← ACARA Digital Technologies Years 7–8

Decoding ASCII binary back into text

To decode ASCII binary, split the bits into groups of 8 (bytes), convert each byte to a decimal code, then look up that code in the ASCII table to find the letter.

Part of: ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 21

Understanding Decoding ASCII binary back into text in ACARA Digital Technologies Years 7–8

What it does: Each group of 8 bits is one byte and stands for one character. This message has three bytes, so it decodes to three letters (CAT). Once a byte is converted to decimal (01000011 = 67), the ASCII table shows that 67 is the letter C. ASCII code 65 is the capital letter A. B is 66. Decode the bytes in order, left to right. 72 is H and 105 is i, so the message is Hi. 01001111 is 79, which is O. 01001011 is 75, which is K. Keep the bytes in the same order: OK. ASCII code 32 is the space character. It separates words in a decoded message.

Common mistakes: ASCII characters are stored in bytes, so the message must be split into groups of 8 bits, not 4. It should say: split into groups of 8 bits. In 01000001 the 1s are in the 64 and 1 place values, so the decimal is 64 + 1 = 65. It should be: decimal = 64 + 1.

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This module's key terms are also in a printable crossword (Modules 128–152) and word search (Modules 69–152).

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Learning
Duo
When decoding an ASCII binary message, how many bits go in each group?
4 bits8 bits (one byte)
Matches
0100100001101001 -> split into groups of 4 bits
↓
Groups of 4 bits are too small. ASCII characters are stored in bytes of 8 bits, so the message should be split into groups of 8.
Applying
Meanings
Term
01001000 -> 72 -> H
Meaning
To decode ASCII binary, split the bits into groups of 8 (bytes), convert each byte to a decimal code, then look up that code in the ASCII table to find the letter.
Practice
01000011 -> 67 -> C
Testing
Gaps
01001000 -> -> H
Check Answer
Spots
1message = 0100100001101001
2split into groups of 4 bits

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Quick Questions

1. When decoding an ASCII binary message, how many bits go in each group?
A. 8 bits (one byte) ✓
B. 4 bits
Each ASCII character is stored as one byte, which is 8 bits. So you split the message into groups of 8.
2. A message has 24 bits. How many ASCII characters does it hold?
A. 24 characters
B. 3 characters ✓
24 bits split into groups of 8 gives 3 bytes. Each byte is one character.
3. What is the correct order for decoding one byte?
A. Letter, then decimal code, then binary
B. Binary, then decimal code, then letter ✓
Decoding goes from binary to a decimal code, then you look up that code in the ASCII table. For example 01001000 -> 72 -> H.
4. What is the first step to decode 0100100001101001?
A. Split it into 01001000 and 01101001 ✓
B. Add up all 16 bits as one big number
Always split into bytes first. Each byte is decoded on its own.
5. Decode the byte 01000001. Its decimal value is 64 + 1 = 65. Which letter is it?
A. A ✓
B. B
ASCII code 65 is the capital letter A. B is 66.

Match the Pairs

01001000 01101001 → Two bytes: split the message into groups of 8 bits before decoding
01001000 -> 72 -> H → Convert the byte to decimal, then look up that code in the ASCII table to get the letter
Decoding step 1: split a long string of bits into bytes of 8 bits each → 01000011 01000001 01010100
Decoding step 3: use the decimal code to look up the character in the ASCII table → 67 -> C
0100100001101001 -> split into groups of 4 bits → Groups of 4 bits are too small. ASCII characters are stored in bytes of 8 bits, so the message should be split into groups of 8.
01000001 = 65 = B → The decimal conversion is right, but the table lookup is wrong. ASCII code 65 is the letter A, not B.

Key Term

01001000 -> 72 -> H
To decode ASCII binary, split the bits into groups of 8 (bytes), convert each byte to a decimal code, then look up that code in the ASCII table to find the letter.

Fill in the Blanks

1. 01001000 -> 72 -> H
Hint: Type the decimal number you get by adding the place values of the 1s in this byte.
2. groups = split(bits, 8)
Hint: Type the number of bits in each group (one byte) that is decoded into one character.
3. 01000010 -> 66 -> B
Hint: Type the letter whose ASCII code is the decimal number shown. Count up from the code for A.
4. =CHAR(72)
Hint: Type the spreadsheet function name that turns an ASCII code number into its character.
5. 24 bits / 8 = 3 letters
Hint: Type how many characters you get when you divide the bits into bytes.

Spot the Error

Question 1
A message = 0100100001101001
B split into groups of 4 bits
C group 1 = 01001000
D group 2 = 01101001
Line B — ASCII characters are stored in bytes, so the message must be split into groups of 8 bits, not 4. It should say: split into groups of 8 bits.
Question 2
A byte = 01000001
B decimal = 32 + 1
C letter = ASCII[65]
Line B — In 01000001 the 1s are in the 64 and 1 place values, so the decimal is 64 + 1 = 65. It should be: decimal = 64 + 1.
Question 3
A byte = 01000011
B decimal = 64 + 2 + 1
C code = 67
D letter = ASCII[76]
E print(letter)
Line D — The decimal code is 67, so the lookup must use 67 (the letter C). 76 is a different character (L). It should be: letter = ASCII[67].
Question 4
A byte = 01010100
B decimal = 64 + 16 + 4
C code = 94
D print(ASCII[code])
E # 84 is the letter T
Line C — 64 + 16 + 4 adds up to 84, not 94. The code must match the sum. It should be: code = 84.
Question 5
A byte = 01001000
B decimal = 72
C letter = ASCII[byte]
D print(letter)
Line C — The ASCII table is looked up using the decimal code, not the binary byte. It should be: letter = ASCII[decimal].

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