Soft synchronization of pitch

The rhythmic drift theme (Risset Chorale) described above was implemented with a special version of granular synthesis, called “partikkel” in Csound. It allows very flexible treatment of small sound fragments, for example the kind of soft synchronization patterns here described. It also allows the same process to happen at any tempo, and if we play a rhythmic pattern very fast, it will start to merge and become a sustained not of a definite pitch. Slow rhythms will be low pitches and fast rhythms is high pitches. Running the rhythmic drift process at a very high tempo, creates tonal pitches that slowly drift towards each other, as if they were trying to agree on a common pitch.

Noise to tone, chaos to order

As mentioned earlier, the signal from the quasars is extremely noisy, but still we use it to distill positional information with millimeter precision. This process of going from noise to something more orderly repeats itself throughout the VLBI system. As an attempt to symbolize this, I have tried to synthesize some sound that goes from noise to a stable tone. This could have been done in a number of different ways. I chose (again) granular synthesis because of its flexibility of control, and the many ways that sound fragments can be distributed in a random or more orderly fashion.  A periodic stream of equal sound fragments will sound like a stable pitch, any deviation in pulse period, pitch, length, shape (etcetera etcetera) will make the sound more or less noisy. With this technique I make a series of chordal events, where each tone in a chord moves individually from noise to tone.

Associative composition

In addition to the more direct methods of sonification that I’ve used in this project, I’ve found it necessary to try to describe the processes of VLBI in a more associative manner. I’ve been trying to figure out how I can make a sound that in a convincing way embeds an essence of the correlation process. The real correlation process of VLBI takes two essentially random signals and slides them over each other to see where there is a tiny bit of a match. When I say essentially random, I mean that if I listen to those two signals, at any playback speed, there will be no perceptual cues to distinguish one from the other. The real correlation process uses thousands of iterations, over which a running sum (residue) is identified, signaling when a match is found. If I were to use this process directly on sound, on signals of a frequency that we can perceive as pitch or rhythm, it would take literally forever and the subtle change when correlation was attained would not make a perceptible difference.

Still, I have felt it essential to capture this part of the process in some way in the sonic work.

Rhythmic correlation, or rhythmic morphing

I tried taking two random rhythms, evaluating the difference between them, and making an algorithm that would allow them to slowly converge towards each other. This could prove a useful musical composition tool sometime, but I was not able to really solve some inherent problems with the approach. It is easy enough to interpolate between two patterns, letting each value in the pattern slowly approach the corresponding value of the other pattern. However, how would you decide which value in one pattern to “couple” to which value in the other pattern? Let’s say that the rhythm patterns may have different length (a reasonable assumption, I think), or perhaps a different number of attacks/events. Then there is no one-to-one relationship between the members of those two patterns. It gets a bit complicated, and as of this time I have not been able to find a solution that I could happily say is perceptually relevant.

Rhythmic drift       

So, my next attempt at this rhythmic correlation process was to simplify the rhythms as much as possible, down to a stream of single events of equal duration. Like a metronome. Starting several instances of this rhythm generator at different tempi, I let them gradually come into synchrony. I implemented this in such a manner that the rhythm clocks has a built in feature of looking for the beats of other clocks, and readjusting its own pace to approach that of the others. This is commonly called soft synchronization, because it is a process that is allowed to unfold over some time, as opposed to hard synchronization where we immediately “snap” to the next beat. This soft synchronization is attained by first adjusting the tempo, then the phase (phase in this context meaning “where in the period between two beats are we now?”). Without phase synchronization, we would have rhythms of the same tempo, but still not playing exactly together. The time it takes for synchronization to be attained can be adjusted (“sync gravity”), typically in this installation, I let the process take a minute or a bit less. To prolong the musical situation, I continuously fade in new unsynchronized events, and fading them out when synchronization has happened. I tend to think of this as a process akin to Jean-Claude Risset’s endless glissando. Using different pitches for each rhythmic voice, it also sounded as a sort of chorale to me, so for my own use I termed this theme a “Risset Chorale”: