Chapter 5 - Perceptual Coding Flashcards
What is perceptual coding?
Reducing the quantity of data used to represent digital audio, reducing the file size.
Name two approaches to perceptual coding.
- Data compression
- Data reduction
Which one is lossless?
- Data Compression
- Data reduction
Data Compression
What is Data Compression?
Reducing file size in a lossless manner, without removing any data. This is achieved using Entropy/ Huffman coding.
What is a common entropy code?
Morse code.
How is morse code used to reduce file size?
Morse code requires less data than binary.
What is Data Reduction?
Reducing file size by removing data in a manner that is unheard.
How does Data Reduction work?
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.
What is the membrane in the ear that has hair cells on it called?
Basilar membrane
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?
High frequencies are sensed closer to the middle ear (outside). Low frequencies at the far end (Inner).
Define the threshold of hearing.
The minimum level at which the human ear can hear a tone.
Explain Masking.
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.
Masking occurs only if the tones are in the same _______.
Critical band
True or false.
Masking is more effective when a lower frequency tone masks a higher frequency one.
True
What is amplitude/ simultaneous masking?
Masking that takes place when two tones are sounded simultaneously.
What is temporal masking?
Masking that takes place when tones are sounded close in time, but not simultaneously.
Name the two types of Temporal masking.
Pre-masking and post-masking
True or False?
Pre-masking occurs when a tone is masked by another tone that ends before the masked signal begins.
False
Name two ways you can reduce the data rate in a data reduction system.
- decrease the sample rate
- Decrease word length
In perceptual coding, word length reduction is done _______ depending on signal conditions.
Dynamically
_______ and _____________ are used to ensure that the resulting increase in Quantization noise is kept as inaudible as possible.
Masking and the Fletcher Munsen equal loudness contours
What is the result of a signal being encoded multiple times.
Noise will be added everytime
Name the six stages of mp3 encoding.
- An existing PCM audio stream.
- The audio is run through an analyzing filter where the audio is divided into 32 sub-bands.
- Sub-bands are grouped into frames. Encoder determines where masking is happening. This determines which frames can have a reduced bit rate.
- Bit allocation. The encoder determines how many bits to encode each frame with.
- All frames are saved as MP3 file.
- On playback, the sub-band frames are recorded into time-domain sections and joined up to recreate an audio stream.
What is Joint Stereo
Info that is the same between channels is encoded in one channel. Info that is different is encoded in the other.
