MAGNETIC TAPE CASSETTE HAVING A PRERECORDED LOW FREQUENCY TONE

Abstract: A magnetic tape cassette (1) for use in a tape recorder that during a recording operation does not provide for erasure of a recording on the tape (2) prior to the tape reaching the recording head. The tape has a prerecorded continuous, single, low frequency signal along the full recordable length of the recording tape. The low frequency signal can be detected subsequent to a recording made on the tape of a conversation, verbal statement or the like to provide an indication that the tape was not altered subsequent to such recording.

Description: Description Magnetic Tape Cassette Having A Prerecorded Low Frequency Tone Technical Field The invention presented herein relates to prerecorded magnetic tape cassettes and, in particular, to such a cassette where the prerecording on the tape is a single, continuous low frequency tone for use in a cassette tape recorder for recording audio signals wherein the cassette tape recorder does not provide for erasure of the tape prior to the tape reaching the recording head during a recording operation. There are situations where it is desirable that a conversation or verbal statement be recorded on magnetic tape for playback at a later date with provision made for determining whether there has been an erasure of any of the previously recorded conversation or verbal statement. One such situation is presented when it is contemplated that a tape recorded conversation will * be used later in a legal proceeding where the authenticity of the recording may be challenged. An objection might be made to the recording in such proceeding on the basis of a showing that events subsequent to the recording presented an opportunity for erasure of some of the original recording or for erasure plus a subsequent recording. It is desirable that a way be found to show that the recording has not been changed to overcome such an objection. Disclosure of Invention It is a principal object of this invention to provide a magnetic tape cassette that can be used to record a conversation or verbal statement wherein the recording can be played back for an examination of the tape output to determine whether there has been an erasure of any of the recorded conversation or verbal statement. Such object is attained by the present invention wherein the magnetic tape cassette to be used to record a conversation or verbal statement is one which has a continuous low frequency tone that is prerecorded on the tape for the full recordable length of the tape with such cassette tape provided for use with a cassette tape recorder that is arranged so there is no erasure of a recording on the tape prior to presentation of the tape to the recording head in the recorder during a record operation. By use of a prerecorded low frequency tone, such tone would not be readily detected if it were played back prior to recording and such tone would not interfere with audio signals subsequently recorded on the tape. The low frequency tone is available for detection subsequent to any recording made using the cassette tape in a recorder that does not provide an erasing function prior to the tape reaching the recording head during a record operation, thereby providing a way for determining whether any portion of the tape was erased subsequent to the recording made of a conversation or verbal statement. Brief Description of the Drawings A better understanding of this invention including its novel features and utility will be obtained upon the consideration of the following detailed description and accompanying drawings wherein: Figure 1 is a showing of a tape cassette having a recordable length of magnetic recording tape; and Figure 2 is a showing of circuitry in block diagram form for detecting the presence of a single frequency signal recorded on a tape cassette of the type shown in Figure 1. Detailed Description Referring to Figure 1 of the drawings, a tape cassette 1 is shown that is of the type that is usable for the practice of the invention disclosed herein. Tape cassettes of the type shown in Figure 1 are available from a number of manufacturers and are suitable for use with known cassette tape recorders. The tape cassette 1, as is known, includes a recordable length of magnetic recording tape, a portion of which is shown at 2. The invention is embodied in a tape cassette of the type shown in Figure 1, wherein the tape 2 contains a recording of a single, continuous low frequency signal that is recorded along the full length of the tape with the tape cassette provided for use with a cassette tape recorder that does not provide a prerecord erase function. The prerecorded single frequency signal on the tape will remain on the tape and will be available for detection subsequent to any recording made with the tape cassette using such a cassette tape recorder. A tape cassette having a low frequency signal that has been prerecorded on the recording tape is unique with respect to its use as a recording tape in a tape recorder that does not erase the prerecorded low frequency signal during a record operation. It is useful in that it provides a tape cassette that can be used for recording a conversation or verbal statement to be used at some subsequent time. It serves to provide such a recording wherein the prerecorded low frequency remains on the tape which can be detected at a time subsequent to the recording made of the conversation or verbal statement to determine whether there has been any tampering of the recording. Any tampering with the recording would result in the creation of a discontinuity in the prerecorded signal which can be detected. The tape cassette of this invention is thus useful for making a recording where the authenticity of the tape may be questioned at some time subsequent to the recording such as in a legal proceeding in which the recording is offered in evidence. For example, a showing that someone had an opportunity to tamper with the recording might be sufficient to cast doubt on the authenticity of the tape. A recording made with a tape cassette of this invention, using a tape recorder that does not have a prerecording erase function, makes it possible to examine the recording tape to determine whether the tape has been tampered creating a discontinuity in the prerecorded low frequency. Figure 2 shows one arrangement whereby a tape cassette of the type described can be examined to determine whether any discontinuity exists in the low frequency signal that is prerecorded on the recording tape. The arrangement requires a magnetic tape player 3 in which the tape cassette to be examined is placed for playback. The output of the tape player 3 is connected to a narrow bandpass filter 4 which passes only signals at the prerecorded low frequency. The output of the filter 4 is connected to an absolute value amplifier 5 which provides a full-wave rectified output of the low frequency signal when detected by the filter 4. The output of the amplifier 5 is connected to a threshold detector 6, which serves to require the output of amplifier 5 to be of a predetermined value before threshold detector 6 provides an output. This prevents the possibility of interpreting low level, noise generated signals from appearing at the output of the detector 6 to falsely indicate a prerecorded low frequency signal is being detected. The output of the threshold detector 6 is connected to an indicating device 7 which can be a light emitting diode. The indication device 7 will thus provide an indication so long as the prerecorded low frequency signal continues to be detected by the magnetic tape player. If there is no termination of the indication provided by the indicating device 7 during a replay of the cassette tape over the length of the recording, such results could be used to overcome an objection made to the authenticity of a tape recording presented in evidence at a legal proceeding. The objecting party would then have the burden of establishing some other basis for placing the authenticity of the tape recording in question. It is desirable that the low frequency that is recorded be of a frequency that would not impair the audio signal that would be recorded when recording a conversation or verbal statement. The use of a low frequency that could not be readily detected by most people would be desirable since its presence on the tape would not be readily detected by someone seeking to alter a recorded tape placing such person in the possible position of recording such low frequency on the tape where an alteration of the recording has been made. It is preferred, therefore, that a frequency of 60 Hertz or lower be used for a signal to be prerecorded on the cassette tape.

Claims: Clai s 1. A tape cassette (1) having a recordable length of magnetic recording tape (2) for use in a tape recorder that during a recording operation does not provide for erasure of a recording on the tape (2) prior to the tape reaching the recording head of the tape recorder, the tape cassette (1) including a continuous, single frequency signal recorded along the full recordable length of said recording tape (2), said single frequency signal being of a frequency that is 60 Hertz or less whereby said single frequency signal remains on said recording tape after said tape cassette has been used for recording in a tape recorder which does not provide for erasure of a recording on the tape prior to the tape reaching the recording head of the tape recorder.

Inventors: KNOWD MICHAEL J, KNOWD, MICHAEL, J.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1986
Main classification: G11B23
IPC codes: G11B20, G11B23, G11B27