CD: Jitter, Errors & Magic Test gear used
This project was made possible by the loan of two pieces of sophisticated test equipment. The first is a CD quality analyzer called the Design Science CD Analyzer, developed by Gordon Rudd in conjunction with Disctronics. It consists of a board that fits in the expansion slot of an IBM-compatible PC, software, and a specially modified Philips CD player. The board takes the HF signal from the player and performs all decoding functions, just like a CD player. Software then analyzes the error types and presents the data in graphic form on the computer display. In addition to measuring error types and rates, the system also measures HF level, tracking level, and asymmetry.
The CD Analyzer distinguishes between each of the CD's error syndromes. These syndromes are called E11, E12, E13, E21, E22, and E23. The designations refer to the severity of errors, in increasing order of severity. E11, E12, and E13 indicate either one, two, or three errant or missing symbols in a block as detected by the first stage of error correction (the C1 decoder). E21, E22, and E23 indicate one, two, or three errant symbols in a block at the second stage of error correction (the C2 decoder). An E23 error is the first stage of error concealment, causing the player to interpolate the missing or errant data.
Test data can be stored on a floppy for later analysis. The system also features a statistical analysis program that compares performance trends over a large number of discs, such as during a CD production run.
The Design Science CD Analyzer is most often used in the quality control stage of CD manufacturing. It is also ideal for record companies who want to verify the quality of CDs made for them by the pressing plant. The Design Science CD Analyzer is available from Design Science, 5245 Sale Avenue, Woodland Hills, CA 91364. Tel: (818) 348-3392.
Jitter measurements were made with a Kenwood DB-3545 Jitter Analyzer, a device used mostly by CD transport manufacturers during quality control. It measures jitter in the HF signal by looking at the time between the falling and rising edges (or vice versa) of I3, the shortest pit or land length on the disc. Ideally, I3 should have a period of 694 nanoseconds (billionths of a second) that remains constant. However, no CD transport is jitter-free, causing the period of I3 to be slightly shorter and longer than 694 nanoseconds. (The eight other frequencies in the HF signal will also be changed.)
The DB-3545 displays the jitter graphically, allowing easy interpretation of jitter distribution and character. Typically, a Gaussian distribution (bell-shaped curve) is seen, as the period of I3 varies around the ideal value. The maximum period of I3 is also displayed continuously in real time with 1-nanosecond resolution. The analyzer also has user-definable jitter limits with a GO/NO-GO display. The DB-3545 measures jitter only in the HF signal from a CD transport, and not in the SP/DIF digital output of a CD player or DAT machine. The DB-3545 sells for $3950 [in 1990—Ed.]. Kenwood makes a complete line of test equipment, including CD and DAT measurement instruments. They can be reached at Kenwood USA Corporation, Communications and Test Equipment Group, 2201 E. Dominguez Street, P.O. Box 22745, Long Beach, CA 90801-5745. Tel: (213) 639-4200.—Robert Harley