spandsp 0.0.6

Noise generation

# What does it do?

It generates audio noise. Currently it only generates reasonable quality AWGN. It is designed to be of sufficiently low complexity to generate large volumes of reasonable quality noise, in real time.

Hoth noise is used to model indoor ambient noise when evaluating communications systems such as telephones. It is named after D.F. Hoth, who made the first systematic study of this. The official definition of Hoth noise is IEEE standard 269-2001 (revised from 269-1992), "Draft Standard Methods for Measuring Transmission Performance of Analog and Digital Telephone Sets, Handsets and Headsets."

The table below gives the spectral density of Hoth noise, adjusted in level to produce a reading of 50 dBA.

Freq (Hz) Spectral Bandwidth Total power in density 10 log_f each 1/3 octave band (dB SPL/Hz) (dB) (dB SPL) 100 32.4 13.5 45.9 125 30.9 14.7 45.5 160 29.1 15.7 44.9 200 27.6 16.5 44.1 250 26.0 17.6 43.6 315 24.4 18.7 43.1 400 22.7 19.7 42.3 500 21.1 20.6 41.7 630 19.5 21.7 41.2 800 17.8 22.7 40.4 1000 16.2 23.5 39.7 1250 14.6 24.7 39.3 1600 12.9 25.7 38.7 2000 11.3 26.5 37.8 2500 9.6 27.6 37.2 3150 7.8 28.7 36.5 4000 5.4 29.7 34.8 5000 2.6 30.6 33.2 6300 -1.3 31.7 30.4 8000 -6.6 32.7 26.0

The tolerance for each 1/3rd octave band is กำ3dB.

# How does it work?

The central limit theorem says if you add a few random numbers together, the result starts to look Gaussian. In this case we sum 8 random numbers. The result is fast, and perfectly good as a noise source for many purposes. It should not be trusted as a high quality AWGN generator, for elaborate modelling purposes.