Lossy Everywhere
- Terminology and concepts to look out for: file size · quality · trade-off
- Relevance to me: Yesterday you learned how compression works. Today you look at where lossy compression is used every day, and where it cannot be used.
- Board Question: Your phone is full. Would you keep 100 sharp photos or 1,000 blurrier ones? Pick one and say what you gave up. You answer this three times today: now, at your table, and on the exit ticket.
Chromebooks closed for now. Lab notebook open.
Lossy Compression in Everyday Use
This section explains where lossy compression is used in everyday life and where it cannot be used. Compression is a way of making a file smaller so that it takes less space and sends faster. Lossless compression makes a file smaller in a way that lets the computer rebuild the original file exactly. Lossy compression also makes a file smaller, but the file that comes back is not identical to the original, because some detail has been removed.
File Size and Quality
File size is the amount of storage space a file takes up, counted in units such as kilobytes (KB), megabytes (MB) and gigabytes (GB). Quality is how much of the original detail a copy keeps. For example, Netflix lists its basic video quality at up to 0.3 GB of data per hour and its high-definition quality at up to 3 GB per hour. The high-definition copy keeps more detail, so its file size is larger. Music works the same way. Spotify lists its Low setting at about 24 kilobits per second and its Very high setting at about 320 kilobits per second. A copy that keeps every detail and stays the same size is not an example of lossy compression, because nothing was removed.
Lossy Compression in Everyday Places
Lossy compression is used in many everyday places. The first is a song on a streaming app. Apple Music offers lossless audio only as a setting the listener turns on, and Apple states that lossless streaming uses significantly more data and more storage space. A song at a lower setting is a smaller file that does not keep every detail of the original recording.
The second is a video that turns blurry when the wifi gets weak. A streaming service stores the same video as several versions, and each version is encoded at a different bit rate. When the connection gets weak, the video player switches to a lower-quality version of the video that needs less data each second. The service is sending a different, smaller version that was prepared in advance.
The third is a photo in a group chat. Google Messages offers two choices when a person sends a photo. The "optimized for chat" choice sends faster and uses less data, and the "original quality" choice sends the photo at its full resolution. Google also states that phone carriers compress photos sent as MMS messages, which can lower image quality.
The fourth is a one-page summary of a long book. A summary is not compression in the technical sense, because no computer method is applied to a file. It works like lossy compression, though. A summary gives only the main points of a text and leaves out the details, and those details cannot be rebuilt from the summary alone.
Trade-offs
Each of these four cases is a trade-off. A trade-off is a choice in which getting more of one thing means getting less of another. In lossy compression, the file size gets smaller and the quality gets lower. Spotify states the trade-off directly: higher streaming quality uses more data, and higher download quality uses more space. A person or a program chose how much quality to give up. The listener picks a setting, the video player picks a version and the sender picks a photo choice. Figure 1 shows the four everyday examples placed along a slider, with a smaller file at one end and higher quality at the other.
Data That Cannot Lose Detail
Some kinds of data cannot lose any detail at all. A bank balance must keep every digit, even if a smaller file size were possible, because a balance of 1,250 dollars with one digit changed is a different amount of money. The National Institute of Standards and Technology (NIST) describes data integrity as the property that data has not been changed, destroyed or lost by accident or without permission. A password has no lower-quality version, because NIST guidelines require a system to check the entire password that a person types. A line of code is not a trade-off case, since nothing in it can be given up. The HTML standard sets exact rules for reading every tag, so a tag with one missing character can make the browser build a different page. Figure 1 lists these three cases below the slider as data that allows no loss.
Two questions apply to any copy of a file. The first asks what detail was lost. The second asks whether that detail matters for the way the copy will be used.
Summary
Lossy compression makes files smaller by giving up detail, and it is used for songs, streaming video and photos in chats. Each use is a trade-off between file size and quality that a person or a program chose. Bank balances, passwords and code cannot lose a single digit or character, so they are never stored with lossy compression.
Sources: ITU-T, T.81 (1992); Pantos and May, RFC 8216 (2017); Netflix, Data use (n.d.); Spotify, Audio quality (n.d.); Apple, Lossless audio (2026); Google, Messages photos (n.d.); Oxford, summary (n.d.); NIST, Data integrity (n.d.); NIST, SP 800-63B-4 (2025); WHATWG, HTML Parsing (2026); CompTIA, Tech+ Objectives (2023); Muggivan, HTML test (2026).
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notebook cue: file size
A video switches to a blurrier picture when the wifi gets weak. What happened to the file size of what is being sent?
Check
The file size of what is being sent got smaller. The player switched to a different, lower-quality version of the video that needs less data each second. You can find this in Lossy Compression in Everyday Places, paragraph 2.
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notebook cue: quality
A one-page summary of a 300-page book is much smaller than the book. What kind of detail is gone?
Check
The summary keeps only the main points, so the details are gone, and they cannot be rebuilt from the summary. A summary is not compression in the technical sense, but it works like lossy compression. You can find this in Lossy Compression in Everyday Places, paragraph 4.
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notebook cue: trade-off
A song is saved as a smaller file. Name what was gained and what was given up.
Check
The song gained a smaller file size, so it uses less data and less space. It gave up quality, because the smaller file does not keep every detail of the original recording. You can find this in Trade-offs and Lossy Compression in Everyday Places, paragraph 1.
-
notebook cue: file size
A video switches to a blurrier picture when the wifi gets weak. What happened to the file size of what is being sent?
Check
The file size of what is being sent got smaller. The player switched to a different, lower-quality version of the video that needs less data each second. You can find this in Lossy Compression in Everyday Places, paragraph 2.
-
notebook cue: quality
A one-page summary of a 300-page book is much smaller than the book. What kind of detail is gone?
Check
The summary keeps only the main points, so the details are gone, and they cannot be rebuilt from the summary. A summary is not compression in the technical sense, but it works like lossy compression. You can find this in Lossy Compression in Everyday Places, paragraph 4.
-
notebook cue: trade-off
A song is saved as a smaller file. Name what was gained and what was given up.
Check
The song gained a smaller file size, so it uses less data and less space. It gave up quality, because the smaller file does not keep every detail of the original recording. You can find this in Trade-offs and Lossy Compression in Everyday Places, paragraph 1.
Lossy Compression in Everyday Use
This section explains where lossy compression is used in everyday life and where it cannot be used. Compression is a way of making a file smaller so that it takes less space and sends faster. Lossless compression makes a file smaller in a way that lets the computer rebuild the original file exactly. Lossy compression also makes a file smaller, but the file that comes back is not identical to the original, because some detail has been removed.
File Size and Quality
File size is the amount of storage space a file takes up, counted in units such as kilobytes (KB), megabytes (MB) and gigabytes (GB). Quality is how much of the original detail a copy keeps. For example, Netflix lists its basic video quality at up to 0.3 GB of data per hour and its high-definition quality at up to 3 GB per hour. The high-definition copy keeps more detail, so its file size is larger. Music works the same way. Spotify lists its Low setting at about 24 kilobits per second and its Very high setting at about 320 kilobits per second. A copy that keeps every detail and stays the same size is not an example of lossy compression, because nothing was removed.
Lossy Compression in Everyday Places
Lossy compression is used in many everyday places. The first is a song on a streaming app. Apple Music offers lossless audio only as a setting the listener turns on, and Apple states that lossless streaming uses significantly more data and more storage space. A song at a lower setting is a smaller file that does not keep every detail of the original recording.
The second is a video that turns blurry when the wifi gets weak. A streaming service stores the same video as several versions, and each version is encoded at a different bit rate. When the connection gets weak, the video player switches to a lower-quality version of the video that needs less data each second. The service is sending a different, smaller version that was prepared in advance.
The third is a photo in a group chat. Google Messages offers two choices when a person sends a photo. The "optimized for chat" choice sends faster and uses less data, and the "original quality" choice sends the photo at its full resolution. Google also states that phone carriers compress photos sent as MMS messages, which can lower image quality.
The fourth is a one-page summary of a long book. A summary is not compression in the technical sense, because no computer method is applied to a file. It works like lossy compression, though. A summary gives only the main points of a text and leaves out the details, and those details cannot be rebuilt from the summary alone.
Trade-offs
Each of these four cases is a trade-off. A trade-off is a choice in which getting more of one thing means getting less of another. In lossy compression, the file size gets smaller and the quality gets lower. Spotify states the trade-off directly: higher streaming quality uses more data, and higher download quality uses more space. A person or a program chose how much quality to give up. The listener picks a setting, the video player picks a version and the sender picks a photo choice. Figure 1 shows the four everyday examples placed along a slider, with a smaller file at one end and higher quality at the other.
Data That Cannot Lose Detail
Some kinds of data cannot lose any detail at all. A bank balance must keep every digit, even if a smaller file size were possible, because a balance of 1,250 dollars with one digit changed is a different amount of money. The National Institute of Standards and Technology (NIST) describes data integrity as the property that data has not been changed, destroyed or lost by accident or without permission. A password has no lower-quality version, because NIST guidelines require a system to check the entire password that a person types. A line of code is not a trade-off case, since nothing in it can be given up. The HTML standard sets exact rules for reading every tag, so a tag with one missing character can make the browser build a different page. Figure 1 lists these three cases below the slider as data that allows no loss.
Two questions apply to any copy of a file. The first asks what detail was lost. The second asks whether that detail matters for the way the copy will be used.
Summary
Lossy compression makes files smaller by giving up detail, and it is used for songs, streaming video and photos in chats. Each use is a trade-off between file size and quality that a person or a program chose. Bank balances, passwords and code cannot lose a single digit or character, so they are never stored with lossy compression.
Sources: ITU-T, T.81 (1992); Pantos and May, RFC 8216 (2017); Netflix, Data use (n.d.); Spotify, Audio quality (n.d.); Apple, Lossless audio (2026); Google, Messages photos (n.d.); Oxford, summary (n.d.); NIST, Data integrity (n.d.); NIST, SP 800-63B-4 (2025); WHATWG, HTML Parsing (2026); CompTIA, Tech+ Objectives (2023); Muggivan, HTML test (2026).
Each table votes: 100 sharp or 1,000 blurry. The table does not have to agree. It writes the count for each side and what each side gave up.
Chromebooks stay closed while your table talks. When the discussion form pops up, open it. It locks your Chromebook until you submit, so finish your conversation first.
DO NOT CLICK OPEN FORM FROM A LAB COMPUTER
Open the discussion formThis opens during your class period.
Use this form at your table. At the start of class, you answered today's Board Question: Your phone is full. Would you keep 100 sharp photos or 1,000 blurrier ones? Pick one and say what you gave up. Talk with your table about it, the three positions and the three statements. Then answer on your own.
MC 1. (Where you stand) At the start of class, you answered this question: Your phone is full. Would you keep 100 sharp photos or 1,000 blurrier ones? Pick one and say what you gave up. After talking with your table, which position is closest to yours now?
- 100 sharp photos, because the details matter more.
- 1,000 blurrier photos, because having more pictures matters more.
- It depends on what the photos are for.
CR 1. (Board Question · answer 2) Think about the Board Question again. Did you reach the same conclusion you had at the start of class, or a different one? Say so in one sentence. Then write your answer as a claim: (1) State your answer as if it is a fact. (2) Support it with evidence from the passage. (3) Explain how that evidence supports your claim.
Write your answer in your lab notebook, labeled CR 1.
MC 2. (Statement 1) A bank could store account balances with lossy compression to save space.
- Agree
- Disagree
MC 3. (Statement 2) A streaming app lowers video quality when the internet connection is slow.
- Agree
- Disagree
MC 4. (Statement 3) A one-page summary of a book works like lossy compression.
- Agree
- Disagree
CR 2. (My view) Write one thing you said or thought at your table.
Write your answer in your lab notebook, labeled CR 2.
CR 3. (A different view) Write the view at your table that was most different from yours. If everyone agreed, write the strongest argument against your table's answer.
Write your answer in your lab notebook, labeled CR 3.