Capabilities of VCRs when recording high-quality sound, maintenance and repair. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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Recently, many of our readers have been interested in "home theater" systems, for which in our country VHS video cassettes are mainly used to play not only images, but also high-quality (Hi-Fi) stereo sound. The problems associated with this will be discussed in the published article.

The rapid growth of interest in "home theater" systems abroad and in Russia has stimulated manufacturers of audio and video equipment to significantly expand the range of all its types. In 1997-98 Many models of such devices have appeared on sale in the most affordable price categories for buyers. Despite the emergence of new media and sources of video and audio information, such as DVD, satellite channels, video CDs, etc., VHS video cassettes with stereo sound accompaniment remain the main source of home theater programs in Russia. The range of films with DOLBY SURROUND PROLOGIC (DPL) and music programs has also been growing rapidly in recent years and, given the appearance on sale of stereo video players at a price of $ 200 ... 300, it can be argued that the corresponding equipment has become relatively affordable and is quickly gaining popularity in the most various parts of Russia.

When implementing fundamentally new technical directions in practice, consumers and service departments have a lot of very different questions, and it is not always possible to get answers to them. And in the conditions of Russia there are a number of nuances that are not typical for other countries. For example, the presence of a language barrier in most of the population. For more than a dozen years now, foreign films have been watched in our country either fully dubbed or with offscreen translation, and not with short credits, as is customary in many European countries. For "home theater" systems, the quality of sound accompaniment of films is especially important, so the overlay of sound during dubbing is a very responsible operation. Poorly done (technically) dubbing has already become a source of frustration for many of our home theater system owners.

Let us briefly dwell on some general technical issues concerning such systems. Historically, the first was the "home" version of the DOLBY STEREO surround sound system, which has been used in cinemas for more than a decade. This version was called DOLBY SURROUND PROLOGIC, or DPL for short. The essence of her work is to decode the stereo sound track (SOUNDTRACK) of films recorded using the DOLBY STEREO system, and obtain four channels from it: left, right, center and effects. SOUNDTRACK DOLBY STEREO have most of the most popular movies. The corresponding logo has probably been seen by many fans of video films at the end of the credits for them.

The development of the DOLBY STEREO system was the so-called experiment of engineer Tom Holman (Tom Holman), abbreviated as the THX system, conducted at LUCASFILM. THX, used in home theater systems, uses the same stereo sound as DPL, but its decoder and components are subject to special technical requirements according to LUCASFILM specifications. Two more artificially formed channels have been added: ultra-low-frequency and additional rear. Various frequency response corrections and other improvements have been introduced. As a result, according to the company's engineers, watching movies becomes more realistic and spectacular.

Technical solutions of THX are patented by the company, therefore, equipment manufacturers pay license fees for the right to apply the corresponding logo. As a result, THX-based "home theater" components are significantly more expensive than corresponding DPL components. The technical details of the system are discussed in more detail in [1].

There are a number of other, as a rule, digital "home theater" systems: DOLBY DIGITAL (AC-3), DTS, THX5.1, etc. their own names. YAMAHA was especially successful in this, developing the "CINEMA DSP TM" processor (US patent No. 5.261.005; YSS-245 chip, 64 pins) for DPL phonograms and the "TRI-field CINEMA TM" modification for DOLBY DIGITAL (AC-3) .

However, on closer inspection, "advanced" "home theater" systems in Russia are difficult to use. The reason is the almost complete lack of sources of programs for them. Therefore, at the moment, almost a monopoly position in the market of "home theater" systems is occupied by DPL.

What sources for it are available to mass consumers in Russia at the present time? Terrestrial television, to the best of the author's knowledge, has no definite plans for the introduction of stereo sound transmissions even in the distant future. The existing domestic satellite channels, including NTV+, remain monophonic. Foreign satellite television is difficult to access due to the language barrier.

Consequently, programs remain on various video discs - LD, VIDEO-CD, DVD and some others, and on VHS and S-VHS video cassettes. However, there are not so many dubbed films on discs, and their prices and playback equipment still preclude their wide use in Russia. Therefore, we repeat, programs on VHS video cassettes have received overwhelming distribution in our country. The price of blank E-180 cassettes is less than two dollars, which is many times less than that of any discs (besides, we especially appreciate the possibility of free rewriting).

Unfortunately, the sound quality of a large number of dubbed videos, to put it mildly, leaves much to be desired. According to the author's observations, a large number of sold cassettes is, in the application to the "home theater", an obvious defect (the estimate was made for the local market). There are several typical manifestations of this "quality" of domestic video products:

on the cover of the so-called "licensed" cellophane-sealed cassettes, the Hi-Fi STEREO or DOLBY PROLOGIC logo flaunts; in fact, only the usual monochannel of sound is present on the recording, however, the owners of Hi-Fi VCRs immediately recognize such fakes - the corresponding indicators do not turn on;

there are stereo channels on the recording, however, both contain the same information, i.e. mono mode with variations in quality from that corresponding to ordinary video recordings to quite high (for example, many SOYUZ cassettes are characterized by such pseudo-stereo sound, on the music collections of which only part of the material is stereophonic) ;

There is a stereo mode, but the thunderous voice of the translator practically makes it impossible to activate the central channel, you have to reduce its level to the limit (usually 12...16 dB).

And finally, professional dubbing, combined with really high quality sound, is quite rare, and oddly enough, on almost all films in the NTSC system (here, obviously, the impossibility of using the simplest technique with adders on two resistors also affects).

Separately, there is a question about the technical quality of the sound and the compatibility of recordings via Hi-Fi stereo channels. A distinctive feature of sound recording with rotating heads is the high sensitivity of video recorders to deviations in the parameters of the read signalogram from standard values. Regardless of the reason that caused such deviations, their manifestation is reduced to the appearance of a crackle, the periodic switching off of Hi-Fi channels, and the need for frequent manual adjustment of tracking. The visibility of crackling and other interference in "home theater" systems is so great that it simply does not make sense to talk about other parameters of sound reproduction quality until the interference is eliminated.

There are quite a few reasons why certain videos are incompatible with a particular Hi-Fi VCR. The first in this series is the wear and tear of recording equipment at replicating studios, since at a rare of them they monitor its technical condition in the application specifically to recording Hi-Fi sound accompaniment. The studio staff usually only regularly performs preventive cleaning of video heads and CVL elements, which is completely insufficient. The fact is that most of the available home video recorders provide a much shorter operating time through Hi-Fi sound channels than through video. In particular, according to the information collected by the author, the upcoming production of new home video recorders, in which video consumers will not have problems with the quality of Hi-Fi sound, is in the range of 600 ... 1500 hours, depending on the model of video recorders, magnetic tapes and operating conditions. Trouble-free operating time on the video channel is at least two to three times longer. This information was obtained from the results of instrumental measurements of the parameters of more than a hundred different video recorders operating in one of the large replicating studios in Rostov-on-Don.

Bearing in mind the operation of the equipment in the recording mode with the preservation of the parameters of the Hi-Fi sound signalogram, it is possible to single out the most reliable models in this respect from the selection made by the author. In the first place in terms of operating time are PANASONIC VCRs: NV-F55AM, NVFS88EE, NV-FS200EG, AG-5700E and other devices of various companies equipped with MATSUSHITA type A or B CVLs (with a tape rollback motor) according to the classification in [2]. Then come the PANASONIC models: NV-HD100AM, NVHD100EE, AG-5260E. PANASONIC devices: NV-HD650AM, NV-HD650EE, NV-HD750AM have a significantly lower resource, and especially low for relatively new and cheap PANASONIC video recorders: NV-SR70AM, NV-HP10RAM.

As for the equipment of other developers, the author has information only on a small number of JVC models (HRJ727MS - two copies, HRS6900EE - one, HR-S7000EG - one) and individual copies of SONY and HITACHI. Their operating time of 600...800 hours corresponds to PANASONIC devices: NV-HD650, NV-HD750.

Compatibility is also influenced to a certain extent by different approaches of equipment manufacturers to the mutual arrangement of video and audio heads on the top cylinder of the BVG. The fact is that there are two options for their location: with a shift of 42o (138o) and 60o (120o) between the axes. The reason for this presumably lies in the patent-licensing area. Angular spacing 138o is supported by JVC, SONY, PHILIPS, SABA, UNIVERSUM and some others. Heads of MATSUSHITA, MITSUBISHI, BLAUPUNKT and others are carried by 120o.

On fig. 1a schematically shows the relative position of the video and audio heads on the upper cylinder when viewed from above. The sound head ГЗ1 “flies up” to the beginning of the signalogram at the point 0о, and the video head VG120, which is lagging by 138 or 1 °, demagnetizes the thin upper layer of the tape on a part of the recorded audio track (Fig. 1, b and c). At standard speed in PAL/SECAM systems, the width of the video and audio tracks is 49 microns, the spatial shift between them is about 10 microns (138o shift) or 16 microns (120o shift). Consequently, the audio and video tracks on the tape do not match (Fig. 1b), and already initially there is an incomplete compatibility of about 6 μm (approximately 12%) between the two head arrangements. For new and finely tuned equipment, it does not matter, JVC VCRs play back recordings made on PANASONIC devices without problems, and vice versa. A completely different picture is observed in worn-out or inaccurately tuned equipment.

In my opinion, the shift of the dynamic trajectory of the sound head relative to the corresponding track on the signalogram of about 50% (24 microns) can be considered critical. Practice shows that such a shift occurs very often. Among users of Hi-Fi VCRs, there are a wide variety of rumors and speculation about this. Some of them are not without foundation. In particular, it has been noticed that some purchased videos on many modern Hi-Fi video players are played with a crash or no Hi-Fi channels at all, while the same recordings on expensive VCRs sound quite normal.

To clarify the situation, the author carried out a number of measurements and analyzed the design features of the popular PANASONIC-NV-SR70AM video players (six devices). The device (hereinafter referred to as the 70th for brevity) is a multi-system single-speed video recorder with standard functions for this class of technology. LPM is made on a cast chassis, like most PANASONIC models of the SD, HD series. The top cylinder is a VEH0714 with four heads (two for Hi-Fi audio and two for standard mode video). BVG drive - with the lower arrangement of the engine. The preamplifier (REF.NO.500 block) is assembled on BA7180AFS microcircuits (20 pins, video section) and BA7743FS (24 pins, Hi-Fi section) from ROHM, the image channel (REF.NO.300 block) is based on AN3553NFBP LSI (84 pin) from MATSUSHITA and a TL8949P CCD from TOSHIBA (16 pins). The Hi-Fi STEREO channel is located on a separate board (block REF.NO.4500). As in many other models of PANASONIC VCRs, the Hi-Fi sound submodule is designed and manufactured by MACLORD / HSS & HSL (this is the chip marking). In the case under consideration, it is made on the LSI XLH7773AKS (80 pins) of this company. The control and auto-regulation system (REP.NO.6000 block) is based on the MN67434VRSY microprocessor (84 outputs). The BVG electric drive is made on the AN3814K chip from the MATSUSHITA company, the drive shaft is based on the BA6871S from the ROHM company, and the refueling engine control system is based on the latter's BA6887. The video player uses a transformer power supply with linear stabilizers on transistors 2SD1273, 2SD1275. The popular PANASONIC-NVHP10RAM video player can also be called practically identical to the one under consideration, including in appearance.

In the process of testing several instances of video players, a number of shortcomings were identified. The first thing that caused surprise was the low accuracy of setting the control heads in position, that is, the distance between the clearance of the control head and the end of the recording line (in VHS format it is 79,244 mm). There was an impression that either the corresponding technological operation was not carried out at the assembly plant, or the influence of the "human" factor during assembly (we are talking about a specific batch of six copies of video players) affected. Visually, this is manifested in the absence of Hi-Fi channels and the presence of noise on the image in the first seconds after loading the cassette and turning on the playback of test cassettes. In addition, recordings made on these video players may not always be played back with good quality in FREEZE FRAME mode even on three- and four-head VCRs.

To eliminate this shortcoming, it is necessary to align the control head in position, for which you need to use an exemplary test cassette (you can, for example, record a "white field" signal on a new VCR of the highest possible quality). During adjustment, the X input of the oscilloscope is connected to the control point TP2001 - "HEAD.SW", and the Y input is connected to the TP3001 point - "ENV".

Before starting work, it is necessary to restore the K1101 jumper located on the vertical power supply board on the right side of the case (at the factory it is bitten after the adjustment is completed). In the absence of a special screwdriver for adjustment (VFK0330 - FINE ADJUSTEMENT GEARDRIVER), loosen the two screws securing the control head platform, turn on the test cassette playback mode, with the thumb and forefinger of the left hand, turning the platform around the axis, find the position at which the level of the envelope of the brightness signal maximum. Without releasing the platform, carefully and alternately fix the fastening screws with a screwdriver in the right hand. Then the jumper is removed, which activates the auto-tracking system of the auto-regulation system and checks the operation of the device with various cassettes. After starting playback, the image should appear immediately without interference, Hi-Fi channels, depending on the specific video recording, may turn on immediately or after a few seconds.

A very serious shortcoming of some instances of video players is the inaccurate setting of tracking on Hi-Fi sound tracks by the auto-tracking system. The fact is that the detected signal (envelope) from the output of the brightness channel preamplifier serves as an exemplary signal for it, and the correctness of the Hi-Fi sound heads following the corresponding tracks of the signalogram is determined only by the accuracy of their installation on the upper cylinder and the manufacture of the heads themselves. Unfortunately, in many video players tested by the author, these accuracies are clearly insufficient, i.e., no adjustment of the guide posts can be used to obtain accurate passages of video and audio heads along their tracks. One gets the impression that MATSUSHITA has different approaches to the production of equipment of various price categories in terms of tolerances for units and parts of equipment. However, this is only the personal opinion of the author. It is very difficult to eliminate the inaccuracy of the installation of the heads on the upper cylinder without special equipment, therefore, the owners of the video players in question are recommended to set the tracking manually by the absence of crackling in the Hi-Fi sound channels.

Another feature that negatively affects the quality of sound reproduction on Hi-Fi channels of the 70s video players is the relatively high threshold for the operation of their phonogram quality identification system. In other words, in cases where most VCRs do not yet turn off the Hi-Fi stereo channel, the 70s turn on the playback mode of the "normal" audio channel (from the linear track). It should be noted that the level of magnetization of sound tracks in various purchased video cassettes varies widely. According to the experimental measurements of the author, the spread can reach 10 ... 16 dB (in terms of the voltage at the output of the preamplifier of the control video recorder). The threshold for the identification system "bad / good" - video equipment manufacturers do not standardize linear / Hi-Fi channels in any way, so there are many examples when the same record is played differently on specific working devices ("NORM" or "Hi-Fi "by indicator). In this regard, the 70th is insensitive (high response threshold), and, perhaps, this was not done by chance, bearing in mind the previously stated assumption about different accuracy classes of the equipment. And the consequences when playing FM signals with low levels are similar to those with uncertain reception of VHF FM stations.

Unfortunately, we have to continue the list of shortcomings of simple and cheap video players. Regarding the 70th (and HP10), for example, it is necessary to note the absence of a very important trimming resistor VR7 ("REC-Y" - the brightness signal recording level), provided by the developers, instead of which there is a jumper, and some other important elements of the path of the image and sound channels.

How to improve the recording quality of a Hi-Fi sound track by selecting the brightness signal recording current will be described in the following publications. I would only like to point out the possibility of a significant "life extension" of the upper cylinders in this way. In general, you can make Hi-Fi stereo recordings of very high quality even with worn heads.

An extremely limited interpretation of the shortcomings of the considered video players should be emphasized - after all, only six copies were tested, and serious violations of the technology were revealed during mass production.

The results of testing some common models of VCRs for high-quality sound reproduction and recording will also be discussed in the following publications. They (results), in the opinion of the author, are very interesting and may be of interest to many lovers of magnetic sound recording. In particular, without comment, they are invited to draw their own conclusions on measurements of the recording-playback path of the JVC-HRJ627MS VCR based on the JCP0056 LSI (JVC) when a meander-like signal is applied, as shown in Fig. 2.

Now consider some interesting cases of repairing such VCRs.

There is a blocked cassette in the JVC-HRJ727MS VCR. When connected to the network, the operation of the engines was heard for several seconds, after which the device switched to standby mode. When the cover was removed, it turned out that the tape on the locked cassette was wound up to the end (a transparent leader cord is visible on the left), which indicated a malfunction in the system for identifying the state of the cassette receiver.

As is known, in VHS VCRs, this system is capable of detecting the beginning, end, and breaks of a tape in a cassette. The sensors are infrared photodiodes (left and right), the emitter is the central LED. In most models produced before the beginning of the 90s, the photodiodes of the sensors are placed directly on the sides of the cassette receiver, and the central LED is placed on a special stand in the center of the LPM. Recently, many companies, for technological reasons, place them on the main board of the VCR (under the LPM), and the light fluxes pass to the cassette and are captured after it through prisms, and light and photodiodes of various designs are used. This approach eliminates manual soldering operations and reduces the cost of production.

In the model under consideration, it is the second approach that is used: the Q605 photodiode, located on the main board and connected according to the circuit shown in fig. 3.

The initial bias voltage is applied to it through the resistor R688 in all modes except standby. The identification signal from it through the resistor R611 goes directly to the microprocessor of the CMOS structure of the control system IC602 (HD6433927F from HITACHI). You can check the health of the Q605 photodiode by connecting a high-resistance oscilloscope or voltmeter to it. By turning on the VCR in the network and pressing the OPERATE button, in some way they block the light flux entering it. A good photodiode should provide a level 0 voltage of no more than 1 V (dark state) and a level 1 voltage of at least 3,5 V (illuminated state).

In a faulty device, level 0 exceeded 2 V, i.e., almost corresponded to level 1, and level 2 was normal (about 5 V). Since it is rather difficult to find the necessary replacement for a miniature photodiode, the initial bias current of the photodiode was reduced: instead of a frameless resistor R688, an OMLT-0,125 resistor with a resistance of 27 kOhm was included. This does not affect the operation of the VCR in any way, since the resistance of the microprocessor input at pin 32 is several hundred kilo-ohms. In another copy of the JVC-HR-J727MS VCR with the same malfunction, it was necessary to further increase the value of the resistor R688 - up to 51 kOhm.

The considered method of restoring performance can also be applied to other models of the JVC line of 1995-1997: HR-J627MS, HR-P80A, HR-P90, HR-J429EE, HRJ329EE, HR-J229EE - as well as for PHILIPS VCRs, "filling "which are made at the JVC factories: VR-755/55, VR-355/55, VR-255/55 (in all listed models, the reference designations, types and ratings of the elements correspond to those indicated in Fig. 3).

Recurring problems with no sound recording on the line channel have been observed in several PANASONIC-NV-F55EE (and NVHD100AM) VCRs. The reason for the failures in all cases was the absence of the RF bias current in the signal applied to the recording head, due to a defect in the R4002 tuning resistor ("A.BIAS"). It is better to replace it with some kind of sealed resistor, for example, SFC-1a (b or c) with parameters of 200 kOhm and 0,25 W.

After replacing the resistor, the selection of the optimal bias current is required, it is checked in a manner generally accepted for sound tape recorders. Measure the RF bias voltage on the recording head (pin 3 of the P4001 connector on the main board of the VCR, as indicated in the fragment of the circuit shown in Fig. 4) with a high-impedance oscilloscope at various values ​​​​of the resistance of the resistor R4002.

A sinusoidal signal with a level of 100 ... 150 mV and a frequency of 1 ... 3 kHz is fed to the input of the linear audio channel and recorded. Playing back the recorded recording, the value of the bias voltage is found, at which the level of the sound signal at the output is maximum. The optimal bias voltage swing on pin 3 of the P4001 connector is usually in the range of 25 ... 35 V (rms value - 8,5 ... 10,5 V).

Literature

1. Scheibal P., Klaus O. THX. - STEREO & VIDEO, 1995, No. 7-8, p. 66-69.
2. Petropavlovsky Yu. VHS format video equipment. Repair strategy. - Radio, 1995, No. 8, p. 12-15.

Author: Yu.Petropavlovsky

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