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Converting

— By Paul McGowan —
Happy labor day. A day set to honor those of us that work for a living, a day of rest begun way back in 1894. We started the conversation yesterday about Analog to Digital Converters and wanted to discuss how they work. There are a number of different types of A/D converters and since this isn't so much a class as it is a means for us to help each other gain an overview of what's going on, I thought I'd explain the workings of a very simple A/D, then skip all the others and go deeper on the kind that might matter to any of us in high-end audio. Just for reference, the basic types of A/D Converters out there include:
  • Parallel ADC
  • Successive Approximation
  • Integrator based
  • Sigma Delta 1 bit
The end game will be to understand the last of the bunch, the 1 bit A/D Converter. It's going to give us both DSD and PCM out and it's the basis of what we use in the NuWave Phono Converter. But first, let's look again at what we're trying to do. Then we'll get into how we'll accomplish this. We will start with the basis of most modern ADC's, PCM. If you'll recall yesterday's postI briefly reminded you that there aren't any such thing as 1's and 0's - those are just euphemismsfor ON and OFF - and those changing states, when used in a group called a word, represent a discrete voltage level. Take a look at this drawing: ADC Note the vertical lines or "slices" of the Original Analog Signal. Then look below to the next image and note the reconstructed version of the first image. This is how it looks when we go A/D then back again to D/A using the discrete voltage levels I spoke of. The space between each vertical pair of lines is the result of a single word in PCM being converted to a single voltage. Note that in the Signal After ADC+DAC picture instead of the nice gentle musical signal, we now have sharp transitions between the vertical lines - each of the transitions representing the output voltage of one PCM word. This is what it looks like to take a sample with an A/D and then reproduce it with a DAC. Doesn't look the same, does it? No, but this is exaggerated to help you understand what's going on. When we're actually done with the signal they'll look identical - but for purposes of this discussion, stare at the drawings a bit and tomorrow we'll jump in deeper.

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