Noise – Part 3
Recording audio
To record and mix the audio signal, they started using decimation, this is the reverse process, oversampling with downsampling and quantization bit depth. The signal is recorded at a high sample rate and quantization bit depth, for example 176.4 or 192 kHz with a 24 bit bit depth, and removing some of the samples using a digital filter is “compressed” to the CDDA standard – 44.1 kHz, 16 bits. This approach can slightly reduce quantization noise.
Below is an illustration of the algorithm for decimating a discrete signal with a factor of 2. Red dots indicate samples, solid lines – a continuous signal, representing these samples. Above is the original signal. In the middle, the same signal after filtering on a digital low pass filter. Below is the decimated signal.
Dithering
Dithering (dithering) – A method of mixing pseudo-random noise when digitizing or playing a sound signal. This technology has two purposes:
linearization of the quantizer / requantizer transfer function;
decorrelation of quantization errors.
Quantization noise has a correlation, that is, a relationship with the main signal. This creates parasitic harmonics that follow the waveform. They affect perception creating a “diffuse” sound. Correlation can be removed by adding specially patterned noise to the main signal, thus converting the correlated quantization noise to ordinary white noise. This increases the overall noise level a bit, but is good for perception.
Dithering in the image processing example: before and after
Dithering in the image processing example: before and after
Noise modeling
Noise Shaping (NS) technology can significantly reduce noise introduced during quantization, re-entrapment, and dithering.
Noise modeling works like this: the quantized signal at the input is compared to the signal at the output of the requantizer, a difference (error) is formed, which is subtracted from the main signal. This compensates for distortions introduced by the requantizer and during the dithering process. A so-called feedback is formed, which seeks to compensate for the error in the input and output of the requanter. This technology works like negative feedback in an op amp, except that all conversions are done digitally.
Here’s a diagram of a first-order requantizer, but as a rule, requanters are used up to order 9-12.
Here’s a diagram of a first-order requantizer, but as a rule, requanters are used up to order 9-12.
This technology has its drawbacks. Using NS introduces a large amount of noise in the high frequency region, making it necessary to apply a low pass filter, with a cutoff frequency close to the upper cutoff frequency. In practice, together with NS, dithering is also always used, the result of their joint work is much better by ear.
Dynamic item matching
Dynamic Element Matching (DEM) is a technology that generates various signal levels at the DAC output. It looks like a cross between a single bit and multi-bit DAC. DEM is used to reduce deterministic errors when using sigma-delta modulation (SDM). These errors, like quantization noise, are highly correlated with the signal at the one-bit modulator output and therefore significantly affect the perception of the audio signal.
This technology also reduces the requirements for the analog filter, because the waveform is close to the reproduced waveform even before filtering. DEM is implemented with several pins connected to a common bus, which form the output signal of the DAC.
