Worksheet

Snow on the Screen: Sending an Underwater Robot's Picture

Grade 8 practice: a robotics club decodes on-off pixel pulses and tests pool-pump noise to explain why digital signals are more reliable than analog.

Grade 8ScienceInteractive · Printable

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1 NGSS standard
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11 questions
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Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Matching, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

Students decode and model a picture's brightness sent as a voltage and as on-off pulses.
Students use noise-test data to explain why digital signals carry information more reliably.
Students explain the noise limit beyond which a digital signal can also fail.

How to use

Work through the pulse code in paragraph 3 together with one example, then let students use both tables on their own and discuss the claim in question 11.

Curriculum

NGSS standard covered

MS-PS4-3

Explain why digital signals carry information more reliably than analog signals.

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Snow on the Screen: Sending an Underwater Robot's Picture

Compare analog and digital pixels sent through a long tether.

Part 1: Two ways to send a pixel

Read the passage, then answer questions 1 to 4.

The Robot in the Deep End

1The robotics club at Fernhill Middle School is building a small underwater robot to explore the deep end of the town pool. The robot carries a camera, and a cable called a tether, 25 m long, connects it to a laptop at the poolside. The tether carries the camera's picture to the laptop as an electric signal. Club members Kenji Oduya and Paz Albright noticed a problem on their first test dive: whenever the pool's pump was running, white specks like snow flickered across the picture. Their adviser, Ms. Ingrid Tallis, explained that the pump's motor adds random changes in voltage, called noise, to signals in nearby cables.

2A camera picture is made of thousands of tiny squares called pixels. The robot's camera gives each pixel a brightness level from 0 (black) to 15 (white). The club's first version sends each brightness level as an analog signal: one voltage, 0.3 V for each level. So level 4 is sent as 1.2 V and level 15 as 4.5 V. The laptop divides the voltage it receives by 0.3 and rounds to the nearest whole level.

3The second version sends each level as a digital signal: a group of four pulses, each either on (5 V) or off (0 V). The four pulses stand for 8, 4, 2 and 1, in that order, and the laptop adds the values of the pulses that are on. For example, off, on, off, on stands for 4 + 1 = 5. The laptop reads any pulse above 2.5 V as on and any pulse at 2.5 V or less as off.

4To compare the two versions, the club placed a test card with a known pattern of gray squares on the pool floor and sent the robot down. For each test, the laptop checked a 1,000-pixel patch of the picture. A pixel counted as wrong when the laptop showed a level different from the one on the card. The club tested both versions with the pump off and on its low and high settings, using the same tether, card and lighting. Ms. Tallis measured the largest noise spike on the tether for each setting.

5Kenji then wondered whether a longer tether would let the robot reach the far end of the pool. The club added a 25 m extension, for 50 m in all, and repeated the high-pump test. The longer cable ran close to the pump room and picked up stronger spikes. Paz recorded every result in Table 1.

Use paragraph 3. The robot sends the four pulses on, off, on, on.

The laptop shows brightness level .

Use paragraph 3. Match each group of four pulses to the brightness level it stands for.

off, on, on, off
on, on, off, off
off, off, on, on
on, off, off, on

A. Level 12
B. Level 9
C. Level 6
D. Level 3

Use paragraph 2. A pixel on the test card is level 6. On its way up the tether, a noise spike raises the analog voltage by 0.6 V.

The laptop shows this pixel as level .

The same level 6 pixel is sent the digital way, as off, on, on, off. The same spike raises each of the four pulses by 0.6 V. Use paragraph 3. What does the laptop show?

A. Level 6, because each pulse is still on the same side of 2.5 V
B. Level 8, because the spike adds about two levels to any signal
C. Level 15, because the spike switches the two off pulses on
D. Level 7, because the spike adds one level to the pulses

Part 2: The pump test

Use Table 1 and paragraphs 4 and 5 to answer questions 5 to 11.

Table 1: Wrong Pixels in a 1,000-Pixel Patch
Pump setting	Tether (m)	Largest spike (V)	Analog: wrong pixels	Digital: wrong pixels
Off	25	0.2	15	0
Low	25	0.6	240	0
High	25	1.4	610	0
High	50	2.8	820	130

Use Table 1. Look at the high pump setting with the 25 m tether.

The analog version showed % of the pixels in the patch at the wrong level.

Which statement best describes the pattern in the first three rows of Table 1?

A. Wrong pixels rose for both versions, but the digital count rose more slowly.
B. Wrong analog pixels rose to 610 at the low setting and then held steady.
C. Wrong analog pixels rose as spikes grew; the digital count stayed at 0.
D. Wrong digital pixels fell as the spikes grew, while the analog count held steady.

With the 25 m tether and the pump on high, spikes reached 1.4 V, yet the digital version showed no wrong pixels. Which explanation fits paragraph 3 best?

A. The pump turns off by itself whenever the digital version is sending.
B. A 1.4 V change cannot push a 0 V or 5 V pulse across the 2.5 V line.
C. The laptop deleted the noisy pixels and filled in the card's pattern.
D. Digital pulses travel faster than noise, so the spikes never catch up to them.

With the 50 m tether, the digital version showed 130 wrong pixels. Which explanation fits paragraphs 3 and 5 and Table 1 best?

A. The extra length slowed the 5 V pulses down to the speed of the analog signal.
B. The digital code used fewer brightness levels on the longer tether than on the short one.
C. The longer tether stored some of the pulses, so they reached the laptop late.
D. Some spikes, up to 2.8 V, were large enough to push pulses across the 2.5 V line.

Circle the two results that are evidence that even small noise spikes change an analog picture.

Pump off: largest spike 0.2 V, 15 wrong analog pixelsHigh pump, 50 m: 130 wrong digital pixelsLow pump: largest spike 0.6 V, 240 wrong analog pixelsHigh pump, 25 m: 0 wrong digital pixelsThe test card was placed on the pool floor.

10.

Use the passage and Table 1. Sort each statement by the version of the robot's signal it describes.

Sends a level 10 pixel as one voltage of 3.0 VShowed the card correctly with the pump on high and the 25 m tetherSends a level 10 pixel as on, off, on, offShowed 15 wrong pixels even with the pump offA 0.6 V spike can change the level the laptop shows by 2

Analog version	Digital version

11.

A club member says, "The digital version isn't more reliable. With the long tether it got 130 pixels wrong, so noise ruins both kinds of signals." Do you agree? Write a claim, give evidence from Table 1 and explain your reasoning using what the laptop does with each kind of signal.