Compact Disc Players are divided into 2 main units, namely:
- Data Retrieval Unit (Optical Pick up): functions to retrieve data stored on the disc correctly and accurately.
- Processing Unit: functions to process data taken from the optic pick up.
Disc Player (CD Drive) Block Diagram
Explanation of Block Circuit Diagram
1. Data Capture Unit (Optical Pick Up)
Focus Servo: Works to keep the laser beam focused and right on the disc surface at all times, the focus servo circuit uses a "vertical accumulator" to follow the vibration of the disc which moves out of balance due to gravity / centrifugal force when the disc rotates.
2. Tacking Servo
It works so that the laser beam always follows the digital data track on the disc at all times by moving the actuator in a horizontal direction.
3. Traverse Servo
Considering that the actuator's reach following the digital data flow is very limited, standardization is very necessary, as below:
| Negara | Input | Output | Distorsi |
|--------------------------|-----------------|------------|----------|
| Jepang | Tone bast | Daya penuh | 10% |
| Amerika | Sinusoide | Daya 1/3 | 1% |
| Eropa | Sinusoide | Daya penuh | 1% |
| Perbandingan Daya Output | | | |
| PMPO | 3 – 2 – 1 | | PMPO |
| MPO | 1.5 – 1 – 0.5 | | 1/2 PMPO |
| RMS | 1 – 0.67 – 0.33 | | 1/3 PMPO |Information:
- PMPO: Peak Music Power Output
- MPO : Music Power Output
- RMS : Rotate Mean Square
- Bast Tone: 333 Hz, 1KHz, 10KHz.
- Sinusoid : 1 KHz.
How Disc Reads Information:
Illustration of How Disc Drive Reads
When the laser beam touches the surface of the disc (laser beam diameter 1mm), the laser beam is transmitted by a transparent layer that has a refractive index of 1.5 (refraction of the poly carbonate layer) which is used to reduce the laser beam from 1mm to 1 micrometer, according to the size of the CD pits, the comparison of the area of 1mm pits of the laser beam with 1 micrometer is super a million to 1, therefore if there is a lot of dirt or scratches on the disc, the disc reading will be imperfect because the data will not match. The actual pit height is 0.11 micrometers, but the laser accuracy becomes 0.165 micrometers (0.11 x 1.5) thanks to the poly carbonate layer.
Disc Layer
The light beam that touches the surface of the disc experiences 2 events, namely:
- The laser beam hits a flat area (plan area) therefore 100% of the laser beam is reflected back, thus the photo detector receives the laser with high intensity, resulting in a high amplitude photo detector output.
- The laser beam hits the pits area, therefore the beam is reflected and scattered so that the photo detector only receives 30% of the light intensity, thus the photo detector output has a low amplitude.
From these two events, we get a sinusoidal wave with a relative speed of 1.3m/second, so the sinusoidal waveform appears in the form of high frequency, and in the end the photo detector output is called an RF signal.