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Danilo的数字译码器解码SRC-RAI信号，这是意大利官方每小时一次的信号。通过这个设备，你能够学到最古老的运行的信号之一的抽样时间是如何做到的以及它是如何工作的。

Decode Periodic Signal Transmissions

Danilo’s digital decoder decodes the SRC-RAI signal, the Italian official hourly signal. With this device, you can learn how one of the oldest operating signals of sample time is made and how it works.

Men have been interested in the topic of time and how to measure it for centuries. Famous philosophers and scientists such as Albert Einstein have studied every aspect of this peculiar physical concept. Through his theory of relativity, Einstein introduced essential concepts about the regular flow of time. These matters continue to puzzle us today. For instance, it is impossible to synchronize two watches that are not in the same “condition.”

Think about the GPS satellites orbiting the Earth at an approximate altitude of 20,000 km. Believe it or not,between the two atomic clocks, one on the satellite and one at sea level, there is a daily difference of about 38.9 μ s. It ’s a difference we cannot ignore that ’s due to different gravitational pulls acting on the orbiting clock and its displacement velocity, which is 12 times greater than the one on Earth. Therefore, GPS satellite systems are usually adjusted before launch in an effort to minimize such effects.

When it comes to time, a temporal scale of reference is important. The old definition of a “second”—as a division of the 86,400 parts of the mean standard day —is inadequate. In 1967, the scientific community acknowledged the new definition of  “second.” Back in 1955,English physicians Louis Essen and J.V.L.Parry created the first cesium-beam atomic clock at the National Physical Laboratory in England. In the international system, the “second ” then became SI, the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

After that, numerous metrological laboratories adapted to the new technological concept. For instance, since 1970, The National Electrical Institute (IEN) in Turin, Italy, has had its own coordinated universal time (UTC) that is based on cesium-beam atomic clocks. With the introduction of the “cesium fountain” atomic clock in 2003, the Institute, now called The National Institute of Metrological Research (INRIM), has been at the forefront of time measurement. This type of clock has a precision of 10^–15, which means a 1-s error in 30 million years, provided that it would work for such a long time.

To coordinate the ever-increasing amount of data produced by the atomic-clock-equipped labs throughout the world, we now refer to the International Bureau of Weights and Measures (BIPM) in Paris, which has calculated the time since 1988 using the UTC times of accredited centers, such as the so-called International Atomic Time (TAI), which has become the scientific community ’s official point of reference for time measurements.

In this globalized, computerized world, we cannot ignore the importance of having a universal temporal scale of reference. Think about the financial and stock transactions that are quickly transferred through the telecommunication networks and may change their values in only seconds. Time synchronization is important in everyday life as well. If you think I am exaggerating,let ’s go back for a moment to New Year ’s Day 2000 when a simple time change was supposed to trigger a worldwide catastrophe. Fortunately, no big problems occurred.But to prevent a potential catastrophe, numerous tests were conducted on devices for thousands of hours.

Today, many systems are automatically synchronized through Network Time Protocol (NTP) servers on the Internet. To supplement its divulgation activities, the INRIM has two NTP free access servers, which are set with the institute’s UTC-IEN time. These servers are found at the following addresses: ntp1.ien.it (193.204.114.232) and ntp2.ien.it (193.204.114.233).

THE SRC-RAI SIGNAL (IEN)
In an international community, transmitting the time signal via a radio signal is common. The German DCF77 signal, which works on 77.5 kHz, covers a large part of Europe, including all of England and ranging from part of the Scandinavian countries down to near the North African coasts (2,000 km). The similar Swiss Prangins HBG signal is compatible with DCF77 signal coding.The only difference is that the emissions frequency for the HBG signal is 77 kHz. Like the German signal, it also has a considerable range of 1,500 km. Finally, in England, since 2007, the National Physical Laboratory (NPL) has transmitted the English official signal MSF from the Anthorn station through 17 kW of power working on 60 kHz.

Unlike the aforementioned signals, the SRC-RAI signal is not generated and directly transmitted on air. It is coded and sent to the Radio Televisione Italiana (RAI) station, which broadcasts through its own radio network.


My IEN-SRC project cannot compete with the progress of the hourly signals or with Internet and the NTP service. But the purpose of this project is to study one of the oldest operating signals of sample time (see Photo 1). Even if the signal’s mode of transmission is obsolete, the IEN in Torino is still at the forefront in terms of precision and technological research. Thus, this project is an excellent starting point for studying a signal’s time and frequency. If you’re in Europe, you can receive the radio RAI signal via satellite. Try Hotbird 8 (13.0°E, 11,804.00 MHz, vertical polarization) or Astra 1L (19.2°E, 11,567.50 MHz, vertical polarization).


Let’s examine how the SRC-RAI time signal is made. First, note that it is a modulated frequency shift keying (FSK) signal in an audio frequency band. This means that it is an “audible” signal. As you can see in Figure 1, it consists of two groups of bits that are transmitted on seconds 52 (32 bits) and 53 (16 bits) of each minute. Every bit has a fixed duration of 30 ms. The bit is 2,000-Hz if it is a 0, or a 2,500-Hz tone if it is a 1. A third tone of 1,000 Hz is used for the last six audio synchronization signals. At the head of the group of bits is the ID field (ID-1 and ID-2), which identifies the bits that follow. Bits 16 and 31 of the 32-bit group (P1 and P2) and bit 15 of the 16-bit group (PA) are parity bits that allow you to verify the preliminary accuracy of the data received through the no-parity method.

The first segment of code transmits the time, the entire date, and the day of the week. The second segment transmits the year and two interesting technical parameters. Data is transmitted through the binary-coded decimal (BCD) coding and not with a binary sequence, as you might think.


As you can see in Figure 2, the bit values are placed according to the positions in the transmitted code.For the remaining technical information (SE  –  SI), refer to the tables in Figure 2. A detailed analysis of the two aforementioned parameters shows that the first (SE)warns when time changes from standard to daylight-savings time and vice versa. The second (SI) sees to the warning for the socalled “leap second.” In brief, the leap second represents the power to add or take away 1 s to compensate for variations in the speed of the Earth ’s rotation. If necessary,this compensation is inserted at the end of June or December after 23:59:59 UTC.

From the introduction of the leap second in 1972, up to the last update on January 1, 2009, a total of 34 s have been added to the time scale (TAI). The decision to use (or not use) the leap second is made by the International Earth Rotation Service (www.iers.org). It does so according to the measurements made by the metrological laboratories that it refers to.

To a casual listener, the SRC-RAI hourly signal sounds like a shrill,modulated, 1.5-s sound that ’s followed immediately by five short synchronizing signals. A final signal is transmitted that corresponds to second 00. As I mentioned, the INRIM ’s SRC-RAI signal is directly transmitted on a dedicated frequency to the RAI ’s headquarters and then broadcast in the normal radio-RAI program.This is why it can be received at different times and places on all three RAI stations in both amplitude (AM) and frequency (FM) modulation.The signal ’s broadcast usually takes place 15 times per day (sometimes 20 or more times) in different time bands. Signals at midday, midnight,and generally before the evening news cannot be eliminated.

If the transmission can’t be received directly from Italy, the same radio-RAI signal can be received throughout Europe with a standard digital satellite TV receiver. In that case, you can can obtain the signal via the audio output on the back of the receiver. For more information about the SRC-RAI time signal, refer to the INRIM IEN web site (www.ien.it/tf/time/index.html).

SYSTEM DESIGN


This project was created for educational purposes. I developed the circuit to study the SRC-RAI signal generated by the IEN in Turin (see Figure 3). I don ’t pretend the circuit is highly accurate or perfectly synchronized.Bear in mind that the signal is transmitted via radio or TV satellite. In addition to being an old technology, it produces a delay in the propagation that cannot always be defined and that is often similar to other systems transmitting the hourly signal (see Photo 2).However, the sources of information contained in the SRC-RAI coded and received signal are exact.Therefore, you can use them as a point of reference.


Let ’s move on to the electric diagram.It is separated into two different sections. The first part hinges on the analog integrated circuit LM311,a classic single-supply comparator,which, surrounded by a few passive components, starts processing the input analog signal. This entire first phase basically supplies a TTL squaring of the signal to send it directly to the microcontroller, which handles the entire decoding process.

The core is a Microchip Technology ’s PIC16F628A microcontroller. Compared to the wellknown PIC16F84, this microcontroller has 2 KB of program memory and more independent timer registers, which can be used for programming.The PIC16F628A handles numerous tasks: it reads the header, decodes the input TTL signal,manages the display (including the display light), and drives the open-collector synchronization output.

Many previous FSK signal decoder designs used a selective circuit as an A/D interface —that is, an integrated circuit (e.g., LM567) based on a PLL sample frequency, which literally  “captured ”the allocated frequency. Once the sample frequency was found in the signal, these devices simply produced a TTL output signal. There are a few obvious disadvantages associated with these circuits: the need to have different selective stages (one for every frequency to be found), the need to use laboratory instruments (oscilloscope or frequency counters)for the calibration phase, and a proven instability due to the noises often present in the input signal.Noises in the AM frequency (vanishing noises) and the less invasive FM frequency (over modulation) are the major causes for unsuccessful decoding with the old decoder circuits.

My project features a digital circuit where the task of selecting the FSK tone, as well as its quality, is delegated to the only microcontroller. In terms of functionality, you have notable advantages: calibration isn ’t required, you have stable selectivity,and you have an inexpensive, compact project.

The PIC16F628A microcontroller ’s software was developed with pure assembly code. High-level languages weren ’t used. This refined approach enabled me to use the 2 KB of memory and maximize the part ’s full potential.I used an alphanumeric integrated display that contains a Hitachi HD44780 with a reduced pin (nibble) configuration. The microcontroller manages the display (and backlighting).The PIC16F628 ’s software is posted on the Circuit Cellar FTP site.

I must point out one thing about the only output signal connected to the open collector phase managing the TR2, which is responsible for transmitting a 3-s pulse dead on second 00 of every hour. This signal pulse may occasionally help when connecting an external device to be synchronized with the aid of your SRC decoder.

Finally, note that the circuit includes a voltage regulator. A tested LM7805 voltage regulator supplies an adequate level of stabilization.

FIRMWARE & DECODING


I consider the software to be the most innovative part of this project. As you can see in Figure 4 and Figure 5, the decoding section of the three sample frequencies (1,000, 2,000, and 2,500 Hz) represents the most interesting portion of the firmware. The READ_FREQ routine is responsible for this activity. It can synchronize on the leading edge of the input signal, establishing its period (lambda). If the lambda value is included in an expected frequency range (1,000,2,000, or 2,500 Hz), the routine exits and advises which incoming frequency has been recognized. The range of lambda values that define the frequencies to be recognized has been given a 3% margin to avoid errors in the recognition mainly due to a frequency slippage, which is typical with AM receiving.


In the same routine, several checks are made to verify the correct lambda reading, as well as to reject frequencies that are too high (caused by spikes) or frequencies that are too low (no incoming signal). The first routine using READ_FREQ is the one that reads the principal header,made up of one bit 0 followed by one bit 1. The correct reading of this part of the code is fundamental for all reading all of the data. The routine takes into consideration the exact length of the signals and then estimates the quality through a noise parameter rather than reading errors.

In fact, if the decoding activity is stopped as soon as a  “simple ” defection in reading occurs, it would become impossible. Bearing in mind the nature of the incoming signal source, you cannot expect to have a signal that is totally accurate.

The noise value then becomes an important parameter to value the signal ’s quality. Higher parameters mean more signal interferene.Beyond a certain cluster point, an error in reading occurs and an ID1-ERROR message appears. The wide margin-based reading technique and the quality of the read data are also made possible by the fact that there are only three frequencies to be read and their spacing. When designing the signal standard, it is likely that the IEN had already considered the most adequate frequencies to get a data transmission with the lowest number of potential errors.

Immediately after reading the main header HD-1, the following bits are decoded, which constitute the first segment in a quick succession, one after the other. The generic READ BIT routine carries out this activity depending on the preceding READ FREQ for the frequency identification.

All of the data, which are read in the first segment, will be stored and considered all at once after reading the second segment. When the 32 bits constituting the segment are available,there is a final check, which consists of testing the control of the odd parity residing in the code every 16 transmitted bits. It is useful for auditing the received data.

After the last check on the first segment, the second segment is read with the same procedures and software routine. The second segment is made up of 16 bits, which is half of the previous one. The use of specific routines and the technique based on orderly and recursive calls represents a typical example of a productive way of programming. The remaining software routines don ’t show any particularities apart from the calendar,which autonomously looks after the correct display of the time and the punctual date change for leap years included between 2000 and 2035. After 2035, this device will probably be considered only as a piece of history. However, an undoubtedly interesting feature is the display of the second intercalary “leap second,” which will be able to display the virtual 60 s as soon as the leap second happens. Its display will happen at 1 A.M., not at midnight,because the Italian time zone is 1 GMT.

USER MANUAL
By now you should be persuaded that using the decoder is easy to use and highly reliable. You just need to decide if you want to use a common AM-FM radio or a satellite TV receiver. The former is easier and readily available, but imperfect frequency tuning may blanket the reception. Don ’t worry. A lot of work has been made on the software in the microcontroller in an effort to minimize mistakes as much as possible.A special routine calculates and judges the signal ’s quality to identify data that is also in the presence of reception noises. On the other hand,a TV satellite decoder offers excellent output signal quality but limits portability.

When switching on the device, a welcome message appears on the display. You ’re then asked to choose the source of the signal: or . By selecting the satellite, you tell the device that the source of the signal will be taken from a satellite receiver with a calculated average delay of about 240 ms, which will be automatically adjusted by the software in the microcontroller. This enables perfect synchronization with absolutely no problem.

Once you connect the source of the input signal and feed the circuit,check the signal ’s volume. To access the test routine, hold the Enter key for 2 s immediately after you switch on the device. A graphic slider moves each of the three frequency samples (1,000, 2,000, and 2,500 Hz ) to be recognized. Then,adjust the input volume. The bar moves a bit toward the right or left every time one of the three frequencies used for recognizing the SRCRAI signal (i.e., on 1,000, 2,000 or 2,500 Hz) is identified.

If the volume is too low, the bar on the display won ’t move and will have to be turned on. Once you have terminated the check phase, press Enter again. The device, with the display off, will wait to receive a good SRCRAI signal. As soon as the first two segments of information are received,they will be decoded and displayed. A bar placed in the lower part of the display will move forward, synchronized by six 1,000-Hz beeps.

If problems occur during reception,a warning message will appear. The most common errors reported on the display will have to do with the initial header of the first or second group (HD-1 and HD-2), rather than the noncorrespondence of the control parities of the byte of the information received (P1, P2, and PA). To be honest, during my frequent tests, I found that the reception problems had been limited.

I must point out that the coded SRC hourly signals, which are broadcast in the middle of the day, must sometimes be broken or incomplete for several technical reasons causing the abortion of the reception. Usually there are no problems with signals that are broadcast just before the news or at midnight because an adequate space for a corrected transmission is allotted.

After a positive reception, the decoder’s display shows a clock perfectly synchronized to a millisecond. Because the clock’s functionality is based on the PIC16F628A’s internal oscillator, don’t forget that it won’t be precise 24 h after the synchronization.

The use of an external 10-MHz quartz doesn ’t ensure that all of the prototypes we make have the same precision. For example, some inexpensive quartz may have a margin (variance in the frequency) of  ±50 ppm (0.005%). This is why programming an EEPROM memory location (38 Hex)with an  “adjustment ” value has been allocated. By changing this value,when developing the PIC, it is possible to adjust, in advance or later, the internal routine, which is in charge of the clock ’s adjustment.

Your internal clock is also fully equipped: it updates the complete date, even for leap years until 2035;it automatically updates the time when passing from standard time to daylight saving time and vice versa;and it is a perfect display of the sixtieth second in case of a leap second at 1 A.M. sharp. Central European Time (CET) in Italy is 1 h ahead of Greenwich Mean Time (GMT).


All the other parameters can be displayed using the two special keys on a keyboard. You can page through to the next submenu. You will then know if it is a standard or a daylight saving time. If in the following seven days there will be some changes (and at the end of the month), a leap second will take place. As you can see in Figure 6, the menu enables you to control the display light and perform a reset.

TEST SOFTWARE (DELPHI)
The decoder for the SRC-RAI time signal is equipped with some of the most refined programming technology that can be squeezed out of one small PIC16F628 microcontroller. To thoroughly test your project without having to wait for the signal to be broadcast, refer to the Test-IENSRC.exe file on the Circuit Cellar FTP site. To use the software, you need a PC running Windows XP and a sound card. The program’s main screen is shown in Photo 3.


After you set all the necessary parameters, the signal is ready to be transmitted in an entire block. You can then manually enter the beeps at 1,000 Hz for the transmission.

The software includes a wave format recording of a broadcast time signal. You can use it to test your circuit as well. Please consider this software as a mere test program with no control procedure. Good luck hunting for signals and then decoding and validating the received data!


Author’s note: You can enter GPS coordinates at www.gpscoordinates.eu/show-gps-coordinates.php to see images of the time signal transmission facilities via satellite. Enter 45.015037, 7.639634 for the IEN-IRITI in Torino, Italy. Enter 50.016000, 09.007600DCF77 for the Mainflingen signal in Hesse, Germany. Enter 54.911227, –3.279997 for the National Physical Laboratory, which transmits the Anthorn station in London, England. Enter 46.408300, 6.252500 for the HBG transmitter in Prangins, Switzerland.

Danilo Consonni (www_enigma@lycos.it) is an electronic engineer who lives in Italy, where he studied electronics at the Politecnico di Milano. He develops circuits and writes for several publications based in Europe. Danilo has been active in the computer security field for more than 15 years. You can contact him him on the web at http://xoomer.virgilio.it/www_enigma.

PROJECT FILES
To download code, go to ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2009/225.

SOURCE
PIC16F628A Microcontroller Microchip Technology, Inc. |www.microchip.com