Slowing Twisting in the Wind

Analysis of the signals on the historic Dictabelt suggests that a studio added the acoustic signature of gunfire to distract researchers from the overwhelming strong evidence that a jammer was the source of the loud interference.

No matter what the critics say they cannot make the pulse patterns attributed to gunfire vanish from the acoustic records. These patterns contain very special pulses that distinguish themselves from all the other snaps, crackles or pops. I call these special signals limiting pulses.

Playing a wave file of these limiting pulses at progressively slower speeds provides audible evidence of the special nature of these pulses.

Reducing playing speed dramatically lowers the pitch of the voice and has a similar effect upon a brief heterodyne and the background noise. However, the pitch of the limiting pulses initially resist lowering and change slightly at greatly reduced playing speeds. This demonstration shows that the high frequency contents of the limiting pulses are widely dispersed and extremely rich. These uncommon characteristics are further evidence that these special pulses are the responses of the radio system to impulses generated by the limiting circuit in audio stage of the transmitter.

BBN documented a level of 100 db re 2 X 10^-5 Newton per square meter at the microphone as the threshold for activation of the limiting circuit. This means that ears in the vicinity of that microphone would have heard sounds reminiscent of moderately distant gunfire.

So you have two choices. You can assert that the Dictaphone recorded gunshots on the Dictabelt or a studio added the limiting pulses and made an untrue acoustic record.

Three characteristics of heterodynes tones generated by a FM receiver provide overwhelming strong evidence that a studio altered the contents of the Bowles tape if not the actual Dictabelt. These characteristics are the commonness of heterodyne tones, nonrandom distribution of the heterodyne frequencies and the absence of interaction between the continuous interference with a silent station whose radio signal belonged to a pair that produced the heterodyne tone.

A heterodyne tone occurs when the radio signal strengths of two stations differ by less than the capture ratio of the FM receiver. When a third station is transmitting a continuous signal such as the loud interference then the radio signal strengths of the silent stations must differ from the radio signal strength of the interfering station by more than the saturation threshold while the strengths of the three signal must be less than the capture ratio of the receiver. For these reasons a heterodyne tone in the presence of the continuous interference would be a highly unlikely event.

The audio frequency of a heterodyne tone equals one-half the difference of the frequencies of the radio frequency signals belonging to the two silent stations. In turn the frequency of a quartz crystal determines the radio frequency of each station. The differences in crystal frequencies are randomly distributed over an interval specified by a manufacturing tolerance. These considerations show that the audio frequencies of the true heterodyne tones are randomly distributed over the audio bandwidth of the system. However, the many heterodyne tones that occur briefly before the cessation of the loud interference shows a nonrandom distribution of audio frequencies. The lower frequencies are close to the 1 kHz high frequency cutoff and the higher frequencies extend to about 3 kHz, well into the roll off portion of the bandwidth.

More important the continuous interference does not show abrupt decreases in amplitude as silent stations keyed in nor abrupt increases in amplitude when the silent stations keyed out. Since each heterodyne tone requires the simultaneous transmission of two silent stations there are at least two and perhaps four keying events associated with each tone. Clearly the failure of the Bowles tape to show this interaction of signals in a FM receiver is further evidence of studio alterations.

The Ramsey Panel and the Watson Research Center evaded this evidence of alteration by misapplying principles of AM receiver operation to the FM radio system of the DPD. This deception by the educated and knowledgeable people associated with the NAS elevates the evidence of alteration into the category of proof.

Spectrographic analysis of an early portion of the loud interference reveals the motive for the alteration of the acoustic record. This portion of the interference has a repetition rate of 0.9 kHz or 54,000 RPM.

Spectral Characteristics of the Relaxed Interference

However, the sound of this signal indicates a loud noise. Calculating the spectrum over shorter and contiguous intervals show that the sinusoid persisted with slightly varying frequency throughout the duration of the longer spectrum. These results show that simultaneous amplitude and frequency modulation of audio was the primary component of the loud signal during this interval. This composition describes a frequency modulated heterodyne in the presence of noise and prompted a systematic study of the spectral changes of the loud interference.

On longer time scales, the spectra of the interference show ordered changes. The dominant frequencies fall into two bands. Lower frequencies appear near 375 Hz and the higher frequencies cluster around 1000 Hz. The relative powers in these bands change with time. These characteristics suggest a ratio of relative powers as a measure of these spectral changes. Dividing the relative power in the low frequency band by the sum of relative powers in both bands gives a measure that is independent of the unknown scale factor relating power to spectral magnitude. This ratio provides an absolute measure of power in the low frequency band as a proper fraction of power in both bands. Changes in this ratio measure changes in the spectra of the interference. However, the frequency selectivity of the DPD radio system and widths of the selected bands influence the magnitude of this ratio. Diminishing the ratios by the mean value of the ratios produce measures, called skewness, that are independent of the frequency response of the system and widths of the selected bands. This study used limits of 300 Hz to 450 Hz for the low frequency band and limits of 800 Hz to 1200 Hz for the high frequency band.

A bar graph of skewness shows that the spectrum of the relaxed interference fits the pattern of neighboring spectra.

Spectral Skewness of the Synthetic Interference

More important the skewness of the relaxed interference coincides with the global minimum of the spectra family. This later consideration is overwhelmingly strong evidence that the loud interference is a simultaneous amplitude and frequency modulation of audio.

The spectral study produced two surprises. By construction the mean value of skewness tends toward zero. However, the bar graph shows two distinct means separated by an abrupt discontinuity. Clearly this discontinuity shows a rapid change in either the radio or recording system.

This surprising change occurs between 225 and 228 second from the start of the Bowles tape of Channel-I, which precedes the first observation of the itinerant AGC activity at 233 second. The increase of skewness at the discontinuity shows a considerable shift toward a larger fraction of lower frequencies. Both considerations are consistent with the sudden introduction of intermodulation distortion by excessive AGC activity.

The skewness of the loud interference began with a small dip followed by an abnormally large peak that lead to an extended and deep decline. Following an interruption of the loud interference, the skewness reached its minimum value and produced the spectra of the relaxed interference. The skewness started to increase, ascended to a lesser peak and declined. These changes typify the warmup drifts of vacuum tube oscillators during their first minute of operation.

During the fifties the standard design for jamming sources of audio used two main oscillators, which operated over different portions of the spectrum. Two secondary oscillators modulated the main oscillators and produced simultaneous amplitude and wide band frequency modulation of audio. This combination yielded a filter resistant and non-stationary noise. By the early sixties transistors replaced vacuum tubes in jamming sources and the earlier designs were available on the surplus market.

AGC

Automatic gain control, AGC, counteracts overloading of circuits by excessively strong signals. AGC may act upon either audio or radio signals. The impression of audio on amplitude modulated radio signals limits the rapidity of AGC attenuation. Ten milliseconds represents a reasonable compromise between loss of audio due to sluggish response and inaction upon the explosive syllables of speech. Ideally the recovery time should be comparable with the attenuation time. However, this requirement significantly complicates design. Typically they make the recovery time two or three times longer that the attenuation time by adding a five-cent resistor.

Amplitude modulation, AM, impresses audio on a radio signal by varying its amplitude. Successful recovery of this audio requires AGC to prevent the amplitude of the radio signal from exceeding the capacity of the receiver. Hence, AM receivers use AGC at the RF and IF stages to prevent overloading.

Radio signal strength at the receiver affects the volume of recovered audio. AGC levels differences between strong and powerful stations. However, it does not boost the audio from weak stations. Hence AGC of audio is useful with a AM receiver.

Frequency modulation, FM, impresses audio on a radio signal by varying its frequency. This design permits full recovery of the audio when strong signals overload stages. For this reason, they use very high gain amplifiers without AGC in the IF stages. In fact many FM receivers include a limiter stage between the IF amplifiers and the discriminator, which recovers the audio from the frequency modulated signal. The purpose of this limiter is to overload and suppress amplitude noise.

The frequency deviation of the transmitter determines the magnitude of the recovered audio. Thus for equal deviations the strongest transmitter produces audio whose volume is comparable with the weakest transmitter. This characteristic of a FM system renders AGC control of received audio unnecessary and even harmful.

Intermodulation Distortion

Distortion is the production of frequencies not found in the original signal. Intermodulation introduces new frequencies that are the sum and difference of all frequencies contained in the original signal. By contrast harmonic distortion adds frequencies that are whole number multiplies of the frequencies in the uncontaminated signal.

From these considerations it follows that harmonic distortion cannot and intermodulation distortion may increase the low frequency content of a signal. When the sums of signal frequencies exceed the high frequency cutoff of the system then intermodulation distortion increases the low frequency content of the signal.

Clearly with 1 khz high frequency cutoff of the DPD radio system, intermodulation distortion would decisively increase the low frequency content of the signal.