B1P3 Flashcards
What is perceptual redundancy
Perceptual redundancy refers to the information contained within an audio or visual signal that can be removed without affecting the recipient’s experience of the signal.
What is coding efficiency
Minimising the bit rate for a prescribed video quality; alternatively, maximising the video quality for a defined bit rate
What is distortion
An error that reflects how much the reconstructed version of the source signal differs from the original source
What is meant by bit rate
In digital communications, the number of bits passing a given point in the network in a given amount of time. This is usually one second, so bit rate is normally expressed as the number of bits per second (bps or bit s-1).
What three processing blocks does Pulse code modulation consist of
sampling, quantising and encoding
What is sampling
The process of converting a continuous analogue time signal into a discrete time representation
The signal must be sampled using sampling theorem
What is meant by quantising
The combined process of sampling and quantising is performed by an analogue to digital converter ADC. The number of bits, n, it uses in its quantised representations of the samples. A 3-bit ADC provides eight (23) discrete levels represented by three bits: 000, 001, 010 etc.
What is quantisation noise
the difference between the original and digital signal
What is encoding
The formal process of arranging a sequence of binary data (can also be symbols or characters) into an efficient format for either transmission or storage.
what is Differential PCM (DPCM)
Differential pulse-code modulation DPCM is a variant of PCM
Coverts a source analogue signal into a digital representation but able to achieve lower bit rate by including sample prediction in its coding
Send the receiver the first sample (220) followed by the differences:
So sending 220 +1 +4
Instead of 220 221 225
In predictive coding, both the encoder and decoder predict the next sample
What is the objective of JPEG and MPEG coding
removal of as much statistical and perceptual redundancy as possible, to achieve the highest compression (lowest bit rate) for a given picture quality
What are the 2 stages of JPEG and MPEG compression
Spatial compression – exploits the fact that many real pictures have considerable similarity between neighbouring areas of an image. This is called intra-frame compression
Temporal compression – exploits the fact that in most sequences, very little changes between consecutive frames. This is called inter-frame compression
Is JPEG coding lossy of lossless
De facto lossy compression standard
How does JPEG pre processing work
Image is first divided into a number of equally sized macroblocks to which DCT is then applied. Generally uses 8 x 8 pixel macroblocks
What is DCT
Converts spatial pixel values (measure of brightness and darkness of each pixel) into alternative mathematical representations
What is the d.c coefficient
top left component, and represents the average greyscale value of macroblock
How does JPEG2000 compare to JPEG
Offers bitsteam scalability – which means it can change its representation to satisfy the requirements of an application or receiver
Instead of DCT, it usesd DWT discrete wavelet transform
Greater interactive flexibility than jpeg
What are the 3 MPEG picture types
I-frame (intra-frame)
P-frame(prediction)
B-frame(bidirectional prediction)
What are the features of a I-frame (intra-frame)
- JPEG coded
- Used a reference for random access with MPEG bit streams
- Coded independently without reference to other picture types
- Do not use motion vectors
What are the features of a P-frame (prediction)
• Use motion prediction and compensation to achieve higher compression than I frames
What are the features of a B-frame (bidirectional predicition)
- Interpolated frames between I and P frames in both forward and backward direction
- ‘Fill in; missing frames
- Highest compression
What is perceptual masking
when the composition of sound can alter the ears ability to perceive specific frequencies at specific amplitudes
What is noise masking
combination of frequency masking and temporal masking
What is meant by frequency masking
A perceptual effect where loud sounds mask lower-amplitude sounds and render them inaudible, especially for neighbouring frequencies. At low amplitudes this masking is contained within a relatively narrow band around the masking signal, but at higher amplitudes the masking effect is extended.
