MAGNETIC TAPE RECORDER RECORD AND REPRODUCE ELECTRONICS
After completing this chapter, you'll be able to do the following:
State the two types of record and reproduce electronics used on magnetic tape recorders.
RECORD AND REPRODUCE ELECTRONICS
There are two ways to record and reproduce analog signals. The first way is direct record. It's also called amplitude modulation (AM) electronics. The second way is frequency modulation (FM). Even though direct record and reproduce circuits are much different from FM record and reproduce electronics, they both share the same two very important jobs. They both must:
Take an input signal, process it as needed, and then send it to the record magnetic head for reproduction.
Take the reproduced signal from the reproduce magnetic head, process it as needed, and output it for listening or analysis.
DIRECT RECORD ELECTRONICS
Direct record electronics record input signals onto magnetic media just as the signals appeared at the recorder's input. The only processing an input signal receives is the adding of a bias signal. The added bias signal makes sure the signal stays away from the steps of the magnetism curve. Figure 5-1 shows a basic block diagram of a recorder's direct record electronics.
Figure 5-1. - Direct record electronics.
Direct record electronics has four main parts:
Input pre-amplifier circuit.
This circuit takes the input signal, amplifies it, and sends it to the summing network. It also matches the impedance between the source of the input signal and the magnetic tape recorder.
Bias source. This circuit generates the high-frequency bias signal and sends it to the summing network.
Normally, the frequency of the bias signal will be five to ten times higher than the highest frequency the tape recorder can record.
Summing network. This network takes the input signal and the bias signal, mixes them, and sends the resulting signal to the head driver circuit.
Head driver circuit. This circuit takes the signal from the summing network, amplifies it, and sends it to the record head for recording.
DIRECT REPRODUCE ELECTRONICS
Direct reproduce electronics amplify the very weak input signals from the reproduce head, and send them out for listening or analysis, as needed. Figure 5-2
shows a basic block diagram of direct reproduce electronics.
Figure 5-2. - Direct reproduce electronics.
Direct reproduce electronics consists of three main parts:
This circuit takes the very weak reproduced signal from the reproduce head and (a) amplifies the signal, (b) removes any bias signal that was used during the recording process, and (c) sends the signal to the equalization and phase correction circuit.
Equalization and phase correction circuit. This circuit takes the pre-amplified signal and fixes any Frequency response problems that the reproduce magnetic head may have caused. To better understand this, look at the voltage versus Frequency response graph in figure 5-3.
The top of the graph shows the input signal that comes from the pre-amplifier and the bottom shows the equalization signal generated by the equalization circuit. In the top part of the graph, note how the output voltage level varies as the frequency of the signal varies.
This isn't good. A good output voltage level is one that remains constant as the frequency changes. The equalization signal corrects this problem. Notice that when the input signal and the equalization signal are combined they cancel each other out. This allows a nice flat (voltage versus frequency) output signal to go to the output amplifier circuit.
Figure 5-3. - Equalization process.
Output amplifier circuit.
This circuit takes the signal from the equalization and phase correction circuit and amplifies it for output. It also matches the magnetic recorder's impedance to the output device that is used for listening or recording.
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.
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.
FM REPRODUCE ELECTRONICS
The FM reproduce electronics work just like direct reproduce electronics, with one exception. FM reproduce electronics must first demodulate the original input signal from the carrier frequency before the intelligence can be sent to the output device for listening or analysis. Figure 5-5 shows a block diagram of the FM reproduce electronics.
Figure 5-5. - FM reproduce electronics.
FM reproduce electronics consist of four main parts:
This circuit takes the frequency modulated carrier frequency from the reproduce head and amplifies it.
This circuit takes the output of the preamplifier, stabilizes the amplitude level, and demodulates the signal intelligence from the carrier frequency.
Low-pass filter circuit.
This circuit takes the signal intelligence from the limiter/demodulator circuit and cleans up any noise or left over carrier signal.
Output amplifier circuit.
This circuit takes the output from the low-pass filter and amplifies it for output. It also matches the impedance of the magnetic recorder to the output device.
Q.1 What two types of record and reproduce electronics are used by
magnetic tape recorders?