Sampling
Definition: The process of converting analog data into digital data by taking a series of samples or readings at equal time intervals.
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In a graphical form, an analog signal can be represented as an electrical waveform against time. It is the ADC's job to turn this information into binary values along the same time axis. This is best demonstrated through a simple wave diagram.
In the example above, an arbitrary wave form is plotted on an amplitude vs time graph. This shows the peaks and troughs of the wave which are to be converted into digital data. For this process to work, a few rules must be established before it can convert. The sampling rate must be decided first. This is the number of times the A/D converter samples the incoming waveform each second. This is a fixed value for the entire process, for example 44,100 times a second. The diagram above shows what a zoomed in image looks like of the sample points. The red dots indicate the points which the A/D will measure and store data. The second diagram shows the digital representation of the outcome which is stored as a discrete signal (i.e. not relational to time). This is then what any computer device can understand when manipulating sounds.
The problem with using lower sampling rates is that sometimes too much information is lost in between the samples. This can often miss out a lot of the harmonics of a sound which play a vital role in the timbre of an instrument or the tonality of a song.
This is demonstrated using the two audio files below. They are both from the original file (44.100hz) but one one has sampled to 8khz.
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At lower sample rates, the effect is more obvious but it still follows the same pattern through the entire sampling range.
Depending on the studio, it is quite common to record in 44.1khz as this is the sampling rate used for CD's. Also, a lot more hard drive space is used when recording in a higher sampling rate. Some people argue that they can hear no difference between recording in 44.1khz or 192khz so see no reason to use anything higher. However, it is always down to the individual.
Below is a list of the common sampling rates:
8,000 Hz - Telephone Quality
11,025 Hz and 22,050 Hz - used for lower-quality MPEG audio
32,000 Hz - mini DV
44,100 Hz - Audio CD. MPEG-1 audio. Also for MP3, Video Cd& Super Video CD
48,000 Hz - mini DV, Digital TV, DVD, DAT
96,000 or 192,000 Hz - DVD-Audio, BD-ROM (Blu-ray Disc) audio tracks, HD-DVD (High-Definition DVD) audio tracks