.
The bitrate of a CD is always 1411 kilobits per second. MP3s can range from 96 to 320 Kbps, while streaming services like Spotify offer around 96 to 160 Kbps.
Does bitrate matter?
There is no best bitrate, only the right bitrate for your purpose. If you’re listening to a track, a higher bitrate will equate to high-quality audio, provided the track has not changed, and the sample rate and bit depth are also high.
Audiophiles favour high bitrates. However, if you’re using a higher bitrate track, make sure you have quality hardware to reap the results.
If you’re looking to save on the small storage space available on your computer, a lower bitrate should be acceptable. However, this will come at the expense of lower audio quality.
For sample rate, 44.1 kHz is the best to go with for most music applications. For creating music or other audio for video, 48 kHz is commonly used. A higher sample rate can have advantages for audio professional work or professional music. However, 44.1 kHz remains the most preferred by many professionals.
For the bit depth, 16 bits is perfectly fine for consumer or end-user applications. For mixing, mastering, or professional video editing, a bit depth of 24 bits is good because it ensures better precision and a good dynamic range when editing.