FM RECORD ELECTRONICS
FM record electronics process signals to be recorded differently than direct record electronics. Instead of recording the input signal just as it appears at the recorder's input, FM record electronics use the input signal to vary (modulate) the carrier frequency of a record oscillator. The frequency modulated output signal of the record oscillator then becomes the signal that's actually recorded onto the magnetic media. Figure 5-4 shows a block diagram of the FM record electronics.
SEE Figure 5-4. - FM record electronics.
FM record electronics consist of three main parts:
Input pre-amplifier circuit.
This circuit does two things: (a) it serves as an impedance matcher between the signal source and the magnetic recorder, and (b) it pre-amplifies the input signal.
Record oscillator circuit.
This circuit generates a carrier signal onto which the input signal will be modulated. The input signal is used to vary (frequency modulate) the carrier signal. This is how the input signal gets frequency modulated onto the carrier signal. The output of this circuit is the frequency-modulated carrier signal.
The center frequency of the carrier depends on two things: (a) the bandwidth of the signal you're recording, and (b) the media onto which you're recording.
For magnetic tape, the carrier frequency can be as low as 1.688 kHz for an operating tape speed of 1-7/8 inches per second, and as high as 900 kHz for 120 inches per second.
Head driver circuit. This circuit takes the frequency-modulated output from the record oscillator circuit, amplifies it, and sends it to the magnetic head for recording. The output level of this circuit is set to be just below the magnetic saturation point of the magnetic media.