solderdude
Grand Contributor
You mean that stepped look of a waveform from a HF range when produced by non-OS DACs?
Funny, Fourier analysis says the perfect base tone frequency sinus wave is present there in its pure sinus form. Harmonics of the base tone which are obviously as well present there make the resulting waveform look stepped.
That is ONLY true when the sample-and-hold stairsteps, coming out of an R2R DAC chip, have passed through a very steep post filter.
That filter is not present in external NOS DAC's.
And yes, the slow filter at 60kHz, or there about, are not fitting the criteria of a proper reconstruction filter.
The 'steps' are NOT the harmonics at all, they are quantization errors that need to be removed.
That is what the reconstruction filter is for.
The steps have the same relative amplitude (during a sample width) that a sample point (which is much smaller in time) had during the encoding process. There WAS no 'step' in the sampling process. Merely a point. As a DAC cannot do a 'point' it makes a 'stripe'.
This means ONLY at the beginning of that 'stripe' the sample value is correct. 99.99% behind that point is the incorrect value (considering sine waves are the input).
When oversampling is done the quantization errors are smaller in amplitude and higher in frequency.
Because of this a less steep and less complex analog post filter can be used.