ADPCM Encoder Principles in C#

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223 ADPCM Encoder Principles
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Figure 2-9 is a block schematic of the encoder For each variable described, k is the sampling index and samples are taken at 125 m intervals Input PCM Format Conversion This block converts the input signal s(k) from A-law or m-law PCM to a uniform PCM signal sI(k)
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Figure 2-9 Encoder block schematic (ITU-T Recommendation G726)
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ADPCM output
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Reconstructed signal calculator
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sr(k)
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Input PCM Difference format signal s(k) conversion sl(k) computation d(k) Inverse adaptive quantizer Adaptive predictor
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Adaptive quantizer
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se(k)
a2(k)
Quantizer y(k) scale factor adaptation a1(k) yl(k)
Adaptation speed control
tr(k) td(k)
Tone and transition detector
2
Difference Signal Computation This block calculates the difference signal d(k) from the uniform PCM signal sI(k) and the signal estimate se(k): d1k2 SI 1k2 se 1k2
Adaptive Quantizer A 31-, 15-, 7-, or 4-level nonuniform adaptive quantizer is used to quantize the difference signal d(k) for operating at 40, 32, 24, or 16 Kbps, respectively Prior to quantization, d(k) is converted to a base 2 logarithmic representation and scaled by y(k), which is computed by the scale factor adaptation block The normalized input/output characteristic (infinite precision values) of the quantizer is given in tables contained in the specification, one for each of the various operations (40, 32, 24, or 16 Kbps)
Operation at 40 Kbps In this mode, five binary digits are used to specify
the quantized level representing d(k) (four for the magnitude and one for the sign) The 5-bit quantizer output I(k) forms the 40-Kbps output signal; I(k) takes on one of 31 nonzero values I(k) is also fed to the inverse adaptive quantizer, the adaptation speed control, and the quantizer scale factor adaptation blocks that operate on a 5-bit I(k) having one of 32 possible values I(k) 00000 is a legitimate input to these blocks when used in the decoder due to transmission errors
Operation at 32 Kbps In this mode, four binary digits are used to specify
the quantized level representing d(k) (three for the magnitude and one for the sign) The 4-bit quantizer output I(k) forms the 32-Kbps output signal It is also fed to the inverse adaptive quantizer, the adaptation speed control, and the quantizer scale factor adaptation blocks I(k) 0000 is a legitimate input to these blocks when used in the decoder due to transmission errors
Operation at 24 Kbps Three binary digits are used to specify the quantizer level representing d(k) (two for the magnitude and one for the sign) The 3-bit quantizer output I(k) forms the 24-Kbps output signal, where I(k) takes on one of several nonzero values I(k) is also fed to the inverse adaptive quantizer, the adaptation speed control, and the quantizer scale factor adaptation blocks, each of which is modified to operate on a 3-bit I(k) having any of the eight possible values I(k) 000 is a legitimate input to these blocks when used in the decoder due to transmission errors Operation at 16 Kbps In this mode, two binary digits are used to specify
the quantized level representing d(k) (one for the magnitude and one for the sign) The 2-bit quantizer output I(k) forms the 16-Kbps output signal
Technologies for Packet-Based Voice Applications
It is also fed to the inverse adaptive quantizer, the adaptation speed control, and the quantizer scale factor adaptation blocks Unlike the quantizers for operation at 40, 32, and 24 Kbps, the quantizer for operation at 16 Kbps is an even-level (four-level) quantizer The even-level quantizer for the 16-Kbps ADPCM has been selected because of its superior performance over a corresponding odd-level (three-level) quantizer Inverse Adaptive Quantizer A quantized version d(k) of the difference signal is produced by scaling, using d(k), selecting specific values from the normalized quantizing characteristic given in the specification tables, and transforming the result from the logarithmic domain Quantizer Scale Factor Adaptation This block computes y(k), the scaling factor for the quantizer and the inverse quantizer The inputs are the 5-bit, 4-bit, 3-bit, and 2-bit quantizer output I(k), and the adaptation speed control parameter a1(k) The basic principle used in scaling the quantizer is bimodal adaptation, which is
Fast for signals (such as speech) that produce difference signals with large fluctuations Slow for signals (such as voiceband data and tones) that produce difference signals with small fluctuations
The speed of adaptation is controlled by a combination of fast and slow scale factors The fast (unlocked) scale factor yu(k) is recursively computed in the base 2 logarithmic domain from the resultant logarithmic scale factor y(k) (u stands for unlocked): yu 1k2 11 2
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