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What is Sampling?
To convert analog signals to digital, two processes are required: sampling and quantization.
Sampling refers to the extraction of samples from a continuous analog signal at regular intervals. In this step, the analog signal is momentarily sampled and represented as discrete numerical values for digitization.
Quantization maps the sampled analog values to their closest discrete values. The bit depth determines the number of possible values that the signal can be represented by. Higher bit depths allow for a more refined representation of values, thus enabling a more accurate reproduction of the signal.
Through these processes, the analog signal is represented in a digital form for storage, processing, and transmission, and it can be restored to an audible analog signal for playback
Sampling rate?
The sampling rate refers to the frequency of the sampling process in hertz (Hz).
For instance, a 44.1kHz sampling rate means that 44,100 samples are taken per second. Higher sampling rates can capture a wider frequency range, thus more accurately reproducing higher frequency components. CD-quality audio typically has a sampling rate of 44.1kHz.
Why was 44.1 kHz chosen?
The adoption of 44.1kHz as the CD standard sampling frequency was determined by a combination of factors:
- Sound Quality: A sufficient frequency bandwidth is needed to maintain CD quality. Human hearing generally ranges from 20Hz to 20kHz, so a sampling frequency of at least 40kHz is required according to the Nyquist theorem.
- Video Compatibility: CD formats also had to be compatible with video standards like NTSC and PAL. Considering the field frequencies and clock synchronization coefficients of NTSC and PAL, a compatible sampling frequency above 40kHz was selected.
- Technical Limitations: When CDs were introduced in the 1980s, digital audio technology had not yet fully developed. Higher sampling frequencies would require more storage and computational power, presenting technical limitations. Therefore, the optimal sampling frequency was chosen based on both sound quality and technical limitations.
Taking these factors into account, the CD standard of 16-bit 44.1kHz was adopted.
Why has 48kHz become the current digital standard?
The adoption of 48kHz as the standard in digital audio is related to DAT (Digital Audio Tape):
DAT, developed by Sony, was a tape system designed for digital audio recording and playback. It underwent an analog-to-digital conversion (ADC) process that involved anti-aliasing filters to eliminate high-frequency signals.
These filters aimed to remove signals above 20kHz to prevent distortion. However, it was technically challenging to completely remove signals above this frequency at that time. Considering the performance of the anti-aliasing filters, 48kHz was chosen.
Subsequently, ADAT (Alesis Digital Audio Tape) was developed, designed to record and play multiple audio tracks simultaneously. To maintain compatibility with master DAT recorders, ADAT also used a 48kHz sampling rate.
As a result, 48kHz was chosen for its anti-aliasing filter performance and compatibility, influencing its adoption as the current digital audio standard.