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Digital Image Compression
Published in Edward R. Dougherty, Digital Image Processing Methods, 2020
Subband coding using quadrature mirror filters (QMFs) provides a natural hierarchical structure that is quite similar to the Laplacian pyramid. Unlike the Laplacian pyramid, the number of values stored in the subband pyramid is the same as the number of pixels in the original image. The lowest-firequency subband is a low-pass filtered and subsampled version of the original and is analogous to the base image in the Laplacian pyramid. The other subbands contain bands of higher-frequency components, and they can loosely be interpreted as a more refined representation of the Laplacian residual image information. Reconstruction proceeds by using the lowest-firequency subband as the initial approximation and then successively incorporating the higher-frequency subbands by appropriate upsampling and filtering with QMFs. Due to quantization effects, subband coding typically results in a lossy reconstruction unless a final residual error image is included.
Compression
Published in John Watkinson, The Art of Digital Audio, 2013
Sub-band coding takes advantage of the fact that real sounds do not have uniform spectral energy. The wordlength of PCM audio is based on the dynamic range required and this is generally constant with frequency although any pre-emphasis will affect the situation. When a signal with an uneven spectrum is conveyed by PCM, the whole dynamic range is occupied only by the loudest spectral component, and all the other components are coded with excessive headroom. In its simplest form, sub-band coding works by splitting the audio signal into a number of frequency bands and companding each band according to its own level. Bands in which there is little energy result in small amplitudes which can be transmitted with short wordlength. Thus each band results in variable-length samples, but the sum of all the sample wordlengths is less than that of PCM and so a coding gain can be obtained. Sub-band coding is not restricted to the digital domain; the analog Dolby noise-reduction systems use it extensively.
Compression
Published in John Watkinson, Convergence in Broadcast and Communications Media, 2001
Sub-band coding takes advantage of the fact that real sounds do not have uniform spectral energy. The wordlength of PCM audio is based on the dynamic range required and this is generally constant with frequency although any pre-emphasis will affect the situation. When a signal with an uneven spectrum is conveyed by PCM, the whole dynamic range is occupied only by the loudest spectral component, and all the other components are coded with excessive headroom. In its simplest form, sub-band coding works by splitting the audio signal into a number of frequency bands and companding each band according to its own level. Bands in which there is little energy result in small amplitudes which can be transmitted with short wordlength. Thus each band results in variablelength samples, but the sum of all the sample wordlengths is less than that of PCM and so a coding gain can be obtained. Sub-band coding is not restricted to the digital domain; the analog Dolby noise-reduction systems use it extensively.
An Overview of Digital Audio Steganography
Published in IETE Technical Review, 2020
Hrishikesh Dutta, Rohan Kumar Das, Sukumar Nandi, S. R. Mahadeva Prasanna
In an early work in the field of steganography, data are hidden in audio streams using sub-band coding, LSB coding, and a shift register-based data scrambler [48]. Sub-band coding breaks a signal into a number of bands of different frequencies and encodes each band independently. A scrambler inverts or transforms signals or otherwise employs an encoding technique such that the retrieval of the message becomes difficult for a receiver which is unaware of the encoding technique. The broadband input signal is first separated into 32 equally spanned sub-bands. Then the data to be concealed is embedded into the LSBs of the certain sub-bands which are synthesized into a broadband signal again.