多媒体英文样卷

a) Describe Analysis-by-Synthesis as used in speech coding. Briefly explain what advantage it provides?1.Fixed length binary codes take no account of the symbol statistics and as such achieve no compression. The length of the binary code is the same for each symbol and is determined from:n > log 2Nwhere : N is the number of unique symbols to be coded2.Variable length codes exploit the statistics of the source values to achieve compression by reducing the average number of bits per codewordb) With the aid of a block diagram describe a source-filter model of speech production and explain how each block of the model relates to the physics of speech production.S:The mixing decision choosesperiodic excitation during voicedspeech and random excitation duringunvoiced speech. The excitation isinput to a filter that models the vocaltract, the output of which issynthesised speech.QUESTION 2A – (10 marks)a) Compare Waveform and Parametric speech coders in terms of their operation and bit rate. In what situations would a parametric coder be preferable to a waveform coder? Briefly explain your answer.S:Waveform coders attempt to reproduce the speech waveform rather than parameters of a model. Parametric coders first derive a model of the speech signal and then quantise and transmit the model parameters. Waveform coders generally operate at higher bit rates than parametric coders. Hence, for high band width channels, waveform coders would be preferred as they result in higher speech quality.b) What type of excitation quantisation scheme is employed by the ITU-T G.721 standard? Draw the block diagram illustrating how this is implemented. Comment on the bit rate required.S:1. The G.721 speech coderoperates at 32 kbps. G.721 usesbackward adaptive linearprediction to generate an errorsignal . The error signal is thenquantised with a backwardadaptive quantiser . The processis then reversed in the receiver2.G721 or Adaptive Differential Pulse Code Modulation(ADPCM).This employs direct quantisation of each excitation samplea) What are the key stages involved in the JPEG encoder? Illustrate using a block diagram.S:The key stages are:a. Perform the DCT on a block of 8-by-8 pixelsb. Perform a zig-zag scan of the DCT coefficientsc. Quantise the DCT coefficients using uniform quantisation and the quantisation table.d. Perform entropy coding of the resulting quantised coefficients.For colour, JPEG operates separately on each componentb) Compare the use of the DCT with the DWT for image coding. What is the advantage of the DWT based image compression as used in JPEG2000?S：The DCT is applied to blocks of pixels whereas the DWT is applied to the entire image. The advantage of the DWT approach is that it analyses the entire image rather than separately analysing each block. This leads to increased compression rates. QUESTION 2B (10 marks)a) I wish to quantise a sequence of numbers that are highly correlated. What is the advantage of differential quantisation of these numbers compared with direct quantisation of each number? Explain using by comparing the bits required to compress the sequence: 21, 22, 25, 23, where each number in this sequence is represented by 5 bits.S：Differential quantisation will require less bits in total. For the example sequence, the total number of bits represent the 4 numbers is 4*5=20 bits. The differential sequence, assuming we need to quantise the first sample using all 4 bits, is: 21,(22-21), (25-22), (23-25) = 21, 1, 3, -2. Assuming that the differential values only range from -3 to +3, this is a total of 7 values, which can be represented with 3 bits. Hence, the total bit rate of the differential encoded sequence is 5+3*3=14 bits. Hence, this is 6 bits less than the result needed without differential quantisation.b) In regards to compression, why do you aim to have more B frames than I frames? What is the tradeoff for increasing the number of B frames compared to I and P frames?More B frames mean you get higher compression rates. The tradeoff is that that more B frames you have, the longer the sequence of differentially encoded frames becomes. This means that if there is an error in a transmitted B frame, it then affects a long sequence of video frames. It also means that if you want to skip to one section in the video, you must decode a long sequence of video frames, which might be a problem on a mobile device with limited processing capabilities.。