Decoder for a recorder-decoder system

An improved decoder for a recorder-decoder system wherein a threshold level detector is included in the decoder. The recorder provides for the recording of an audio signal plus a data signal. The data signal includes zero crossing information for sequential increments of time plus an indication of whether the threshold level was met for an increment of the recorded audio. Upon playback the decoder also provides a data signal based on the recorded audio. The threshold level detector of the decoder is higher than that of the recorder. If the recorded data signal indicates the threshold was not met and the same threshold indication is not provided by the threshold level detector of the decorder for the corresponding increment of the recorded audio signal, tampering of such audio increment of the recorded audio is considered to have occurred.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a decoder or verifier for examining an audio 
recording with respect to the presence of evidence indicating whether the 
recording had been tampered subsequent to its recording wherein data 
signals based on characterizing information contained in the recorded 
speech are recorded with the audio on the recording medium for use in 
detecting evidence of tampering. The invention is particularly pertinent 
to the examination of audio recordings wherein the characterizing 
information is based on zero crossing detection. 
There are situations where it is desirable to make a recording of audio 
information, such as speech or speech plus other audio, which will be 
played back at a later time with the recording made in a manner such that 
the authenticity of the recording, if questioned, can be verified 
electronically using a reasonable amount of electronic circuitry. 
Recordings made in such a manner have application in fields such as law 
enforcement, private investigation, journalism, medical, legal and many 
other similar fields where the authenticity of audio recordings may be 
questioned at some time. 
2. Description of the Prior Art 
A tamper protected recording system has been disclosed wherein input audio 
signals are recorded together with a first data signal that includes 
characterizing information derived from the input audio signals with a 
second data signal produced during playback that includes characterizing 
information of the same type obtained from input audio signals, with such 
second data signal derived from the recorded input audio signals. The two 
data signals are compared electronically. If the two data signals fail to 
provide a high level of comparison, the recording is considered to have 
been altered. 
SUMMARY OF THE INVENTION 
A recording and verification system can be provided which characterizes the 
audio signal input by a count of zero crossings during equal and 
sequential increments of time with each count recorded as a part of a data 
signal that is recorded on a recording track that is separate from the 
recording track for the audio signal. A count in the data is followed by 
one bit of data which indicates whether a threshold level was satisfied by 
the audio signal at least once during the count. As taught by the prior 
art, the recorded audio can be examined for tampering by a verifier or 
decoder which provides for playback of the recorded audio signals and the 
data signal with another data signal derived by the decoder from the 
recorded audio, which in this case would include a count of zero 
crossings. If the threshold level data bit in the data signal produced 
during recording for a zero crossing count indicates the threshold level 
was satisfied at some time during the count, the difference between such 
count and the count obtained during playback for the corresponding audio 
portion is determined. If an alteration in the recorded audio signal was 
introduced subsequent to its recording it is evidenced by a count 
difference that is significantly larger than in the case where no 
alteration has been made. 
It has been discovered that the verification provided by the recording and 
verification system described can be circumvented by operating the 
recorder for the system without an audio input to the recorder. A data 
signal would be recorded, and though noise generated by the recorder will 
give rise to zero crossing counts, the data signal would not include a 
combination of a zero crossing count followed by a threshold level data 
bit indicating the threshold was satisfied at some time during the count. 
After the data signal is recorded, the recording medium can then be used 
in a different recorder to record on the audio track. When the audio 
recording is then played back via the verifier or decoder for the system, 
the verifier will not detect any combination of zero crossing counts 
followed by a threshold level data bit in the recorded data signal 
indicating the threshold level was satisfied at some time during the count 
so that no zero crossing count comparisons will be made. Thus, though a 
tampered audio recording is presented to the verifier, it will not be 
detected. 
The deficiency in the recorder-decoder system described, which would allow 
the verification to be circumvented, is eliminated by the invention 
presented herein. The invention provides an improved decoder for a 
recorder-decoder system having a recorder which records an audio input and 
records a data signal that includes a count of zero crossings of the audio 
occurring during sequential increments of time with a threshold 
information bit provided for each count indicating whether threshold level 
was satisfied by the audio at least once during the count, the decoder for 
the system providing for playback of the recorded audio and the recorded 
data signal with a count of zero crossings of the recorded audio made 
during sequential increments of time, the improvement including a 
threshold circuit operable during playback of the recorded audio and 
recorded data signal for detecting whether the recorded audio exceeds the 
threshold level of said threshold circuit, said threshold level being 
higher than the threshold level of the recorder whereby tampering of the 
recorded audio is considered to have occurred if the threshold information 
bit for a count of zero crossings in the recorded data signal indicates 
the threshold level was not met and the same threshold level indication is 
not provided by the threshold circuit of the decoder for the corresponding 
increment of the recorded audio signal.

DETAILED DESCRIPTION 
The recorder portion of a tamper protected system provides for the 
recording on a recording media, such as magnetic recording tape, of an 
audio input together with a data signal containing information. The 
decoder portion of the system uses the recorded audio and data signal to 
determine whether the recorded audio input has been altered subsequent to 
its recording. It is possible to record the audio and data signal on the 
same track or use separate tracks. If the recording is made at a slow 
speed, the use of a separate track is preferred due to the limited 
frequency separation that is possible at the low recording speeds. In a 
known tamper protected recording system the data signal includes such 
information as the periodic recording of the identification number 
provided for the recorder, a recording of real time and a recording of 
characterizing information from the audio signal based on a count of zero 
crossings. Referring to FIG. 1 of the drawing, the recorder, with which 
the improved decoder of the invention presented herein is used, has 
circuitry which includes a bandpass filter 10, an amplifier 12, a zero 
crossing detector 14, a threshold detector 16, two microprocessors 18 and 
20 plus an FSK integrator 22. 
Speech, while having a low dominate frequency, does not contain much 
characterizing information at such frequency. Another factor that must be 
considered is the frequency bandwidth of the tape recording system 
provided for the recording speed being used. The higher the recording 
speed, the greater the bandwidth provided by the recorder. In order that a 
compact recorder can be employed and still provide adequate recording 
time, a low recording speed is used. The circuit of FIG. 1, with respect 
to the bandwidth of the filter 10, is based on the foregoing factors 
wherein the recording speed is 2.4 cm/sec. The bandpass selected for 
filter 10 is 1,000 to 3,000 Hertz and is connected to receive the same 
audio signal that is being recorded by the tape recorder (not shown). The 
output of the filter 10 is applied to the amplifier 12. A bounded or 
clipped amplifier is preferred for use as the amplifier 12 since it 
provides an amplifier that will not go into saturation and will use less 
current which is important for a battery operated recorder with which the 
circuit of the drawing is included. The amplifier 12 is used to provide a 
gain that is sufficient to assure that low level noise signals present in 
the filtered signal, when there is an absence of speech, will be of a 
level sufficient to satisfy the hysteresis presented by the zero crossing 
detector 14 to which the output of the amplifier is connected. The 
amplifier 12 is, of course, not needed if the signal level from the filter 
10 is adequate. While not required in all cases, it is preferred that some 
hysteresis be presented by the zero crossing detector 14 so it will not 
chatter due to noise in the speech signal. The speech signals are normally 
much greater than any background noise. The zero crossing detections made 
by the zero crossing detector 14 are counted by a binary counter. This can 
be provided by the microprocessor 18, which is available, eliminating the 
need for a separate counter circuit. The microprocessor 18 will be 
available since it is needed for other functions relating to the overall 
system in which the present invention is usable. The counts taken by the 
microprocessor 18 of the zero crossings are obtained during equal and 
sequential increments of time wherein such time increment is much less 
than the time it takes to say most words. A time interval of 15.625 
milliseconds is suitable providing 64 counting intervals every second. 
This time interval coupled with the bandpass filter means the count will 
range from about sixteen to forty-eight allowing the count to be 
compressed by using only the count above sixteen as data to be recorded 
with the audio signal. With the count so compressed, only five data bits 
are required for each count which makes for efficient use of data bit 
density. These counts, as provided by the microprocessor 18, are fed to a 
microprocessor 20 which serves to convert the count to frequency shift 
keying (FSK) form for recording of the count on the tape or other 
recording media of the recorder (not shown) as frequency signals wherein 
one cycle of one frequency represents a "1" and another frequency, for 
example, a frequency twice that of the one frequency, is used to provide 
two cycles to represent a "0". To simplify the maintenance of the 
necessary timing that is required, such frequencies have a harmonic 
relationship with the frequency of the clock of the microprocessor 18 
which determines the time interval used for taking the zero crossing 
count. Frequencies of 1024 and 2048 Hertz are used for "1" and "0", 
respectively, and are derived from the clock provided by microprocessor 
20. The microprocessor 20 is also used to time the entry of other 
information for the data signals mentioned earlier in such FSK form for 
recording with the count information and also provide necessary 
synchronizing bits, bits for an error correction code as well as fill 
bits. The FSK signal output of the microprocessor 20 is shaped via the FSK 
integrator circuit 22 before it is presented to the recorder (not shown) 
for recording with the audio input provided to the recorder. Real time has 
been indicated as one of the inputs to be provided as a part of the data 
signal to be recorded. The real time information is maintained by the 
microprocessor 18 which uses a 32,768 Hertz clock crystal as the basis of 
its timing. This allows the microprocessor 20 with its 1.048576 megahertz 
clock to be turned off to conserve power when the system is not being used 
for recording audio input and data signals. 
The threshold detector 16 used in the circuitry of FIG. 1 provides an 
indication to the microprocessor 18 as to whether the audio signal 
received from the filter 10 has satisfied the threshold level of the 
detector 16. A suitable threshold level for the detector 16 is one that 
will be satisfied 50 to 70 percent of the time while speech is presented 
for recording. In the case of a detector 16 having hysteresis, a threshold 
on the order of twenty decibels above the hysteresis of the zero crossing 
detector was found to be suitable. The microprocessor 18 is programmed to 
provide one bit of information to the microprocessor 20 for each counting 
time interval to indicate whether the threshold was satisfied at any time 
during the counting time interval. A "1" can be used to indicate the 
threshold was satisfied and a "0" used to indicate the threshold was not 
satisfied or vice versa. This information bit is passed to the 
microprocessor 20 and is put in FSK data for recording. It is convenient 
to have this information bit follow the bits used to convey the zero 
crossing count made for a count time interval. 
By using the threshold detector 16 to establish a criteria with respect to 
the reliability of the count made of the zero crossings during the count 
time interval, it is possible to make a count of zero crossings upon play 
back of the recorded audio signal to provide a reliable comparison with 
the zero crossing counts recorded via the data signal at the time the 
audio signal is recorded to determine whether any tampering of the 
recorded audio signal has occurred. 
A comparison of the zero crossing counts shows a total maximum difference 
of about forty-eight crossings over sixteen consecutive counting intervals 
when there has been no alterations made to the recorded audio signal. A 
difference is considered only when the threshold information bit indicates 
the threshold of detector 16 was satisfied for that counting interval. The 
presence of any tampering in a sixteen consecutive counting interval 
causes the difference count for the sixteen count intervals to increase 
dramatically to a level of about one hundred or more. Based on the 
foregoing, the audio of a recording that is made in the manner that has 
been described will be considered to have been tampered subsequent to its 
recording if the difference between the zero crossing count obtained for 
sixteen consecutive counting intervals made at the time the audio signal 
is recorded and the count obtained from a playback of the audio is greater 
than forty-five. 
A decoder for the system has been considered which would use circuitry 
similar to that shown in FIG. 1, but without a threshold detector 16, for 
providing the zero crossing counts of the recorded audio when played back 
by the decoder. It was discovered that the tamper protection provided by 
such a system as has been described could be circumvented if the recorder 
is first operated with a magnetic tape cassette in place, but without any 
audio input with the tape cassette then used in a recorder which would 
make an audio recording only. A recording then would be presented by the 
cassette with an audio recording on one track and a data signal on a 
second track containing all the data that was to be recorded such as real 
time and the identification number of the recorder, but no zero crossing 
counts would be present in the data signal followed by a threshold level 
data bit indicating the threshold was satisfied at some time during a 
count. With no zero crossing counts followed by a threshold level data bit 
indicating satisfaction of the threshold level present in the data signal, 
the recording could be presented to the decoder and though the decoder 
would be providing zero crossing counts of the audio, no comparisons would 
be made since the making of a comparison requires the presence in the 
recorded data signal of a zero crossing count plus a data bit indicating 
the threshold level in the recorder was satisfied at least once during 
such count. Thus, a recording made in the manner outlined would circumvent 
the tamper protection provided by the recorder-decoder system that has 
been described. 
The present invention provides a solution to the circumvention problem 
discussed above. The solution resides in an improvement to the decoder for 
the system. Referring to FIG. 2, which shows circuitry used in the 
decoder, the improvement involves the use in such circuitry of a threshold 
detector 36 having a threshold level that is greater than that used for 
the threshold detector 16 used in the circuitry of the recorder for the 
system. With this arrangement each threshold information bit in the 
recorded data signal is examined in the decoder upon playback of the 
recording. If the threshold information bit for a count indicates the 
threshold level was met at least once during the count, the count is 
considered to be a valid. However, if a threshold information bit for a 
count in the recorded data signal indicates the threshold level was not 
met, the threshold information bit that is produced by the decoder upon 
playback of the recorded audio for the corresponding increment of audio is 
examined and if the two threshold information bits are not the same, 
tampering of the recorded audio associated with such bit is considered to 
have occurred. 
The circuitry portion of FIG. 2 that was used in the decoder for the system 
that presented the circumvention problem includes the bandpass filter 30, 
amplifier 32 and zero crossing detector 34 plus the microprocessor 38 
corresponding to the filter 10, amplifier 12, zero crossing detector 24 
and microprocessor 18, all of which are also used in the recorder 
circuitry shown in FIG. 1. The audio signal provided on a magnetic tape to 
be examined is presented to the input of the filter 10. Such decoder 
circuitry also includes a low pass filter 40 to which the recorded data 
signal is presented, an amplifier 42 connected to the output of filter 40 
and a FSK detector 44 connected to the output of filter 42. The FSK 
detector 44 can be a zero crossing detector similar to detector 44 and is 
used to detect the frequencies of the FSK (frequency shift keying) signals 
of the data signal. The time between the zero crossings detected by the 
FSK detector 42 is directly related to the frequency of the data signal 
presented to the low pass filter 40 and is used by the microprocessor 38 
to determine the frequency of the various portions of the data signal. 
With the frequencies of the various portions of the data signal 
established, the various information items presented in the data signal 
can be determined by the microprocessor 38 including the zero crossing 
counts made and recorded as a part of the data signal during the recording 
made of the audio. As in the recorder for the system, the circuitry of 
FIG. 2, including the zero crossing detector 34, provides a zero crossing 
count of the audio signal, which in this case, is the audio that had been 
recorded. Such zero crossing counts, which are also supplied with a 
threshold information bit, when processed by the microprocessor 38, are 
synchronized with the zero crossing counts obtained from the data signal 
presented to the low pass filter 40 enabling a comparison to be made 
between the zero crossing counts recorded as a part of the recorded data 
signal and those obtained from the audio signal presented to the filter 
30. As has been indicated, a comparison counts is considered only when the 
threshold bit for a zero crossing count in the recorded data signal 
indicates the threshold level for threshold detector 16, FIG. 1, had been 
satisfied during such count. 
The threshold detector 36 in the decoder has its threshold level set above 
the threshold level used for the threshold detector 16 of the recorder. 
This serves to provide the basis for an additional check that is made in 
the microprocessor 38 for possible tampering of the recorded audio signal. 
Tampering of the recorded audio is considered to have occurred if the 
threshold information bit for a count in the recorded data indicates the 
threshold level was not met and such threshold information bit is not the 
same as the threshold information bit provided by the decoder for the 
corresponding increment audio. 
While the invention has been described in connection with an exemplary 
embodiment thereof, it will be understood that many modifications thereof 
will be readily apparent to those of ordinary skill in the art; and that 
this specification is intended to cover any adaptations or variations 
thereof. Therefore, it is manifestly intended that this invention be only 
limited by the claim and the equivalents thereof.