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Average read time



Introduction

Average reading time, also known as average access time, refers to the CPU sending commands to the optical drive, the laser head moves from a certain position to a new recording position, to the beginning of the disk The time it takes to read or write information on the sheet surface. Generally consists of four parts: average seek time, average rotation delay, control delay and data transmission time. The average seek time is the time taken to move the head or optical head to a specified position. It is usually used to describe the ability of an optical disk or hard disk to read data. The rotation delay time is the time it takes for the specified sector to move below the head. In different types of optical drives, the rotation speed differs by at least an order of magnitude, and the transmission time refers to the time it takes to read data from or write data to the optical disk or disk.

CD-ROM drive

The CD-ROM drive is a CD-ROM drive, and the carrier that loads data and information is called a CD-ROM. The one that reads or writes data to an optical disc is called an optical drive. Optical drive for short Optical drive is a product that combines optical, mechanical and electronic technologies. In terms of the combination of optics and electronics, the laser light source comes from a laser diode, which can generate a beam with a wavelength of about 0.54-0.68 microns. After processing, the beam is more concentrated and can be accurately controlled. The beam is first hit on the optical disc, and then reflected by the optical disc. , Capture the signal through the photodetector. The internal structure of the optical drive:

  1. Laser head assembly: including photoelectric tube, focusing lens and other components, cooperate with mechanical components such as gear mechanism and guide rail, according to the system signal when the power is on Confirm and read the disc data and transmit the data to the system through the data belt.

  2. Spindle motor: the driving force for the operation of the disc, which provides fast data positioning function during the operation of the disc reading process.

  3. CD tray: the CD carrier in the open and closed state.

  4. Starting mechanism: control the in and out of the disc tray and the starting of the spindle motor. When the power is on, the starting mechanism will make the servo mechanism of the head assembly including the spindle motor and the laser be in In the half-loaded state.

Transfer rate

Sustained Data Transfer Rate (Sustained Data Transfer Rate) is the most basic performance indicator of CD-ROM drive, which directly determines the drive The data transmission speed is usually calculated in KB/s. The data transfer rate of the first CD-ROM was only 150KB/s. At that time, the relevant international organizations set the rate as single speed, and the speed of CD-ROMs that appeared later was a multiplier relationship with the single-speed standard. For example, for a 2x-speed CD-ROM, its data The transmission rate is 300KB/s, the 4x speed is 600KB/s, the 8x speed is 1200KB/s, the transmission rate at 12x speed has reached 1800KB/s, and so on. CD-ROM mainly has CLV (constant linear velocity), CAV (constant angular velocity) and P-CAV (local constant angular velocity) three types of disc reading methods.

Among them, CLV technology (Constant Linem Velocity) is a technology commonly used by optical drives below 12 times speed. CLV technology means that the track arc length read in unit time is equal in the process of moving from the inner track (inner circle) of the disc to the outer track. Since the radius of the inner ring of the CD disc is smaller than the outer ring, the rotation speed of the detection head when it is close to the inner ring is naturally faster than when it is close to the outer ring. Only in this way can the data transmission rate remain unchanged.

CAV technology (Constant Angular Velocity) is a commonly used technology for optical drives above 20 times speed. The characteristic of CAV technology is to keep the rotation speed constant, and its data transmission rate is variable. That is, when the detection optical head reads the data of the inner ring and outer ring of the disc, the data transmission rate will change accordingly. For example, a 20 times speed product may only have 10 times speed in the inner ring, and the data transmission rate gradually increases as the outer ring moves, until it can reach 20 times speed in the outer ring.

P-CAV technology (Partial CAV, local constant angular velocity) is a technology that combines the essence of CLV and CAV. When the detection optical head reads the inner ring data of the disc, the rotation speed remains unchanged, so that the data transmission rate can be increased; and when the detection optical head reads the outer ring data, the rotation speed is increased.

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