For some reason, I've recently discussed audio clicks and other kind of distortion many times with both physical and virtual friends - way more than usual.
Undoubtedly a sign of destiny, it prompted me to write a short post on them.
The situation is always the same. It so happens you've recorded your opus major and it sounds good, but there's annoying clicks and glitches - all noises that go under the name digital distortion. Which of course, sucks.
What gives? Let's see.
Somebody likes it hot
First,, the bad news. By far, the most common reason for having digital distortion is that you have recorded too hot.
It's a common beginner's mistake - thinking that you have to see lots of green and even some yellow on the DAW meter when you record. The result is that the occasional loud transient will go overboard (in the red) and you'll run out of headroom, "clipping the converters".
I've discussed that before, for example in Why DAWs destroy recordings, so no point in repeating. If your levels were too hot when you recorded, you may have clipped your A/D converters and your audio is damaged. Permanently. Well, there are things you can do (and that's an idea for another post) but you just had a character-building experience and the reason for the clicks is clear.
Let's take a short buffer
The second most common reason for clicks and glitches is a short audio buffer. The audio buffer, as you know, is the memory area where the interface and the CPU use as a staging area.
If the CPU/DAW cannot consume samples with enough speed - before the buffer is filled - either the existing samples or the new ones will be discarded (depending on what the interface driver prefers); either case, it'll sound crap.
The same will happen if your CPU/DAW cannot generate samples speedily enough (perhaps because each of them has to go thru a chain of twenty four heavy processing plugins per track) - the interface will be ready to play back, but there's nothing to play back - again, glitches and clicks.
The solution is simple: increase the buffer size and cut on the effects. If you have that many, they don't sound that great anyways.
Clip me right
The next typical culprit is that you have cut and rejoined audio clips. That's one of the best things about a DAW and digital recording - editing is incredibly easy. Don't like that note? Cut it out and replace it! Feel lazy? Copy the first chorus and paste it as the second one. Try that with tape and razor blades!
However, you need to be careful. Each clip is simply a sequence of samples, each one telling the interface DA converter which voltage to generate in the next micro-mini-mini second (at 44.1 KHz sample rate, there's 41000 samples per second, so each sample is 1/41000=about 0.000022 seconds long).
If the clips are near each other and there's significant change in level between the end and start samples (or there's a hole between the two, so that the audio goes from something high to 0 and then again to something high), your DA converters (which in the end have to produce an analog voltage to drive the speakers preamps) simply won't have the physical time to move between the two levels.
In the physical world, it just takes a certain finite amount of time to go from a voltage to another, and if you try to make the DA converter go faster then that, it won't. The resulting analog signal won't be as intended and often produce that kind of "click" distortion.
To fix this kind of problem, ideally you should cut clips when the waveform is crossing the 0 level (you need to zoom in enough in the track to have a good view).
However it's common to simply cut clips so that there's a short overlap between them (even 1 ms is enough, it fits a lot of samples) - when u drag them in place one on top of the other, the DAW will usually create an automatic crossfade (unless you have disabled it), basically making sure that the levels morph seamlessly.
If you need to leave a "hole" between two clips, make sure you place a short fadeout on the clip on the left and a fadein on the clip on the right. Job done. No more clips.
Gentlemen, start your engines
If the clicks seem to appear at random when you play back - they may simply be due to where you start the playback.
That's because, if you start in a sample containing silence or almost silence, there's no problem. But if you start at a sample where there's a significant level, you're once again asking your DA converter to go from zero to a loud voltage very quickly in a single sample time.
Usually too quickly, resulting in distortion.
These clicks will usually not be present at all when you bounce the track (because a bounce will start playing from the beginning, where you usually have a few milliseconds of silence).
Most digital music players always introduce some microseconds of fade-in on purpose, but a DAW is supposed to be as analytical as possible (and you're supposed to know what you are doing), so if you start playback in the middle of transient, you will likely hear clicks.
That's all, folks! To the next!