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首页G.726 ADPCM标准：40/32/24/16kbps自适应差分脉冲编码调制技术文档

G.726 ADPCM标准：40/32/24/16kbps自适应差分脉冲编码调制技术文档

G.726

ADPCM

CCITT

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G.726是国际电信联盟（ITU）下属的国际电信与电话咨询委员会（CCITT）提出的一项标准，专注于数字传输系统的通用方面，特别是针对终端设备的适应性差分脉冲编码调制（ADPCM）技术。这份名为"Recommendation G.726"的文档于1990年在日内瓦发布，特别关注的是40 kbit/s、32 kbit/s、24 kbit/s和16 kbit/s的ADPCM编解码系统。 ADPCM是一种高效的数据压缩编码方法，它通过将信号进行差分量化和编码，减少了数据的冗余，从而在保持音频质量的前提下降低了带宽需求。G.726标准旨在提供一套统一的规范，使得不同速率的ADPCM信号可以在全球范围内实现兼容性和互操作性，这对于当时的长途电话通信尤其重要，因为这可以减少网络的复杂性和成本。在G.726标准中，适应性体现在编码器能够根据信号特性动态调整量化阶数，以优化编码效率。这种灵活性使得该标准适用于各种不同的语音质量需求和带宽条件。推荐建议了详细的编码和解码算法，包括预测编码、量化、编码以及解码流程，确保了音频信号在经过压缩和传输后仍能保持良好的可听性。 G.726标准的制定过程是由CCITT的第十五研究组负责，经过了多次讨论和审议，最终在1990年12月14日得到了CCITT全体会员的认可。CCITT决议号2（Melbourne, 1988）为这一批准提供了框架，强调了标准化的重要性，以便在全球范围内推动电信技术的统一和高效应用。作为一项国际标准，G.726 ADPCM在语音通信领域扮演了关键角色，被广泛应用于电话交换系统、VoIP服务以及早期的移动通信设备中。随着时间的推移，尽管有了更先进的编码技术，G.726仍被许多遗留系统支持，体现了其持久的技术价值。

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8 Recommendation G.726

2.6 Adaptation speed control

The controlling parameter a

(k) can assume values in the range [0, 1]. It tends towards unity for speech signals

and towards zero for voiceband data signals. It is derived from a measure of the rate-of-change of the difference signal

values.

Two measures of the average magnitude of I

(k) are computed:

(k) = (1 − 2

−5

) d

(k − 1)

−5

F[I (k)] (2-5)

and

(k) = (1 − 2

−7

) d

(k − 1) + 2

−7

F[I (k)] (2-6)

For 40 kbit/s ADPCM, F[I

(k)] is defined by:

For 32 kbit/s ADPCM, F[I

(k)] is defined by:

For 24 kbit/s ADPCM, F[I

(k)] is defined by:

| I(k) |

15 14 13 12 11 10 9 8

F[I(k)] 6 6 5 4 3 2 1 1

| I(k) |

76543210

F[I(k)] 1 1 1 0 0 0 0 0

| I(k) |

76543210

F[I(k)] 7 3 1 1 1 0 0 0

| I(k) |

3 210

F[I(k)] 7 2 1 0

Recommendation G.726 9

For 16 kbit/s ADPCM, F[I

(k)] is defined by:

Thus d

(k) is a relatively short term average of F[I (k)] and d

(k) is a relatively long term average of F[I (k)].

Using these two averages, the variable a

(k) is defined:

(k) =











(1 − 2

−4

(k − 1) + 2

−3

, if | d

(k)

−

(k) | ≥ 2

−3

(k)

(1 − 2

−4

(k − 1) + 2

−3

, if y (k) < 3

(1 − 2

−4

(k − 1) + 2

−3

, if t

(k) = 1

1, if t

(k) = 1

(1 − 2

−4

(k − 1), otherwise

(2-7)

Thus, a

(k) tends towards the value 2 if the difference between d

(k) and d

(k) is large (average magnitude

of I

(k) changing) and a

(k) tends towards the value 0 if the difference is small (average magnitude of I (k) relatively

constant). a

(k) also tends towards 2 for idle channel (indicated by y (k) < 3) or partial band signals (indicated by t

(k) =

1 as described in § 2.8). Note that a

(k) is set to 1 upon detection of a partial band signal transition (indicated by t

(k) =

1, see § 2.8).

(k – 1) is then limited to yield a

(k) used in Equation (2-4) above:

(k) =







1, a

(k − 1) > 1

(k − 1), a

(k − 1) ≤ 1.

(2-8)

This asymmetrical limiting has the effect of delaying the start of a fast to slow state transition until the absolute

value of I

(k) remains constant for some time. This tends to eliminate premature transitions for pulsed input signals such

as switched carrier voiceband data.

2.7 Adaptive predictor and reconstructed signal calculator

The primary function of the adaptive predictor is to compute the signal estimate s

(k) from the quantized

difference signal d

(k). Two adaptive predictor structures are used, a sixth order section that models zeros and a second

order section that models poles in the input signal. This dual structure effectively caters for the variety of input signals

which might be encountered.

| I(k) |

F[I(k)] 7 0

10 Recommendation G.726

The signal estimate is computed by:

(k) =

∑

i=1

(k − 1) s

(k − i) + s

(k), (2-9)

where

(k) =

∑

i=1

(k − 1) d

(k − i),

and the reconstructed signal is defined as

(k − i) = s

(k − i) + d

(k − i).

Both sets of predictor coefficients are updated using a simplified gradient algorithm:

for the second order predictor:

(k) = (1 − 2

−8

(k − 1) + (3 · 2

−8

) sgn [p (k)] sgn [p (k − 1)], (2-10)

(k) = (1 − 2

−7

(k − 1) + 2

−7

{sgn [p (k)] sgn [p (k − 2)]

− f [a

(k − 1)] sgn [p (k)] sgn [p (k − 1)]},

(2-11)

where

(k) = d

(k) + s

(k),

f (a

) =







, | a

| ≤ 2

−1

2 sgn (a

), | a

| > 2

−1

and sgn [0] = 1, except sgn [p

(k – i] is defined to be 0 only if p (k – i) = 0 and i = 0;

with the stability constraints:

| a

(k) | ≤ 0.75 and | a

(k) | ≤ 1 − 2

−4

− a

(k).

If t

(k) = 1 ( see § 2.8), then a

(k) = a

(k) = 0.

For the sixth order predictor:

(k) = (1 − 2

−8

(k − 1) + 2

−7

sgn [d

(k)] sgn [d

(k − i)], (2-12A)

for i = 1, 2, . . ., 6.

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