VCRs Working

Helical Recording: This is the system universally used in video recorders. This format has been chosen with a clear objective, to obtain the highest density of information and record as much time in the film shorter. The recording-playback heads are placed or mounted on a carrier cylinder head that is tilted with respect to the longitudinal movement of the tape. The cylinder comprises two concentric drums, one fixed and one rotating mobile, on which are the heads. The time for each head start and end in contact with the tape determines the information content. In the VCR (video recorders), this time coincides with a field of video information. Example: on a track is recorded odd field information, this information is recorded by the head "A", while on the runway nearby, the head "B" prints the odd field, so it records all information of a schedule. This means that in a full rotation of cylinder, two tracks recorded or read and whose contents will be the 365 lines on a chart according to our rules.


The applicability of this method lies in the switching of heads that occurs once per field. To make this switch produces minimal interference; the change is made on recent field lines before vertical sync pulses i.e. each track begins with the start of vertical sync pulse. The cylinder heads are mounted at the same height but 180 degrees opposite. While the head "A" begins to make contact with the tape on the bottom, the head "B" leaves the contact at the top. In the helical scan, you get a high speed tape-head as the tape progresses slowly. The film moves with a speed of about 2 [cm / s], however the front of the tape head is moved to the tangential velocity of cylinders. The cylinder rotates at 1800 rpm in NTSC standard "M" and 1500 rpm in our system Standard PAL "N", this equates to 30 and 25 turns per second respectively. The cylinder has a diameter of 60 [mm] perimeter accordingly be: P = ∏. D = 3.1416 x 60 = 188.49 [mm] = 0.1885 [m]. The head gives a full turn in 1 / 25 [s] in the PAL N, hence the tangential velocity is: Vt = 0.1885 x 25 = 4.71 [m / s] Applying the equation which is discussed, we can determine the frequency response or frequency limit recording. Here: Note: This example has used a distance d = 0.5 [μm] for the gap. In conclusion, the helical recording method for obtaining a high frequency response, which combined with FM recording, produce the necessary response to the luminance signal.

Safety band (zero guard band) for High-density Recording

To avoid interference between adjacent tracks recorded was a clear space between them, known as a band of space security. Obviously the protection band is an advantage from the standpoint of image quality but has the disadvantage of tape waste by the lack of utilization of total surface of the magnetic material deposited. In the non-professional video recorders, it tends to eliminate the said protection band, eventually the system known as high-density recording. The problem with this system lies in the probability that the head to move on the next track, and reading some information from another field, ie unwanted information. This is further complemented with a mechanical problem of great demand for follow leads. To minimize this drawback, to the extent of abandoning the need for a band without recording protection was devised with the use of air-gap heads tilted. Head "A" is constructed with a gap whose angle to the vertical is + , while the head "B" is constructed with an angle -  for the same. The result is that consecutive tracks are recorded with a definite and opposite azimuth. The angle "" is an approximate value of 6 º. During playback head "A" act with maximum efficiency to cover the tracks recorded for the same, just as the head "B" will to walk the tracks recorded by him. Now suppose that one of the heads by mistake advances in monitoring and runs in part by the wrong runway. At various points the gap will be different and even opposing values of magnetic field, so that the average field induced by the cross will be minimal (adopting inclinations are such that the output voltage is almost zero). The head "A" crosses the track itself and part of the track adjacent "B" information "A" will be reproduced correctly, while the information provided by the track "B" will be ignored.

Put another way, the inclinations of the air gaps are equivalent to the effect of a "gap" larger than the actual width. For reference, you can compare with the tilt of the head in an audio recorder and its influence on the reproduction of high frequencies. This technique for eliminating interference from adjacent tracks read by tilting air gap is valid for the luminance signal that modulates a carrier frequency. Treatment of the chrome signal is different and presents problems not solved by this method.

Elementary scheme for recording video

The luminance signal and color are obtained after a separate circuit equivalent to those used in televisions. Being well-defined spectra, the filter does not require special features. If we note that the separation is sacrificed in some frequency response in the luminance signal, as already mentioned. This restriction is part of the permissible limitations for non-professional recording, otherwise they should adopt more complex methods. The color information is limited in its frequency response to 500 kHz. The color signal is applied to a converter or mixer which in turn receives its other input the voltage of a constant frequency oscillator (crystal oscillator) in order to effect the shift of spectrum required, the frequency of 629 KHz. At the output of the converter, we find the color signal with all its properties, i.e. an AM signal suppressed carrier, double sideband, but now centered on the frequency mentioned above. Importantly, we run the entire spectrum and that the sidebands remain far apart and position on the new carrier. As the chrome signal is recorded directly and that their frequencies are low compared to the FM signal containing luminance, both heads opposite azimuth, is not sufficient to eliminate cross-field interference, i.e. between adjacent tracks. This problem and its solution is discussed later as phase shift.

For information on FM luminance records, never mind the linearity or distortions that may occur this signal. For chrome, the situation is different, this message is recorded directly, therefore, be kept good linearity between recording and playback, otherwise they would suffer abnormalities in terms of color hue and saturation. As with audio, for recording color requires a high frequency signal (RF) to be added to the color signal and act by way of pre polarization. In recording this video signal is the same RF carrier frequency modulated with the luminance information. The recording and transcript of the color signal requires lower tolerances than those used for black and white information. For example, small deviations from the color sub carrier, the TV can not recognize the signal and only see black and white image.


Color Reproduction

The recovery of the original signal color conversion is done through a reverse way as in the recording. The imprecision of the mechanical system and the elasticity of the tape, resulting in reproduction as frequency variations. If we call the signal fsc printing, in reproduction we have (±  f'sc f) where f  error of reproduction (according to the values given above is (629 KHz ±  f)). By means of a converter that is applied to the same RF as that used in the recording, it reconstructs the color information in its original value of sub carrier. For the error disappears, a comparator circuit is used, this device compares the output of the mixer with a standard oscillator, the comparator circuit produces a control voltage which changes the oscillator frequency used for conversion, so that the signal has no error output. (4.211 MHz ±  f) - (0.629 MHz ±  f) = 3.582 MHz
 f value has been introduced by adding to the oscillator frequency of 4.211 MHz is the effect of the control voltage. The simplified process used in reproduction.

VHS format - Treatment of the signals

We started from a composite video signal of appropriate amplitude in our case 1 Vpp coming from a tuner or camera. In the case of the VCR (VCRs) actually provides a set consisting of a tuner similar to those used in color televisions, plus an amplifier and its associated detector. The TV signal is transmitted, as we have a carrier frequency, which changes according to the selected channel. But it is always recorded in VHS format. The same signal may be from another VCR in the case of a cassette copy as mentioned before from a camera. We also understand that the audio signal as sound Interporto (IS) has previously been separated, to be derived by another route and recorded on tape in the traditional form of audio recorders in an auxiliary runway.

Recording process - The luminance signa

The composite signal entered, is separated in the same way that a television in three parts namely: luminance signal or sync signal or color sub carrier signal. These signals are separated in the same way that a TV through a system of filters and traps, each is then entered in various circuits for processing. The luminance signal is input separately after a controlled gain amplifier, as you know these amps are used to control and maintain the level of the signal. Then passed to other amplifiers, from this point takes the signal to a sync separator, sync that is used to servo circuits, recording the pulses of control and processing circuits of the color signal. The standard signal (standard) 1 Vpp video has, however, for various reasons, this signal can vary in amplitude, therefore we must ensure a constant level for the proper functioning of the later stages. That is, the signal must be maintained within well defined limits, regardless of variations in input voltage. But no one should act to normal variations of average image brightness.

A system that simply reacts to the signal average or peak to peak amplitudes, reduce the gain when the average brightness and amplify it were high during dark scenes. The result would be a uniform brightness, gray, regardless of the content of the scene. What the system should correct a total amplitude error and not dependent variations of the scene. Consequently the above, it can not be taken as reference material program, the only thing that can be taken as reference is the horizontal sync pulse. For which it has a sync separator. The AGC detector operates as the type triggered the TV. The AGC cocked black compares the value of the signal with the sync. Then in some VCR is used or trap adds another color to remove all traces of the chrome signal. The filter used has a cutoff frequency of 3.38 MHz, the effect of this filter on the luminance signal is to reduce the definition to some 240 horizontal lines. In other words increasing the pixel size reduces the definition or picture quality. The signal continues, being entered into a circuit called emphasis, its function is to improve the signal to noise ratio. As the luminance signal is frequency modulated, and in all cases of frequency modulation, it tends to degrade the response to higher frequencies, degrade signal to noise ratio. The signal to noise ratio of a frequency modulated signal depends on the relationship between carrier frequency and signal frequency, and consequently when the frequency information increases the signal to noise ratio gets worse.

This high-pass filter increases the gain at high frequencies producing an effect of over-drive, commonly known as "Overshot". After the playback will be made the reverse process to restore the relationship to information correct amplitudes. Here are two circuits called latch and trimmer respectively, whose objectives are: Reset the zero reference level, which is lost in pre-emphasis circuit (coupling capacitors); The level of continuous change with average brightness of the scene and should be transferred during the processes-modulation and frequency modulation. On some machines interlocking precedes the pre-emphasis circuits, so that the peaks are generated at this stage are not determinants of interlocking dc level. Peak cut black and white. This allows us to have a maximum level of black and white, not allowing the signal "Y" on module to the VCO (voltage controlled oscillator) from which we obtain the FM signal. The signal thus obtained is entered as mentioned at the VCO, obtaining the corresponding output FM signal or carrier whose center frequency is 3.8 MHz, in all cases, with a sweep of 1 MHz this signal, via band pass filter whose center frequency is 3.8 MHz is applied to the head amplifiers, to be recorded on the tape.

vcr recording

Reproduction of the luminance signal

The signal coming from the head amplifier is composed of two frequencies:

•    The color signal with a center frequency of 629 KHz or, 627 MHz, according to the standard and
•    FM signal containing the luminance information with a frequency sweep from 3.4 to 4.4 MHz, between the extremes of synchronism and white of the scene.

For the different processes, it needs the information previously separate, voltage output from the head amplifiers, resulting two paths, one a ceramic filter to achieve high-pass luminance signal (FM) and the second band pass filter for color tension. The signal Y separated enters a controlled gain amplifier to adjust levels. The output of the amplifier enters a circuit called "DOC" or "DROP OUT", whose function is to eliminate noises that appear in reproduction. These sounds may be due to: mechanical interference, electronic interference sources or oscillators switched servo control stage, the wear and tear of the tape, all these noises appear as small white dots on the screen in the form of drops. As we know, the sounds are very short duration transients.

To achieve effective implementation of the function uses a level detector, a delay line of 64 microseconds and an electronic key switch. The operation is as follows: the signal at the terminals of the key switch are on one hand the direct signal and the other the signal stored in the delay line for the previous stroke, lack of information detected by the corresponding circuit, during that time the key is switched reproducing information from the previous line ended noise becomes the normal condition. This process is feasible because of the redundancy of information, i.e. the great similarity between two consecutive lines. The corrected signal is amplified again and proceeds to detect it with a circuit similar to those used in FM radio receivers. Then he proceeds to de-stress and to interlock signal to regain the level of zero, and finally amplified again and sent the signal to a mixer, to add color signal. The information thus obtained can be used directly by entering the TVC video input, or is applied to a modulator for transmission to the TCF, to enter by the RF input.

Process of recording color signal

Once separated color signal through a band pass filter whose center frequency is 3.58 MHz and 1.2 MHz bandwidth, the signal is attached to an amplifier gain control (AGC), this amplifier operates in a manner that allows information to exceed certain limits, causing congestion and deterioration of image quality. Then the signal is again filtered, which removes any trace of undesirable such as radio frequency of 4.5 MHz for sound (IS). This signal is inserted into a first converter or converter MAIN, which causes the frequency shift required.

The mixer requires another signal for operation of 4.211 MHz, which in this case stems from a second converter whose behavior explained shortly, allowing the displacement of the spectrum to 629 kHz. For these values of frequency, as clarification is important to mention that change according to the rule, therefore generically continue using the values of 629 KHz, 4.21 MHz and 3.58 MHz, having made that reservation. Like all the output converter circuit yields the sum and difference frequencies, a resonant circuit through which we want to adopt. The second converter receives at its inputs the following voltages: The first, coming from a crystal oscillator at 3.582 MHz and phase control. The phase control is effected by a phase comparator circuit which teem BURST pulses from the channel or camera.

Second, compilation is somewhat more complex. As a starting point, this signal is obtained by multiplying the horizontal frequency, by reference to the horizontal sync pulses. The frequency can be obtained 320 fH = 5 MHz or 160 fH = 2.5 MHz This tension is obtained from a VCO and delivered in the form of bursts or flashes when it is present the horizontal pulse. Then the tension enters a frequency divider circuit, this frequency divided by 8 if the incoming signal is 5 MHz or 4 if the input is 2.5 MHz in both cases the resulting frequency is 625 KHz . Part of this signal is fed back, to a phase comparator to effect control over the VCO frequency and keep as stable as possible.

One reason to perform an oscillator frequency of 4 or 8 times higher than is necessary, so you can get a more stable frequency drift frequency of 50 Hz in the oscillator will also appear divided on exit. After the division is performed the phase shift, this procedure is an encoding that is different for each system (PAL or NTSC). This point will be expanded later. This form of signal processing makes every head knows what the track that corresponds to read. Supplemented to the control pulses that are recorded in the bottom of the tape as an auxiliary signal, the control pulses are used not only for the operation of the circuit controlling the tape speed but also will position each head on the track right at the appropriate moment. It also reduces interference between adjacent tracks color.

Finally, after the phase shift is entered the second converter, also called sub-converter or converter side, where the output is obtained by the sum of the two frequencies [3.582 MHz + 625 kHz = 4.21 MHz] this tension resulting is usually separated by ceramic filters to finally enter the first converter (MAIN CONVERTER) as was mentioned above. At the start of the first converter we have full color information, ie a spectrum of double sideband and suppressed carrier centered on the new frequency of 629 KHz and a symmetrical bandwidth of ± 500 kHz. This output signal is achieved through a band pass filter, before being amplified and attached to a mixer circuit which will join the luminance signal (FM). The information emanating from the mixer output is applied to the heads through the respective rotary transformers.

Reproduction of the color signal

As discussed above the color signal undergoes heterodyne (Mixing) taken a carrier frequency of 629 KHz. The tension that comes from the head is a composite, made up of the FM signal containing luminance information, but (mixed-together), the color information, centered at 629 KHz. The latter signal is separated by a band pass filter with a bandwidth of ± 500 KHz and amplified by a controlled gain stage. This processor is the same used in the recording, unless reversed input filters in place of the 3.58 MHz filter (recording) is placed on 629 KHz (for playback) and added some stages. Once separated, the color information signal is applied to a controlled gain amplifier for the equalization of levels, then some more elaborate machines are usually placed another band pass filter to remove any trace of the FM signal. Then inserted to the first converter also called general converter or converter (MAIN CONVERTER). In this circuit the two signals entering the aforementioned of 629 KHz 4.21 MHz and a variety latter complex or elaborate, as in the recording process. From the controlled gain amplifier Burst Media is removed from a separate circuit for the phase control oscillator 3.58 MHz. The 3.58 MHz signal phasing is inserted to the second side converter or converter to be mixed with 625 KHz signal coming from the phase shift circuit. The 625 KHz signal from a controlled oscillator multiplies the horizontal frequency by reference horizontal pulses for regulation. The oscillator delivery [320 fH fH or 160] by the machine. This frequency obtained is divided by 8 (eight) or 4 (four) as appropriate. Obtained as a result of 625 KHz. This oscillator is a circuit that takes some of the output for refueling through a phase comparator for controlling the oscillator. This procedure is the same as made in the recording. The output of this circuit is 625 kHz phase enters the phase shift, which performs the reverse process in the recording made to achieve reconstruct the color signal with all its original properties.

Information gathered, as mentioned Entering secondary converter to beat it (mixing) with the RF of 3.58 MHz as a result of the mixture gives the sum and difference of those adopting the sum, 4, 21 MHz, using a band pass filter. After passing through the filter, the voltage is applied to the main converter where it multiplies the read voltage (629 KHz) of the output color obtained through a band pass filter, the frequency difference, 3.58 MHz, and is the signal color formed. With the respective amplitude modulation, suppressed carrier and bandwidth ± 500 kHz; characteristics of the chrome information. We now need amplifying and eliminate interference that occur. The killer is a circuit that disables the offset amplifier operation when it has lost the phase reference is the signal is lost identification, under these conditions is reproduced in black and white.

Head amplifiers and switching signals

The amplification system and switching heads: As is known should be considered two different situations in the explanation (RECORD) and reproduction (playback). Burning-in control terminal that switches electronic keys (REC) is a voltage of 12 V, this will close the keys grounded lower terminal heads, which in turn connects to mass input amplifiers reproduction, eliminating the source of internal noise in integrated circuits that can be induced at other stages. In the terminal control key (PLAY) the tension becomes zero, maintaining and ensuring that the key is open. The tension to be recorded is obtained at the output of MIXER (adder) FM signals modulated with luminance and color information with the respective phase shift centered at 629 KHz. This signal can not be incorporated in the head in this way, as explained in the introduction or the beginning of recording, the signal heads should be incorporated, must be a current or voltage proportional to the information recording. For all this, the step required is a voltage-current converter circuit between the mixer and the headers.

According to what is observed in the layout of the circuit, both heads are parallel, so there is no switching in recording and a head that is not supported on the belt despite being energized and no function either generates interference. The head is magnetizing the air. Reproduction: In this situation appears to excite the required voltage and close the electronic key (PLAY) and the potential vanishes (becomes zero) closing keys (REC). Under these conditions, the upper end of the headers is earthed and lower terminals connected to the amplifier head ready for playback. Having opened the keys (REC), the inputs of these amplifiers are open, allowing the entry of the information is read from the tape. Then, we must go through a stage of switching heads, an electronic key role in detecting mutation in which is supported on the tape head, ie the active head. The key switch heads (HEAD SW) receives the information directly from the carrier cylinder heads, through a strain called "PG". It is important to emphasize that in this way prevents the head off (the air is), do not inject the circuit noise or interference. There has been an express mention, but the heads are attached to the electrical circuit through a rotary transformer, where the static winding is connected to the switching circuit and connected to rotational head.

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Phase rotation

Recall that the fields consist of a signal read by the head "A" and another read by the head "B", or adjacent parallel tracks. If there is an error reading from any of the heads (CROSS TALK), there will be an interference, which translates to the screen, which are colored lines going from bottom to top, magnified depending on the colors red and blue. Sure, this happens if the correction is not made adequate or appropriate treatment of the color signal recording and playback. This process is already mentioned and called phase shift. The method starts from the principle that the color information between consecutive lines or close to any image, is very similar (the television picture has a high rate of redundancy). If we draw a perpendicular imaginary anywhere on the screen, is easy to see that between adjacent lines of a frame the color is almost the same. Is taken as the first example a machine working in NTSC, this system no phase change in the color information, therefore the process is simpler.

This reasoning is also to refer to the BURST phase, for which the color information   takes the stage and to further simplify the explanation we consider a saturated color field, i.e. a full screen image or the same color other than white or black. For example red. By appropriate switching, the phase of the color information and BURST, will be changed, with an increase of 90 º line by line, differing both heads so that the increase for the head "A" is positive and increasing the torque Head "B" is negative. The statements shown in the table below where there are phases (rotational vectors) representing the color voltage on each line. While the phase signal is sent to the head "A" line by 90 degrees each, which is sent to the head "B" is late 90th. We understand progress, draw opposite to the waters of the clock. We know this is all we are doing to eliminate cross-field interference. So it is important to know which is the reading of the headers, the own and cross country.

The corrective effect is achieved by proceeding in reverse, making recover the original information, but now with the contents of interference. The result, each of the reports themselves have the same phase as in the recording, while the interferences have opposite phases between consecutive lines. So far, there is no evidence that this procedure is done to eliminate or minimize interference. But it's really efficient, because, assuming the interference of equal magnitude, but opposite, when the time of observation the box on the screen, the human eye resolved in an additive, making the cancellation of interference. In conclusion, the eye as a result of the persistence in the retina, which is canceled or minimizes interference. They say minimize, because in an image with movement (an actual scene), we can not ensure equality of interference between consecutive lines, but they are very similar (redundant information).

In the PAL system the phase shift process is somewhat different, due to the phase change of the system itself. This causes additional problems that we try to explain in simple terms. The turns are introduced into one of the heads, in this case the "B" and are the same as those made for the NTSC system, the former remains unchanged. So far everything seems easier, but the complication comes next. In the first section we discussed the signal phases as it originates, then the two lines is the signal applied to each head, the "A", not the additional rotation features. The next two rows are the phases representing the respective reading with crossed-field interference. Finally, in the last two rows, one has the reverse rotation and re-composition of the signal. This restores the original phase information, while interference again be out of phase, but with a difference, now is the opposite phase every two lines. The result is the same additive effect on the retina of the eye cancel interfering information. All these processes of phase shift, do not completely eliminate the interference of cross-field color should be complemented with the inclusion of a comb filter. The comb filter is not limited to processors implemented in color, but there is a much more general.

This type of filter can separate frequency spectra that reach the entry form intertwined. A basic circuit and consists of an adder and a delay line. In the NTSC system must take into account the phase shift introduced to the chrome signal, as a consequence of the adder at the entrances will have information from two consecutive lines where information of adjacent lines is in phase opposition. As a result, the output of the adder will have eliminated the vestiges of interference and duplicated head own information.

We should clarify that the combination of two adjacent lines is possible because the information content changes very little from one another online. In the PAL system the process of eliminating this is something more complex, given that we are interfering information that is out of phase every two lines, which means using for the cancellation of the delay line of a tH duration 2.



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