One of the reasons Class D amplifiers are considered by most to be digital (technically they are not) is the need to encode the continuous analog input signal into another language to work. So it may be correct, in many minds, to classify this amplifier type as something other than analog because of the encode/decode process. However, I would disagree.
Class D’s language, Pulse Width Modulation (PWM), is very far removed from true digital audio (called PCM) and is more closely related to DSD or 1 bit audio. In a PCM language the analog voltage is converted to a number which can then be stored in a computer. Without a magic decoder ring that understands this numeric language, true digital audio cannot be understood or reproduced. When we think of DSD, we get closer to analog because the stream of bits can be converted back to analog without a magic decoder ring.
In other words, if you take PCM and put it into your preamp you get nothing out but nasty noise. DSD and PWM (very closely related) fed directly into a preamp produce music! In fact, when DSD is recorded onto a disc a type of PWM is used to record that DSD data and, if you take either out of the disc or hard drive they are stored on, you can play them directly onto a stereo.
So we don’t typically refer to something as digital unless it’s based on a binary (2) system of on and off. With only on and off you must use a numeric system that represents values that must be decoded. I have always thought it a mistake that DSD has the word digital in its naming – because you can make an argument either way – and the fact both DSD and PWM can be streamed as analog without conversion is the key for me not referring to it as digital.
If you remember from yesterday’s post we learned that PWM (and DSD) actually have 3 states, as opposed to 2. It is the addition of this third state (time) that clearly differentiates both PWM and DSD from digital audio.
Why is today’s post focused so heavily on our understanding of both DSD and PWM as coded analog instead of digital? Because getting a clear picture of the difference between the two will help us understand just how this works.
Lastly, I wanted to point out the similarity between DSD and PWM because these are the two most analog sounding technologies I know of.