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bandwidth at that frequency, and compared to the threshold. Fig.33 indicates the audibility of 16-bit triangular probability-density (TPD) flat dither noise assessed in this manner, dearly showing the dither to be audible in the 2-6kHz frequency range: this is similar to the result achieved by Stuart in fig.2 of his AES paper.34 We can use the error-audibility model to assess the validity of these claimed limits to jitter audibility. Consider the case where an audio tone is corrupted by spectrally white jitter. Fig.34a shows that for a 100% DAC reproducing a 0dB, 20kHz sinusoid, rectangular probability density (RPD) jitter of peak amplitude 180ps should be on the threshold of audibility, although it should be noted that the error level reduces as the audio sinewave frequency is reduced. This can be compared to fig.34b for an impulsive DAC, where the error signal associated with 550ps peak jitter noise still lies below the audibility curve. It should be noted that the error curve in this case is constant with audio frequency (this diagram was obtained using 100Hz). The problem with making predictions about the audibility of jitter artifacts using noise-like jitter is that the error tends to be spread across the audio band; more stringent jitter specifications are required when the jitter is sinusoidal. Fig.35a shows the worst-case 100% ])AC jitter error resulting from a 22kHz audio signal and 18.5kHz jitter - only 20ps peak jitter is required for audibility. The 75ps limit for an impulsive DAC occurs when reproducing a low-fre- 34 J.R. Stuart, "A Search for Efficient Dither for DSP Application" presented at the 92nd AES Convention, Vienna, March 1992, Preprint 3334. |
quency tone, and jitter sidebands are coincident due to reflection about DC (fig.35b). Are there any circumstances under which these critical combinations of audio and jitter signals could occur at the same time? In the "Jitter in the Digital Audio Interface" section we have shown that digital audio interface jitter can be highly correlated with the transmitted audio data - and, when it is remembered that digital filters with delays up to a few milliseconds often follow interface receivers before D/A conversion (causing the jitter to precede the associated audio signal at the DAC), such combinations may indeed occur. We now progress to an examination of simulated DAC errors due to band-limited interface jitter Fig.36a shows the jitter-error spectrum tot a 100% DAC reproducing a 0dBFS, 20kHz audio signal with an interface RC time constant of 40ns; the PLL cutoff frequency is set to infinity so that any jitter on the interface won't be attenuated before reaching the DAC. The error contains discrete frequency components rising from the noisefloor - this is because bandwidth-limited interface jitter can contain components that are well correlated with the audio signal (see "Interface Noise"). Fig.36b shows a similar simulation for an impulsive DAC fed a 0dBFS, 9.8kHz audio signal; in this case, an interface time constant of 50ns results in an audible error. Of course, most interface receivers will employ PLL falters such that interface jitter above the PLL cutoff frequency is attenuated. This results in the jitter error forming "skirts" around the audio signal; examples for t00% and impulsive DACs reproducing full-scale 7kHz sinewaves are shown in fig.37. The PLL second-order filter cutoff has been set to 1kHz, |
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