ASTi was selected to provide the high-fidelity aural cue platform for a series of trainers to be certified for FAA Level D operation. The program includes a variety of trainers including a Beechcraft Beechjet 400A, King Air 200, and Cessna Citation Jet. All aircraft types are serviced by a common ASTi Digital Audio Communications System (DACS) processing platform to provide the audio environment simulation.

ASTi also provided the communications simulation environment within the same DACS processing platform. This simulated all aircraft radio, navigation aids, intercom, and warning tone generation for the aircraft; and provided instructor radio, intercom, and monitoring facilities. This project example, however, focuses on the aural cue portion of the system.

The required list of sounds for a typical Level D certification includes:

• Engine: starter, ignition, run-up, normal operation, abnormal operation, shutdown, and malfunction conditions
• Auxiliary Power Unit (APU): run-up, normal running, shutdown
• Landing gear: touchdown, skidding, brakes, tire bursts, air rush, up-lock, down-lock
• Runway: rumble, joints
• Aero noise: aero dynamic hiss
• Wing flaps: actuation, aero-effects
• Pumps: fuel boosters, hydraulic
• Weather: rain, hail, thunder
• Wipers
• Cabin equipment: fans, gyros
• Air conditioning noise
• Crash
• Other aircraft-dependent "spot" sounds (e.g., electrical breaker engagement)

FAA Level D certification requirements place a much greater importance on accuracy of sound. The intention of today's high-fidelity simulators is to immerse the crew in an experience that so very closely replicates the aircraft behavior and environment to the extent that it becomes "real". And sound is a vitalÑoften subjectiveÑelement of this experience. Psychoacoustics play a vital role in assessing whether something "sounds right" to any given individual. However, this is only part of the problem ASTi, as manufacturers of simulation audio systems, must help solve. The FAA Level D requirement, as it applies to sound, requires both a measured performance, in the form of spectral plot matches to aircraft data, and for subjective approval and sign-off by an aircraft-type experienced crew.

Level D specification also imposes greater demands on the analog audio replay system (amplifiers and loudspeakers), in addition to the core audio generation platform. Finally, that audio generation platform must be successfully integrated with the simulator's host control system.

For this program, 10 audio output channels were required, providing a high level of flexibility in the localization and directionality of the required sounds. ASTi provided the initial, "first-cut" sound models which the customer engineers would later refine on the actual trainer platform. This allows the customer to gain greatly from the experience of the ASTi engineering team and past aural cue programs, with the added bonus of ensuring efficient modeling practices and optimal model structure using the Model Builder software toolkit.

These "first-cut" sound models were based on customer-supplied aircraft sound recordings, aircraft physical data, other support material, as well as ASTi's eleven years of sound and aural cue modeling experience. ASTi performed sound tuning for the models on an individual, sound-by-sound basis using various, highly-specialized techniques (e.g., spectral and time domain analysis, complex sound mask filtering, etc.), greatly simplifying our customer's workload in tuning the models after installation on the target training device.

Integration with the simulator's host control system was facilitated by Model Builder's flexibility in accepting high-level state parameters such as engine RPM(s), engine thrust, weight-on-wheels, airspeed, weather conditions, etc. This flexibility results in an absolute minimum of additional host coding. A full Interface Control Document (ICD) for this program is available from ASTi upon request.

After installation of the DACS system, ASTi provided on-site engineering support to review the model, and provide initial on-trainer tuning. Final model tuning and sign-off with the end-customer was performed by the customer, with full knowledge that ASTi support could be called upon should the need arise.

This particular project continues to be on-going, and, to date, five systems have been ordered and delivered. A significant number of additional orders are expected in the future.