Sometimes my mouth runs faster than my common sense and caution, research and fact checking are sacrificed for the heat of the moment. Such was my proclamation that there is no such thing as a stepped staircase waveform at the output of a DAC. Instead, said I, there are a series of pulses that look more like lollipops and I presented a picture to prove my point. While the pulses comment is indeed true, I left out “the rest of the story”. Sorry.
John Atkinson, editor of Stereophile Magazine, was kind enough to send me a note reminding me that most multi-bit DACS also have a sample and hold circuit built into their outputs that actually makes the stair steps happen. You’ll recall that in my post Hold that thought I explained the function of a sample and hold circuit on the input of the A/D converter. That same type of circuit also exists at the output of many DACS so when the “lollipop” pulse happens, the level of that pulse is maintained by such a circuit and the result of that action is ….. wait for it ……. a stair step. Take a look at this picture of the raw output of a DAC, from a Stereophile article of the Audio Note CD player:
Note the steps? So this happens because there is a pulse that goes to a specific height and once established a sample and hold circuit keeps that level constant until the next pulse.
Once you have this stepped waveform, it’s a pretty simple matter to remove all the jaggie steps with a simple filter and the result is a perfect sine wave.
And one last thought as long as we’re continuing to poke the proverbial box.
I have said before that if you have a vinyl disc and make a 44.1kHz/16 bit copy of that analog vinyl disc, the sound will be indistinguishable from the original. This is because with vinyl or a tape recorder, you have a limited scope recording (vinyl and tape has limited everything relative to a CD) and capturing that limited recording on a CD is easy. The opposite isn’t true.
A live event, recorded on a high resolution system, cannot be accurately copied on a lower resolution system such as a tape recorder or vinyl record.
And on that one, I have thought it through for quite some time.
Paul McGowan
