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IV. TIMING EVIDENCE FROM MATCHING FEATURES
A private citizen, Steve Barber of Mansfield, Ohio, voluntarily wrote
to the Committee that he was convinced from his own listening that there
are clear instances in which phrases recorded on Channel II tape were
distinctly audible on the Channel I tape as well. This is quite naturally
explained by assuming that the motorcycle with the open microphone (Channel
I) was near another police radio receiving a transmission from Channel II,
so that transmissions over Channel II would issue from its loud speaker and
be picked up by the open microphone and rebroadcast on Channel I. In
addition there are simultaneous broadcasts by the dispatcher onto Channels
I and II. Both kinds of cross talk are perfectly clear in many cases. The
existence of such identical portions of speech on both channels would allow
one to establish precise time synchronizations between specific portions
of the two recordings. The specific time synchronizations would not apply
to the recordings in their entirety, because Channel I ran continuously
during the period of interest while Channel II was sound activated and
operated intermittently. However, such matching features would enable one
to determine the relative timing between many events on Channel I and other
events on Channel II.
Barber identified several such matching sections on the two tapes.
Four of them are quite clear, but they occur several minutes after the
assassination and involve various police communications connected with the
follow-up to the shooting; however, they demonstrate clearly that there was
cross talk from Channel II to Channel I. As will be seen, they also
provide a clear demonstration of Channel I heterodynes suppressing the
recording onto Channel I of cross talk from Channel II, which suppression
we later also show exists in the interval containing the impulses and shows
that the cross talk was recorded through a radio receiver. Two events are
especially important for fixing the time of the section of tape analyzed by
BRSW and WA relative to the assassination. The first is a 4-second
fragment of speech that overlaps the conjectured 3rd and 4th BRSW shots on
Channel I. Barber there identifies a phrase, which he says begins with the
words "hold everything..." as identical to the phrase "...hold everything
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secure until the homicide and other investigators can get there...,"
clearly recorded on Channel II. The significance of this proposed match is
that the section on Channel I is concurrent with the last two of the
conjectured BRSW shots, whereas on Channel II that communication is part of
a clear sequence of emergency communications that followed the shooting and
occurred approximately one minute after the assassination. It is, in fact,
part of Sheriff Decker's instructions to his men in response to the
assassination. This time synchronization, if correct, would prove that
BRSW/WA conjectured shots were unrelated to the sounds of the assassination
gunshots. The section of Channel I recording with the BRSW/WA conjectured
shots would then correspond to a period of time well after the
assassination.
The second crucial event is the transmission "You want me...Stemmons",
which occurs several minutes after the assassination and is clearly
intelligible on both channels. It provides a common reference point for
timing events on the two channels. We used it to determine whether the
section of the recording containing the conjectured shots occurred before
or after Chief Curry instructed the motorcade to "Go to the hospital."
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IV-1. Sound Spectrograms
Initially, the poor quality of the "hold everything..." portion of the
recording made it appear unlikely that a convincing interpretation of the
badly garbled speech on Channel I could be made and the Committee was aware
of the power of suggestion, or cueing effect, in which a listener to a
garbled message will often be convinced he has heard what he has been
coached to hear.
For these reasons, arrangements were made through Bruce Koenig and
others of the FBI Technical Services Division for members of the Committee
to utilize the Division's excellent sound analysis equipment to obtain
sound spectrograms ("voiceprints") of the relevant communications on
Channels I and II. The spectrograms were prepared under the supervision of
Committee members. The sound spectrograms first reproduced were from tape
recordings kindly provided by James C. Bowles, Radio Dispatcher Supervisor
at the time of the assassination, but a sound spectrogram with a similar
pattern for the "...hold everything..." phrase on Channel I was also made
from a tape supplied by James Barger, essentially identical to that used in
the analysis of BRSW; later sound spectrograms were also made from new high
quality magnetic tape copies of the original Channel I Dictabelt and
Channel II Audograph disc. A sound spectrogram is a plot with elapsed time
along the horizontal axis, with frequency along the vertical axis and with
the darkness of the trace representing the intensity at that frequency.
Since the interpretation of sound spectrograms depends on continuous
gradations in darkness, copies in a printed report lose clarity. For this
reason photographs of the sound spectrograms will be retained in the
National Research Council files.
We began by making sound spectrograms of two of the later proposed
matching sections of speech. The match is clear, and establishes
unambiguously that identical portions of speech can be identified on both
channels. One of these matches is shown in Figure 3, and it demonstrates
conclusively that there was cross talk between the two channels. We then
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made spectrograms of the crucial ''hold everything" sections. As discussed
in greater detail in Appendix B. Figure 4 is a photograph of composite
sound spectrograms for the full four second message. The beginning of the
"...hold everything..." phrase is approximately at zero on these time
scales and the impulses for the BRSW conjectured grassy knoll shot occur
beginning approximately at the arrow marked 145.15s (the time of the
conjectured grassy knoll shot on the BRSW time scale) and the WA impulses
occur 0.2 seconds earlier. As discussed in Appendix B. the black dots mark
27 corresponding features on the two channels.
It is apparent from Figure 4 that there is a marked correlation
between parts of the sound spectrograms of the two channels, even though
the Channel I recording has much more noise. The correlation becomes much
more impressive when the spectrograms of the two Channels are compared in
detail. The correlation is particularly striking when one realizes that
only the initial second of the "...hold everything..." phrase can be heard
clearly on Channel I, yet the sound spectrograms contain numerous matching
features for the entire three and a half second sequence; note for example
the impressive match in the final segment from T = 3.2 to 3.6 seconds. In
all cases of matching features it is clear from the text of the messages
and from the signal intensities that a signal from Channel IT was
duplicated on Channel I and not the reverse.
The sound spectrograms present much more convincing evidence in the
present case than in their application to speaker identification. There,
words spoken at different times, supposedly by the same speaker, are
compared and a trained interpreter is often required to explain why the
subjective match in significant. In the present case, the need is to
identify two identical messages extending over a three and a half
l
second interval. Not only must individual parts of the two sound spectra
be alike but they must occur at exactly correct time intervals and with
exactly matching frequencies. The existence of these required time and
frequency correlations between the two channels imposes rigid constraints
on the messages to be matched. Furthermore, all sounds that appear on both
Channel I and II are useful in correlating the channels even though some
are not spoken words. For example in listening to Channel II it is
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apparent that there is an intermittent tone that contributes to the flat
portions common to Channels I and II. However, this tone varies in both
amplitude and frequency and is al so useful in correlating the two
channel s .
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IV-2. Analysis of Sound Spectrograms of "Hold Everything..."
The Committee used three methods in addition to visual inspection to
determine whether these critically important sound spectrograms of Channels
I and II contained signals from the same source. The studies are described
in greater detail in Appendix B and were made on the sound spectrograms
shown in Figures 4 and B-3.
In the first method we identified characteristic features that were
present on both spectrograms of Figure B-3 and then determined the relation
between the times of occurrence of the two sets of features. Twenty-seven
features were selected (indicated by the black dots in Figure B-3); a brief
description of each is given in Table B-1 in Appendix B along with its
frequency and time coordinates. The existence of correlations between the
two spectrograms over a long time interval can be demonstrated by plotting
T', the time coordinate of the Channel I spectrogram, as a function of T",
the time coordinate of the corresponding characteristic on Channel II. The
results are shown in Figure 5. The marked linearity of the plot shows that
the similar characteristics of the sound spectrograms of the two channels
follow the same time sequence, as they must for one to be cross talk from
the other. As described in Appendix B-l, a linear fit to the recorded
points gives the equation in Figure 5, and the slope of the line or ratio
of recording speeds is l.O59+0.002, which corresponds to a
(5.9+0.2)% net difference in the recording speed. The ratio of
recording speeds independently inferred from the measured frequency ratios
of the same points is 1.064+0.006, a value fully compatible with that
obtained from the time sequence. As discussed in Appendix B-1, the
probability of obtaining such close agreement by random occurrence of the
features at their observed average spacing would be about 2.1 x 10~13,
and the probability of randomly obtaining such good agreement on the
frequency ratio of the points is about 2 x 10-1°
The second method, which provides further confirmation of the
correctness of the identification of the two patterns, is the calculation
of the effective relative speeds from the frequency ratios for five
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sections with particularly well defined frequencies on the two channels, as
discussed in Appendix B-1. Such a calculation gives a ratio of recorder
speeds of 1.062+0.005 in excellent agreement with the value in the
preceding paragraph. Alternative analyses to minimize subjective errors in
the pattern recognition are also discussed in Appendixes B-2 and B-3.
To help in the visual recognition of similarities of the two patterns,
sound spectrograms have been made with the speed of Channel I effectively
changed by 6.7%. The results are given in Figure 4. Both frequencies and
times of the two channels now appear to be quite compatible.
A third approach to the investigation of whether Channel II segments
are recorded onto Channel I along with the acoustic impulses was taken by a
third member of the Panel and two collaborators. The Channel I and Channel
II recordings were digitized and the short-term acoustic spectra were taken
and stored in a digital computer. The printouts of these spectra are
similar to Figures 3 and 4 and are shown in Figures B-4, B-5, and B-6.
These digital spectrograms were computed directly from magnetic tapes and
did not involve the use of the FBI sound spectrogram equipment. Many of
the features observable in the analog spectrograms of Figure B-3 can be
seen in B-6, but no use was actually made of the spectrogram patterns.
instead, the actual data were used to test certain hypotheses, without
human intervention. An objective measure of similarity of two spectral
matches is obtained from the cross correlation coefficient, defined in
Appendix B-4. This cross correlation coefficient would be reduced if one
of the recordings were played at the wrong speed, or if the recording at
one time were compared with the same or a different recording at a
different time.
The first cross correlation coefficients were made from the same
Channel I and II recorded copies that were used in preparing Figures 3, 4,
B-1, and B-2. It was found that the biggest peak for the cross correlation
coefficient occurred for a relative warp (or speed ratio) of 1.06, in
agreement with the other two manual approaches for comparing Channels I and
II; a 1% deviation of warp from optimum diminished the peak substantially.
Unfortunately, that Channel II copy contains many repeats caused by the
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Gray Audograph machine in playback. Accordingly another tape copy was
prepared by members of the Committee directly from the original Audograph
plastic disk itself and by the use of a standard turntable and tone arm,
thus producing a tape without compensation for the fact that the disk was
originally recorded at constant linear track speed. It was this tape that
was used in preparing the sound spectrograms shown in Figures B-4, B-5, and
B-6. Figure 6 gives the cross correlation coefficient for the "hold
everything..." segments when the relative speed was selected to give the
largest peak and the 750 correlation coefficients were obtained by sliding
2.50 sees of Channel I along 10.00 sees of Channel II, 0.01 sees at a time,
using frequencies in the band 600 Hz to 3500 Hz. For comparison the cross
correlation coefficients of the unambiguous segment "You want...Stemmons't
are plotted in Figure 7. The shape of the peak is very similar to that for
the "hold everything..." segment. The background is somewhat smoother,
simply because there is less noise in Channel I at this time. Channel I,
however, in neither case gives a perfect reproduction of Channel II. It
has lost some of the high and low frequencies, and as one would expect
there are tones present on Channel I that are not on Channel II.
The marked narrow peaks of the cross correlation curves clearly show
by an objective test that the "hold everything..." segment of Channel II is
present on Channel I at the same location as the acoustic impulses. There
is no doubt that the voice (and other) sounds of Channel II are present on
Channel I to an accuracy in location corresponding to a few milliseconds.
We find these three sets of results to be overwhelming evidence that
the "hold everything" sections of the two recordings are traceable back to
a single acoustic signal from Channel II. If there is no overrecording on
Channel I (as we later show to be the case), the correspondence between
these two recordings of "hold everything..." would be conclusive evidence
that the events analyzed by BRSW/WA were not the assassination shots, since
we know from Channel II that the "hold everything" transmission was made at
least 50 seconds after the Chief instructed the motorcade to "Go to the
hospital." We will discuss in Section IV-4 the possibility of there having
been an overrecording on Channel I and our conclusion that there was not.
Indeed, the digital analyses in themselves are used in Section IV-4 and
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Appendix D to demonstrate that the Channel II cross talk on the Channel I
recording was already present at the Channel I radio receiver and was not
added later in copying or as an overrecording
.
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IV-3. Timing of Channel I and Channel II Events
In the previous section, a synchronization between events on Channels
I and II simultaneous with the conjectured shots was obtained by detailed
analysis of sound spectrograms. Other examples of matching features on the
two channels, occurring several minutes after the assassination, are so
much clearer that no special technical procedures are required to establish
synchronizations in these parts of the recordings -- simple listening is
sufficient to eliminate all doubt about these synchronizations. By timing
both recordings backwards from the time of these matches, it is possible to
relate the times of events in the critical portions of both recordings,
independent of the correspondence established in the previous section.
The clear match that occurs closest to the assassination is "You
want...Stemmons," which occurs on Channel II several minutes after the
Chief said "go to the hospital." Figure 3 shows a sound spectrogram of the
match. Since Channel II was sound activated and recorded intermittently,
we obtain a lower bound on the time between these two transmissions by
timing the tape between them. Any halts in the recorder-would cause the
tape time to be less than the actual clock time between these
transmissions.
Time intervals were measured using two different sets of tape
recordings. First, we used the tapes obtained from Bowles to time events
in critical portions of the recordings. Since relative time between
Channels I and II is all that is of significance in this comparison of
events, time in this set of measurements was made in somewhat arbitrary
Channel I elapsed time units. The timing was difficult to do because there
were "repeats" (see Appendix C) on the Channel II magnetic tape and speed
differences between segments of it and the Channel I tape. Appendix C
describes how these timings were made and how compensations for repeats and
speed differences were accomplished. The results of the spectrogram
analyses just discussed were used to obtain the speed correction (a factor
of about 1-.06). The durations of repeats were determined from strip charts
of the signal level as a function of time.
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The result of these timings, also given in Appendix C is that:
a) On Channel II, "Go to the hospital" occurs at least 189 seconds
before "You want me...Stemmons."
b) On Channel I the portion of the tape on which BRSW/WA found
"shots" occurs 171 seconds before "You want me...Stemmons."
c) Since Channel II operated intermittently, any time that elapsed
while the recorder was stopped would increase the 189 second interval
between "Go to hospital" and "You want me...Stemmons." There were five
places where the recorder could have stopped.
By this analysis, the last of the conjectured shots occurred at least
20.9 seconds after Chief Curry issued his instructions "Go to the
hospital;" therefore, they could not have been the shots of the
assassination.
After the preceding analysis of the tapes obtained from Bowles was
completed, the Committee gained access to the original Gray Audograph and
Dictaphone recordings. These were transcribed, as described in Appendix C,
onto tape, with care taken to minimize the 60 Hz hum that was added to the
signal and to ensure that no skips or repeats were introduced in the tape
recording of either channel. No break interrupted the Channel II
recordings as was the case for the Bowles tapes. These recordings, of
course, did not eliminate the effects of the intermittent operation of
Channel II, and time interval measurements are still lower bounds. The 60
Hz hum from the original recordings was used to determine the relationship
between playback speed and original recording speed and to convert the
measured-elapsed time intervals to real elapsed time units. (Recall that
arbitrary Channel I elapsed-time units were used for the first set of
measurements made on the Bowles tapes.) It was easy to make this
correction on Channel II, but difficult on Channel I, because the Dictabelt
was in poor condition. The conversion method is described in Appendix C.
Except for this speed-time correction 9 obtaining comparable measurements of
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the time intervals between critical events on
common "You want...Stemmons" transmission was straightforward.
Channels I and II and the
The result of these timings made on tapes obtained directly from the
original recordings, also given in Appendix C, is that:
a) On Channel II, "Go to the hospital" occurs at least 206 seconds
(real time) before 'lYou want me...Stemmons.'t
b) On Channel I, the portion of the Dictabelt on which BRSW/WA found
Shots occurs 178 seconds (real time) before llYou want me...Stemmons.tl
By this analysis,the last of the conjectured shots occurred at least
30.9 seconds (real time) after the instructions "Go to the hospital". This
measurement is believed to be more accurate than the one obtained from the
Bowles tapes, since the tapes obtained from the original recordings showed
no evidence of skips, repeats, or breaks.
Both of these results confirm the previous finding from the sound
spectrograms that the section of tape in which BRSW/WA found "shots"
recorded events that occurred after the assassination. Note that the
results from timing events do not require a match between the two
recordings of "hold eveything," but they do not preclude such a match.
Halts in the recorder would increase the time between the conjectured shots
and Chief Curry's instructions. Furthermore, any delay between the
assassination and the instructions "Go to the hospital" would increase the
discrepancy between the timing of the conjectured shots and the actual
assassination.
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IV-4. Possibility of Superposed Recordings
The Committee has considered seriously the possibility that the
impulses analyzed by BRSW/WA might have been overlaid at a later time by
the "hold everything..." message. Conceivably such an overrecording could
have occurred by an accidental knocking backwards of the Dictabelt or the
recording head by about one minute in the first minute following the
assassination or by the substitution (either accidentally or deliberately)
of a new Dictabelt copy for the original, with the copy being made by audio
coupling while a Channel II recording was playing in the background. The
Committee found conclusive evidence that this was not the case. The
evidence is of four kinds: (1) physical examination of the Dictabelt for
indications of overrecording or of substitution of a copy for the original;
(2) the unlikely nature of any of the highly contrived scenarios required
to provide such an undetectable overrecording either accidentally or
deliberately, (3) the compatibility of the timing implied by the "hold
everything..." identification with other firmly established evidence, and
(4) the conclusive acoustic evidence on the Dictabelt itself that the cross
talk recordings were made through a radio receiver with automatic gain
control. These different forms of evidence are discussed in Appendix D,
where all are shown to be compatible with the recordings being made at the
same time and some are incompatible with the hypothesis of later superposed
recordings by audio or direct electrical coupling. Only the evidence of
category (4) will be reviewed in this section.
The digital analyses of the sound spectra can be used to demonstrate
that the Channel II imprint on the Channel I recording was already present
at the Channel I receiver and was not added later in the recorder or as an
overrecording. The by radio nature of Channel II cross talk is
demonstrated by its detailed behavior in the presence of Channel I
heterodynes when another Channel I transmitter is keyed on with a more
powerful carrier signal. The frequency offset between the two carriers
gives rise to a heterodyne tone in the Channel I recording. However, the
Channel I receiver was fitted with automatic gain control (AGC) to hold the
output level approximately constant; as a result, the cross talk signals
decreases in intensity in a few tens of milliseconds (as does any residual
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transmission from the original stuck-mike transmitter). At the end of the
Channel I heterodyne, the AGO gradually increases the receiver gain, and
signals on the stuck-mike transmission increase in intensity in the
recording. An excellent probing signal for the Channel I gain would be a
Channel II steady tone acoustically coupled from the field loudspeaker to
the stuck-mike transmitter. This would come in at constant level, and the
variation in level on the Channel I recorder should mimic the AGC action
if the Channel II signals were present in this way. Inspection of the
digital spectrogram of Figure B-4 (and digital tabulations of the data)
show that numerous Channel II brief tones have constant level from
beginning to end. However, a crucial demonstration is provided by the
Channel I heterodyne beginning in Figure B-6 at time 32.02 seconds. The
underlying Channel II brief tone is clearly substantially reduced in
intensity at the beginning of the Channel I heterodyne, and gradually grows
back when the Channel II brief tone results after the Channel I heterodyne
ceases. More detail is available in the two digital plots of Figures B-7
and B-8. This behavior is validated by similar Channel II brief tones
underlying Channel I heterodyne signals in the "You want me...Stemmons"
phrase and in a phrase "I'll check...", likewise present on both channels.
This is discussed in further detail in Appendix D along with other evidence
that has led the Committee to conclude that the acoustic impulses
attributed to gunshots were recorded about one minute after the President
was shot.
Representative terms from entire chapter:
cross talk
