Chapter 5 - Perceptual Coding

Question 1

What is perceptual coding?

Answer 1

Reducing the quantity of data used to represent digital audio, reducing the file size.

Question 2

Name two approaches to perceptual coding.

Answer 2

• Data compression

- Data reduction

Question 3

Which one is lossless?

• Data Compression
• Data reduction

Answer 3

Data Compression

Question 4

What is Data Compression?

Answer 4

Reducing file size in a lossless manner, without removing any data. This is achieved using Entropy/ Huffman coding.

Question 5

What is a common entropy code?

Answer 5

Morse code.

Question 6

How is morse code used to reduce file size?

Answer 6

Morse code requires less data than binary.

Question 7

What is Data Reduction?

Answer 7

Reducing file size by removing data in a manner that is unheard.

Question 8

How does Data Reduction work?

Answer 8

Data is removed according to how we perceive sound. In order to identify what data can be safely removed, a perceptual coder must compare the audio against a psychoacoustic model of our hearing system.

Question 9

What is the membrane in the ear that has hair cells on it called?

Answer 9

Basilar membrane

Question 10

In the ear, the cells on the membrane respond to different frequencies depending on where they are located along the membrane. Where are high and low frequencies sensed?

Answer 10

High frequencies are sensed closer to the middle ear (outside). Low frequencies at the far end (Inner).

Question 11

Define the threshold of hearing.

Answer 11

The minimum level at which the human ear can hear a tone.

Question 12

Explain Masking.

Answer 12

When a tone sounds, there is a theoretical lifting of the minimum audio threshold in the local frequency range around the tone. if there is another tone nearby in frequency and slightly softer, it could be masked by the louder tone.

Question 13

Masking occurs only if the tones are in the same _______.

Answer 13

Critical band

Question 14

True or false.

Masking is more effective when a lower frequency tone masks a higher frequency one.

Answer 14

True

Question 15

What is amplitude/ simultaneous masking?

Answer 15

Masking that takes place when two tones are sounded simultaneously.

Question 16

What is temporal masking?

Answer 16

Masking that takes place when tones are sounded close in time, but not simultaneously.

Question 17

Name the two types of Temporal masking.

Answer 17

Pre-masking and post-masking

Question 18

True or False?

Pre-masking occurs when a tone is masked by another tone that ends before the masked signal begins.

Answer 18

False

Question 19

Name two ways you can reduce the data rate in a data reduction system.

Answer 19

• decrease the sample rate

- Decrease word length

Question 20

In perceptual coding, word length reduction is done _______ depending on signal conditions.

Answer 20

Dynamically

Question 21

_______ and _____________ are used to ensure that the resulting increase in Quantization noise is kept as inaudible as possible.

Answer 21

Masking and the Fletcher Munsen equal loudness contours

Question 22

What is the result of a signal being encoded multiple times.

Answer 22

Noise will be added everytime

Question 23

Name the six stages of mp3 encoding.

Answer 23

1. An existing PCM audio stream.
2. The audio is run through an analyzing filter where the audio is divided into 32 sub-bands.
3. Sub-bands are grouped into frames. Encoder determines where masking is happening. This determines which frames can have a reduced bit rate.
4. Bit allocation. The encoder determines how many bits to encode each frame with.
5. All frames are saved as MP3 file.
6. On playback, the sub-band frames are recorded into time-domain sections and joined up to recreate an audio stream.

Question 24

What is Joint Stereo

Answer 24

Info that is the same between channels is encoded in one channel. Info that is different is encoded in the other.