Sampling rate and resolution
Two Properties, Two Dimensions of Quality
A digital sound recording is defined by two independent properties: sampling rate and sample resolution. Together, they determine both the quality of the recording and the size of the resulting file. Improving either property increases quality - but also increases storage requirements.
| Property | Definition | Unit | Affects |
|---|---|---|---|
| Sampling rate | The number of samples taken per second | Hz (or kHz) | How accurately the wave shape is captured over time |
| Sample resolution | The number of bits used to store each sample | bits per sample | How precisely each amplitude value is measured |
Understanding Each Property
Sampling Rate
The sampling rate is the number of amplitude measurements taken every second, measured in hertz (Hz) or kilohertz (kHz). One hertz equals one sample per second. A higher sampling rate means more snapshots are taken of the wave, capturing rapid changes in the sound more accurately.
If the sampling rate is too low, high-frequency sounds - those that complete many cycles per second - may not be captured correctly. The standard for CD-quality audio is 44,100 Hz (44.1 kHz). Voice calls typically use 8,000 Hz, which is sufficient for speech but discards much of the detail present in music.
Sample Resolution
Sample resolution (also called bit depth) is the number of bits used to store each individual amplitude measurement. With n bits per sample, 2n different amplitude levels are available. A higher resolution means finer gradations between levels, producing a more precise measurement of the wave's height at each sample point.
With 8-bit resolution, 256 possible amplitude levels are available. With 16-bit resolution, 65,536 levels are available. The difference between the true amplitude and the nearest storable level is called quantisation error. A higher resolution reduces this error, resulting in a cleaner, more accurate recording.
How the Two Properties Work Together
Sampling rate and sample resolution are independent: changing one does not affect the other. However, both contribute to the overall quality of the recording, and both increase file size when raised. Real-world audio formats balance the two:
- Telephone voice: 8,000 Hz / 8-bit - compact, low quality, sufficient for speech.
- CD audio: 44,100 Hz / 16-bit - high quality, widely used for music.
- Professional studio recording: 96,000 Hz / 24-bit - very high quality, used in production and mastering.
The combined effect on file size is multiplicative: doubling the sampling rate doubles the file size, and doubling the resolution also doubles it. Increasing both doubles the file size twice over (i.e. multiplies by 4).
Key Takeaways
- Sampling rate is the number of samples taken per second, measured in Hz. A higher rate captures the wave shape more accurately over time.
- Sample resolution (bit depth) is the number of bits per sample. With n bits, 2n amplitude levels are available - more bits means finer precision.
- Both properties are independent: sampling rate controls accuracy over time; resolution controls precision of each measurement.
- Increasing either property improves quality but increases file size proportionally. Increasing both multiplies the increase.
- CD-quality audio uses 44,100 Hz and 16-bit resolution - a widely used balance of quality and file size.