Live convolution with Kjell Nordeson

Session at UCSD March 14.

Kjell Nordeson: Drums
Øyvind Brandtsegg: Vocals, Convolver.

Contact mikes

In this session, we explore the use of convolution with contact mikes on the drums to reduce feedback and cross-bleed. There is still some bleed from drums into the vocal mike, and there is some feedback potential caused by the (miked) drumheads resonating with the sound coming from the speaker. We have earlier used professional contact mikes, but found that our particular type did have a particularly low output, so this time we tried simple and cheap piezo elements from Radio shack, directly connected to high impedance inputs on the RME soundcard. This seems to give very high sensitivity and a fair signal to noise ratio. The frequency response is quite narrow and “characteristic” to put it mildly, but for our purposes, it can work quite well. Also, the high frequency loss associated with convolution is less of an issue when the microphones have such an abundance of high frequencies (and little or no low end).

IR update triggering

We have added the option of using a (midi) pedal to trigger IR recording. This allows a more deliberate performative control of the IR update. This was  first used by Kjell, while Øyvind was playing through Kjell’s IR. Later switched roles. Kjell notes that the progression from IR to IR works well, and that we definitely have some interesting interaction potential here. The merging of the sound from the two instruments creates a “tail” of what has been played, and that we continue to respond to that for a while.
When Kjell recorded the IR, he thought it was an extra distraction to have to also need to focus on what to record, and to operate the pedal accordingly. The mental distraction probably is not so much in the actual operation of the pedal, but in the reflection over what would make a good sound to record. It is not yet easy to foresee (hear) what comes out of the convolution process, so understanding how a particular input will work as an IR is a sort of remote and second-degree guesswork. This is of course also further complicated by not knowing what the other performer will play through the recorded IR. This will obviously become better with more experience using the techniques.
When we switched roles (vocal recording the IR), the acoustic/technical situation became a bit more difficult. The contact mikes, would pick up enough sound from the speakers (also through freely resonating cymbals resting on the drums, and via non-damped drum heads) to create feedback problems. This also creates extra repetitions of the temporal pattern of the IR due to the feedback potential. It was harder to get the sound completely dry and distinct, so the available timbral dynamic was more in the range from “mushy” to “more mushy” (…). Still, Kjell felt this was “more like playing together with another musician”. The feeling of playing through the IR is indeed the more performatively responsive situation, overpowered by the reduction in clarity that was caused by the technical/acustical difficulties. Similarly, Øyvind thought it was harder because the vocals only manifest themself as the ever changing IR, and the switching if the IR does not necessarily come across as a real/quick/responsive musical interaction. Also, delivering some material for the IR makes the quality of the material and the exact excerpt much more important. It is like giving away some part of what you’ve played, and it must be capable of being transformed out of your own control, so the material might become more transparent to it’s weaknesses. One can’t hide any flaws by stringing the material together in a well-flowing manner, rather the stringing-together is activated by the other musician. Easily, I can recognize this as the situation any musician being live sampled or live processed must feel, so it is a “payback time” revelation for me, having been in the role of processing others for many years.

Automatic IR update

We also tried automatic/periodic IR updates, as that would take the distraction of selecting IR material away, and we could more easily just focus on performing. The automatic updates shows their own set of weaknesses when compared with the manually triggered ones. The automatic update procedure essentually creates a random latency for the temporal patterns created by the convolution. This is because the periodic update is not in any way synchronized to the playing, and the performers do not have a feedback (visually or auditively) on the update pulse. This means the the IR update might happen offbeat or in the middle of a phrase. Kjell suggested further randomizing it as one solution. To this, Øyvind responds that it is already essentially random since the segmentation of input and the implied pulse of the material is unrelated, so it will shift in an unpredictable and always changing manner. Then again, following up on Kjells suggestion and randomizing it further could create a whole other, more statistical approach.  Kjell also remarks that this way of playing it feels more like “an effect”, something added, that does not respond as interactively. It just creates a (echo pattern) tail out of whatever is currently played. He suggested updating the IR at a much lower rate, perhaps once every 10 seconds. We tried a take with this setting too.

Switching who has the trigger pedal

Then, since the automatic updates seems not to work too well, and the mental distracion of selecting IR material seems unwanted, we figured, maybe the musician playing through the IR should be the one triggering the IR recording. This is similar (but exactly opposite) to the previous attempts at manual IR record triggering. Here, the musician playing through the IR is the one deciding the time of IR recording, and as such has some influence over the IR content. Still he can not decide what the other musician is playing at the time of recording, but this type of role distribution could create yet another kind of dynamic in the interplay. Sadly the session was interrupted by practical matters at this point, so the work must continue on a later occation.


kjellconvol1 kjellconvol1

Take1: Percussion IR, vocal playing through the IR. Recording/update of IR done by manual trigger pedal controlled by the percussionist. Thus it is possible to emphasize precise temporal patterns. The recording is done only with contact mikes on the drums, so there is some “disconnectedness” to the acoustic sound.


kjellconvol2 kjellconvol2

Take2: Vocal IR, percussion playing through the IR. Recording/update of the IR done by manual trigger pedal controlled by the singer. As in take 1, the drums sound going into the convolver  is only taken from the piezo pickups. Still, there is a better connectedness to the acoustic drum sound, due to an additional room mike being used (dry).


kjellconvol3 kjellconvol3

Take3: Percussion IR, automatic/periodic IR update. IR length is 3 seconds, IR update rate is 0.6 Hz.


kjellconvol4 kjellconvol4

Take4: Percussion IR, automatic/periodic IR update. IR length is 2.5 seconds, IR update rate is 0.2 Hz.

Other reflections

IR replacement is often experienced as a sudden musical change. There is no artifacts caused by the technical operation of updating the IR, but the musical result is more often a total change of “room characteristic”. Maybe we should try finding methods of slowly crossfading when updating the IR, keeping some aspects of the old one in a transitory phase. There is also a lot to be gained performatively, by the musician updating the IR having these transitions in mind. Choosing what to play and what to record is an effective way of controlling if the transitions should be sudden or slow.

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