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| 型號： | μSAP70372-B04 |
| 廠商： | NEC Corp. |
| 英文描述： | 8 Bit Single Chip Microcontrollers(8位單片微控制器) |
| 中文描述： | 8位單片機(jī)微控制器（8位單片微控制器） |
| 文件頁數(shù)： | 16/51頁 |
| 文件大?。?/td> | 132K |
| 代理商： | ΜSAP70372-B04 |
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16
CHAPTER 1 GENERAL
The interface of ADPCM is based on the 64-kbps PCM code stipulated by ITU-T Recommendation G.711. This
is because the 64-kbps PCM of Recommendation G.721 has been widely used as an international standard telephone
quality coding method, and because ADPCM was developed as a standard to improve the compression rate.
The 64-kbps PCM of ITU-T Recommendation G.711 limits the bandwidth to 0.3 to 3.4 kHz by using a bandpass
filter and codes analog speech signals sampled at 8 kHz into 8-bit digital signals by means of non-linear quantization.
The 64-kbps PCM has two modes:
μ
-law mode (used in Japan and North America) and A-law mode (used in Europe).
ADPCM is a standard for further compressing the speech codes compressed by means of 64-kbps PCM.
Figure 1-2. Concept of ADPCM
PCM code
64 kbps : 8 kHz
×
8 bits
Compression
Expansion
ADPCM code
32 kbps : 8 kHz
×
4 bits
16 kbps : 8 kHz
×
2 bits
The compression/expansion processing of ADPCM is described next.
The compression processing of ADPCM is performed by a PCM-to-linear conversion block and an ADPCM
compression block, as shown in Figure 1-3.
The expansion processing of ADPCM is performed by an ADPCM expansion block, linear-to-PCM conversion
block, and synchronous coding correction block, as shown in Figure 1-3. The expansion processing prevents
degradation of the sound quality by repeatedly compressing and expanding speech data, by using the synchronous
coding correction block, when incorporated to an existing digital (64-kbps PCM) communication network.
Figure 1-3. Organization of ADPCM Processing
Compression processing
14 bits
PCM code
ADPCM code
8 bits
PCM-to-linear
conversion block
ADPCM
compression block
Internal variables
Synchronous coding
correction block
8 bits
8 bits
16 bits
Linear-to-PCM
conversion block
ADPCM
expansion block
Expansion processing
32 kbps : 4 bits
16 kbps : 2 bits
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