GB2055239A - Verification of video recording - Google Patents

Abstract

An arrangement for verifying that recording onto multiple parallel tracks of a magnetic tape is actually taking place in a helical-scan recorder which includes a first and second pickup heads (740,742) located at fixed positions downstream from the headwheel. Each head has a gap oriented to provide significant response from alternate ones of the parallel recorded tracks on the tape. A detector (804, 810, 814) coupled to the pickup heads processes the received signals for producing a warning signal in the absence of a received signal from either of the pickup heads. <IMAGE>

Description

SPECIFICATION Verification of video recording This invention relates to an arrangement for verifying that recording is taking place in a helicalscan tape recorder.

In electronic news gathering (ENG) applications, the operator of a portable camera records the information on a portable tape recorder. Since news events generally happen only once, it is important to the news service to be sure that the event is properly recorded. It is desirable to verify that the tape recorder used to record the video produced by the camera is in fact recording the video. For this purpose, it is desirable to warn the operator in the event that recording does not take place.

Generally speaking, modern video tape recorders operate by mechanically scanning one or more recording heads bearing magnetic gaps across the tape to achieve high-frequency response without extreme tape speed. One of the most common forms of failure to record involves a loss of coupling between the head and the tape due to foreign matter or dirt. Such a decoupling will cause failure to record, notwithstanding that signals are applied to the head. Consequently, schemes which verify application of signal to the recording head will not necessarily be indicative of failure to record. What is needed is a playback head which is responsive to the recorded video signal.

Playback of the recorded signal can be accomplished by a playback head associated with the scanning recording head. The playback head can be mounted on the headwheel for rotation therewith and can scan across the tape. With such an arrangement, the recorded video may be detected and processed to provide an indication of recorded signal level which may be further processed by comparison with a reference level representing the minimum acceptable recorded signal level to provide the operator with an indication when the recorded signal level is below the minimum.

Arranging the playback head on the rotating headwheel in a quadraplex system increases the weight of the headwheel, and requires additional slip ring contacts or rotary transformers, and is for that reason undesirable. A stationary head cannot be used, because no matter how the gap in the playback head is oriented (whether perpendicular or parallel to the direction of motion of the tape) it scans across the recorded track in a direction orthogonal to the direction of original recording and no substantial variation in flux appears across the gap which may be sensed.

In a helical-scan recorder, the playback head may be mounted on the rotating headwheel or associated with the recording heads, but this is subject to the same objections relating to weight and slip rings as in the case of a quadraplex recorder.

In a helical-scan recorder a single fixed pickup head having a gap oriented parallel to the record track can be used in conjunction with a detector and an indicator to provide recording verification.

However, where multiple parallel tracks are used, a single head may simultaneously scan more than one track. When this occurs, the amplitude of the output signal from the single head may have random fluctuations due to the addition of randomly phased signals from the multipe scanned tracks. These random amplitude variations can falsely indicate a failure to record.

A A recording verification arrangement adapted for use with a helical-scan tape recorder constructed in accordance with the principles of the present invention includes first and second record heads adapted to be scanned across a tape.

The first and second record heads include first and second gaps disposed at first and second aximuth angles, respectively, relative to the direction of relative motion between the first and second record heads and the tape. These heads record first and second parallel tracks on the tape, at the first and second azimuth angles, respectively. First and second playback heads are disposed at fixed locations adjacent the tape at a point at which the first and second parallel recorded tracks appear after recording. Each playback head includes a gap disposed at an angle relative to said first and second azimuth angles, respectively for producing first and second verification signals indicative of the presence of the first and second recorded tracks, respectively.A detector is coupled to the playback heads for processing the first and second verification signals and for producing a warning signal in the absence of either of the first or second verification signals.

IN THE DRAWINGS: FIGURE 1 illustrates a tape transport mechanism useful as an aid in understanding the invention; FIGURES 2, 5 and 6 illustrate recorded tracks on a tape and a pickup head; FIGURE 3 illustrates an amplitude-time plot of the response of the head of FIGURE 2; FIGURE 4 is a block diagram illustrating electrical connections to the head of Figure 2; FIGURE 7 illustrates recorded tracks on a tape and two pickup heads located in accordance with the principles of the present invention; FIGURES 8 and 9 illustrate in block diagram form electrical connections to the heads of FIGURE 7, in accordance with the principles of the invention.

FIGURE 1 illustrates generally the support plate and main transport portions of a helical-scan tape recorder. Baseplate 10 bears a tape supply reel 12 and a take-up reel 14. A tape 16 leaves supply reel 12 and passes in succession over a tensioning spring 18 and a roller 20. The tape leaving roller 20 passes between a pinch roller 22 and a capstan 24 and thence to a guide post 28 which leads the tape onto a headwheel 30. Between capstan 24 and guide post 28, tape 16 passes across an erase head 26. Tape 16 leaves headwheel 30 guided by a second post 32 and passes between a drive capstan 34 and a pinch roller 36 before being guided onto take-up reel 14.

In normal operation, rotation of headwheel 30 causes one or more heads associated with headwheel 30 to scan a path illustrated as 38 across tape 16.

In a helical-scan tape recorder a pickup head illustrated as 40 in FIGURE 1 may be mounted in a stationary position as on a post 42 on the take-up side of headwheel 30. The gap (not shown in FIGURE 1) of head 40 is substantially perpendicular to the direction of the tape motion and will provide response which can be sensed and processed in order to provide an indication of recording.

In FIGURE 2, magnetic tape 210 is illustrated, at the top of which a control pulse track 212 may be recorded and at the bottom of which an autio track 224 may be recorded. The tape moves to the right as illustrated in FIGURE 2 with respect to head 40 in which a gap 242 is formed. Head 40 is connected as a playback head and is fixed to the body of the recorder as described in conjunction with FIGURE 1. A record head (not shown) has just finished laying down video track 214. Another track is about to be laid down next to track 214.

The track laid down previous to tracks 214 is track 216, and previous to that 218 and so on. The track presently overlying head 40 is track 234.

Between tracks 214 and 216 is a guardband 220, between tracks 216 and 218 is a guardband 222 and between track 218 and track 234 is a guardband 236.

At the instant shown, gap 242 is approximately parallel to the transitions illustrating the magnetic signal recorded on track 234, and responds to the signal in the track. Since the video information is frequency modulated as it is applied to the recording heads, the information contained in the magnetic tracks includes a carrier frequency and its diebands. At the instant shown in FIGURE 1, track 234 is substantially centered on head 40, and head 40 will respond with a maximum signal representative of the FM carrier signal.

As the tape moves to the right, the upper part of gap 242 will overlie a portion of guardband 236. With increasing tiem and tape motion, gap 242 will overlie progressively more of guardband 236 and progressively less of track 234, with the result that the signal picked up by head 40 will decrease. At a time T1 at which gap 242 lies entirely over guardband 236 and has no portion overlying either track 21 8 or track 234, the response of head 40 will be zero. A plot of the signal response as a function of time is illustrated in FIGURE 3. In FIGURE 3 the response is illustrated as being at a maximum at a time TO corresponding to the time illustrated in FIGURE 2.

At a later time T1, corresponding to a time at which gap 242 completely overlies guardband 236, the signal level is illustrated as being zero. As time increases, movement of the tape will bring portions of track 218 under gap 242, and the response will again begin to increase until T2, at which time gap 242 lies completely across track 218 and provides a maximum response. When gap 242 completely overlies guardband 222 in which no signal is recorded, the signal will again decrease to zero ad illustrated at time T3 in FIGURE 3.

FIGURE 4 illustrates generally in block form circuitry suited for providing an indication of "recording" or "nonrecording" to the operator of the tape recorder of FIGURE 1. In FIGURE 4, stationary pickup head 40 is illustrated as being coupled to an amplifier 402 for buffering the head and amplifying the signal to a level suited for amplitude detection. The output of amplifier 402 is coupled to an amplitude detector illustrated as a block 404. AM detector 404 may be, as is known, little more than a simple diode rectifier. The output of AM detector 404 is coupled to an integrator illustrated as a block 406, which will ordinarily include a capacitor. The output of block 406 is coupled to the noninverting input of a comparator designated generally as 408, the inverting input of which is coupled to a reference voltage source illustrated as a battery 410.Battery 410 provides the reference voltage against which comparator 408 compares the integrated output of AM detector 404. When recording is occurring, the output of integrator 406 will be a maximum in the selected polarity, which in this case is assumed to be positive. Thus, the output of comparator 408 during normal recording will be high. When the heads producing video tracks 214, 21 6, 21 8, etc.

are clogged, or there is some problem with the electronic signals supplied to the recording heads, so that tracks 214, 216 and 218, etc. are void or not recorded, the output of comparator 408 will be low.

The output of comparator 408 could be used for a fault indication without more, but would indicate a fault or "not recording" condition even when the recorder is not set for recording function even though energized. In order to key the output of comparator 408 to driving of the recorder headwheel and movement of the tape, a set of contacts S2 may be associated with switch S1 by which the recording function (i.e., the movement of the tape and scanning by the heads) is initiated.

Contacts S2 are coupled between a logic 1 or "high" voltage source and one input of each of AND gates 412 and 414. The output of comparator 408 is applied directly to one of the inputs of AND gate 412 and by way of an inverter 416 to the other input of AND gate 414.

The output of AND gate 412 is coupled to a green "recording" lamp 418 and the output of AND gate 414 is coupled to a red "not recording" lamp 420.

In operation, when the "record" function is implemented by closing switch S1, ganged switch S2 also closes. If a new tape has been put in place, gap 242 will for the first few moments receive no input signal because it does not overlie a scanned track. Consequently, the output of integrator 406 is low, comparator 408 produces a low output and one input to AND gate 412 will therefore be low and green lamp 41 8 will not be lit. Since the output of comparator 408 is low, the output of inverter 41 6 is high, and therefore both inputs of gate 414 will be high, whereby the red "not recording" lamp 420 will be lit.

As soon as a scanned track passes under head 40, the FM carrier will be sensed and amplifier 402, detected by detector 404 and integrated by integrator 406 to produce a high input to comparator 408. The output of comparator 408 will switch to a high condition, whereby both inputs to AND gate 412 will go high, lighting green lamp 41 8. At the same time, inverter 41 6 will apply a low to an input of AND gate 414, deactivating gate 414 and extinguishng the red "not recording" lamp 420.

If two adjacent tracks 502 and 504 have no guardband therebetween, as illustrated in FIGURE 5, a head 40 may have a response at the time that it overlays one of the tracks. However, in that interval in which the head straddles two or more tracks, there is a possibility of zero output or a large output depending upon the phasing of the carriers in the two adjacent tracks. Angling the gap as illustrated in FIGURE 6 will increase the response of the head to the recorded signal when overlaying one track, but will not help the problem of randomly occurring lack of output due to the random phasing of the carrier in the two adjacent tracks.

In accordance with the principles of the present invention in those helical-scan recorders in which the gaps are angled with respect to the scanning direction to provide isolation between the two adjacent tracks notwithstanding the absence of a guardband, a stationary head arrangement can be used without reduction in the signal level resulting from random phasing of the carrier.

In the embodiment of the invention shown in FIGURE 7, an audio track 724 is provided for at the bottom of tape 710. A video track 714 is being recorded at the instant shown, and immediately adjacent "A" track 714 is a "B" track 716. Immediately adjacent track 71 6 is another "A" track 718, and adjacent that another "B" track 720. The tape motion is to the right, as in the case of FIGURE 2. The recording head (not shown) which is recording track 714 has the gap of its head angled with respect to the direction taken by the track. The angle which the gap of the recording head generating track 714 makes with the longitudinal direction of the track is 70 less than 900. The other head (also not shown) in the headwheel which is scanning tape 710, and which just prior to the instant shown laid down track 716, has its gap canted the other way, also by 70.These heads are designated A and B, respectively, and produce alternate A and B tracks the recorded portions of which are offset by plus or minus 70 as illustrated in FIGURE 7.

The angle 70 is selected as a multiple of the angle at which the first null in the response of angled head occurs, so that the normal A and B pickup heads (not shown) will be fully responsive to their respective tracks and yet, as is known, be relatively unresponsive to the adjacent track. For example, where the lower deviation sideband reaches a frequency of 8 megahertz, and the headwheel rotates with a velocity providing the recording and normal playback head with a 456 inch-per-second velocity (11.5 meters/sec) with respect to the tape, the longest wavelength on the track is approximately 57 microinches (1.44 x 1 0#6meters). The angle made by the gap of the pickup head relative to the longitudinal direction of the track at the first null is approximately 0.6720.The eleventh null occurs at 70. Thus, the track A pickup head will respond principally to track A and reject track B and the track B pickup heads will respond to track B and reject track A information.

In the aforementioned system, stationary heads 740 and 742 can be used in much the same manner as in the arrangement of FIGURE 1. In particular, as the tape moves across heads 740 and 742, head 740 will respond to the A tracks and head 742 will respond to the B tracks.

Because of the angling of the gaps in heads 740 and 742 to match the azimuth angle of the recorded tracks, head 740 will not respond substantially to the B tracks and head 742 will not respond substantially to the A tracks.

Consequently, head 740 effectively has a guardband between A tracks which consists of each of the B tracks and head 742 has a guardband between the B tracks which consists of the A tracks. Thus, neither head 740 nor head 742 will be responsive to energy in either of the tracks adjacent the track to which it is responsive, and therefore the signal of those adjacent tracks cannot cancel the desired signal.

Since head 740 responds only to the A track and 742 responds to the B track, the information may be used to provide the operator with the information in its separated form (i.e., two green lights, one for track A and one for track B, and similarly for the red lights) or additional logic can be used to combine the signals and provide a composite green or red indication.

The speed of the tape across stationary heads 740 and 742 is only about 5 inches per second (12.7 cm/sec), notwithstanding the high headwheel speed, and the wavelength as calculated above yields a 97 kilohertz response at heads 740 and 742. Thus, the AM detector need only have a bandwidth in the vicinity of 100 kc, which is much lower than the 8 megahertz frequency at which the signal was recorded. AM detection of the signal provides a signal amplitude which pulsates regularly with time, as illustrated in FIGURE 3.

Instead of using an integrator and comparator as in FIGURE 4, the arrangement of FIGURE 8 may be used. A stationary head 840 is coupled to an amplifier 802, the output of which is coupled to an AM detector 804 and an integrator 806. At the output of integrator 806, a 30 Hertz signal is produced during normal scanning, as each track to which the head is responsive passes under the head.

Instead of responding to the amplitude of the signal picked up from the recorded tracks by the verification head, the control circuit may respond to the frequency components. In normal operation of a helical-scan recorder, two heads alternately scan the tape for the duration of one field (1/60 second under NTSC standards). Thus, each of the two heads scans 30 times per second. Since in normal operation the heads scan alternately, the total number of scans/second is 60. With both heads recording properly, therefore, there is a 60 Hz component to the envelope of the signal detected from the verification head in normal operation. When one of the heads fails to record, however, there is only a 30 Hz signal. The control circuit may use filters to discriminate between 30 and 60 Hz signals to provide an indication or may simply respond to the 30 Hz component.In FIGURE 8, a verification head 840 is coupled by a buffer amplifier 802 to an AM detector 804 and associated integrator 806. The output of integrator 806 is a 60 Hz signal as illustrated by waveform 808 in normal operation. A 30 Hz filter 810 coupled to integrator 806 will not respond to signal 808. During a failure of one of two heads to record, a signal such as is illustrated by waveform 812 will be produced at the input of filter 810.

Filter 810 responds to the 30 Hz signals and coupled them to a further amplitude detector or comparator 814, which triggers at a predetermined level of 30 Hz component of signal 810 to produce a "not recording" control signal.

The control signal is amplified by amplifier 815 and applied to a red recording lamp 816.

In the arrangement of FIGURE 9, the outputs of 30 Hz and 60 Hz filters 910 and 922 are compared in a comparator 920 to produce the control signal for amplification by amplifier 915 and application to lamp 916. Such an arrangement insures that variations in signal levels or timing which might cause improper operation in an absolute-amplitude system as in FIGURE 8 are normalized so that only relative amplitude differences can affect the indication.

Other embodiments of the invention will be apparent to those skilled in the art.

Claims (4)

1. A recording verification arrangement adapted for use with a helical-scan tape recorder which recorder includes first and second recording means adapted to be scanned across a tape, said first and second recording means including first and second gaps disposed at first and second azimuth angles respectively, relative to the direction of relative motion between said first and second recording means and the tape for recording first and second parallel tracks on said tape, at said first and second azimuth angles, respectively, the verification arrangement comprising:: verification head means including first and second playback heads disposed at fixed locations, said playback heads being adjacent said tape at a point at which said first and second parallel recorded tracks appear after recording for producing first and second verification signals indicative of the presence of said first and second recorded tracks, respectively; and detection means coupled to said verification head means for processing said first and second verification signals and for producing a warning signal in the absence of either said first or second verification signal.

2. An arrangement according to Claim 1 wherein said detection means comprises amplitude detection means and said verification signals have a relatively large amplitude during correct recording and a relatively low amplitude during failure of recording.

3. An arrangement according to Claim 1 wherein said detection means comprises amplitude detection means for providing an output signal having a predetermined frequency component indicative of normal recording.

4. An arrangement according to Claim 3, wherein said detection means further comprises a frequency selective detector coupled to said amplitude detector for producing said warning signal in the absence of said predetermined frequency component of said output signal.