Abstract: The purpose in this study is constructing digital filter with application of modern microprocessor systems, learning methods of automatic projecting and perfecting skills of programming in language of assembler. In this study, the microprocessor of firm MICROCHIP PIC16C711 is used which has possibilities of analog signal’s processing. DAC is not a part of the microcontroller and it is carried out separately, its entrances are connected to port B. The program written in language of assembler is universal for all microcontrollers of MICROCHIP having a possibility for analog signal’s processing. It is necessary to configure ports and to consider specification GPR (General Purpose Register) and special registers according to the documentation on demanded model.

INTRODUCTION

Digital signal processing is an arithmetical processing in real-time sequences values of signal amplitude (Concha et al., 2003) defined through equal temporary gaps. Examples of digital processing are signal filtering, convolution of two signals, calculation values of correlation function two signals and amplification, restriction or transformation of signal or forward/inverse fourier transformation.

The analog signal processing, traditionally used in all radio engineering devices is cheaper mode of achievement to demanded results in many cases. When high accuracy of processing, diminutiveness of the device and stability of its characteristics in various temperature conditions are required digital processing appears the unique good enough solution. The example of an analog filtering is showed in Fig. 1. The operational amplifier used in the filter allows to expand a dynamic range of handled signals. The form of amplitude-frequency characteristics of the filter is defined by sizes R2, C. For the analog filter, it is difficult to provide high value of good quality, characteristics of the filter strongly depend on a temperature condition. Components of the filter bring additional noise to resultant signal (Grossman and Morrisett, 2000).

Fig. 1:	Analog filtering of a signal

Analog filters are difficult for retuning in a wide frequency range. The major feature of the digital filter lies in the fact that with selection of the certain means of factors of difference equation describing output signal is possible to transformation input signal which will be impossible by using the analogue filter. The device realizing digital filtering will have compact sizes and will not demand special technical adjustment.

THE REVIEW OF SIMILAR DEVICES

Filters can be classified on their frequency characteristics. Figure 2 a-c shows the characteristics of the low-pass filter, the high-pass filter and zonal filter. The basic function of any filter consists in weakening the signals that lie in certain band of frequencies in bringing in them various phase shifts or in introducing a time delay between input and output signals.

By of active RC-filters, it is impossible to gain ideal forms of frequency characteristics in the form of rectangular with strictly constant transfer factor in bandwidth, the endless decay in a strip of suppression and endless steepness of recession at transition from bandwidth to a strip of suppression. Designing active filter always represents searching compromise between the ideal form of the characteristics and complexity of its realization. It is called as a problem of approximation (Hyde, 2003). In many cases, the requirements to quality of a filtration allow to manage the elementary filters of the 1st or 2nd order.

Fig. 2:	Typical characteristics fo filters

Designing a filter in that cases is reduced to a choice of the scheme with the most suitable configuration and to the follow-on computing means of element, ratings for concrete frequencies. The analysis of a condition and tendencies of development of the theory and practice of a digital filtering show that the basic used methods are frequency selection of signals and an optimum (adaptive) filtering:

The newest engineering realizations both traditional and no conventional problems of digital processing of signals often used different technical-scheme decisions. The greatest attention of participants of section of digital filtration were on algorithms of multihigh-speed processing of complicated signals which contain as quick as slow varying components. Such algorithm should provide preliminary division of fast and slow components with downturn of sampling rate of slow components and their follow-on processing.

Modern computational capabilities allow to solve in a mode of real-time and problems of a multivariate filtration, it is essentially more complex than the digital filtration of one-dimensional signals which are carried out by means of alarm processors or multiprocessing systems. The block diagram PIC16C711 is shown in Fig. 3.

One shall list basic elements of the scheme; ALU where the elementary arithmetical operations are carried out and also logic operations; the accumulator (W) where, data of results of calculations are stored of outcomes of operations and also bits of choice of bank of memory; the register of indirect addressing (FSR) through which in directing one can turn to the RAM; memory (RAM), organized by 8-bit special registers and register of general-purpose, the working area of the user makes 68x8; the 13-bit counter of commands (Program counter), inferior 8 bats come from register PCL, seniors 5 of register PCLATH; EPROM, it makes 1K 14 bit cells of memory; the stack is 8 layered, length of a word is 13-bits during interruption contents of the counter of commands kept automatically in a stack; two bidirectional 8-bit ports A and D; 4 channel 8-bit A/D and timer 0 counter.

The chosen variant of design completely satisfies to the technical project. The microcontroller+photoelectric effect allow creating the flexible and high-powered system which meets the requirements of microdiminlature, high-speed performance, quality and simplicity of execution. Comparing the given microcontroller with the majority of modern microcontrollers, we can tell that it has rather low cost and access in the market of Ukraine.

In modern processing systems of signals high trank capacity of data and ADC for increasing precision of calculations is required also for these purposes, it is necessary that ALU could calculate operations with a floating point. The chosen microcontroller does not have given properties but owing to the speed and flexibility can reach the aims that are in the technical project (Smith, 2003). The architecture of the processor is constructed in such a manner that it is capable to execute a command for one cycle, except operations of conditional and unconditional transitions. It means that time of running, the program will be short and sampling rate of ADC will satisfy to a condition of signal’s processing of a qualitative telephony. The assembler is very simple; it is necessary to learn 35 commands. Last advantage allows to lower the time of preparation for completing the project.

SYNTHESIS OF THE PROJECTED DEVICE

The block diagram of type of realization DF is shown in Fig. 4. Parameters of parts of DF are the following:

Proceeding from this structure and also factors for each part we shall make difference in equations; the 1st part:

the 2nd part:

the 3rd part:

DESIGNING PROGRAMS IN THE ASSEMBLER`S LANGUAGE

Before writing the program, it is necessary to consider the specification of ROM microcontroller. The organization of ROM is shown in Fig. 5. It is visible that the beginning of the program (Wagner and Barr, 2002) of the user can be located below 0004h address (a 0000h address a vector of dump), 0004h (a vector of interruption). Before programming as a rule, one registers unconditional transition to the program of the user and unconditional transition to the program of processing interruption to given addresses. As in case of the, system the device carrying out inquiry of interruption is one so to the address of 0004h, it is possible to register the beginning of the program of processing interruption. By programming, it is necessary to consider the specification of banks of the RAM.

The structure of the RAM is shown in Fig. 5 and 6. By considering registers of the RAM every time, it is necessary to switch using the program between banks of the RAM, depending on that in what bank there is a register. The assembler delivered by the manufacturer, allows to all registers to appropriate the certain name instead of the address. It is convenient for making program (Walker and Morrisett, 2000). First, we shall write the program of processing of interruption.

Fig. 3:	Block diagram PIC16C711

We shall bring the program shown in the Table 1 with addresses of ROM and comments. The program realizing mathematical model of the filter, we shall begin with 000E h address and we shall place in Table 2.

Operations NOP are intended for synchronization of work of ADC with the program. Also for this purpose, four commands have been added before operations, above current mean of readout. In the made algorithm, the given corrections were not stipulated. Time for the program, realizing the digital filter is 49 micro sec. The interval of discrete sampling of a signal is 24.6 micro sec accordingly sampling frequency is 41000 Hz.

Fig. 4:	The block diagram of the digital filter

Fig. 5:	Organization of ROM PIC16C71

Table 1:	The program of processing of interruption with addresses and comments

Fig. 6:	Organization of ROM PIC16C71

Table 2:	The program realizing mathematical model

Fig. 7:	Conclusion of processor PIC16C711

In the program, two transitions are used: one is for zeroing the RAM another for returning the program to a place of start ADC. It is possible to exclude the operation NOP by using modern PIC processors (Fig. 7).

DEVELOPMENT AND THE DESCRIPTION OF THE BASIC SCHEME

For technical realization of DF following elements are necessary:

Description DAC: In the capacity of DAC, the microcircuit of firm Analog Devices AD 7302 is used. Data DAC has following basic characteristics: AD 7302 Double, 8-bit DAC which works from the unipolar power supply with a pressure +2.7 to +5.5 V.

Fig. 8:	Conclusion of DAC AD 7302

Fig. 9:	Scheme of the generator

AD 7302 has the interface, compatible with a parallel output of microprocessors and Digital alarm processors (DSP). The interface possesses high-speed registers and double buffered logic. Data are loaded at the raised level on CS or WR. By means of pin ~A/B, one chooses one of 2 built DAC.

Power supply unit 220 V, 50 Hz: The brief characteristic of DAC interesting for a given problem (Fig. 8). In Fig. 9, there is a simple scheme of a power unit 3 V (a current in loading is 200 A) with automatic electronic protection against overloading (I2 = 250 A). The level of a pulsation of a compliance voltage does not exceed 1 B. In the scheme as a source of a basic pressure light color diode HL1 is used. Transformer T1, it is possible to get from unified series TH but it is better to use the most small-sized TH1-127/220-50 or TH2-127/220-50. Types of transformers with a secondary winding for 5-6 V, reactors C1 type κ 50-35 will be suitable as well. The scheme uses integrated stabilizer DA1 for normal work of a microcircuit, it is necessary that the entrance pressure exceeds output voltage not <3.5 V. It reduces efficiency of the balancer due to a thermal emission on a microchip by low output voltage the capacity lost in a power supply unit will exceed given one in loading. The necessary output voltage is established by tuning resistor R2. The microchip is established on a radiator.

CONCLUSION

According to results, we can estimate errors connected with numbering a signal. The basic error will be brought by limited with 8-bit wire ADC, the more levels of quantization the better. Also because of absence of operations with a floating point, division into two odd numbers will give inexact result. For numbering analog signals, the condition Fd ≥ 2F is necessary. In practice Fd ≥ 2, 5 F is higher for better quality of a transferred signal. The spectrum of a speech signal of a qualitative telephony makes 0.3-3.4 κH according to the established international standard. The digital filter designed during research, meets the requirements of processing not only signals of qualitative speech but also signals with the top frequency of a spectrum 15.