Case Study : Development of a Portable CD Player


  1. Problem Statement
    Purpose : Introduce the portable CD player.
    Topics : Scope and Objectives of the case study.

  2. Generation of System Requirements
    Purpose :Generate System Requirements through goals,scenarios and use cases.
    Topics :Identify system requirements;Goals and scenarios; identify actors and use cases; activity diagrams for use cases.

  3. Simplified Models of System Behavior
    Purpose : Develop basic interactions for fragments of system behavior.
    Topics : Main subsystems; Component Interactions.

  4. Simplified Models of System Structure
    Purpose : Develop simplfied models of system architectures.
    Topics : Map System structures as Class Diagrams.

  5. Creating the Logical Design
    Purpose : Create logical design alternatives.
    Topics : Mapping models of system behavior onto system structure alternatives.

  6. Creating the Physical Design
    Purpose : Create physical design alternatives.
    Topics : Assess technology options.

  7. Evaluating and Ranking of System Design Alternatives
    Purpose : Evaluate and rank system design alternatives.
    Topics : Design Alternatives for the CD Player components

  8. System Optimization and Tradeoff Analysis
    Purpose : Identify factors affecting system performance.
    Topics : Design Considerations while designing the Player.

  9. References and Web Resources

Problem Statement

Prior to the early 1980s, audio devices (e.g., cassette tape) reproduced an audio signal using mechanical or electrical contact with the recording media (i.e., the playback mechanism must physically contact the media, causing wear to both). The quality of the audio signal is poor, in part, because the system has no means of differentiating noise, distortion, and damaged media from the original recorded signal. [CD Player]

The appeal of compact disc and compact disc player technologies (developed in 1982; first introduced to the market place in the mid 1980s) stems from their use of digital audio signals, where noise and distortion can be separated from the audio signal. The compact disc player as a sound reproduction device fulfills the loop begun in the recording studio, returning the audio signal back to its original analog form.

Lightweight portable compact disc players (e.g., the Sony walkman) are designed ao allow anyone access to studio-quality sound nearly anywhere. This is possible due to inexpensive skip-free protection technology that provides enhanced mechanical protection against vertical shock and digital-based shock recovery.

Purpose. The purpose of this (relatively simple) example is to demonstrate the extent to which high-level systems concepts and UML notation/semantics can be used to describe the functionality of a working portable CD player. Key issues are:

One interesting aspect of this application is that a very precise standard for CDs was developed as a prerequisite to CD players.

We will explore the extent to which portable CD player sub-systems are loosely coupled and easily reconfigurable, perhaps as a prelude to subsystem resuse in a product line of CD players.

Generation of System Requirements

In the following two-loop procedure,

  1. Loop 1 : Establish Goals and Scenarios and Initial Use Case Modeling.

  2. Loop 2 : Detailed Use Case Modeling.


Use case development is a prerequisite to the identification of objects/subsystems and object/subsystem operations. A use case describes a single goal and all the things that can happen (i.e., scenarios) as the user attempts to reach that goal.

Goal 1. The CD Player must be portable.

Goal 2. The CD Player must be resistant to active/rigorous motion.

Goal 3. Users must be able to tell when the CD Player is on/off.

Goal 4. The CD Player must allow for insertion and removal of the CD.

Goal 5. The CD Player must work with a headphone sets.

Goal 6. The CD Player must work with home stereo systems.

Goal 7. The CD player must have a good interface


The actors are as follows:

System Boundary. The system boundary is defined by the portablt CD player itself. The music fan, his/her CDs, battery and AC/DC power sources, and optional broadcast devices are all external systems.


Our initial use case diagram has four actors and five use cases.

[CD player1]

Notice that the "Play CD" use case is coupled to the "Install CD" and "Turn CD Player On/Off" use cases. We also expect that "Play CD" will be expanded to include all of the functionality that will be needed for the CD player human-computer interface.


Use Case 1 : Carry CD Player.

Use Case 2 : Turn CD Player On (or Off).

[CD player1]

Use Case 3 : Install CD into Player.

[CD player1]

Use Case 4 : Play CD.

[CD player1]

Use Case 5 : Eject CD.

[CD player1]


These are high level system requirements which are categorised under different types of requirements.

Portability Requirements

  1. The CD player dimensions must be small enough to be hand-held.
  2. The CD player must be water resistant.
  3. The CD player must have a hand strap attachment for holding the player.

Performance Requirements

  1. The CD player must work with all compact discs, including CD-R/RW compilations from your CD recorder deck or PC
  2. The CD player must include a memory system that protects against skipping caused by vibration.
  3. The response time between the user command and the system must be appropriate.
  4. The player's construction (chassis material) must be robust to avoid vibrations.
  5. The circuit board and components must be securely mounted.

User Interface Requirements

  1. Play feature must include "skip", "play" and "pause". A more advanced portable CD player might also include "random" and "intro".
  2. A multi-digit display must show the track number, elapsed time and programming information.
  3. The CD player should come with a behind-the-neck in-the-ear headphones with a single cord.
  4. The CD player must have a "line out jack" that allows for connectivity to the amplifier/receiver of your home audio system.

Ergonomics Requirements

  1. The CD interface layout must be easy to understand and self-explanatory.
  2. The buttons should be placed at appropriate distance from each other.
  3. The disc drive and drawer must operate smoothly and quickly.
  4. If the CD player is a multi-disc player it should have multi disc random play and programming.

Power Requirements

  1. Rechargeable batteries can be recharged inside the player by using an AC/DC adapter.
  2. The player can also be powered from a standard AC electrical outlet using the AC/DC adapter (included).
  3. Up to 36 hours continuous playback with 2 AA batteries; rechargeable battery capable with optional nickel cadmium/nickel metal hydride batteries
  4. The auto power off feature saves batteries.

Expanded Description of "Play CD" Use Case
Goals and Scenarios

Goal 1. The user/music fan in this case wants to locate the buttons on the interface.

Goal 2. The CD Player must play music.

Goal 3. Users should be informed what music is playing.

Goal 4. Users should be able to select a (particular) music track.

Goal 5. Users should be able to directly access part of a music track.

Goal 6. Users should be able to adjust the music volume.

Goal 7. Users should be able to stop (or pause) playing of a music track.

Goal 8. The user should be able to make Playback options

Goal 9. The CD player must have radio functionalities.


The actors are as follows:

System Boundary. The system boundary is defined by the portablt CD player itself. The music fan, his/her CDs, battery and AC/DC power sources, and optional broadcast devices are all external systems.

[CD player1]

Baseline (Textual) Use Cases and Activity Diagrams

Use Case 1 : Access Music Tracks.

[CD player1]

Use Case 2 : Playback Modes.

[CD player1]

Use Case 3 : Adjust Volume.

[CD player1]

Use Case 4 : Radio Functions.

[CD player1]

Use Case 5 : Stop the Player.

Simplified Models of System Behavior


The Compact disc player have the following main subsystems:

  1. Transport System. The transport system is composed of servo/controls, the disc drive, and the laser pickup. Together, these subsystems orchestrate the mechanical operations of the CD player.
  2. Audio Data Processing System. The audio data processing system covers all of the other functionality in the CD player (i.e., digital-to-analog conversion; demodulation and error correction; output filtering). Display System. The display system acts as the human-machine interface.
  3. Electronic Packaging System. The electronic packaging system protects the transport and audio data processing system from the "real world" elements.
  4. Power System. Provides power to the mechanical and electrical systems.

[CD player1]

[CD player1]

[CD player1]

We will elaborate on these subsystem descriptions in the system structure and physical design sections.

Simplfied Models of System Structure

The compact disc player contains two main subsystems: the transport system and the audio data processing system. The transport system orchestrates the mechanical operation of the player and includes such items as the spindle motor, laser pickup, lens focus, servo mechanism, and the user interface. The audio data processing section covers all other player processes.


[CD player1]




Once the data samples have been read off the disc by the transport system, they are fed into the audio data processing system. This system consists of the demodulation and error detection circuits, error concealment and demultiplexing circuits, digital-to-analog converters, and the output filter.

[CD player1]

Creating the Logical Design

The purpose of the logical design is to determine the different ways in which the system can work, independent of implementation technology. Relevant logical simulation questions include:


[CD player1]


[CD player1]

Creating the Physical Design

A CD is a fairly simple piece of plastic, about (4/100) of an inch (1.2 mm) thick. Most part of CD consists of an injected-molded piece of clear polycarbonate plastic. During manufacturing the plastic is impressed with microscopic bumps arranged as a single, continuous long spiral track of data. Once the clear piece of polycarbonate is formed a thin reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then stamped on the acrylic layer.

[CD player1]

Cross Section Of a CD

A CD has a single spiral track of data, circling from the inside of the disc to the outside. The fact that the spiral track starts at the center indicates that the CD can be smaller than it's regular size (4.8 inches) if desired.

[CD player1]

The CD Spiral Track

The Data tracks are very small approximately 0.5 micros wide, each track separates from the next by 1.6 microns. The elongated bumps that make up the track are 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high.

[CD player1]

CD Internals


[CD player1]

Internals of a CD Player
Transport System The transport system has the following main components

A CD player employs a readout mechanism using a semiconductor laser. A non-contact means is required to read data, maintain the laser beam's focus and track the pit spiral. The result is a highly sophisticated pick up, utilizing complex optical devices within the servo loops. The servos pick use electrical signals from the pick up to control motors to mechanically adjust the pickup's position horizontally and vertically relative to the disc surface. A spindle motor is used to rotate the disc with constant linear velocity, thus the disc may vary it's speed depending on where the pick up is located underneath the surface. In another servo loop, information from the data itself is used to determine to maintain the correct rotating speed and the proper output data rate. A microprocessor monitors the users controls and their interface to the circuits of the CD player. The various modes of player operation are controlled by software. Subcode data plays an important role in directing the pick up to the proper disc location. Once the data is recovered from the disc the player must decode the audio information to reconstruct the audio signal. The data received is demodulated, errors are detected and corrected by decoding the error correction algorithm. This digital audio signal must be converted to appropriate audio analog signal. This is done by D/A converters and low pass filters. A final output circuit is nothing more than a buffer to the outside world. It ensures that the player's line level output is appropriate to drive external amplifiers with a minimum amount of analog distortion. The process can be viewed as a simple block diagram

[CD player1]


CD Player : Transport System Technologies

CD Player : Audio Data Processing System

Once the data samples have been read off the disc by the transport system, they are fed into the audio data processing system. This system consists of the demodulation and error detection circuits, error concealment and demultiplexing circuits, digital-to-analog converters, and the output filter.

Compact Disc Technology

The specifications for the compact disc and compact disc players were jointly developed by Sony, Philips, and Polygram in 1982. This specification is contained in their standards document referred to as the Red Book. A summary of this standard is shown in Table 1.


Playing time:

74 minutes, 33 seconds maximum


Counter-clockwise when viewed from readout surface

Rotational speed:

1.2–1.4 m/sec. (constant linear velocity)

Track pitch:

1.6 Ám


120 mm


1.2 mm

Center hole diameter:

15 mm

Recording area:

46 mm – 117 mm

Signal area:

50 mm – 116 mm


Any acceptable medium with a refraction index of 1.55

Minimum pit length:

0.833 Ám (1.2 m/sec) to 0.972 Ám (1.4 m/sec)

Maximum pit length:

3.05 Ám (1.2 m/sec) to 3.56 Ám (1.4 m/sec)

Pit depth:

~0.11 Ám

Pit width:

~0.5 Ám



Standard wavelength:

l = 780 nm (7,800 ┼)

Focal depth:

▒ 2 Ám



Number of channels:

2 channels (4 channel recording possible)


16-bit linear

Quantizing timing:

Concurrent for all channels

Sampling frequency:

44.1 kHz

Channel bit rate:

4.3218 Mb/sec

Data bit rate:

2.0338 Mb/sec

Data-to-channel bit ratio:


Error correction code:

Cross Interleave Reed-Solomon Code (with 25% redundancy)

Modulation system:

Eight-to-fourteen Modulation (EFM)

Table 1. Compact Disc Technology Specifications

Evaluating and Ranking of System Design Alternatives

Design Considerations(not complete)

A portable CD player has some considerable advantages over the audio media. They can be specifications such as flat response, low noise, low distortion, absolute speed stability and accuracy or operational features like small size, long playing time, indexing of data and programmability. Tradeoffs and design alternatives for a better product can be achieved by studying the following design structures.

The design alternatives should evaluate some important criteria's:



The type of CD being used (CD-R/CD-RW,CD-I etc.)

System Optimization and Tradeoff Analysis

Tradeoff while deciding the suitable interface:

Cost tradeoff

References and Web Resources