INDEX SIGNAL DETECTING SYSTEM

Abstract: An index signal detecting system for detecting an index signal recorded in a predetermined track area in the high speed playback mode is disclosed. According to this index signal detecting system, the rotary head is wobbled in the direction intersecting the track so that in the high speed playback mode, the index signal can always be picked up by the rotary head.

Description: This invention relates generally to an index signal detecting system for detecting an index signal recorded in a track area separate from those of a video signal and a PCM (pulse code modulated) audio data so as to carry out a program search and so on for use with a rotary head type recording and/or reproducing apparatus such as an 8 mm video tape recorder (VTR) or the like. The so-called 8 mm VTR (video tape recorder) can employ, in addition to a recording mode for recording an audio signal such that it is frequency-modulated and mixed with a color video signal under the condition that the audio signal is separable from the color video signal in frequency, such an optional recording mode in which the audio signal is pulse-code-modulated and recorded in an area separate from that of the color video signal so as to form one skewed track together. Fig. 1 schematically illustrates one example of a rotary head assembly used in the 8 mm VTR and Fig. 2 illustrates a tape format thereof. Referring to Fig. 1, record and reproduce rotary magnetic heads HA and HB are arranged to differ from each other in their azimuth angles of recording gaps and mounted on a rotary drum 1 with a distance of 180° therebetween in a rotation angle. The rotary magnetic heads HA and HB are rotated at a frame frequency (30 Hz) in the direction shown by an arrow 3H in the state that they slightly protrude from the periphery of the rotary drum 1. A magnetic tape 2 is wrapped around the periphery of the rotary drum 1 over an angular range of 221° in a helical fashion and is transported at a constant speed during recording and normal playback in the direction shown by an arrow 3T. Accordingly, as shown in Fig. 2, a signal is recorded in such a manner that tracks 4A and 4B each having a corresponding length to 221° of rotation angle of the rotary head are alternately formed on the tape 2 by the rotary heads HA and HB. On an area AP of about 36° of rotation of the rotary head (including a margin to record a PCM audio signal in the so-called after-recording mode and a guard band area) from a time point at which the rotary heads HA and HB start to scan the tracks 4A and 4B, there is recorded a pulse-code-modulated and time-base-compressed audio signal associated with one field period of the video signal. On a succeeding area AV of 180° of rotation angle of the rotary head, there are recorded a color video signal and an FM (frequency-modulated) audio signal of one field period, and a tracking pilot signal. The remaining area of 5° of rotation of the rotary head is assigned as a head detaching area used when the rotary head HA or HB detaches from the tape 2. As described above, the 8 mm VTR is capable of recording and/or reproducing the PCM audio signal. So, this feature of the 8 mm VTR attracts particular attention and it has previously been proposed to use the color video signal recording area AV as the PCM audio signal recording area so that the 8 mm VTR can be used also as an exclusive apparatus for recording and/or reproducing a PCM audio signal (see U.S. Patent NO. 4,542,419, issued on Sept. 17, 1985). The above mentioned previously proposed technology effectively utilizes the fact that the area AV of 180° of head rotation where the video signal and so on are recorded is five times as long as the PCM area AP of 36° head rotation angle. Then, the area AV is equally divided by 5 to form 5 divided segment track areas AP2 to AP6 shown by to in addition to the original PCM audio signal track area AP1 shown by per each of the tracks 4A and 4B, as shown in Fig. 3. Then, the PCM audio signal of one channel, that is, the PCM audio signal of one field period with the compressed time base is recorded in and/or reproduced from each of 6 segment track areas AP1 to AP6. Accordingly, in this case, the audio signal of one channel can be recorded and/or reproduced by the area unit so that 6 audio signal channels can be recorded and/or reproduced, thus providing a recording time (capacity) 6 times as long as the prior art recording time (hereinafter this technique will be referred to as a multi-PCM mode). In the multi-PCM mode, the PCM signal processor may be a signal processor capable of processing a signal of one channel and which is used in the prior art 8 mm VTR because the PCM signal is recorded and/or reproduced at the unit of every segment track area. By the way, the track format of the above mentioned 8 mm VTR will be described more fully with reference to Fig. 4. Referring to Fig. 4, from the right-hand side at which the rotary head begins to scan the magnetic tape 2, a top portion corresponding to 5° head rotation is assigned as a tracing start area 11. An area of the latter half of this tracing start area 11 and of 2.06° (corresponding to 3H periods where H is the horizontal period) head rotation is assigned as a preamble area 12 which becomes a clock run-in area synchronized with a PCM data succeeding thereto. Next to the preamble area 12, there is provided a PCM data recording area 13 of 26.32° head rotation, in which a time-base-compressed PCM audio signal is recorded. An area next to the PCM data recording area 13 and of 2.06° (3H) of head rotation is assigned as a postamble area 14 which is used as a margin to cope with a case where the recording position is displaced in the after-recording mode and so on. A next area of 2.62° of head rotation is assigned as a guard band area 15 between a video signal area and the PCM data area 13. Next to the guard band area 15, there is provided a video signal recording area 16 of one field amount of over 180° head rotation. Then, an area next to the video signal recording area 16 is assigned as a head detaching area 17 of 5° head rotation extent. Fig. 5 illustrates a track format of the multi-PCM mode more in detail. One segment tract area for the PCM audio signal is exactly the same as the PCM audio area in the track format of the 8 mm VTR. Referring to Fig. 5, this track format of the multi-PCM system is formed of a tracing start area 21, a preamble area 22, a PCM data area 23, a postamble area 24 and a guard band area 25. This track format is assigned to each of the segment tract areas AP1 to AP6. The PCM data is recorded such that the data "1" and "0" are modulated and then recorded on the tape 2. In the 8 mm VTR, for example, the data "1" is frequency-modulated as a signal having a frequency of 5.8 MHz and then recorded, while the data "0" is frequency-modulated as a signal having a frequency of 2.9 MHz and then recorded. In the preamble area 12 or 22 and the postamble area 14 or 24, there are recorded the data of all "1" that is, the signal having the frequency of 5.8 MHz. As a method for carrying out the program search in the 8 mm video tape recorder mode and the multi-PCM mode, the assignee of the present application has previously proposed a technique in which an index signal is recorded, for example, in the postamble area 14 or 24 in the respective track formats as described above and this index signal is used as the program search. According to this previous proposal, the index signal can be recorded and/or reproduced by the rotary head so that it is unnecessary to provide an exclusive stationary head for the index signal. Further, the index signal can easily be recorded and/or erased in the after-recording mode. The above mentioned program search using the recorded index signal is frequently carried out in the so-called high speed playback mode in which the tape is transported at high speed. In this case, the rotary head HA and HB scan a plurality of recording tracks as shown in Fig. 6. Since the rotary heads HA and HB differ from each other in the azimuth angles of their recording gaps, in the high speed playback mode, the respective rotary heads HA and HB pick up the signal only from either one of the tracks 4A and 4B. Therefore, reproduced outputs of both the heads HA and HB become as shown in Figs. 7A and 7B. In Figs. 7A to 7D, reference letter TA designates a scanning locus of the rotary head HA and reference letter TB designates a scanning locus of the rotary head HB, respectively. The envelope waveforms of the outputs from the rotary heads HA and HB in the respective scanning loci TA and TB are illustrated in Figs. 7A to 7D. In the high speed playback mode, although the drum phase servo can be applied to the rotary heads HA and HB, the tracking servo is not applied to the recording track so that the scanning position of the rotary head relative to the recording track pattern is not determined. However, after the high speed playback is started and the tape is started running, the scanning phase of the rotary head relative to the recording track pattern is determined so that the envelope waveform is repeated at the period corresponding to the scanning phase. Fig. 7A shows an example of an envelope waveform of the output from the rotary head at a certain scanning phase when the tape speed is an even multiple of the normal tape speed and Fig. 7B shows an example of an envelope waveform of the output from the rotary head at a certain scanning phase when the tape speed is an odd multiple of the normal tape speed, respectively. When the tape speed is exactly the integer multiple of the normal tape speed, as shown in Figs. 7A and 7B, the envelope waveforms of the reproduced outputs from the respective rotary heads HA and HB become fixed. On the other hand, if the tape speed is not an integral multiple of the normal tape speed but has added an offset value α to thereby make the tape speed an integer multiple ± α (0 < α < 1), the envelope waveform is repeated at a predetermined period. For example, if α = ± ½, the envelope waveform becomes an envelope waveform repeated every 2 rotations (4 scans) of the rotary drum 1 as shown in Fig. 7C. Whereas, if α = ± ¼, the envelope waveform becomes envelope waveforms which are repeated every 4 rotations (8 scans) of the rotary drum 1 as shown in Fig. 7D. When the envelope waveforms of both the rotary heads HA and HB become the same, as,for example, shown in Fig. 7B in which the tape speed is the odd multiple of the normal tape speed, if the position at which the rotary head scans the index area is displaced to the position at which the rotary head scans the track having a different azimuth angle from that of the rotary head, the envelope output becomes substantially zero as shown by the hatched area representing the index signal in Fig. 7B. Thus, the index signal can not be reproduced at all. Further, when the tape speed is an even multiple of the normal tape speed (in Fig. 7A), if the hatched area in Fig. 7A is taken as the index area, the index signal cannot be picked up at all from the reproduced output from the rotary head HB. This defect can be avoided if the tape speed has added to it the particular offset value α as shown in Figs. 7C and 7D. As shown by the hatched areas in Figs. 7C and 7D, although the reproduced output becomes zero in the index area portion during one particular scan, a reproduced output of more than a predetermined level can be obtained in such index area portion during other scans. However, in order to afford such offset value α, it becomes necessary to move the tape using the capstan in the high speed playback mode. Also, the higher the tape speed becomes due to a tape slip or the like, the most difficult becomes the tape speed control. With a tape speed which is an integer multiple of the normal tape speed, as another method for avoiding the output reproduced from the index area by the rotary head from becoming zero, it may be considered to control the scanning phase such that the output from the index area does not become zero. In that case, when the reel motor is servo-controlled or the tape is transported by the capstan, the capstan motor must be servo-controlled. Further, it is very difficult to apply this method to the multi-PCM mode. Accordingly, it is an object of this invention to provide an improved index signal detecting system which can remove the defects encountered with the prior art. According to the present invention, there is provided an index signal detecting system for use in a helical scan type recording apparatus, the system comprising: a) rotary transducing means; b) tape driving means for advancing a recording tape in transducing relation to said rotary transducing means; and c) an index detecting circuit connected to said rotary transducing means for deriving index informations out of reproduced signals obtained from said rotary transducing means, characterised by the provision of d) wobbling means for wobbling relative positions of said rotary transducing means and recorded tracks formed on said recording tape so that the rotary transducing means cross said recording tracks. As will become apparent from the following description, the invention can provide an improved index signal detecting system in which an index signal can always be positively reproduced by a rotary head in the high speed playback mode and which is suitable to provide an improved index signal detecting system suitable for use with a helical scan type tape recording apparatus. The above and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings, throughout which like reference numerals designate like elements and parts. In the drawings:- Fig. 1 is a diagram showing one example of a rotary head assembly of the so-called 8 mm video tape recorder to which the present invention can be applied; Figs. 2 and 3 are diagrams respectively showing recording track patterns thereof; Figs. 4 and 5 are diagrams respectively used to explain practical track formats thereof; Fig. 6 is a diagram used to explain a tracing locus of a rotary magnetic head in the high speed playback mode; Figs. 7A to 7D are diagrams respectively used to explain the outputs of the rotary heads in the high speed playback mode; Fig. 8 is a block diagram showing one example of a recording and/or reproducing apparatus to which this invention is applied; Fig. 9 is a block diagram showing other example of a main part of the present invention; and Fig. 10 is a block diagram showing another example of the recording and/or reproducing apparatus to which the present invention is applied. Now, the present invention will hereinafter be described with reference to the drawings. Fig. 8 is a block diagram showing one example of an apparatus for recording and/or reproducing a signal to which this invention is applied. In this embodiment, the recording and/or reproducing apparatus is the above mentioned 8 mm video tape recorder. Referring to Fig. 8, a head change-over switching circuit 31 is adapted to alternately connect its movable contact to the illustrated position and to the position opposite to the illustrated position at every half rotation of the rotary drum in response to a switching signal RFSW. A switch 32 is adapted to change the switching signal RFSW in the 8 mm video tape recorder mode and the multi-PCM mode. That is, the switch 32 is connected to a fixed contact N in the 8 mm video tape recorder mode. A pulse generator 42 is provided to produce one pulse at every one revolution of a drum motor 41 for driving the rotary heads HA and HB. A pulse PG having a frequency of 30 Hz indicative of the absolute rotary phases of the rotary heads HA and HB generated from this pulse generator 42 is supplied to a rotary head switching signal generating circuit 43 from which a square wave signal SC having a duty ratio of 50% is derived. In the 8 mm VTR mode, this square wave signal SC is supplied through the switch 32 to the switching circuit 31 as the switching signal RFSW and also to a timing signal generating circuit 45. In the multi-PCM mode, this switch 34 is changed to be connected to a fixed contact M . At that time, the square wave signal SC from the switching signal generating circuit 43 is supplied to a phase shifting circuit 44 and thereby phase-shifted by 36° x (n - 1) where n is an integer corresponding to the segment track number so that n = 1 for the segment track area AP1, n = 2 for the segement track area AP2, ... n = 6 for the segment track area AP6). The signal phase-shifted is supplied through the switch 32 to the switching circuit 31 as the switching signal RFSW. The switching signal RFSW is further supplied to the timing signal generating circuit 45 which generates a PCM area signal SA indicative of the segment track area specified. This PCM area signal SA is supplied to a PCM audio signal reproducing and processing circuit 55. The playback mode will be described next. In this playback mode, the drum phase servo is effected with the pulse signal PG taken as a reference though not shown. In the normal playback mode, the reproduced signals derived from the rotary heads HA and HB are supplied through playback amplifiers 51A and 51B to the switching circuit 31. When the switching circuit 31 is changed in position by the switching signal RFSW, the video signal and the FM audio signal from the area AV are both supplied to a video and FM audio signal reproducing and processing circuit 52, while the PCM data from the area AP is supplied to the PCM audio signal reproducing and processing circuit 55. In the video signal and FM audio signal reproducing and processing circuit 52, the video signal is demodulated and then delivered to an output terminal 53, while the FM audio signal is also demodulated and the demodulated output is delivered to an output terminal 54. On the other hand, in the PCM audio signal reproducing and processing circuit 55, the reproduced signal is waveform-shaped to digital data of "1" and "0" by a digital signal reproducing circuit 56. Thereafter, the digital data is supplied to a PCM processor 57 in which the digitl data undergoes the demodulation, error-detection, error-correction and so on. Then, it is reconverted to left channel and right channel analog audio signals and then delivered to output terminals 58L and 58R, respectively. Further, in the multi-PCM playback mode, the switching signal RFSW is phase-shifted by a shifting amount corresponding to the specified segment track area, the signal picked up from the specified segment track by the rotary heads HA and HB is supplied to the PCM audio signal reproducing and processing circuit 55 which reproduces the audio signal similarly as described above. At that time, the outputs from the video and FM audio signal reproducing system 52 are muted by means not shown. A description will be given of the case in which the index signal is detected in the high speed playback mode. Also at that time, the drum phase servo is applied to the rotary heads HA and HB. Accordingly, in exactly similar manner to the normal playback mode and the multi-PCM playback mode, the switching signal RFSW is generating, the PCM area signal SA is generated from the timing signal generating circuit 45 and the signal reproduced from the PCM area is supplied through the switching circuit 31 to the PCM audio signal reproducing and processing circuit 55. At that time, the signal from the digital signal reproducing circuit 56 is supplied to a gate circuit 61 and the gate circuit 61 is supplied from the timing signal generating circuit 45 with a gating signal which opens the gate circuit 61 during an interval in which the rotary heads HA and HB scan the index area portion. Thus, from the gate circuit 61 there is derived the output which is reproduced from the index area portion by the rotary heads HA and HB. The output from this gate circuit 61 is supplied to a demodulating circuit 62 in which it is demodulated to the data of "1" or "0". The demodulated data from the demodulating circuit 62 is supplied to an index signal detecting circuit 65 which is formed of a "0" detection circuit 63 and a counter 64. When the fact that a data "0" continuing more than a predetermined time is detected, the detecting circuit 65 generates a detected output indicating the presence of the index signal. In Fig. 8, reference numeral 71 designates a capstan. The tape 2 is pinched by the capstan 71 and a pinch roller 72 and thereby the tape 2 is transported. A capstan drive motor 73 is adapted to switch the tape speed between the normal playback mode and the high speed playback mode in the following manner. Specifically, a change-over switch 76 for that purpose is provided whose movable contact is connected to a fixed contact or terminal N in the normal playback mode and to a fixed contact or terminal V in the high speed playback mode, respectively. Further, the capstan drive motor 73 is provided at its rotary shaft with a frequency generator 74 which generates a frequency signal FG corresponding to the rotation speed of the capstan drive motor 73. The resulting frequency signal FG is supplied through an amplifier 75 to one input terminal N of the switching circuit 76. The frequency signal FG passed through the amplifier 75 is supplied through a frequency dividing circuit 77 to another input terminal V of the switching circuit 76. The output from this switching circuit 76 is supplied to a servo circuit 78 from which a motor drive control voltage is derived. This motor drive control voltage is supplied through a motor drive circuit 79, an adding circuit 80 and an amplifier 81 to the capstan drive motor 73. The switching circuit 76 is changed in position by a mode signal MD applied to a terminal 83. To be more concrete, in the normal playback mode, this mode signal MD is, for example, "0" so that the movable contact of the switching circuit 76 is connected to the input terminal N , while a switching circuit 84 which will be described later is made off. Accordingly, in this normal playback mode, the frequency signal FG is supplied as it is to the servo circuit 78. Thus, the capstan drive motor 73 is supplied with a speed servo signal such that the frequency of the frequency signal FG becomes a predetermined frequency f₀, thus transporting the magnetic tape 2 at the normal playback speed. Subsequently, in the high speed playback mode, the mode signal MD becomes "1" so that the switching circuit 76 connects its movable contact to the input terminal V . Further, since this mode signal MD is supplied to a frequency-dividing ratio setting circuit 82, in this high speed playback mode, the frequency-dividing ratio of the frequency-dividing circuit 77 is set to 1/n by the setting circuit 82. Accordingly, at this time, the switching circuit 76 generates a signal which results from dividing the frequency signal FG by n . This signal is supplied to the servo circuit 78. Thus, the speed servo is applied to the capstan drive motor 73 in such a manner that the frequency of the frequency-divided signal becomes a predetermined frequency f₀. That is, the tape 2 is transported at a speed n times that of the normal playback mode. Further, in this high speed playback mode, the switching circuit 84 is turned on by the mode signal MD. Then, a wobbling signal from a wobbling signal generator 85 is supplied to the adder 80 and thereby superposed upon the motor drive voltage. Because of this wobbling signal, the tape 2 is not transported at a constant speed so that the tracing locus of the rotary heads HA and EB on the tape 2 does not become rectilinear but is wobbled in the direction intersecting the scanning direction. Since the index signal is generally written in the index area during a time period of 3 to 10 seconds at the normal tape speed, if the tracing locus is wobbled as described above, an index signal of more than a predetermined level can be detected from at least one index area of a plurality of tracks corresponding to 3 to 10 seconds. Accordingly, it is possible to avoid the head output from always becoming zero unlike the prior art. While in the embodiment shown in Fig. 8 the wobbling signal is superposed upon the motor drive voltage, alternatively, another arrangement is possible as in Figure 9, in which the wobbling signal is supplied through the switching circuit 84 to an adding circuit 86 in which it is added with the output voltage from the servo circuit 78 in the high speed playback mode. Further, the wobbling signal may be a signal which has no correlation to the rotation period of the capstan drive motor 74. In the high speed playback mode, it is alternatively possible that the wobbling is applied to the rotation of the capstan motor in the following manner. Fig. 10 is a block diagram of one example of a reproducing system which is provided with another embodiment of this invention. In this embodiment shown in Fig. 10, like parts corresponding to those of Fig. 8 are marked with the same references and will not be described in detail. In this embodiment, there is provided no special signal source for effecting the wobbling unlike the first embodiment shown in Fig. 8. In this embodiment, in the high speed playback mode, the frequency-dividing ratio set by the frequency-dividing ratio setting circuit 82 is alternately changed to 1/N and 1/M at intervals of from several times to several tens of times period of the vertical frequency signal. That is, if the frequency-dividing ratio is changed at, for example, 6 vertical periods, at every three rotations of the rotary heads HA and HB, the frequency-dividing ratio is changed-over to 1/N and 1/M (N ≠ M). Accordingly, each time the rotary heads HA and HB are rotated three times, the tape 2 is alternately transported at the tape speed N times and M times as high as the normal tape speed with the similar effect to that of the first embodiment in Fig. 8 being achieved. The period at which the frequency-dividing ratio is changed is determined by the responsive speed of the capstan motor. According to this embodiment, unlike the first embodiment shown in Fig. 8, a special signal source for the wobbling signal is not required. Also, the setting of the frequency-dividing ratio can be controlled by use of a micro-computer so that the manufacturing cost can be avoided from being increased and the apparatus of the invention can be realized with ease from this standpoint. In the above mentioned embodiments, the reason that in the high speed search mode the frequency signal FG is divided and the capstan drive motor is servo-controlled by the divided signal is to obtain a reproduced picture of the video signal in this high speed search mode. In the high speed search mode to detect the index signal, such servo is not necessary. In other words, it is alternatively possible that a constant voltage is applied to the capstan drive motor, the capstan drive motor is allowed to run freely, the tape is transported at the high speed and the wobbling is applied to the rotation of the head drive motor. When the servo is applied to the capstan drive motor as set forth above, the video signal is reproduced at high speed, while when the index signal is not detected, the switching circuit 84 is turned off. Only when the index signal is detected, is the switching circuit 84 turned on. While in the above mentioned embodiments the capstan motor is controlled, it is needless to say that this invention can also be applied where a reel motor is controlled. Further, in this invention, it is possible for the tape running speed not to be controlled but a rotary head is mounted, for example, on a bimorph leaf plate and this bimorph leaf plate is supplied with a control voltage, whereby in the high speed playback mode, each head can be wobbled in the direction perpendicular to its scanning direction at every predetermined period or at random. Further, in the above mentioned embodiments, since the postamble signal is a signal having the frequency of 5.8 MHz which is data" 1" s , the signal having the frequency of 2.9 MHz which is data "0" s is used as the index signal. In this case, however, the index signal is not limited to the above mentioned signal but a signal having a pattern which does not appear as the data can be used. Furthermore, as the index signal, it is of course possible to record coding data. If the coding is carried out as described above, it is possible to record as the index signal, in addition to data indicative of a starting point of a tune, data indicating the intermediate portion of the tune and other data such as tape speed, time information and so on. In addition, the present invention is not limited to the 8 mm VTR but can also be applied to an index signal detecting system used in a rotary head type reproducing apparatus. According to the present invention as set forth above, in the high speed search mode, the rotary head does not draw a definitely determined tracing locus relative to the recording track pattern but is wobbled in the direction perpendicular to the scanning direction so that the head output can be prevented from always becoming zero in the recording area portion of the index signal. Thus, it becomes possible to make the probability of detection of the index signal uniform. The above description is given on the preferred embodiments of the invention but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims: 1. An index signal detecting system for use in a helical scan type recording apparatus, the system comprising: a) rotary transducing means; b) tape driving means for advancing a recording tape in transducing relation to said rotary transducing means; and c) an index detecting circuit connected to said rotary transducing means for deriving index informations out of reproduced signals obtained from said rotary transducing means, characterised by the provision of d) wobbling means for wobbling relative positions of said rotary transducing means and recorded tracks formed on said recording tape so that the rotary transducing means cross said recording tracks. 2. Index signal detecting system according to claim 1 wherein said tape driving means includes a capstan and a pinch roller, and said wobbling means is provided with a capstan driving circuit for said capstan. 3. Index signal detecting system according to claim 1 or 2, wherein said capstan drives said recording tape at higher speed than normal recording speed in the case when the detection of said index informations is required. 4. Index signal detecting system as cited in claim 3, wherein said recorded tracks include digital data, and said index information is derived from reproduced digital data from said rotary transducing means at said index detecting circuit. 5. Index signal detecting system as cited in claim 4, wherein said wobbling means include a wobbling signal generator which supplies a wobbling signal to said capstan driving circuit.

Inventors: OKADA, HIROSHI C/O SONY CORPORATION, TAKAHASHI, TAKAO C/O SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1986
Main classification: H04N5
IPC codes: G11B15, G11B27, G11B20