Duped by a Dub

For decades students of the acoustic evidence have suspected that the crosstalk and tones became imprinted upon a dictabelt during copying a tape of the historic Dictabelt. Surprisingly, the NAS published clear and irrefutable evidence that they found the alleged Decker crosstalk on tape of a recopied dictabelt.

The Ramsey Panel varied the speed of their Channel-II playback of the "hold everything . . . " portion of the Decker broadcast and noted the compression to maximize matching of selected Channel-II frequencies with Channel-I frequencies. They found a compression of 0.911 to 0.913 maximized cross-correlation with the Channel-I segment containing pulse patterns attributed to gunfire.

Figure 1. Reference: Report of the Committee on Ballistic Acoustics, Page 107

BB&N studied the frequency compression caused by the Dictaphone and measured an error of 5 percent. Their measurements agree closely with spectral analysis of the segment containing the pulse patterns attributed to the third shot. The Bowels tape of the Dictabelt is the source of this segment.

The spectrographically measured 57.3 Hz for the power line frequency yields a compression ratio of 0. 955.

Figure 2. Spectral Analysis of the Channel-I pulse patterns attributed to the third shot

The Ramsey Panel used a phonograph to play the Audograph disk of Channel-II. This method eliminated repeats and skips but introduced a varying frequency scale. A spectral analysis of the concluding words of Decker's "hold everything secure" broadcast and an unmistakeable power line hum show no error in composite playback/recording speeds of the Audograph and phonograph disk. Apparently they targeted the Decker broadcast and adjusted playback rate of the phonograph to produce a 60-Hz hum.

Figure 3. Spectral Analysis of the hum following the concluding words of the "hold everything secure" message

Since tape recorders operate with near clock like precision, the Dictaphone bears sole responsibility for altering frequencies. So taping a playback of the historic Dictabelt compressed frequencies by a factor of 0.955. Playing the tape had negligible effect. However, if they recorded a new dictabelt from the compressed tape then the net compression during playback would be 0.955 multiplied by 0.955. This product is 0.91205 and falls exactly within the range of reported compressions to maximize matching.