Glossary definition of 'Super Noise Shaping'

(SNS) A family of noise shaping algorithms that can operate at sample rates between 44.1 kHz and 192 kHz and is used to reduce longer word lengths to standard 16 bits, while retaining a high degree of perceived dynamic efficiency and very low noise.

SNS offers four different algorithms which increase the degree of shaping. SNS spectra are adjusted automatically to provide optimum subjective advantage at each different sample rate. The four SNS algorithms are designated SNS1 to SNS4, in increasing order of the degree of shaping. The spectra of the four SNS algorithms are shown below. Note that, unlike some noise shaping algorithms, SNS spectra are adjusted automatically to provide optimum subjective advantage at each different sample rate and wordlength. The spectra are shown below for 16-bit output, at 44.1kHz, 48kHz and 96kHz sample rates only.

SNS1 provides only a very small subjective noise advantage, but only applies limited noise-lift at quite high frequencies. In many applications (particularly those where the program material is already quite noisy) this type of shaper is very often preferred.

SNS2 is a happy medium. It provides a good amount of subjective lowering of the noise floor, but with addition of only moderate amounts of high-frequency noise. It also has the advantage that the noise floor remains subjectively white, even when artificially amplified.

SNS3 and SNS4 are ‘optimal’ shaper designs – their shaping is quite extreme in order to get the maximum theoretical subjective improvement in noise performance based on an average human low-field sensitivity curve. This results in the addition of larger amounts of high frequency noise.

It is difficult to assess the difference in sound between different noise shapers for any given program material, since their effects are at very low amplitudes (the 0dB line on the plots below represents flat dither with an RMS noise amplitude of about –93.4dBFS). It is tempting to audition noise shapers by using a low signal level and boosting the shaper output by tens of dBs in the digital domain prior to monitoring (using, for example, the ‘microscope’ feature on Prism Sound test equipment). Using this method it is easy to hear that the noise floor of more extreme shapers is clearly not white – switching, say, from SNS1 to SNS4 sounds like shhhhh..ssssss as the noise is shifted towards the higher frequencies. However, this is not really a meaningful test since the sensitivity of the ear at different frequencies is very dependent on level, and the design of the more extreme shapers is in any case intended to render the noise floor completely inaudible at normal listening levels. Ultimately, the only ‘right’ choice of noise shaper is the one which sounds best for the material.

The Prism Sound SNS logo shown above is found on many of the world's finest CDs, and is recognised as a standard of technical excellence.






see also:

•     Dither
•     Noise shaping