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

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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.

1.
Use paragraph 3. The robot sends the four pulses on, off, on, on.

The laptop shows brightness level .

2.
Use paragraph 3. Match each group of four pulses to the brightness level it stands for.
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 .

4.
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?
Question 4 options

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 settingTether (m)Largest spike (V)Analog: wrong pixelsDigital: wrong pixels
Off250.2150
Low250.62400
High251.46100
High502.8820130
5.
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.

6.
Which statement best describes the pattern in the first three rows of Table 1?
Question 6 options
7.
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?
Question 7 options
8.
With the 50 m tether, the digital version showed 130 wrong pixels. Which explanation fits paragraphs 3 and 5 and Table 1 best?
Question 8 options
9.
Circle the two results that are evidence that even small noise spikes change an analog picture.

10.
Use the passage and Table 1. Sort each statement by the version of the robot's signal it describes.
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.