myths and facts about aliasing

A recent trend in the audio producer scene seems to be to judge an audio effect plug-in just by analyzing the harmonic spectrum, which is usually done by throwing a static sine-wave right into the plug-in and then look at the output with a FFT spectrum analyzer afterwards. In this article I’m going to talk about what this method is capable of and where its limitations and problems lie and that aliasing gets confused with a lot of other phenomenons quite often. I’m also clearly showing that this method alone is not sufficient enough to judge an audio plug-in’s quality in a blackbox situation.

a harmonic spectrum plot showing quantization noise, harmonic distortion and aliasing effects

NastyDLA – final teaser and release info

the gate/expander in use

written by susiwong

A basic gate has a single parameter, the threshold – when the level is above the threshold the signal passes unchanged, when the level drops below the threshold the signal gets switched off, simple as that. Attack time ideally should be as fast as possible without causing clicks or distortion, so it’s preset to a sensible compromise with most gates, a few good gates even offer you a choice of two settings. Knee, hold and release determine shape and speed of the fade out, release is responsible for the overall decay time, knee changes the behaviour around the threshold level, helping you avoid the dreaded “motorboating” effect where the gate switches on and off rapidly. Think BSS or Drawmer gate vs Alesis compressor …

Hold simply specifies the “reaction time” from the moment the signal passes the threshold till the begin of the gain reduction – critical to preserve as much meat as possible from drums or keeping guitar decay intact. This is mostly what separates the good from the bad and the ugly. Last is the “range” or “floor” parameter, it sets a certain minimal volume to which the signal gets attenuated when dropping below the threshold, instead of being muted completely. Very helpful when you need to reduce the background noise between a singer’s phrases for example, much less obtrusive than muting the track completely. Set the floor so the background noise gets masked well enough by the music, often 3dB or 6dB are enough. This technique is also known as downward expansion, paired with a longer release and soft knee it’s often used for distorted guitars (with slow decay), too.

Some good gates offer sidechain filters allowing you to “zero in” on the important part of a complex signal, take a tom mic of a multi-miced drumset for example, where a lot of similar signals (bleed) are fighting for control. Difficult even with sidechain, impossible without. Worth noting that these filters do NOT influence your audio signal, only the signal used for detection, hence the name sidechain. And finally an external sidechain allows you to even borrow a signal from another channel to trigger your gate – the creative options are huge.Unfortunately not all hosts have this implemented in a user-friendly way. One popular example is tightening up the bass by triggering its gate from the kick.