Audio Test File Generator Specifications



The goal was to produce a generator of audio frequency sine wave signals of the highest possible quality that may be used as a frequency standard for testing all components in the path of the signal. A very thorough specification follows, with the aim to increase the usefulness of the signal generator for sound engineers.




The sine wave generator

In the quest to create "the perfect wave", the floating point calculations that produce the sine wave employ 80 bits of internal precision. Each signal generated is a perfectly symmetrical series of whole waves so as to end up with a negligible DC component. Whatever DC remains is removed, so that for any given frequency of 10 seconds duration or less, the DC level is guaranteed to be 0 before dithering is applied. Dithering will add a minuscule amount. The actual DC level before correction is pre-calculated on a sample by sample basis, so a slight delay may be noticed when generating signals approaching 10 seconds in duration. Since DC correction is not applied to signals of a duration greater than 10 seconds, no delay is entailed in this case.

The following are details of distortion and noise readings taken on 10 second files at 44100 samples per second. It should be noted that these commonly used frequencies have significantly higher THD and noise than, for example, most of the musical notes because of aliasing due to the fact that they share a common denominator with the sampling rate. For example, a sine wave of C6 (1046.502261 Hz) at 0 dB, averaged over a 10 second test file and under the same test setup, has a THD+N of only 0.00011% and a SNR of about 114 dB. Compare that to the results below for 1000 Hz. As a testament to the effectiveness of dithering, simply average that same 1046.502261 Hz (C6) signal over 120 seconds and you will find the THD+N has now dropped to 0.00008% and SNR > 122 dB. (SNRs are for a bandwidth of 22kHz)



The sweep wave generator

The sweep generator employs a swept sine wave function and does not include DC correction. (Significant DC can accumulate in a sweep signal but attempts to remove it would result in unacceptable distortion.) It can sweep up or down. The minimum and maximum frequencies and attenuation values allowed are the same as for the sine wave generator. The minimum sweep duration is 0.1 seconds and the maximum is 120 seconds. Duration is rounded up to the nearest multiple of 128 samples. A linear taper of 128 samples is applied to the end of the signal. During taper down, the frequency is held at the ending frequency requested. The default setting sweeps through 6 octaves from C2 to C8.


The white noise generator

The latest version (1.06) of the Audio Test File Generator has a white Gaussian noise generator. The pseudo-random number generator employs the "Mother-Of-All" generator invented by George Marsaglia and provides an excellent even distribution. The code for this can be found at http://www.agner.org/random/. The overlaid Gaussian window is a codification of the Box–Muller transform in its basic form to provide a zero-centered normal distribution of the samples. The output was thoroughly tested to ensure uniform distribution with a Gaussian curve.


The pink noise generator

The pink noise generator employs an algorithm by Andrew Simper of Vellocet, a C++ implementation derived from the code provided by the following people mainly from the music-dsp mailing list: Allan Herriman, James McCartney, Phil Burk and Paul Kellet and the web page by Robin Whittle: http://www.firstpr.com.au/dsp/pink-noise/. Maximum signal level is nowhere near 0 dB, since considerable head room is given to ensure there can be no clipping. Determining the signal level of noise is problematic because of its random nature. However, if you employ attenuation you can be assured of accuracy of relative signal levels over the long term. Its "peak" frequency is below 1 Hz, a thing you can verify by averaging a 120 second file using a very large FFT size.

Warning - pink noise contains frequencies below the range of hearing that may damage your speakers at very high levels. Turn down the bass if you want to pump up the sound, or watch your speaker cones for excessive movement.

From Wikipedia: "Pink noise, also known as 1/f noise, is a signal or process with a frequency spectrum such that the power spectral density is proportional to the reciprocal of the frequency. There is equal energy in all octaves (or similar log bundles). In terms of power at a constant bandwidth, 1/f noise falls off at 3 dB per octave. At high enough frequencies 1/f noise is never dominant. (White noise is equal energy per hertz.) 1/f noise occurs in many physical, biological and economic systems. Below are images showing spectral views of the signals produced by the Audio Test Signal Generator


Pink noise falls off at 3 dB per octave...




440 Hz sine wave without dithering...




Same signal with dithering applied...