Abstract: This article in view of the present IP telephone and the network state of play, proposed that the IP telephone plan which unified for the core software and hardware and introduced the speech coding basic principle by the DSP technology; According to the IP telephone’s characteristic, determined that takes the IP telephone’s encoding algorithm by the G .728 code standards.
Key word: IP telephone; DSP; Speech coding
Introduction
The traditional telephone network is by the circuit switching way transmission voice signal, it needs the basic band width is 64kbit/s. Statistics indicated that in the normal call situation, probably only then 40% time to have a time, other timing circuit is occupies spatially, the network band width use factor is not high. Along with computer technology’s unceasing development, the internet (Internet) unceasing consummation, becomes the most important mailing address particularly based on packet switching’s data communication. But must in transmit the pronunciation based on the IP grouping network, must carry on the special handling to the simulation voice signal, after causing processing the signal may suit in faces in the connectionless grouping network to transmit, this groups the pronunciation technology. This article introduces is one kind based on the TMS320VC5409 IP telephone design.
G..728 code standard
The speech coding technology is one of IP telephone’s core technologies, code quality quality direct relation IP telephone’s connection quality.
The G .728 standard’s speech coding algorithms are the 16kbit/s sounder code standards, uses the low latency codebook drive linear prediction (LD-CELP) technology. What linear predictor use is the feedback backward auto-adapted technology, the predictor coefficient is carries on the renewal according to the previous pronunciation quantized data, therefore the algorithm latency is short, is 0. 625ms, is equal in 5 sampling point time, this is also the G .728 frame size time. Because use feedback auto-adapted technology, therefore the predictor coefficient cannot transmit, what needs to transmit is only the drive signal quantification value, is also the codebook index value. The G .728 standard’s speech coding algorithm’s codebook altogether has 1024 vectors, the index must occupy 10 bits, therefore its bit rate is 10/0.625=16kbit/s.
The G .728 standard’s speech coding’s main feature includes:
* the algorithm latency is short, is only 0. 625ms;
* a group coding delay is smaller than 2ms;
* the transmission bit rate is 16kbit/s;
the *MOS value is 4. 173, has achieved the two-way communication quality.
Because the G .728 standard’s speech coding algorithm’s latency is short, the speech transmission bit rate may satisfy the IP telephone’s application request, therefore we select G. 728 standard’s speech coding algorithms take the IP telephone’s encoding algorithm.
Hardware system design
System’s leading role uses the DSP high speed data handling ability fully, reduces computer CPU the burden; The pronunciation input and the output system also alone separate, like this may well and DSP carries on the data transmission, simplifies the nonessential middle link, reduces the latency. Finally, through the high speed PCI main line, data transfer for computer. System’s overall diagram as shown in Figure 1, the various modules’ concrete function see Table 1 to show.
Figure 1 system diagram
DSP and FLASH correspondence
Because the TMS320VC5409 I/0 connection voltage is 3. 3V, but the AM29F101B connection voltage is 5V, therefore needs to carry on the voltage in the connection part to transform, and AM29F101B selects patches or strips of land as worth saving for seed the signal () and the output enables the signal () to need the address decoding. These work complete by piece of complex programmable logical component (CPLD).
Because the AM29F101B connection speed is slow, therefore between TMS320VC5409 and the AM29F101B connection must insert the software waiting status, must insert specifically the software waiting status number may obtain by the data book computation or is when the assignment of aircraft by the experiment obtains. Between TMS320VC5409 and AM29F101B interface circuit as shown in Figure 2.
Figure 2 DSP and FLASH interface circuit
Between DSP and ADC, DAC correspondence
This system selects the G.728 standard’s speech coding algorithm needs 8kHz the sampling speed, therefore here we are the highest sampling rate or the switching time are not lower than 8kHz to ADC and the DAC request.
According to voice signal’s characteristic, we select TI Corporation’s TLC32044 chip, this was one piece integrated ADC and the DAC function chip. Its highest transformed speed is 19.2kHz, the transformation figure is 14, the input voltage belt scope adjustable, has the standard synchronized serial port, but also has the input filter and the output restructuring filter, like this may omit analog filter’s design. TMS320VC5409 and TLC32044 interface circuit as shown in Figure 3.
Figure 3 DSP and DAC, ADC interface circuit
Figure 4 DSP and twin port RAM interface circuit
Figure 5 PC19025 twin port RAM interface circuit
DSP and between twin port RAM correspondence
In order to manifest the PCI main line speed quick merit, we select speed quick twin port RAM CY7C133-25, the maximum transmission speed is 25ns. Twin port RAM in the DSP data space’s address mapping is 8000H-87FFH.
What here needs to stress is the twin port RAM BUSY signal. We do not use this signal, because we carry on the operation separately to the twin port RAM different part, therefore has avoided any conflict which possibly occurs, therefore has omitted the BUSY signal, the BUSY signal is hanging. Electric circuit’s voltage transformation and the address decoding complete similarly by CPLD.
PCI9052 and between twin port RAM correspondence
The DSP duty is completes the pronunciation the code and the decoding, then again carries on the data exchange through the PCI main line and the computer. Here we have used PCI connection chip PCI9052. Therefore, the question turned between DSP and the PCI9052 correspondence. Between DSP and PCI9052 (capacity for 2k×16bit) carries on the data exchange with piece of twin port RAM.
Because PCI9052 has 5 local address spaces and 4 partial equipment selects patches or strips of land as worth saving for seed the signal, therefore the segment becomes quite simple, we only need twin port RAM to map a local address space, then use selects patches or strips of land as worth saving for seed the signal to connect twin port RAM, finally (R/W) and the correspondence connects the PCI9052 read-write signal twin port RAM the R/W sum. This has omitted periphery logic circuits and so on address decoding.
Software system design
The IP telephone system must realize the algorithm is the speech coding algorithm, but must complete with computer’s data exchange.
The software part’s leading role is to the speech coding signal which needs to transmit carries on the pack with the embedded operating system’s in TCP/IP agreement, transmits again through computer’s on network card the data through Internet the receiving end, and the TCP/IP package which receives returns to original state into the original speech coding signal, finally carries on the pronunciation decoding through the PCI main line transmission to DSP.
Conclusion
The practice proved that this system design plan’s principal advantage is not high to the computer hardware’s request, the processing speed is quick, the pronunciation input-output system integrates alone, signal-to-noise ratio (SNR) is high, has suitable being suitable with the promoted value.
Reference:
1. Extends brilliant, Lai Pingzhang, IP telephone technology and application, people’s posts and telecommunications publishing house, 1999
2. Wang Bingxi, speech coding, Xidian University Publishing house, 2002
3. Broom corn millet jade, IP network telephone technology, people’s posts and telecommunications publishing house, 2000
4. The forest space, Yang Xia, Zhang Guangzhao, the IP telephone system pronunciation terminal’s design and realizes, telecommunication technology, 2000 .6
