No Evidence of Engine Sounds on the Dictabelt

At 60.9 seconds after the start of the Bowles tape of channel I, a station broadcasted a brief message that said, seventy-five place. Toward the end of this message we hear two beeps and the start of the transmission from the open microphone. These concurrent broadcasts provide an opportunity to measure the loudness of the interfering signal at the open microphone.

An oscillograph of the brief message shows compressed audio, which is evidence of an excessively loud voice. Following the beeps, the audio from the open microphone begins with one-half the amplitude of the brief message. However, due to the beeps it is unclear whether the transmission from the open microphone was a solo or a simultaneous broadcast. In about one second the audio from the open microphone abruptly doubles and shows that it began as a simultaneous transmission. Contrary to the general belief that the open microphone picked up background sounds, these measurements indicate that the signal at the open microphone was louder than typical foreground sounds and comparable with an over driving voice.

In the initial phase of their work, Bolt Beranek and Newman Inc. (BB&N) filtered out periodic signals from their tape of the Dictabelt. They demonstrated the effectiveness of autocorrelation and an adaptive filter to recognize and remove periodic signals from a high-fidelity tape recording of a motorcycle. These proven methods failed to filter the loud interfering signal from the recording of the Dictabelt. BB&N attributed failure of their filtering techniques to distortions introduced by the transmitter and an accumulation of random noise on the Dictabelt from repeated playing. These rationalizations fail for two reasons. First distortion of any periodic signal produces more periodicity. Second the intelligibility of barely audible voices show the insignificance of accumulated scratches.

Briefly the interference sounds repetitious. An oscillograph of this signal, from 234.8 to 236.2 second on the Bowles tape, shows a repeating pattern of strong pulses. A closer examination reveals irregular intervals between adjacent pulses. This repetitious sound lacks the regular timing, which characterize the sound of a Harley motorcycle. By contrast the intervals between adjacent motorcycle pulses repeat with clockwork precision. Spectrographs of these signals highlight their differences.

Harley Motorcycle

The prominent line at 196 Hz corresponds to a repetition rate of 11760 rpm per cylinder. For a motorcycle with two cylinders this rate represents a reasonable engine speed of 5880 rpm. Secondary lines at slightly lower frequencies arise from the damping action of the engine block and the muffler system. Frequency modulations of the signal due to changes in engine speed cause the dense cluster of lesser lines. The full scale frequency range of 2.756 kHz fits the spectrum of the repetitious signal and facilitates comparison.

Repetitious Signal

The narrow frequency response of the DPD radio system accounts for some features of this spectrograph. A typical low frequency cutoff of 300 Hz explains the attenuation of lower frequencies. Higher frequencies begin a gentle decline at 1 kHz and decrease more acutely above 2 kHz. The dip in magnitudes near 750 Hz is common to spectrographs and indicates a narrow band siren filter.

A glancing comparison of the spectra reveals slight overlap. However the spectrum of signals from motorcycle moving at 11 mph would be shifted farther left than the spectrum of the speeding Harley. Clearly if engine sounds were present then the separation of spectra would have allowed an electronic filter to detect their presence.

The line at one twentieth of full scale is common to all spectrographs of signals from the Dictabelt. This line appeared before the interference began and continued after the noise abated. Its unchanging frequency of 119 Hz identifies the line as representing power supply ripple. The inaudibility of this obvious line shows the ability of Fourier analysis to extract periodic signals from interfering noise.

Adaptive Filter

An electronic filter separates one frequency from another. However it cannot determine whether any given frequency belongs to the desired or the interfering signal. Hence the character of the signals determine the effectiveness of filtering.

When the signals have most frequencies in common, filtering is useless. Effectiveness of filtering improves as the number of common frequencies decrease.

An adaptive filter uses autocorrelation to identify the simple spectrum of a periodic signal and adjust its performance to track changes in the period. Hence the techniques employed by BB&N were highly effective at recognizing and filtering out the sounds of engines.

Autocorrelation

Periodic signals repeat the same pattern at regular intervals. This characteristic enables recognition of periodicity by sliding an oscillograph over a copy of itself. A coincidence of signal shapes shows periodicity and the time interval between one coincidence and the next equals the period. Normally people perform this test mathematically and call it an autocorrelation.

Magnitude Spectrogram

A magnitude spectrogram plots magnitude on the y axis against frequency on the horizontal axis. Usually this plot presents the short term spectrum of the signal. Under these conditions, it produces a snapshot of the spectrum. This enables a person to flip a stack of short term spectrograms and view the changes in the spectra as a motion picture.

Oscillograph

Many wave players include an oscillograph, which plots amplitude on the vertical axis against time on the horizontal axis. On this graphical representation of the voice dense black is indicative of average magnitude and the thin vertical lines show voice peaks. Normally the peaks of an uncompressed voice extend three times higher than the average. On this particular message the peaks of the first two words are only slightly more than the average. This shows a high degree of audio compression. The peaks of the third word are more separated from the average and represent less compression.

A heterodyne begins after the second word. This event marks the start of a simultaneous transmission. Presence of the unmodulated carrier reduced the frequency deviation of the composite signal and the decreased the average and peak magnitudes of the third word. These considerations show that audio compression occurred after reception by the Channel-I receiver.

Periodic

Periodic signals, no matter how complicated, have simple frequency spectra. They consist of narrow-band sinusoids whose frequencies are whole number multiples of a fundamental frequency. The amplitudes and phases of the narrow-band sinusoids contain all the complications of the periodic signal.