tical DAC would be able to create such an output pulse train. Nevertheless, the impulsive model fairly approximates a heavily oversampled PDM DAC (eg, Philips Bitstream) subject to a jitter process, where each 44.1kHz audio sample is represented by many discrete output pulses, each of unit amplitude and each subject to the same timing error. Referring to fig.23b, the continuous error signal en due to one audio sample An jittered by tjn can be written: Equation 19: en = An [δ (nts-tjn) - δ (nts)]
where δ(t) corresponds to a Dirac impulse at time t. Equation 20:
Eimp(f) =
∑n=ØN-1
en e-i2πfnts Now if we assume that jitter interval tjn is very much less the sampling interval ts, then: Equation 21:
Eimp(f)
=
i2πf∑n=ØN-1
An
tjne
-i2πfn/fs Thus, the error spectra are simply the Fourier Transforms of the product sequence Antjn scaled to rise with frequency. Equation 21 can be compared to the error spectra obtained for the 1000/0 DAC model (Equation 18); intermodulation between signal and jitter frequencies is still expected in the impulsive error spectra, but the intermodulation components will have different weights than those obtained from the 100% model.
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This is best illustrated by comparing error spectra for the two models when a 1kHz tone is jittered by wideband white noise (fig.27). The error spectra for the 100% model are essentially flat with frequency, the impulsive error rises with frequency at 6dB/octave (the exact frequency at which the two curves intersect depends upon the audio signal frequency and the statistical properties of the jitter noise). The accuracy of the impulsive DAC jitter error model can be confirmed by comparing simulated (fig.28a-c) and measured (fig.29a-c) results for the same signals used in "100% Sample DAC" (fig.26a-c). The measured results, made using a Meridian 203 Bitstream DAC, clearly show good agreement (within 2dB) with the simulations. The jitter injection interface was also employed to determine that the cutoff frequency of the Meridian PLL was higher than 10kHz. The same measurement made on another Bitstream D/A
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