Teacher Notes on Description-Writing Exercises

Exercise 3: The Compact Disk (CD)

Context for this case:

Prerequisites:
  • Segmented CD description cut into 14 pieces.
  • Large figures (4) and headings to guide rebuilding.
  • Alternative paragraph versions (optional).

Cognitive Apprenticeship Features:
  • Facilitates coaching on value of reader signals and how to rebuild by using them.
  • Externalizes role recognition of every descriptive piece.
  • Builds cognitive maturity.

Supporting References:
Relevant CA Content Standards  
Goal:
To have students reconstruct a coherent, adequate technical description from scrambled parts (as they read them aloud). The description topic is a compact disk (CD) and its player, the text parts are paragraph length (coarse grained), and four figures help guide the reconstruction.
Strategy:
This is the first of several exercises in which students use the features and signals embedded in a good technical description to rebuild it from its scrambled (paragraph-sized) parts. The description's headings and figures provide the framework for publicly reassembling the description step by step, as a class project, like a verbal jigsaw puzzle of which each student has a piece.

MACAULAY BACKGROUND:
As with Exercise 2, this CD description borrows two of its four figures from technical illustrator David Macaulay's The New Way Things Work (Boston: Houghton Mifflin Company, 1998), pp. 336-337. Macaulay liked so well his large drawing that shows how a laser reads a spinning compact disk (Fig. 3) that he reprinted it in color on his book's title page. For background on Macaulay, the URL for his book's web site (largely promotional), his relation to collaborator Neil Ardley, and his strategy of drawing important features realistically but unimportant ones whimsically, see the Strategy section of annotated Exercise 2.

THE FIGURES:
The descriptive text of this exercise refers to four figures that you need to show to students while they work on the exercise. The first was donated for educational use by a now-defunct Eastman Kodak website and is linked from the table below. The second comes from a copyrighted, quasi-educational commercial site freely viewable on the World Wide Web (URL below). The last two copyrighted figures come from David Macaulay's The New Way Things Work. The figures, together with the list of section headings, complement and help visually organize the text of the compact-disk description. You might want to point out that Fig. 4 here is really a schematic diagram, like Fig. 2 of Exercise 2, not a realistic rendering. And in Fig. 3, the artist places the point of view inside the CD player looking up toward the spinning disk overhead, a revealing but unusual perspective.

For Exercise 3 the four figures are:

Fig.  Caption             Source

 1    A compact disk      Kodak educational figure.
      has 3 layers.
     
 2    Magnetic disks      www.howstuffworks.com
      store data on    
      circular rings
      of iron oxide.

 3    Reading a compact   Macaulay, p. 336 (and inside cover)
      disk involves
      following its
      spiral track of
      pits with a laser
      beam.

 4    The optical read-   Macaulay, p. 337
      out system turns
      the pits and
      lands into digital
      off and on signals.

HOW TO USE THIS EXERCISE:

(A) (Optional) Role Recognition.
You can use Exercise 3 for more practice in recognizing the features of a description and their roles, if you wish. Below I provide a student version with text (left column) and feature prompts (right column), which you can pair with the fully annotated version to drill role recognition (as with Exercises 1 and 2). I prefer to use this exercise for text reconstruction, however (next).
(B) Text Reconstruction (Background).
In On the Surface of Discourse (London: George Allen and Unwin, 1983), text linguist Michael Hoey reports (pp. 3-6) how he and E. O. Winter found that 55% of 229 computer science, chemistry, and engineering students "demonstrated beyond dispute an ability to reconstruct a [technical] discourse out of a jumble of sentences" by attending to text features and continuity signals installed by the author. The 11% of students who failed completely at this were all ESL learners.
Furthermore, reconstructing a description from fragments is a surprisingly "authentic" technical writing exercise. I point out to my students that at work I often receive (many) separate parts of a future description, perhaps at different times from different people. Like the students with Exercise 3, I have to build a useful, coherent large description by paying careful attention to what each fragment says and how it complements the other fragments. (The student fragments are known to be a complete set and previously edited, of course, while on the job the description fragments are often incomplete and need much editorial improvement.)
(C) Text Reconstruction (Process).
Below I provide a "segmented" version of the student CD description. It has the same text (without the scaffolding) as before, but marks (---) divide it into 14 (roughly) paragraph-sized chunks. The descriptive chunks omit the headings, which appear in a separate list for you to use as the project outline. But the text chunks are large enough to each contain several signals or rhetorical clues about each chunk's intended role in and contribution to the overall description.
  • Print out
    the segmented version of the CD description and cut it into pieces (of paper) along the marks (---) indicated.
  • Enlarge
    each text chunk on a photocopy machine for easier in-class reading and sharing. Everyone can participate more easily if the description parts (that you assemble as puzzle pieces on the wall) have print big enough to read from across the room. Alternatively, use less enlargement and reassemble the description on a big table.
  • Scramble
    the text pieces so that their original order is hidden.
  • Distribute
    the (enlarged) text chunks randomly, one to each student (or perhaps to each pair of students).
  • Read each piece aloud
    and try to find its best place. Use its internal rhetorical features (as mentioned on the guidelines) as clues and the list of headings and the figures as an organizing "target" framework, a broad outline of the intended result. Student discussion may perform most of this work in some classes; you will need to provide considerable leadership and encouragement in others since this is valuable but unfamiliar territory for many students. In some cases, even reading their text chunk aloud for classmates to consider may challenge the student who holds it.
  • Post
    each text chunk on a wall or blackboard (with little pieces of tape or Post-it notes) as students decide on its preliminary role and place. But don't tape the paper sheets to each other unless you can easily undo them, because changing the order and grouping is a natural and appropriate part of reconstructing the whole description from its parts.
  • Adapt
    the growing description as new pieces of the puzzle are read and reviewed. As in real life, first guesses may need to be revised to accommodate later arriving chunks of text that clarify the overall structure of the description that you are (re)building.
  • Review
    the emerging whole as the last pieces fall into place, as you would with any puzzle. This approach enables students, cooperatively, to "write" a long, complex technical description using important, real-world design principles (the guidelines), without having to compose each separate piece of prose. It shows "actively" how the pieces of a good description have features that knit together to form a coherent pattern intended to help readers use the text well. Since the students must focus on those same text features to rebuild the description, they come to see why writers bother to deploy them.
Case:
Student version [headings only, for outline]:

Description Case 3:  Compact Disk



Overview


General Shape

     Size

     Capacity

The Layers


The Groove

     One-track Design

     Moving the Groove

The Pits


The Optical Read-Out System

Student version [with scaffolding]:

Description Case 3:  Compact Disk

     Description                                  Analysis

Overview

A compact disk (CD), like a              FEATURE:
phonograph record, stores                WHY:
information physically for
electronic replay, and, like a
phonograph record, it stores the
information along a single spiral
groove on the side of a plastic
circle.

In contrast to a phonograph              FEATURE:
record, however, a CD                    WHY:
  * stores the information in
    digital (on/off) rather than
    analog (variable shape) form,
    and
  * is read by reflected (laser)
    light, rather than by vibrating
    a needle that travels along the
    groove.

General Shape

     Size
A compact disk is a circle of            FEATURE:
clear plastic (polycarbonate)            WHY:
about 12 cm in diameter and
1 mm thick, with a 1.5-cm
diameter hole in the center.
CDs are stamped from a mold that
leaves a spiral track lined with
pits (little dents) on the CD's
bottom side (details below),
while the top side is smooth.

     Capacity
The surface area of a CD is just         FEATURE:
less than twice (1.77 times) as          WHY:
large as the surface area of a
3.5-inch magnetic "floppy" disk.
But because the pits store
information much more densely
than the iron oxide particles on
a floppy disk, a CD holds at
least 350 times more data (at
least 500 Mbyte on a CD, only
1.44 Mbyte a magnetic disk).

The Layers                               FEATURE:
                                         WHY:
CDs consist of three layers (see
Fig. 1):
(a) The bottom layer is the              FEATURE:
    stamped, grooved plastic.            WHY:
(b) Above that lies a very thin
    film of aluminum (or chrome-
    aluminum alloy).   This
    metallic film reflects any           FEATURE:
    light entering the disk from         WHY:
    below, except where the pits
    fall in the plastic layer.

(c) Above the reflective metal
    layer is a coat of acrylic
    lacquer that protects the
    metal from scratches and
    oxidation.  It also allows
    printing descriptive labels
    safely on the top side of
    the disk.

The Groove

     One-track Design
The groove on the bottom surface
of a CD is a single channel that
spirals outward from the center
to the edge.  This track is              FEATURE:
thinner than a human hair and            WHY:
several kilometers long.
(The spiral differs from the             FEATURE:
many concentric rings of iron            WHY:
oxide (see Fig. 2) that store
information on a magnetic disk.)

     Moving the Groove
The spindle through a CD's center
hole (see Fig. 3, d) connects
the disk to a variable-speed             FEATURE:
motor (unlike the constant-speed         WHY:
motor on a phonograph turntable).

The disk turns (clockwise) about
500 revolutions/minute (e) when          FEATURE:
the reading laser beam is at the         WHY:
center, but only about 200
revolutions/minute when the beam
reaches the outer edge.
This causes the track to pass
over the read-out system (f),
which gradually moves from the
center to the edge, at a
constant linear speed, to help
reliably detect the pits.

The Pits

The moving spiral track is lined         FEATURE:
with pits (dents) and flat spots         WHY:
("lands").  These vary in size
and placement in a sequence that
represents the information stored.
The pit sequence can digitally           FEATURE:
encode text, images, computer            WHY:
programs, or the left- and right-
hand audio signals of a stereo
sound recording.

Additional pits
  * give location and timing
    information (for player
    display), and
  * control the motor speed
    so that the reading rate
    remains constant.

The Optical Read-Out System

Two lenses and a semi-transparent        FEATURE:
(partially silvered) mirror              WHY:
(see Fig. 4) direct the laser
beam from below at the track on
the spinning CD.

If the laser beam strikes a pit          FEATURE:
on the track (g), it is not              WHY:
reflected.  The light-sensitive
photodiode (detector) below the
mirror sees no beam and produces
no signal.
If the laser beam strikes a land
between pits on the track (h),
it reflects back straight through
the mirror to the photodiode
below.  This detector then
produces an electric signal.

For compatibility with other
electronic equipment, a special
reversing circuit (a "not gate")
then turns these pit
interruptions into ON signals
(binary 1s) and turns the land
reflections into OFF signals
(binary 0s).

Student version [segmented, no scaffolding]:

Description Case 3:  Compact Disk

                                       ---
A compact disk (CD), like a
phonograph record, stores
information physically for
electronic replay, and, like a
phonograph record, it stores the
information along a single spiral
groove on the side of a plastic
circle.
                                       ---
In contrast to a phonograph
record, however, a CD
  * stores the information in
    digital (on/off) rather than
    analog (variable shape) form,
    and
  * is read by reflected (laser)
    light, rather than by vibrating
    a needle that travels along the
    groove.
                                       ---
A compact disk is a circle of
clear plastic (polycarbonate)
about 12 cm in diameter and
1 mm thick, with a 1.5-cm
diameter hole in the center.
CDs are stamped from a mold that
leaves a spiral track lined with
pits (little dents) on the CD's
bottom side (details below),
while the top side is smooth.
                                       ---
The surface area of a CD is just
less than twice (1.77 times) as
large as the surface area of a
3.5-inch magnetic "floppy" disk.
But because the pits store
information much more densely
than the iron oxide particles on
a floppy disk, a CD holds at
least 350 times more data (at
least 500 Mbyte on a CD, only
1.44 Mbyte a magnetic disk).
                                       ---
CDs consist of three layers (see
Fig. 1):
(a) The bottom layer is the
    stamped, grooved plastic.
(b) Above that lies a very thin
    film of aluminum (or chrome-
    aluminum alloy).   This
    metallic film reflects any
    light entering the disk from
    below, except where the pits
    fall in the plastic layer.
                                       ---
(c) Above the reflective metal
    layer is a coat of acrylic
    lacquer that protects the
    metal from scratches and
    oxidation.  It also allows
    printing descriptive labels
    safely on the top side of
    the disk.
                                       ---
The groove on the bottom surface
of a CD is a single channel that
spirals outward from the center
to the edge.  This track is
thinner than a human hair and
several kilometers long.
(The spiral differs from the
many concentric rings of iron
oxide (see Fig. 2) that store
information on a magnetic disk.)
                                       ---
The spindle through a CD's center
hole (see Fig. 3, d) connects
the disk to a variable-speed
motor (unlike the constant-speed
motor on a phonograph turntable).
                                       ---
The disk turns (clockwise) about
500 revolutions/minute (e) when
the reading laser beam is at the
center, but only about 200
revolutions/minute when the beam
reaches the outer edge.
This causes the track to pass
over the read-out system (f),
which gradually moves from the
center to the edge, at a
constant linear speed, to help
reliably detect the pits.
                                       ---
The moving spiral track is lined
with pits (dents) and flat spots
("lands").  These vary in size
and placement in a sequence that
represents the information stored.
The pit sequence can digitally
encode text, images, computer
programs, or the left- and right-
hand audio signals of a stereo
sound recording.
                                       ---
Additional pits
  * give location and timing
    information (for player
    display), and
  * control the motor speed
    so that the reading rate
    remains constant.
                                       ---
Two lenses and a semi-transparent
(partially silvered) mirror
(see Fig. 4) direct the laser
beam from below at the track on
the spinning CD.
                                       ---
If the laser beam strikes a pit
on the track (g), it is not
reflected.  The light-sensitive
photodiode (detector) below the
mirror sees no beam and produces
no signal.
If the laser beam strikes a land
between pits on the track (h),
it reflects back straight through
the mirror to the photodiode
below.  This detector then
produces an electric signal.
                                       ---
For compatibility with other
electronic equipment, a special
reversing circuit (a "not gate")
then turns these pit
interruptions into ON signals
(binary 1s) and turns the land
reflections into OFF signals
(binary 0s).
                                       ---
Annotated version:

Description Case 3:  Compact Disk

     Description                                  Analysis

Overview

A compact disk (CD), like a              FEATURE: comparison
phonograph record, stores                WHY: same role, parts
information physically for
electronic replay, and, like a
phonograph record, it stores the
information along a single spiral
groove on the side of a plastic
circle.

In contrast to a phonograph              FEATURE: contrast
record, however, a CD                    WHY: different behavior
  * stores the information in
    digital (on/off) rather than
    analog (variable shape) form,
    and
  * is read by reflected (laser)
    light, rather than by vibrating
    a needle that travels along the
    groove.

General Shape

     Size
A compact disk is a circle of            FEATURE: specifics
clear plastic (polycarbonate)            WHY: relevant to making
about 12 cm in diameter and
1 mm thick, with a 1.5-cm
diameter hole in the center.
CDs are stamped from a mold that
leaves a spiral track lined with
pits (little dents) on the CD's
bottom side (details below),
while the top side is smooth.

     Capacity
The surface area of a CD is just         FEATURE: contrast
less than twice (1.77 times) as          WHY: significance of parts
large as the surface area of a
3.5-inch magnetic "floppy" disk.
But because the pits store
information much more densely
than the iron oxide particles on
a floppy disk, a CD holds at
least 350 times more data (at
least 500 Mbyte on a CD, only
1.44 Mbyte a magnetic disk).

The Layers                               FEATURE: order of (dis)assembly
                                         WHY: large to small
CDs consist of three layers (see
Fig. 1):
(a) The bottom layer is the              FEATURE: parts
    stamped, grooved plastic.            WHY: show relation among them
(b) Above that lies a very thin
    film of aluminum (or chrome-
    aluminum alloy).   This
    metallic film reflects any           FEATURE: specifics
    light entering the disk from         WHY: show role
    below, except where the pits
    fall in the plastic layer.

(c) Above the reflective metal
    layer is a coat of acrylic
    lacquer that protects the
    metal from scratches and
    oxidation.  It also allows
    printing descriptive labels
    safely on the top side of
    the disk.

The Groove

     One-track Design
The groove on the bottom surface
of a CD is a single channel that
spirals outward from the center
to the edge.  This track is              FEATURE: comparison
thinner than a human hair and            WHY: explains data density
several kilometers long.
(The spiral differs from the             FEATURE: contrast
many concentric rings of iron            WHY: show role
oxide (see Fig. 2) that store
information on a magnetic disk.)

     Moving the Groove
The spindle through a CD's center
hole (see Fig. 3, d) connects
the disk to a variable-speed             FEATURE: contrast
motor (unlike the constant-speed         WHY: show role
motor on a phonograph turntable).

The disk turns (clockwise) about
500 revolutions/minute (e) when          FEATURE: specifics
the reading laser beam is at the         WHY: relevant to making
center, but only about 200
revolutions/minute when the beam
reaches the outer edge.
This causes the track to pass
over the read-out system (f),
which gradually moves from the
center to the edge, at a
constant linear speed, to help
reliably detect the pits.

The Pits

The moving spiral track is lined         FEATURE: specifics
with pits (dents) and flat spots         WHY: terminology
("lands").  These vary in size
and placement in a sequence that
represents the information stored.
The pit sequence can digitally           FEATURE: comparison (implicit)
encode text, images, computer            WHY: among pit roles
programs, or the left- and right-
hand audio signals of a stereo
sound recording.

Additional pits
  * give location and timing
    information (for player
    display), and
  * control the motor speed
    so that the reading rate
    remains constant.

The Optical Read-Out System

Two lenses and a semi-transparent        FEATURE: parts
(partially silvered) mirror              WHY: show relations
(see Fig. 4) direct the laser
beam from below at the track on
the spinning CD.

If the laser beam strikes a pit          FEATURE: specifics
on the track (g), it is not              WHY: relevant to making
reflected.  The light-sensitive
photodiode (detector) below the
mirror sees no beam and produces
no signal.
If the laser beam strikes a land
between pits on the track (h),
it reflects back straight through
the mirror to the photodiode
below.  This detector then
produces an electric signal.

For compatibility with other
electronic equipment, a special
reversing circuit (a "not gate")
then turns these pit
interruptions into ON signals
(binary 1s) and turns the land
reflections into OFF signals
(binary 0s).

Extended Activities:
Besides the primary activities explained above, you can have students pursue several secondary activities with the descriptive text of Exercise 3. These optional activities reinforce and (slightly) extend this exercise's original goals.
(1) SPELLING.
I always spell "compact disk" with a final "k," the standard in computing dictionaries and the spelling used by David Macaulay in The New Way Things Work as well. Students may notice (or you can point out) that on digital audio compact disks, the spelling printed on the lacquer layer is "disc" with a final "c." You can use this discrepancy to explain that every major publisher of technical descriptions (research departments, government agencies, publishing houses) has a style guide (a book of spelling, punctuation, and format rules) and settles such issues by appealing to the rules in their local guide. Different style guides yield different answers, as shown with "disc" and "disk."
(2) COMPARISONS.
As the middle of the description-writing guidelines points out, comparisons and contrasts often improve the usefulness of technical descriptions. They relate new features to familiar ones from other situations, and they clarify the significance of features whose value or contribution might otherwise remain obscure. (See also the "contrast class" discussion in the teacher notes on Exercise 0, Strategy, Part (4).)

Many comparisons and contrasts appear in the compact-disk description. You can ask students to find each one and discuss its role in the description, either on the (enlarged) text chunks used to reconstruct the description or on a separate copy of the whole text distributed just for that purpose. To help start this process, I include here a list of the most important comparisons and contrasts in Exercise 3:

     Comparison                             Paragraph

...like a phonograph record...                1 (twice)
In contrast to a phonograph record            2
...rather than analog...                      2
...rather than by vibrating a needle          2
...less than twice...as large                 4
...much more densely than                     4
...thinner than a human hair                  7
The spiral differs from...                    7
...unlike the constant-speed motor            8

(3) REWRITING, REVISING.
For every paragraph in a technical description, alternative versions exist that could have been used. Some are clearly better or worse that the original, while others involve tradeoffs that improve the description in one way but weaken it in another. (For background on the real-life relevance of text revision to laboratory science, see the teacher notes on Exercise 9.)

You can introduce students to this important idea that good writing demands revising, and revising requires carefully comparing the relative merits of alternative chunks of text. I have used paragraph 3 (on disk size) as the focus for such extended activity. You can present and discuss alternative versions of this paragraph (below) most easily in large-print format or by projecting them to large size on the wall.

  • Version 1.
    The original version of the size paragraph (reproduced here) contains helpful signals for readers. But with only two sentences, each is very long (and hence harder to read). Have students count the words in each sentence and compute the total (shown below).
    A compact disk is a circle of
    clear plastic (polycarbonate)
    about 12 cm in diameter and
    1 mm thick, with a 1.5-cm              27 words
    diameter hole in the center.
    CDs are stamped from a mold that                 total = 56 words
    leaves a spiral track lined with
    pits (little dents) on the CD's
    bottom side (details below),           29 words
    while the top side is smooth.
    
  • Version 2.
    You can ask able students to rewrite Version 1 using many short(er) sentences instead of just two long ones. You can also present less able students with an alternative text (shown here) that illustrates such a short-sentence revision. Have students count the words in each sentence in this (or their own) revised version and again compute the total. For the text shown here, each sentence is much shorter and easier, but the total paragraph size is actually 10% longer than the original. This tradeoff is typical, and students can discuss the new version's strengths and weaknesses compared to Version 1.
    A compact disk is a circle of
    clear plastic (polycarbonate).         10 words
    It is about 12 cm in diameter
    and 1 mm thick.                        11
    It has a 1.5-cm
    diameter hole in the center.            9
    CDs are stamped from a mold.            6
    The mold leaves a spiral track on                total = 63 words
    the bottom side (details below).       12
    The spiral track is lined with
    pits (little dents).                    9
    The CD's top side is smooth.            6
    
  • Version 3.
    One technique to improve the usefulness and clarity of complex descriptions is to make information more explicit with overt lists (as suggested in the third part of the description-writing guidelines). You can ask able students to rewrite Version 1 (or 2) using an overt list of compact-disk features to make the text both clearer and shorter. You can also present less able students with an alternative text (shown here) that illustrates such an overt-list revision. Have students count the words in this revised version and again compute the total. For the text shown here, list format reduces the total word count to 47, which is 20% shorter than the original version. Lists must be used thoughtfully, of course, but this result typifies how they can improve technical descriptions.
    A compact disk is a circle of
    clear plastic (polycarbonate)
    with a
     * 12-cm diameter,
     * 1-mm thickness,
     * 1.5-cm diameter hole in
       the center,                                   total = 47 words
     * spiral track on its bottom
       side (stamped from a mold), and
     * smooth top side.
    The spiral track is lined with
    pits (little dents, details below).
    
Note:
This exercise most closely supports the following Common Core State Standards.
Reading:
Grades 6-8, RST6-8.1 "Cite specific textual evidence to support analysis of science and technical texts."
Grades 6-8, RST6-8.5, "Analyze the structure an author uses to organize a text, including how the major sections contribute to the whole and to an understanding of the topic."
Grades 6-8, RST6-8.7 "Integrate quantitative or technical information expressed in words in a text with a version of that information expressed visually (e.g., in a flowchart, diagram, model, graph, or table)."
Writing:
Greades 11-12, W11-12.2a, "Introduce a topic; organize complex ideas, concepts, and information so that each new element builds on that which precedes it to create a unified whole; include formatting (e.g., headings), graphics (e.g., figures, tables), and multimedia when useful to aiding comprehension."
Grades 11-12, W11-12.2b, "Write informative/explanatory texts...develop the topic thoroughly by selecting the most significant and relevant facts, extended definitions, concrete details, quotations, or other information and examples appropriate to the audience's knowledge of the topic."
Grades 11-12, W11-12.5, "Develop and strengthen writing as needed by planning, revising, editing, rewriting, or trying a new approach, focusing on addressing what is most significant for a specific purpose and audience."

Contact: T. R. Girill trgirill@acm.org