London, June 2010

The second performance of codex IX, and the first with the revised spatialization software system took place at Hall Two, King’s Place, London in June 2010. The revised software had been tested under load in the SIAL Sound Studios, and the latency problem appeared to have been solved. Other revisions to the software included the implementation of IRCAM’s Spat spatialization engine (Jot, Jullien et al.

1995-2010), and enhancements to the user interface with the development of

‘spatialization channels’ that allow real-time user control via a choice of cartesian X, Y, Z or AED (azimuth, elevation, distance) parameters. OSC control (Wright 1997) was also implemented for all spatialization parameters to enable complex real time manual or automated control.

The performance system configuration for the London performance was an evolution from the Brisbane performance, with a Yamaha M7-48 digital mixing console used in place of the analogue console. Post fade direct outs were again used to feed the spatialization computer (again operated by Jeffrey Hannam), this time sent via analogue expansion cards installed in the M7. Significantly, the outputs from the spatialization computer were this time routed back to the M7 on another input layer

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and sent to speakers via the M7’s sixteen mix busses. It would have been preferable to interface the spatialization computer to the console digitally and avoid two conversion stages⁹, but the venue’s console was pre-configured with analogue expansion cards, so in order to avoid the expense of hiring digital expansion cards the spatialization system was configured with A/D and D/A converters and the extra conversions tolerated. Since it is a simple matter to store and recall states on the M7, the contingency on this occasion was an alternate recallable mixer state that fed the instrument inputs directly to the mix busses. This loopback technique (routing the spatialization computer outputs back through the console) is effective as a failsafe when using a digital console, at the cost of doubling the number of mixer channels required.

9 As well as the A to D and D to A conversions being redundant, conversion resolution was compromised by conversions taking place after attenuation for balance.

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Figure 11 – codex ix revised signal flow incorporating loopback OSC

Control

Yamaha M7-48 Digital Mixing console

Ensemble Inputs

Mix Outputs 1 - 16 Post-fade Direct Outputs

Spatialization Computer

To Speakers Loopback to Console

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One immediate benefit of the software revisions that had been implemented after the Brisbane performance was the inclusion of a graphic display of speaker locations. This allowed a pictorial representation of the system design to be sent to the King’s Place technical personnel in advance, which considerably simplified the process of communicating the required configuration and expedited the venue’s installation planning.

Figure 12 – SIAL Spatialization System speaker location display

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The speaker configuration was slightly modified from the Brisbane performance in that only two overhead speakers were specified, and two speakers were added at floor level in the centre of the circle of performers. This was done to provide better spatial integration of electronic sounds with the acoustic output of the ensemble, without exceeding the sixteen-output limitation of the spatialization computer’s routing matrix.

The system was installed in the venue and calibrated the day before the performance, and initial sound checking revealed uneven panning resolution in the vertical axis resulting from the centre ring of four speakers being positioned approximately 500mm below their required height. The accommodating venue crew made the adjustment, and vertical resolution was improved considerably. Further listening tests determined that clear spatial articulation was only possible with the aperture parameter of the IRCAM Spat spatialization engine set to its minimum value of ten degrees. I had hoped this parameter would be useable as a performance variable to define spatial fields rather than point sources, but this was unable to be achieved in context. With the aperture set to minimum, however, precise positioning and smooth motion were achieved in three-dimensional space.

The updated software performed well in the audio domain. Motion was smooth and clearly defined and there was no audible latency. The software control mechanism on the other hand exhibited an errant behavior that would impact performance control. As testing proceeded, the system became progressively less

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responsive to control input, eventually exhibiting latency of several seconds between user input and system response. A system reboot restored the control responsiveness to normal, but the progressive decline continued to occur. It was evident that even with a reboot immediately before the performance there would be significant deterioration over the duration of the piece, and this would limit the controllability of the system.

Rehearsal time with musicians was limited to a single three-hour call on the day of the performance. Further technical rehearsal was impossible due to scheduling constraints and the need to manage musicians’ workload in the lead up to the concert.

This only allowed time for microphone positioning, gain setting and input equalization, as well as overall system equalization. The musicians were able to rehearse while this was taking place, but there was insufficient time for spatialization strategies to be tested or rehearsed. A similar spatialization strategy to that employed in the Brisbane performance was implemented, although the enhancements to the real time control capabilities of the spatialization system and the resolution of the audio latency problem meant the quintet groups could be placed in motion rather than left static, and all inputs could be manually spatially controlled or placed in automated trajectories.

Twenty minutes before the start of the concert, a final line check revealed that four of the twelve inputs to the spatialization computer were no longer receiving signal. The inputs in question were all being sent to the computer by means of a

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standalone A/D converter connected to the lightpipe input of a Metric Halo 2882 audio interface. The eight analogue inputs to the interface continued to function, but rebooting the entire system and changing the lightpipe cable failed to rectify the problem with the optical inputs. codex IX was not scheduled until the second half of the concert programme, so further attempts to reinstate the missing channels were made at interval. After repeated unsuccessful attempts, the missing signals spontaneously reappeared, and the system again appeared to be functioning normally.

Without a clear understanding of what had gone wrong or why it had righted itself, I decided to start the performance in ‘safety mode’, using a console preset that directly assigned inputs to outputs, then gradually transition to the spatialization system if it appeared stable.

The performance therefore began with static spatial amplification, with each musician localized to their actual position. As the spatialization system appeared to be functioning correctly, I transitioned instruments across to it channel-by-channel, commencing with the soloists, moving through the trio and finally the quintet. The result was the unintended addition of an extra layer of formal structure to the performance in the gradual progression from static to dynamic spatialization.

Unfortunately this detracted from the effectiveness of the spatial articulation of the composed form of the work.

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Figure 13 – codex IX rehearsal, King's Place, London, June 2010¹⁰