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CMX608 参数 Datasheet PDF下载

CMX608图片预览
型号: CMX608
PDF下载: 下载PDF文件 查看货源
内容描述: [Integrated Input and Output Channel Filters]
分类和应用:
文件页数/大小: 70 页 / 3411 K
品牌: CMLMICRO [ CML MICROCIRCUITS ]
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RALCWI Vocoder  
CMX608/CMX618/CMX638  
6.2.  
FEC and LLR Decoding  
The Robust ALCWI 3600 bps Vocoder contains a Forward Error Correction (FEC) Encoder and an FEC  
Decoder, which provide robust performance in a noisy channel. The interleaving of data and an unequal  
error protection approach is used to provide effective protection against errors. A recursive, systematic,  
convolutional encoder is used with a CRC to encode the data and Viterbi decoding with either “hard  
decision” or “soft decision” metrics is used in the FEC Decoder. CRC decisions are used to improve the  
quality of data regeneration in cases where the bit frame is greatly distorted by channel errors. In "soft  
decision" mode, the incoming data is encoded with four bits representing the level of each "hard" bit as a  
LLR (Log Likelihood Ratio), ranging from 0x0 for a '0' to 0xF for a '1'.  
6.3.  
DTX and SID Functions  
To provide DTX functionality, the voice encoder contains a Voice Activity Detector (VAD) and the voice  
decoder contains a Comfort Noise Generator (CNG). The VAD uses several analyzed voice parameters  
(spectral envelope, pitch, energies of the signal’s decomposed components, etc.) to estimate the features  
of the incoming signal and to switch the encoder to the silence or comfort noise transmission (SID) mode,  
in the event of a non-voice signal being detected. If DTX mode is enabled, the voice decoder checks for  
the possibility of an SID frame before the received frame is handled as a full Vocoder data frame. If SID is  
detected, the decoder generates a “comfort noise” in accordance with the transmitted spectral envelope  
and gain parameters.  
6.4.  
Single Tone Transfer  
The Vocoder is designed to encode and decode voice, not continuous tones. If a siren or single tone is  
the only audio input present, it will not be reliably encoded/decoded through the Vocoder. To overcome  
this, the CMX608/CMX618/CMX638 includes a facility that detects the presence of a single tone at the  
audio input and encodes it in a special frame. When a special frame is received, the tone is regenerated.  
This facility is not available on the CMX638 when used in full-duplex mode.  
Single Tone Transfer is enabled in the encoder by setting bit 12 in the VCTRL register ($11) to '1' and in  
the decoder by setting bits 7 and 8 in the VCTRL register ($11) to '1'. This feature can be independently  
enabled/disabled in the encoder or decoder. This is necessary as, when enabled, the decoder detection  
can false with voice.  
6.5.  
Slip Management  
Slip, or rather time-slip, is the effect that occurs when two asynchronous real-time systems must  
communicate with each other. Two crystal controlled systems with the same value crystal will run at  
basically the same speed, but there will be a slight difference due to inaccuracies. The accuracy of a  
crystal depends on its quality and therefore its cost. Popular crystals have an accuracy better than 100  
parts per million (ppm).  
Taking a crystal accuracy of 100 parts per million as an example; this is the same as 1 part in 10,000.  
Given we are using a sample rate of 8000 samples per second, to make calculations easy, we can  
assume then that the accuracy of the CODEC is going to be approximately +/- 1 sample per second. With  
two CODECs in the system, the worst case accuracy is therefore +/- 2 samples per second.  
There are a number of ways of controlling slip:  
Eliminate it altogether  
Slip can be eliminated by deriving a common clock over the whole system and using PLL techniques for  
supplying local clocks. In a digital radio system operating with fixed time slots for transmission, a  
reference clock could be derived from the slot timing, this could then be used to control a PLL, which in  
turn generates the master clock for the CODEC (this could be the external CODEC for a CMX608, or the  
Vocoder itself - in a CMX618/CMX638 using the internal CODEC).  
2014 CML Microsystems Plc  
57  
D/608_18_38/11  
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