Noise – Part 2
Also, poor sound engineers love to shake and level everything using limiters and compressors, the principle of which is based on reducing the dynamic range.
Almost all samples go through all this torture. Even when using a simple EQ, the signal passes through a digital filter, which introduces rounding noise by at least half a quantization step. During final mixing, all samples are collected in a sequence, respectively, the noise from each being added to the noise from another resampling. But that’s not all: during playback, the DAC adds its own noise and rounding noise. Can you imagine what really remains of the useful signal?
Noise control techniques
To remedy this unfortunate situation, special noise reduction technologies have been developed. Let’s see the most basic.
Oversampling
Oversampling technology has been used since the days of multi-bit DACs to compensate for losses caused by noise. The principle of oversampling is that intermediate samples are added to existing discrete samples, repeating the approximate waveform. Intermediate samples are calculated by mathematical interpolation or filled with zero values and transmitted to a digital filter. Generally, both approaches are called interpolation and the digital filter is called interpolation. The simplest interpolation method is linear interpolation, and the simplest digital filter can be a low-pass filter.
Below is an illustration of an interpolation algorithm for a discrete signal with a factor of 2. Red dots indicate the original signal samples, solid lines – a continuous signal, representing these samples. Above is the original signal. In the middle is the same signal with inserted zero counts (green dots). Bottom: interpolated signal (blue dots: interpolated sample values).
At first they started using only oversampling with an increase in frequency, for example from 44.1 to 176.4 kHz. Subsequently, oversampling was used with an increase in the sampling frequency and an increase in the quantization bit depth; This process is called recantization.
Although oversampling introduces rounding noise, it also reduces overall noise density by expanding the dynamic range of the signal, and post-processing of the signal will have less impact. Each doubling of the sample rate expands the dynamic range by approximately one quantization step (6 dB) minus the rounding noise.
Just to be able to use oversampling, they began to produce multi-bit DAC chips that supported up to 192×24 digital stream on input. DSP (digital signal processor) -based hardware upsamplers also appeared.
Of course, the use of oversampling technology improved the characteristics of the audio signal, but the situation did not change drastically: the noise level remained high. Therefore, other technologies began to be applied.
