J. Robert Ashley

Test signals for music reproduction systems

Experiments with both long, continuous tones and short, transient tones have shown that the human ear is insensitive to relative phase differences between fundamentals and overtones. Short notes from a piano and from a drum are studied here to show that the phase requirement for music reproduction is that the group velocity of the system be constant in the relatively narrow regions surrounding the fundamentals and overtones. The phase velocity across the audio spectrum does not have to be constant, thus easing the requirements on loudspeakers, crossover networks, and tape recorders. Pulse-testing schemes require minimum-phase behavior of a system for simple interpretation and therefore tend to overtest a music reproduction system. Random noise can be used as a test signal if elaborate processing equipment is available. However, the sinusoid is still the best test signal for determining distortion and relating device performance to theory.

Near carrier noise in TWT amplifiers

Near carrier amplitude and phase noise modulations have been measured for low noise (receiving type) and medium power (20 w) TWT amplifiers at 6 GHz. The measured noise is greater than predicted by the noise figure measured 30 MHz from the carrier. Below 20 kHz, at least part of the excess can be accounted for by phase modulation products coherent with the power line frequency.

Hybrid computer integration of partial differential equations by use of an assumed sum seperation of variables

The majority of numerical solution methods for partial differential equations by either analog or digital methods involve some form of finite differences technique, integral transforms, or Monto-Carlo methods. On the other hand, the most common classical analytical approach is based on some form of separation of variables and series expansions. The motivation for the research presented in this paper was to investigate the possibility of using the classical separation of variables approach as a basis for an efficient computational algorithm. The method studied was developed with a hybrid computer implemention in mind due to the ease in on-line operation in engineering design applications although it could be used for digital computation also.

A sonic tour of symphonic concert halls

The Koss CM‐1030 speakers designed by the author will be described to justify their choice for the demonstration at the 104th meeting. The 100‐liter bass section was synthesized to a 30‐Hz BB4 alignment. Dual closed‐box 800 ml mid‐range systems are crossed over at 300 Hz. Dome tweeters cover the spectral ranges of 2 to 6 to 15 kHz. These speakers have much lower intermodulation distortion than most commercially marketed speakers. Telarc® digitally mastered recordings will be used for most of the sonic examples. These recordings have been made with three omni‐directional microphones and little equalization. A recording from the current best concert hall in the world, Powell Hall in St. Louis, will illustrate the accuracy of the recording and playback process. Another good hall, Symphony Hall in Boston, will show why the musicians prefer halls such as these. The cold, steely string quality of many modern halls will be sonically illustrated by the Atlanta Symphony. Slides will illustrate the physical causes of sonic differences.

Loudspeaker requirements for electronic organs in churches

When visiting a strange church, we can identify the organ as a pipe organ or an electronic organ with just a few minutes of listening (and without visual inspection). A run on the pedal organ will show the usual organ loudspeaker to cease radiating below 60 Hz. Modulation distortion will show that the all important middle octave (261–525 Hz) is being radiated from the same cone as the low tones. The lack of high‐frequency power radiation is noticeable. We deem these faults are caused by the fact that organ loudspeakers have not been designed, they have just happened. Dr. J. E. Bensen reported (in 1959) an experiment in the Sydney (Australia) Town Hall which proves this need not be true. We have repeated Dr. Bensens experiment in the First Methodist Church, Colorado Springs, Colorado, and verified his conclusion. To prove that the recent loudspeaker revolution can make the electronic organ viable, we use a Moog Synthesizer to simulate several pedal and great organ stops.

Another approach to time scaling the analog computer

on the left-hand side. In the computer, this boils down to selecting the size of the integrator capacitors. Since most of the nonlinearity in f (x, t) is accounted for by terms such as x, x~ or X.,2, students quickly grasp that changing the time scale does not bother the multipliers which generate these terms. Pointing out that the functional dependence on t in f ( x, t ) is usually in the form of a time delay unit makes it relatively easy to convince seniors that the time delay setting must be synchronized with time scale changes. Working with seniors and graduate students (especially those who have not yet sweated through Ashley’s analog ordeal) who want to use the computer to do &dquo;homework&dquo; for another course (control theory, for example) has pointed out another educational problem regarding scaling. The first time the student tries to use a multiplier, he will come to me with