Hybrid subband coder/decoder method and apparatus
First Claim
1. In a subband coder including means for processing an input electrical signal having a frequency band of signal components by separating it into plural frequency subbands of signal components which subband are separately encoded into corresponding binary-valued digital signals that are subsequently combined for transmission over a common digital communication channel, the improvement comprising:
- hybrid encoding means which encodes the signal components of at least one of said subbands in accordance with a backward adaptive encoding algorithm and which encodes the signal components of at least one other of said subbands in accordance with a forward adaptive encoding algorithm.
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Abstract
The subband channels of a subband coder/decoder utilize different encoding/decoding algorithms so as to maximize overall signal transmission quality using a digital signal processor having limited digital memory capability. In the exemplary embodiment, a digitized 180-2900 Hz voice band signal is divided into four octave-spaced subbands. The highest frequency subband (1450-2900 Hz) is encoded/decoded using a block companded pulse code modulation (BCPCM) algorithm while each of the lower frequency subbands is encoded/decoded using an adaptive pulse code modulation (APCM) or an adaptive differential pulse code modulation (ADPCM) algorithm. The resulting "hybrid" vocoder tends to maximize the quality of transmitted voice signals while yet permitting implementation within constrained digitial memory capacity and/or constrained transmission channel bandwidth.
92 Citations
18 Claims
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1. In a subband coder including means for processing an input electrical signal having a frequency band of signal components by separating it into plural frequency subbands of signal components which subband are separately encoded into corresponding binary-valued digital signals that are subsequently combined for transmission over a common digital communication channel, the improvement comprising:
hybrid encoding means which encodes the signal components of at least one of said subbands in accordance with a backward adaptive encoding algorithm and which encodes the signal components of at least one other of said subbands in accordance with a forward adaptive encoding algorithm. - View Dependent Claims (2, 3)
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4. A hybrid subband electrical signal processor comprising:
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filter means having an input for accepting digital electrical signals representing a band of frequency components and having plural outputs each producing digital subband signals representing signal components of the input electrical signal within different frequency subbands; first digital encoder means for receiving the digital subband signals from one of said outputs and for encoding such received subband signals in accordance with a backward adaptive encoding algorithm which determines a current quantization step size based only on past signals using a first capacity memory to produce a first encoded digital signals; second digital encoder means for receiving the digital subband signals from anohter of said outputs and for encoding such received subband signals in accordance with a forward adaptive encoding algorithm which determines quantization step size based on future as well as past signals using a second capacity memory, larger than said first capacity memory, to produce second encoded digital signals; and multiplex means for combining said first and second encoded digital signals into a common string of digital signals for transmission over a digital signal communication channel. - View Dependent Claims (5, 6, 7, 8, 9)
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10. In a subband signal processing method for processing an input electrical signal having a frequency band of signal components by separating it into plural frequency subbands of signal components which subbands are separately encoded into corresponding binary-valued digital signals that are subsequently combined for transmission over a common digital communication channel, the improvement comprising:
encoding the signal components at least one of said subbands in accordance with a backward adaptive encoding algorithm and encoding the signal components of at least one other of said subbands in accordance with a forward adaptive encoding algorithm. - View Dependent Claims (11, 12)
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13. A hybrid subband electrical signal processing method comprising:
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accepting digital electrical signals representing a band of frequency components and producing plural outputs each having digital subband signals representing signal components of the input electrical signal within different frequency subbands; receiving the digital subband signals from one of said outputs and encoding such received subband signals in accordance with a backward adaptive encoding algorithm to produce first encoded digital signals; receiving the digital subband signals from another of said outputs and encoding such received subband signals in accordance with a forward adaptive encoding algorithm to produce second encoded digital signals; and combining said first and second encoded digital signals into a common string of digital signals for transmission over a digital signal communication channel. - View Dependent Claims (14, 15, 16, 17, 18)
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Specification