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An introduction to audio/video compression Prepared by :: Bhatt shivani (130450112002)
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Introduction Review digital video. Need for compression. Lossy and lossless compression. How to apply techniques to still video (jpeg).
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Digital Video Samples represent pixel values of a video frame. Typically 480x640x3=1M (approx) per frame Sample size 8-10 bits typically.
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Video datarates With 8 bits samples at 25 frames per second we have a datarate of 200 Mbits per second. Broadcast video (10 bit and includes audio) 270 Mbits per second.
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Lossless Compression Achieve datarate reduction without discarding information. Run Length Coding – Eg: 5 5 5 5 5 5 – May be coded 5 6
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Lossless Compression Entropy (Huffman) Coding Assign shorter codes to the more probable values. Possible when there is statistical bias in image data.
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Lossy Compression Achieving datarate reduction by discarding information. Missing out samples from data stream/reducing sample size generally noticeable.
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Example DCT Basis Pictures Picture broken into 8x8 pixel blocks (typically). Any 8x8 pixel (picture) block made by adding different proportions of basis pictures. So store the proportions.
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DCT basis functions (Basis pictures).
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Original Picture
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Using one (dc) basis picture
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Using 6 basis pictures
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Video compression Most 8 x 8 blocks do not contain high frequency components. Code high frequency components using less bits (introduces noise).
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Video compression We are left with a sequence of values. Zig-Zag scan. Many values of zero towards the bottom right hand corner. Use RLE to reduce the datarate further.
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Thank you …
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