The complex simple delay

By mic on 3/24/2012

I use the words "simple delay" to describe the simplest of practical recording effects – an effect that produces one repetition of the input signal with some delay in time and usually with some decay in amplitude. More complex delays can have additional parameters: feedback, delay and decay sweeps, multiple "taps" to output, etc. Even simple delays, however, can be very interesting and the following are example settings that can produce great results.

Bounce

First and foremost, although a simple delay will produce only one repetition of an input signal, in contemporary practice simple delays are actually fed with two input signals: one for the left channel of a stereo recording and one for the right channel. There are actually two input signals, two repetitions with two time delays and two amplitude decays, and two output signals. Nothing says we have to use the same amount of delay and decay in both the left and right channels. By using significantly different amounts of time delay in the left and right channels, for example, we can create a perception of a signal repetition that bounces from one channel to the next. Take the following settings:

Left channel: delay – 400 ms, decay 20% (i.e., 80% of the original signal remains)
Right channel: delay – 800 ms, decay 40%

The 400 ms difference between the delays in both channels is large enough to create two completely separate sound repetitions (the human ear would typically interpret anything over 100 ms as separate). The larger decay is combined with a larger delay, creating the perception of a decaying echo. The decays and delays are approximately related, as there is 20% decay for every 400 ms delay (although the decay in a realistic echo should probably be exponential). We have thus created a two repetition echo, one that jumps from, say, the middle (if the original signal is in the middle), to the left, and then to the right.

Simple spatial perception

Say that we are in a good size room or hall, but not necessarily in the middle. Sound will bounce from the walls and come back, perhaps at different times and perhaps with different amplitude, as in the bounce delay above. If the room is really large and we are way off the middle, the difference in the time delays of the reflected sound would be very noticeable. The difference in decay will probably not be as noticeable, unless the walls are markedly different. In a smaller room, we could end up with the following.

Left channel: delay – 300 ms, decay 60%
Right channel: delay – 400 ms, decay 60%

The difference in time delay between the left and right channel is a barely noticeable (for most it will probably be unnoticeable). There is no difference in decays. The end result will be a hint of some spatial positioning, but just a hint. Of course, a room has four walls, but there are only two channels to work with.

Presence

Everyone is familiar with the slapback delay, which has a really short delay – less than 20 ms – and creates the perception of a "phased" signal. We could also use the term "phaser", but that one is usually reserved for a simple delay that allows delay sweeps – gradual changes in the delay time. If the delay time in the slapback delay is really small – 5 ms – and there is little decay, there will be the perception of sound presence. That is, the sound will be louder and closer. We could make this even more interesting if we change the left / right perception of that presence.

Left channel: delay – 0 ms, decay 0%
Right channel: delay –5 ms, decay 0%

We can even use the settings

Left channel: delay – 0 ms, decay 0%, phase is same
Right channel: delay –5 ms, decay 0%, phase is inverted

although some will argue whether or not the difference in phase in the two channels is noticeable.

Chorus

Finally, we can change the perception of a single sound at play. We will use larger delays than the ones used to create presence above.

Left channel: delay – 30 ms, decay 0%
Right channel: delay –12 ms, decay 0%

Although a real chorus will allow at least for delay sweeps, this simple delay may just sound close enough to a chorus.

authors: mic

delay
DSP
Author
mic

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