الاستماع الى التسجيل - الاجهزة الترانزستورية - تسجيل الشريط والاسطوانات المجسمة

Listening to the recording - Transistor devices
- Tape recording and stereophonic discs .. Electronics

Listening to the recording

In order to listen to the recording, the tape is wound on the original reel and then passes in front of the air gap of the recording head at the same speed at which it was recorded before (185). The passage of the magnetic tape in front of the recording head causes a change in the magnetic field of the air gap in it, which in turn leads to the generation of an electromotive force in the coil surrounding the iron core. In short, we can say that in the case of recording, the passage of sound signals in the coil leads to the generation of a changing magnetic field in the tape. As for listening, this changing magnetic field leads to the regeneration of sound signals again, and in many cases the signals are not only recorded by the microphone, but also by other recording devices, and it is also possible to record and listen to television programs with sound and image, but such devices are more complex than audio recording devices.

In general, recording devices include about four stages that are read to each other by resistors and capacitors (186). It is worth noting that improving the loudness level and efficiency of the recording device requires the use of other circuits such as push and pull circuits.

Recording and listening

The same head can be used for both recording and listening on different recording tapes, and the same amplifier can be used for both

185 - Listening to the recording.
186 - Thermic valves used in listening.

The two operations when separated from the input and output circuits in the device (187). When the switch is in the recording position (T), the microphone output is connected to the amplifier. When the switch is in the listening position (A), the output of the recording head coil is connected to the amplifier input, which then feeds the loudspeaker or speaker. This device usually includes two outputs, one of which is dedicated to the recording head issued from a previous stage. In this case, the recording process does not require the use of the full power of the amplifier, which is necessary when listening only.

Recording level indicator

Recording devices are equipped with an indicator that shows the appropriate recording level (188). It is noted in this figure that the connection point S goes towards the audio amplifier, while the diode works to rectify the audio signals, thus creating a negative bias voltage between the network and the cathode, which leads to a decrease in the value of the anode current, and an increase in the positivity of the deflecting plates in it. It is also noted that the electrons emitted from the cathode collide with the electrode called the target, and that the control of this electronic radiation is done by adjusting the potential of the deflector plates. Accordingly, the output is related to the average sound power, where we can control the recording level by means of the loudness control in the device.

187 - Recording/listening switch.
188 - Recording level indicator.
189 - High frequency bias oscillator.

The recording quality can be improved by using a high frequency bias in the recording head. The power output stage can also be used as a bias oscillator. In Figure (189), we notice that the capacitor S A acts as a coupling capacitor (horn) of the oscillator coil and also works to connect the remaining oscillations with feedback from the coil to the control network S A. As for the high-frequency bias, it is taken to the recording head via the circuit consisting of the resistor M and the capacitor S2. To erase the recording, the order of the iron particles spread on the recording tape must be cancelled and returned to their previous random position, and this is done by a permanent magnet. However, it is preferable to use a high-frequency scanning signal where the bias oscillator gives a strong signal obtained from the two ends of the secondary coils (190). In this case, the tape passes before recording, in front of the scanning head where the oscillating magnetic fields move the iron particles spread on the surface of the tape and make them in a random position. The output of the recording head, during listening, is characterized by being irregular over a wide range of frequencies, but it reaches its maximum when the frequency of the signals to be heard is within the range of 2500 oscillations per second ( 191). It is noted in Figure (191) that the reactance of capacitor S A is small for high frequency signals, while capacitor S? amplifies these signals. Therefore, adjusting the variable resistor mm3 helps control the output of the recording head when listening.

190 - Erasing an old recording.
191 - Frequency discriminator and resonant control circuit.

Transistor devices

Figure (192) shows the circuits of a pocketable recorder. When recording (T), it is noted that the microphone feeds the amplification circuits with audio signals via capacitor SA. Then, the signals pass after being amplified through the resistor MA. In the case of listening (A), the recording head coils feed the amplifier circuits with signals via capacitor SA - There is no HF or erase bia

These devices do not contain a high-frequency oscillator or a scanning bias. The scanning process is carried out by a permanent magnet that rearranges the iron particles during the recording process so that they take a specific direction. A small motor powered by a voltage of 6 volts is also used in the process of pulling the tape.

192 - Transistor recorder.