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Modulation Aided Recording On Magnetic Tape

Select Channel Instead of Changing Cassette
Patent application no. -  395/BOM/96

 This invention relates to recording of information on magnetic tape and retrieving the recorded information from magnetic tape. Present method of recording information on magnetic tape is; The information is converted into an electrical signal. The electrical signal is amplified and then fed to magnetic writing head that converts the electrical signal into magnetic field corresponding to the electrical signal. These tiny magnetic fields are stored on a tape coated with magnetic material. Passing the magnetic tape over reading head retrieves the information. The tiny magnetic fields recorded on the tape create corresponding electrical signal in the coil of the reading head. This electrical signal is amplified and fed to output device like loudspeaker to reproduce the information recorded on the magnetic tape. 
Invention (What is New)
The invention is recording more than one information signals on a magnetic tape. The information signals are converted into electrical signals and are used as modulating signals to modulate carrier signals of higher frequencies. More than one carrier signals are modulated by different information signals. Each information signal modulates one carrier signal.
All the modulated carrier signals are recorded simultaneously on a magnetic tape. The signal recorded on magnetic tape is resultant of mixture of all carrier signals.

formation is retrieved by reading the information on magnetic tape by magnetic reading head. Signal red by this head is resultant signal mixture of all carrier signals recorded on the tape.

An adjustable tuning circuit filters carrier signals and passes only carrier signal it is tuned to. The selected carrier signal is amplified and demodulated to obtain the modulating information signal.
This recording and retrieving is done by one of the two types of modulation. (Frequency Modulation and Amplitude Modulation)
Modulation Aided Recording

In Figure 1

S1, S2 and S3 are sources of information signals, like microphones; each source provides one information signal in electrical form that is to be recorded on magnetic tape.

Blocks 1, 4, and 7 are information signal amplifiers. Output of each amplifier is fed to separate electrical signal modulators.

Blocks 2, 5, and 8 are electrical signal modulators. Each signal modulator modulates the carrier signal fed to it by using information signal as a modulating signal. All modulators modulate carrier signals of different frequencies.

Blocks 3, 6, and 9 are carrier signal generators. Each carrier signal generator generates one carrier signal of specific frequency that is fixed for each generator and each carrier signal is fed to one modulator for modulating the carrier signal. The output signals of modulators are carrier signals, each of different frequency and each modulated by different information signal. At this stage there are more than one carrier signals each of different frequency and each modulated by different information signal.

The output signals of signal modulators are fed to block 10 that amplifies the signals. Output of block 10 is resultant signal of mixture of more all modulated carrier signals. Output of block 10 is recorded on magnetic tape by a magnetic tape writing head.
Shown Here is block diagram for retrieving the signal

Block 1 is a reading device (playback head for reading magnetic tape)

Block 2 is an amplifier that amplifies electrical signal retrieved by the reading head. The retrieved electrical signal is resultant of mixture of all carrier signals that are recorded on the tape, each carrier is modulated by one information signal that was recorded on the magnetic tape.

Block 3 is an adjustable tuning circuit (Same circuit that is used in radio receiver to select one broadcasting station transmitted at specific frequency). The tuning circuit selects one carrier signal of specified frequency for which it is tuned by the user. All other signals are suppressed, except one signal of specified frequency to which the adjustable tuning circuit is tuned.

The selected carrier signal is amplified and demodulated in block 4. Block 4 is a demodulating circuit that demodulates the selected carrier signal to obtain the modulating signal. (Demodulating circuit is same circuit that is used in radio receiver to demodulate signal of the frequency selected by the user)

Output of block 4 is an original modulating signal that was used to modulate the selected carrier signal at the time of recording it on magnetic tape. The obtained signal is the information in the form of electrical signal.

Block 5 is a signal amplifier that amplifies output of block 4. Output of block 5 is the original information signal. This signal is fed to output devices for reproduction (block 6).
Modulation Aided Recording
A receiver comprises of two de-modulators to retrieve the two signals transmitted on one carrier wave. Each de-modulator demodulates different signal providing outputs of two signals those were modulated at the time of transmission