“I’m curious: If you have a pure sine wave that starts with a negative half-cycle, that means the signal polarity is negative, right? But what if you snip off the first half-cycle of that sine wave in your DAW’s waveform editor; does that mean the signal polarity is now positive? There’s no real reason or relevancy for this question, except that I’m perplexed by it. Please help me get some sleep.”
We’ve pushed this question to the top of the queue so we can get it answered and get this inSync reader some sleep.
Much of the contents of your question necessitates the need to make some semantic distinctions. Let’s break it down.
First, we need to make sure we understand the distinction between polarity and phase of signals. Polarity speaks more to how the positive and negative wires are connected. Phase speaks more to a relative TIME differential between two signals.
The polarity of a signal can, in practice, be related to what happens in the first half-cycle, but in audio polarity usually just refers to how the connection is wired. In an unbalanced line keeping the positive (or hot) lead consistent all the way through a system (including at the mic) means the polarity should always be correct. In a balanced system this just means treating the positive (hot) and negative (cold) leads appropriately. See WFTD Absolute Phase for a little more depth on this issue.
When you speak about snipping off the first half-cycle of a waveform in a DAW, it then becomes important to know what happens to the remaining waveform and what you are relating that to. For example, let’s say you have two tracks of the same waveform that are completely in phase with each other. Snipping off the first half-cycle of one of their waveforms isn’t going to change its polarity or phase relationship with the other tracks UNLESS you then move it in time so it starts at the same time as the other one that didn’t get snipped. This is a critical point to understand. Phase relationships are time based. It doesn’t matter about starting in the negative or positive half of the wave. It’s about the time relationship relative to other sources. Polarity isn’t based on time relationships or where in the cycle the waveform starts. Sure, you can shift a wave in time 180 degrees and effectively reverse its polarity, but it’s not technically the same thing. And even if you clip off the first half waveform of a signal you still haven’t changed the fundamental nature of the rest of the signal unless you either move it in time relative to other sources or reverse the polarity of the rest of it.