Wednesday, June 18, 2008

Ferranti Mark 1 computer music

The BBC is reporting that it has unveiled the oldest known recording of computer generated music. This recording was generated by the Ferranti Mark 1 computer at the University of Manchester in the Autumn of 1951. The Ferranti Mark 1 was the first computer to have a memory device that allowed it to run software programs. Previous computers of the day ran hardwired programs which were much more difficult to program.

The baudline spectrogram visualizer created the image below of the historic Ferranti Mark 1 computer music:

There are two interesting spectral features visible in the spectrogram.

The first feature is the frequency folding at 1012 and at 2024 Hz. These mirror spectral images can be artifacts of a sample rate conversion with poor anti-alias filtering or they could be caused by modulation (AM or FM) of the audio output. Modulation is the likely explanation because the audio output is suspected to be a simple wire connection to one of the Ferranti's register bits. Toggling a register bit at a CPU clock frequency at about 1012 Hz could create similar modulation sidebands. The Ferranti Mark 1 had a standard instruction time of 1.2 ms and a multiplication instruction time of 2.16 ms which is near a 1 kHz clock rate. On a related note, mystery signal #4 has similar modulation side-banding and the Khoomei Acoustic Analysis blog post has similar frequency folding.

The second spectral feature of interest was discovered with baudline's harmonic measurement bars. The harmonics of the musical notes are all missing their fundamental frequency. The exact cause is not known but a phantom fundamental is not easy to generate by bit twiddling. An analog filter or the frequency response of the loudspeaker could have caused this fundamental removal. Diode rectification is not the cause because the |x| absolute value operation moves the fundamental and it also moves the harmonics. On a related note the musical output is monophonic and has an apparent one octave range.

For technical information about the Ferranti Mark 1 computer see:

Friday, March 14, 2008

Cassini - Eerie Saturn Radio Emissions

The baudline scientific visualizer was used to investigate some eerie Saturn radio emissions captured by the Cassini spacecraft. NASA believes that the source of these radio waves are related to the auroras near the poles of Saturn. The 27 minute radio emission signal was collected by Cassini's radio and plasma wave instrument and has been compressed down to a 73.5 second audio file for playback.

This signal looks and sounds a lot like Earth VLF chorus with fading blobs of spectrum moving up and down in frequency. The large blocks visible throughout the spectrogram are interesting looking artifacts that could be synthesis or compression related. Another interesting artifact are the horizontal scan lines that can be seen in the zoomed in spectrogram image below:

The NTSC-like horizontal scan line artifacts could be synthesis based or they could be related to how the Cassini sensors operate. Baudline's periodicity bars measured the scan lines to have a repetitive spacing of 0.1487 seconds which when multiplied by the 5000 sample becomes 743.5 samples. Adjusting for a 73.5 second to 27 minute file expansion, a reciprocal factor of 22.04, the number of samples becomes 16386.7 samples which is very close to 16384 a power of 2 and a popular buffer size.