In electronics, signals are divided into: analog, discrete and digital. To begin with, everything that we feel, see, hear, for the most part, is an analog signal, and what the computer processor sees is a digital signal. It doesn't sound very clear, so let's deal with these definitions and how one type of signal is converted to another.

In the electrical representation, an analog signal, judging by its name, is an analogue of a real value. For example, you feel the temperature of the environment constantly, throughout your life. There are no breaks. At the same time, you feel not only two levels of "hot" and "cold", but an infinite number of sensations that describe this value.

For a person, “cold” can be different, it can be autumn coolness and winter frost, and light frosts, but “cold” is not always a negative temperature, just like “warmth” is not always a positive temperature.

It follows that the analog signal has two features:

1. Continuity in time.

2. The number of signal values ​​tends to infinity, i.e. an analog signal cannot be accurately divided into parts or graduated by dividing the scale into specific sections. Methods of measurement - based on the unit of measurement, and their accuracy depends only on the division of the scale, the smaller it is, the more accurate the measurement.

Discrete signals are signals that are a sequence of reports or measurements of some quantity. Measurements of such signals are not continuous, but periodic.

I'll try to explain. If you have installed a thermometer somewhere, it measures an analog value - this follows from the above. But you, actually following his testimony, receive discrete information. Discrete means separate.

For example, you woke up and found out how many degrees are on the thermometer, the next time you looked at it at noon, and the third time in the evening. You don't know how fast the temperature changed, evenly, or in a sharp jump, you only know the data at that point in time that you observed.

It's a set of levels, like 1 and 0, high and low, yes or no. The depth of reflection of information in digital form is limited by the capacity of the digital device (logic set, microcontroller, processor, etc.). It turns out that it is ideal for storing Boolean data. An example, we can give the following, for storing data of the type "Day" and "Night", 1 bit of information is enough.

Bit- this is the minimum value for representing information in digital form, it can store only two types of values ​​1 (logical one, high level), or 0 (logical zero, low level).

In electronics, a bit of information is represented as a low voltage level (close to 0) and a high voltage level (depending on specific device, often coincides with the supply voltage of a given digital node, typical values ​​are 1.7, 3.3. 5V, 15V).

All intermediate values ​​between the accepted low and high levels are a transition region and may not have a specific value, depending on the circuitry, both the device as a whole and the internal circuitry of the microcontroller (or any other digital device) may have a different transition level, for example, for 5 -tivolt logic, voltage values ​​from 0 to 0.8V can be taken as zero, and from 2V to 5V as unity, while the gap between 0.8 and 2V is an indefinite zone, in fact, with its help, zero is separated from unity.

The more accurate and capacious values ​​you need to store, the more bits you need, let's give an example table with digital display of four values ​​of the time of day:

Night - Morning - Day - Evening

For this we need already 2 bits:

In the general case, analog-to-digital conversion is the process of converting a physical quantity into a digital value. digital value is the set of 1s and 0s accepted by the processing unit.

Such a transformation is necessary for the interaction of digital technology with the environment.

Since the analog electrical signal follows the shape of the input signal, it cannot be written digitally "as is" because it has an infinite number of values. An example is the process of recording sound. It looks like this in its original form:

It is the sum of waves with different frequencies. Which, when expanded in terms of frequencies (for more details, see Fourier transforms), one way or another, can be approximated to a similar picture:

Now try to represent this as a set like "111100101010100", quite difficult, isn't it?

Another example of the need to convert an analog value into a digital one is its measurement: electronic thermometers, voltmeters, ammeters and other measuring instruments interact with analog values.

How does the conversion take place?

First, look at a typical analog-to-digital conversion circuit and vice versa. We will return to it later.

In fact, this is a complex process that consists of two main stages:

1. Signal discretization.

2. Quantization by level.

Signal discretization is the definition of time intervals at which the signal is measured. The shorter these intervals, the more accurate the measurement. The sampling period (T) is the length of time from the beginning of data reading to its end. The sampling rate (f) is the reciprocal of:

After reading the signal, it is processed and stored in memory.

It turns out that during the time that the signal readings are read and processed, it can change, thus, the measured value is distorted. There is such a theorem of Kotelnikov and the following rule follows from it:

The sampling rate must be at least 2 times the frequency of the sampled signal.

This is a screenshot from Wikipedia, with an excerpt from the theorem.

To determine the numerical value, level quantization is necessary. A quantum is a certain range of measured values, averaged to a certain number.

Those. signals ranging from X1 to X2, is conventionally equated to a certain value of Xy. This is reminiscent of the division value of an arrow measuring instrument. When you take readings, you often compare them to the nearest mark on the scale of the instrument.

So with level quantization, the more quants, the more accurate measurements and the more decimal places (hundredths, thousandths, and so on) they can contain.

More precisely, the number of decimal places is rather determined by the ADC bit depth.

The picture shows the process of signal quantization using one bit of information, as I described above, when a high level value is received when a certain limit is exceeded.

On the right, the signal is quantized, and recorded as two bits of data. As you can see, this fragment of the signal is already divided into four values. It turns out that as a result, a smooth analog signal turned into a digital “stepped” signal.

The number of quantization levels is determined by the formula:

Where n is the number of bits, N is the quantization level.

Here is an example of a signal broken into more quanta:

From here it is very clearly seen that the more often the signal values ​​are taken (the higher the sampling rate), the more accurately it is measured.

This picture shows the conversion of an analog signal to digital form, and to the left of the y-axis (vertical axis) is an 8-bit digital record.

Analog to digital converters

ADC or ADC can be implemented as a separate device or be built into .

Previously, microcontrollers, for example, the MCS-51 family, did not contain an ADC, an external microcircuit was used for this, and it became necessary to write a subroutine for processing the values ​​of an external IC.

Now they are in most modern microcontrollers, for example, the AVR AtMEGA328, which is the basis of most popular ones, it is built into the MK itself. On the Arduino language Reading analog data is as easy as using the AnalogRead() command. Although it is not in the microprocessor that is installed in the equally popular Raspberry PI, so not everything is so simple.

Actually there is big number options for analog-to-digital converters, each of which has its own advantages and disadvantages. To describe which within the limits of this article does not make much sense, since this is a large amount of material. Let us consider only the general structure of some of them.

The oldest patented ADC variant is Paul M. Rainey's "Facsimile Telegraph System," U.S. Patent 1,608,527, Filed July 20, 1921, Issued November 30, 1926. This is a 5-bit direct conversion ADC. From the name of the patent, thoughts come that the use of this device was associated with the transmission of data via telegraph.

If we talk about modern direct conversion ADCs, they have the following scheme:

From this it can be seen that the input is a chain, which at their output give a signal when crossing some threshold signal. This is bit depth and quantization. Who is even a little strong in circuitry, saw this obvious fact.

Who is not strong, then the input circuit works like this:

The analog signal is fed to the "+" input, to all at once. The outputs with the designation “-” receive a reference voltage, which is decomposed using a chain of resistors (resistive divider) into a number of reference voltages. For example, the series for this chain looks like this ratio:

Uref=(1/16, 3/16, 5/16, 7/16, 9/16, 11/16, 13/16)*Uref

In parentheses, separated by commas, it is indicated which part of the total reference voltage Uref is fed to the input of each input voltage.

Those. each of the elements has two inputs, when the input voltage is signed «+» exceeds the input voltage with the “-” sign, then a logical unit appears at its output. When the voltage at the positive (non-inverting) input is less than that at the negative (inverting) input, then the output is zero.

The voltage is divided in such a way that the input voltage is divided into the required number of bits. When the input voltage is reached, a signal appears at the output of the corresponding element, the processing circuit outputs the “correct” signal in digital form.

Such a comparator is good for data processing speed, all elements of the input circuit work in parallel, the main delay of this type of ADC is formed from the delay of 1 comparator (they still work in parallel) and the encoder delay.

However, there is a huge disadvantage of parallel circuits - this is the need for a large number of comparators to obtain a high-capacity ADC. To get, for example, 8 digits, you need 2 ^ 8 comparators, and this is as many as 256 pieces. For a ten-bit (in arduino, a 10-bit ADC, by the way, but of a different type), you need 1024 comparators. Judge for yourself about the feasibility of such a processing option, and where it may be needed.

There are other types of ADCs:

successive approximation;

delta-sigma ADC.

Conclusion

The conversion of an analog signal into a digital one is necessary for reading parameters from analog sensors. There is a separate type of digital sensors, they are either integrated circuits, for example DS18b20 - its output is already a digital signal and it can be processed by any microcontrollers or microprocessors without the need for an ADC, or an analog sensor on the board on which its converter is already placed. Each type of sensor has its pros and cons, such as noise immunity and measurement error.

Knowledge of the principles of transformation is mandatory for everyone who works with microcontrollers, because not everyone even modern system such converters are built in, you have to use external microcircuits. For example, we can cite such a board, designed specifically for the Raspberry PI GPIO connector, with a precision ADC on the ADS1256.

This type of equipment is quite difficult to attribute to any particular category of equipment. By the way, this is why the converters considered here are very difficult to find in well-known online markets: it is not clear in which product category to search - among capture devices, among tuners or among converters? At the same time, these devices are closest to the category of converters, since their only task is to convert one type of signal to another. And how it will be possible to use the devices is a purely personal matter, and depends on the tasks and skills of the user.

Design and specifications

The converters in question are supplied in the same blister packs, and at first glance they do not differ from each other. Only an inconspicuous inscription-designation of the model will help you figure out which converter is in front of you.

Another thing is the back of the package. Here you don’t need to read anything, just look at the connectors that are visible under the transparent packaging.

The first device, with the model designation ET110, is designed to convert a standard computer RGB signal from a VGA interface (a 15-pin connector, otherwise called D-sub) into a today's standard digital signal routed via an HDMI connector. D-sub video outputs are available in video cards of personal computers, laptops, and other video signal generation devices.

The second converter, ET111, is busy converting the ancient composite signal to digital, which is also output via HDMI port. Absolutely every VCR, game console or video camera of previous generations was supplied with such "tulips".

Finally, the third converter, with the index ET113 (I wonder why not 112?), as can be seen from its connectors, digitizes the YPbPr component signal that goes through ordinary coaxial cables with "tulip" connectors. Such video outputs are available for game consoles, some VCRs and media players, even modern ones.

The housings of the devices are made of plastic and consist of two halves, firmly linked with latches. In order to find out the location of these latches, we had to pretty much mangle the case of one of the converters. And yet, it was possible to disassemble the devices. Their design turned out to be extremely similar, which is not surprising, given the fact that the main electronic component that digitizes video is the same production chip.

Our devices are not commonplace adapters with soldered through outputs, they are completely independent devices, the electronics of which work according to active circuit, that is, it requires power. To do this, each of the converters under consideration has one more "tail" - standard USB to be connected to the USB port of the TV or other device. In extreme cases, an ordinary five-volt battery, the so-called powerbank, is also suitable, which are now bred in abundance.

Main specifications converters are shown in the following table:

This and other information can be seen on.

Connection and operation

From the outside and even more so technical description devices, it becomes completely clear how to connect converters. Nevertheless, here are diagrams of typical applications of apparatuses.

As you can see, in each of the schemes, the end point is digital tv or a projector. But the question arises: any modern projector or TV - with the rarest exceptions - is necessarily equipped with all video inputs, both digital and various analog ones, including even the "computer" D-sub. Of the information display devices devoid of analog inputs, the author can only remember some highly specialized monitors, like those that are installed on the "shoe" of video cameras or cameras. What prevents the average user from connecting an old VCR or laptop to modern TV directly, through the cables and adapters supplied with the TV? For what reason will he choose to connect through a separate separate device, which, moreover, also supply power?

Per ordinary user we have nothing to say, but with the “unusual” user, everything is not so simple. The specifics of the section " Digital video”, in which this article is published, requires you to remember about capture devices. This is where the real difficulties begin: video capture devices are divided not only into hardware or software, stationary or portable. One of the main distinctive features of any capture device is the type of signal that this device is able to receive and convert. Finding a universal device that has all the necessary inputs and supports all possible video standards is extremely difficult. Especially now, when capture devices are equipped with a single input. And that, of course, is HDMI. Thus, having one HDMI signal capture device and several multi-standard converters that convert any video into digital standard, the user will be able to digitize absolutely any source - a VHS tape recorder or camera, a game console of previous generations, a Blu-ray player or a media player, a laptop, an old personal computer and so on, up to the ultrasound diagnostic apparatus.

But enough theory, I would like to consider those few aspects that are generally possible to study here. And the first of them, the most important, concerns the delay in signal processing and transmission. After all, the converters in question can be used as adapters between a game console and some kind of video signal display device (TV, projector). And what factor is important in the game, for example, in a shooter or racing and other simulators? Of course, the reaction of the player.

We will not play, let a satisfied consumer play, but we will calculate the delay that may be present during signal transmission. The converters in question work according to an active scheme, here any incoming signal undergoes full processing, converting to another standard on the fly. And this, even theoretically, takes time.

To begin with, we will assemble a sort of spontaneous stand for the test. Let's connect a laptop to the TV using its VGA (D-sub) video output, and use the ET110 device as a signal converter. It is according to the same scheme, which is given above in the first example. As a result, the laptop received a second screen, which displays the same information that is displayed on the main display. Now let's start playing a special video on the laptop, which has a frequency of 60 frames per second. Here, in the video, there is a rotating arrow that makes one revolution per second, as well as a rectangle moving along the upper scale, which also runs its way in one second. It remains to film the resulting test bench, and video recording will be conducted at the same frequency of 60 frames per second. Here is the result:

In this video, it is clearly seen that the signal delay is 7 sectors out of 60, that is, about 1/10 of a second. Whether it's a lot or a little, we don't know. game consoles never got involved. However, in the races that were run on big screen from the same laptop through this converter, such a delay was not felt in any way. Perhaps the masters of online shooters will be able to notice some kind of annoying lag, but, frankly, I don’t really believe in it.

The next question, which is also the final one in the study of such simple (but at the same time complex) devices, is the preservation of detail when transcoding a signal. The ET111 device, which digitizes composite video, is useless to study in this regard. There can be no question of any detailing - this ancient standard is too merciless with a video signal, in which both the pixel is not square, and the aspect ratio of the frame is “wrong”, overscan areas that are not visible on “tube” TVs, and even interlacing memorable, which halves the vertical detail. Taking the opportunity, we captured some vintage VHS tapes by connecting an ET111 converter to the composite video output of the tape recorder and passing the signal into a capture device with a single HDMI input. The quality (more precisely, what VHS can provide) turned out to be quite up to par, no worse than during direct viewing from a tape recorder on a TV.

But the other two devices are quite interesting to study from the point of view of detail - are these converters really not cheating with the signal, do not compress it, for example, twice, processing it, and subsequently stretching it to Full HD?

The easiest way to check this assumption is to play a special test video file and then capture the video stream. In the case of the ET110, the laptop will play the file, and the signal is output through its VGA output, passed through the converter, and fed into the capture device. In the second case, the source will be a media player equipped with component video outputs. The test video file contains many lines one pixel thick, which are located at the same distance from each other. The capture results can be seen below.

The different brightness of the frames is explained by the different range of video outputs (the laptop has a "computer" brightness range), and the different clarity is also easy to explain: we remember that the video signal entering the converter undergoes full processing - here they are, the results of this processing, on freeze frames.

conclusions

What are the purposes for which these inexpensive devices, which are full-fledged analog signal converters, are supposed to be used various formats into one digital one, supported by all modern display devices without exception? As already mentioned, they may be required if the TV does not have the required input. Or even in such banal situations as the lack of necessary adapters (one of the author's TVs was received in a limited configuration, as a result of which all its analog inputs were not available due to the lack of special branded adapters).

But still, the option of capturing a video signal that has a variety of standards seems to be more convincing. And even preferable, given the considerable cost of various capture devices. Of course, an ideal output that will suit everyone would be a signal converter similar to one of the ones under consideration, only having three types of input connectors at once - VGA, composite, and component. But such a decision, apparently, is not at all included in the plans of marketers.

Analog to Digital Converters (ADC) are devices designed to convert analog signals to digital. For such a conversion, it is necessary to quantize the analog signal, i.e., limit the instantaneous values ​​of the analog signal to certain levels, called quantization levels.

The ideal quantization characteristic has the form shown in Fig. 3.92.

Quantization is the rounding of an analog value to the nearest quantization level, i.e. the maximum quantization error is ±0.5h (h is the quantization step).

The main characteristics of the ADC include the number of bits, conversion time, non-linearity, etc. The number of bits is the number of bits of the code associated with the analog value that the ADC can produce. Often people talk about the resolution of the ADC, which is determined by the reciprocal of the maximum number of code combinations at the output of the ADC. Thus, a 10-bit ADC has a resolution of (2 10 = 1024) −1, i.e., with an ADC scale corresponding to 10V, the absolute value of the quantization step does not exceed 10mV. Transformation time t p - time interval from the moment given change signal at the input of the ADC until the corresponding stable code appears at its output.

Typical conversion methods are as follows: analog value parallel conversion and serial conversion.

ADC with parallel conversion of the input analog signal

The parallel method compares the input voltage to n reference voltages simultaneously and determines which two reference voltages it lies between. In this case, the result is obtained quickly, but the scheme turns out to be rather complicated.

The principle of operation of the ADC (Fig. 3.93)

With U in = 0, since for all OS the voltage difference (U + - U -)< 0 (U + , U − - напряжения относительно общей точки соответственно неинвертирующего и инвертирующего входа), напряжения на выходе всех ОУ равны −Е пит а на выходах кодирующего преобразователя (КП) Z 0 , Z 1 , Z 2 устанавливаются нули. Если U вх >0.5U, but less than 3 / 2U, only for the lower op-amp (U + - U -) > 0, and only at its output does the voltage + E pit appear, which leads to the appearance of the following signals at the outputs of the CP: Z 0 \u003d 1, Z 2 \u003d Z l \u003d 0. If U in > 3 / 2U, but less than 5 / 2U, then a voltage + E pit appears at the output of the two lower op-amps, which leads to the appearance of code 010 at the outputs of the CP, etc.

Watch an interesting video about the work of the ADC:

ADC with serial input signal conversion

This is a sequential counting ADC, which is called a tracking ADC (Fig. 3.94).
The ADC of this type uses a DAC and an up/down counter, the signal from which provides a change in the voltage at the DAC output. The setting of the circuit is such that the approximate equality of the voltages at the input Uin and at the output of the DAC −U is provided. If the input voltage U in more tension U at the output of the DAC, then the counter is switched to the direct counting mode and the code at its output increases, providing an increase in the voltage at the output of the DAC. At the moment of equality of U in and U, the count stops and the code corresponding to the input voltage is removed from the output of the up/down counter.

The sequential conversion method is also implemented in the ADC time - pulse conversion (ADC with a linearly varying voltage generator (CLAY)).

The principle of operation of the considered ADC fig. 3.95) is based on counting the number of pulses in the time interval during which the linearly changing voltage (LIN), increasing from zero value, reaches the level of the input voltage U input. The following designations are used: SS - comparison circuit, GI - pulse generator, Cl - electronic key, Сч - pulse counter.

Time point t 1 marked in the timing diagram corresponds to the beginning of the measurement of the input voltage, and time point t 2 corresponds to the equality of the input voltage and the CLAY voltage. The measurement error is determined by the time quantization step. The Kl key connects the pulse generator to the counter from the moment the measurement starts until the moment U in and U clays are equal. Through U SC denotes the voltage at the input of the counter.

The counter output code is proportional to the input voltage. One of the disadvantages of this scheme is the low speed.

ADC with double integration

Such an ADC implements the method of sequential conversion of the input signal (Fig. 3.96). The following designations are used: CS - control system, GI - pulse generator, Sch - pulse counter. The principle of operation of the ADC is to determine the ratio of two time intervals, during one of which the input voltage U is integrated by an integrator based on the op-amp (the voltage U and at the output of the integrator changes from zero to the maximum value in absolute value), and during the next - integration of the reference voltage U op (U and varies from the maximum value in absolute value to zero) (Fig. 3.97).

Let the input signal integration time t 1 be constant, then the longer the second time interval t 2 (the time interval during which the reference voltage is integrated), the greater the input voltage. The KZ key is designed to set the integrator to the initial zero state. In the first of the specified time intervals, the key K 1 is closed, the key K 2 is open, and in the second, the time interval, their state is reversed with respect to the specified one. Simultaneously with the closure of the key K 2, the pulses from the pulse generator GI begin to flow through the control circuit of the control system to the counter SC.

The arrival of these pulses ends when the voltage at the output of the integrator is equal to zero.

The voltage at the output of the integrator after the expiration of the time interval t 1 is determined by the expression

U and (t 1) = − (1/RC) t1 ∫ 0 U in dt= − (U in t 1) / (R C)

Using a similar expression for the time interval t 2 , we obtain

t 2 \u003d - (R C / U op) U and (t 1)

Substituting here the expression for U and (t 1), we get t 2 \u003d (U in / U op) t 1 whence U in \u003d U oa t 2 / t 1

The code at the output of the counter determines the value of the input voltage.

One of the main advantages of the ADC of this type is high noise immunity. Random surges in the input voltage, which occur within a short time, have almost no effect on the conversion error. The disadvantage of the ADC is its low speed.

The most common are the ADCs of the microcircuit series 572, 1107, 1138, etc. (Table 3.3)
The table shows that the parallel conversion ADC has the best performance, and the serial conversion ADC has the worst.

We offer you to watch another worthy video about the operation and device of the ADC:

Today we will talk about how to quickly and accurately digitize old video cassettes using a computer, camcorder or other do-it-yourself equipment at home. We will also consider simple techniques for self-digitizing audio and video recordings or how to convert an analog signal into digital

Over the past period, the owners of video systems have accumulated great amount archives. Of course, if we are talking about films or TV shows, then they can be found on actual media, but not everything can be found. For this reason, many people continue to keep an old video player, which is used to watch "tape rarities."

Magnetic tape, unfortunately, is short-lived: it gets old, the magnetic layer collapses over time, the recording first loses its quality, and then is not suitable for viewing at all. If you do not give due importance to this, one day it will turn out that the unique wedding ceremony, filmed on cassette and carefully stored in the allotted place so that it can be viewed with trepidation on the next anniversary of your own wedding, is hopelessly damaged

As VCRs slowly fall into disuse, giving way to DVD and Blu-ray players, you want to be able to watch home videos with a player.

In addition, there is an opinion that film cassettes cannot store lossless video for a long time; over time, the recording quality inevitably begins to deteriorate. Any dubbing also degrades the quality of analog video. But a film that has been digitized can be rewritten many times without loss of quality.