MODD / Mr. Hansen 10/27/2006

Name: _________________________

Test #2a

No calculator allowed today! Do not spend too much time on any single problem.

Part I: Essays (16 pts. each).

1.(a)	Give 2 advantages of analog representations over digital. Write approximately a sentence for each.
(b)	Give 2 advantages of digital over analog. Write approximately a sentence for each.

2.(a)	Explain in a few sentences how we might use a 256-color palette to save space when storing the information contained in a 24-bit-per-pixel image. That is, give an overview of how this would be accomplished. Try to use the term color map in your answer.
(b)	Part (a) sounds like a fair amount of work. After all, it is certainly easier to store 24 bits per pixel to represent true RGB colors and not have to worry about a palette at all. State (roughly) how many bytes per pixel will be saved by using the palette in part (a).
(c)	Why is the amount of storage that is saved not precisely 2/3 of the size of the original 24-bit-per pixel image file?
(d)	For what types of images would the procedure in part (a) produce unacceptable results?

Part II: Problems/Short Essays (15 pts. each). Show work if you desire partial credit.

3.	A radio station is broadcasting an analog FM signal with a power of 100,000 watts. The power of the noise is 56 dB down (i.e., –56 dB relative to the broadcast signal). Find
	(a) the S/N ratio as an absolute number
	(b) the S/N ratio in the usual units that we prefer (don’t forget to write the units!)
	(c) the power of the noise (don’t forget to write the units!)
4.	Fill in the gaps in the following table.
		Power ratio		dB
		5,000,000,000:1		+97
		2,000,000,000:1		+93
		1,000,000,000:1		______
		250,000,000:1		______
		80,000,000:1		______
		40,000,000:1		+76
		20,000,000:1		______
		10,000,000:1		+70
		6,000,000:1		______
		3,000,000:1		+65
		1,000,000:1		______
		5000:1		______
		1000:1		______
		250:1		______
		100:1		+20
		10:1		______
		1:1		______
		1:3		–5
		1:8		______
		1:10		–10
		1:50		–17
		1:100		______
		1:100,000		______
		1:200,000		–53
		1:1,000,000		______
5.	Match each vocabulary term to its description. Note that you also must FILL IN THE BLANKS to complete the definitions. Scoring: 2 points per blank, with the worst one counting only 1 point.
	____ steganography		(A) the science of ___________ data within seemingly unrelated data
	____ error detection		(B) the science that fuses ___________ technology with the study of protein folding, ___________ , etc.
	____ bioinformatics		(C) a technology for which ___________ and VIN check digits are examples
	____ error correction		(D) a technology for which the Hamming codes are examples
6.	Compute
(a)	the Levenshtein distance between A and F (hint: convert each hex digit to binary first)
(b)	the Levenshtein distance between FF and D
(c)	43 mod 11

Part III: Educational Exercise (14 pts. total).

On the following pages you will find part of an ASCII table. After you have extracted the bytes from the meaningless boldface text, you can use the ASCII table to decode the true, hidden message. The rule is as follows: For any word containing 7 or more letters, the first letter will be either a vowel or a consonant. If a vowel, write “4”; if a consonant, write “5.” The next word immediately after that word will start with either a letter A through F, in which case you will write that letter, or some other letter of the alphabet. If the initial letter is “O,” write a zero, but otherwise write down the number of letters in the word.

Example: Prestidigitation is the silliest thing I can imagine entering into. The first word that is 7 characters or longer is “prestidigitation,” which starts with a consonant. Write: 5 The next word (“it”) does not begin with O or A-F but is 2 letters long. Write: 2 The next word that is 7 characters or longer is “silliest,” which also begins with a consonant. Write: 5 The next word (“thing”) has 5 letters. Write: 5 The third word of length 7 or greater is “imagine,” which begins with a vowel. Write: 4 The initial letter of the very next word is E. Write: E Since there are no more words of length 7 or greater, we are finished. If you look at all the characters we have written down (52554E in hex), you will see that according to the ASCII table we have spelled out the word RUN. This is not a very efficient way to encode secret messages, but it is extremely hard for an adversary to figure out! I am sure that you can imagine clever ways of hiding data inside bitmaps or MP3 files so that nobody would ever figure out what is going on without gathering vast quantities of sample data. And guess what? The message RUN wasn’t even encrypted. It was merely hidden. (Although ASCII is a code, it is so widely used that nobody could really call it cryptic.)
Byte value (hex)	Character	Byte value (hex)	Character
41	A	53	S
42	B	54	T
43	C	55	U
44	D	56	V
45	E	57	W
46	F	58	X
47	G	59	Y
48	H	5A	Z
49	I	5B	[
4A	J	5C	\
4B	K	5D	]
4C	L	5E	^
4D	M	5F	_
4E	N	60	`
4F	O	61	a
50	P	62	b
51	Q	63	c
52	R	64	d

7.	ASCII is the ___________ ___________ ___________ for ___________ ___________ .
8.	Using the hidden-message protocol described above, find the word that is hidden in the boldface text below. Raise your hand for free hints if you get stuck. (You must first be familiar with the rules described above.)
	Inverted protocol is indisputably found in densely occupied areas or in earlier types of blogs where people write a great number of things.
	Byte value (hex)	Character	Byte value (hex)	Character
	41	A	53	S
	42	B	54	T
	43	C	55	U
	44	D	56	V
	45	E	57	W
	46	F	58	X
	47	G	59	Y
	48	H	5A	Z
	49	I	5B	[
	4A	J	5C	\
	4B	K	5D	]
	4C	L	5E	^
	4D	M	5F	_
	4E	N	60	`
	4F	O	61	a
	50	P	62	b
	51	Q	63	c
	52	R	64	d
Bonus	(1 pt.) Why does every executable DOS and Windows file begin with 4D5A in hex?
	(1 pt.) What character would the VIN code use to indicate the answer to #6(c)? _____