Numerical Betacam

Presentation

Introduced by the firm Sony in 1993, the DIGITAL Betacam ( numerical Betacam French ) seems the successor Numérique worthy Betacam SP. The massive use of integrated circuits specific, associated with recent technological advances as regards digital processing of the data with wideband and with the evolution of the magnetic medium (bands Metal), made it possible to develop Magnétoscope S in accessible components Numérique S for a production company or a Television channel.

The DIGITAL Betacam from Sony is a format reducing in a factor 2 the flow of numeric signal 4: 2: 2 coded out of 10 bits. Certain models of video tape recorders DIGITAL Betacam are compatible in reading with the cassettes Analogique S recorded on a machine Betacam SP.

A new transport of band developed especially for the numerical recording, authorizes an intensive request of the machine. In order to reinforce the effectiveness against cloggings, the DIGITAL Betacam is equipped with an automatic system of cleaning on the level of the fixed and rotary heads; the band itself is also subjected to a permanent cleaning. The reading rate can be adjusted from + 15% to make it possible for example to synchronize between them two video tape recorders reading the same program.

The drum of the heads

The Diamètre of the drum of the heads which is of 74.5 mm for the analogical video tape recorders with the format Betacam SP was increased to 81.5 mm to allow a speed of sufficient writing, with an angle of wrap of 180°. The recorded total flow (video and audio) is of 125.58 Mbits/s. the number of revolutions of the drum it was also increased by a factor of 3 to reach 75 rotations per seconds.

On the drum 14 heads are laid out: 4 for the recording, 4 for the reading (Dynamic Alignment), 4 heads known as of “confidence” to read the audio and video signals during their recording and finally, 2 heads for obliteration.

The models allowing compatibility in reading with the format Betacam SP have 4 additional heads rises on two blocks of the drum. This series of video tape recorders detects thanks to the hole of identification of the cassette if this one is with the format DIGITAL Betacam or Betacam SP. The video tape recorder commutates automatically and adapts speed of band and number of revolutions of the drum in order to obtain the optimal follow-up of track.

Configuration of the rotary and stationary heads

4 play-back heads advanced (Advance) make it possible to read the video signal present on the tape some screen S before the recording heads. This function known as the “preone” makes it possible to make a modification on a program recorded without requiring more than one video tape recorder. In recording mode, the old signal read by these advanced play-back heads can indeed be recovered and treated by external equipment (Mélangeur, correct colorimetric…), then re-recorded exactly in its place of origin. This very interesting function must be the subject of a great precaution of use because the operation is irreversible.
4 heads known as “heads confidence” (Confi) read the tracks after the recording heads in order to allow the checking of the recording in progress (with a light temporal shift).
4 other heads allow the reading in analogical Betacam.
4 heads allow the numerical recording of the data
2 heads are used for obliteration of the audio data and video

In mode of normal reading of a cassette to the numerical Betacam format, the play-back heads advanced are used. In mode assembles or insert, the play-back heads advanced read the signal before obliteration (pre-read) and the heads confidence read the recorded signal.

With regard to the stationary heads, one finds the configuration of traditional Betacam to knowing, a general erase head, a recording head of the CTL, a location and erase head of Time-Code, a recording head and of location of Time-Code.

Bandage used

The band used for the format DIGITAL Betacam is a band with metal particles lying thickness similar to that used in Betacam SP. However, it was optimized in order to record the spectrum code of channel used in DIGITAL Betacam. The tape speed of the band is about 5% slower in the format DIGITAL Betacam than in the format Betacam SP (96.7 m/s compared with 101.5 m/s for Betacam SP): the maximum duration of recording is thus increased to 124 minutes.

Format of recording

So that the answer of a recorder is optimal in the high frequencies it is paramount that the air-gap of the play-back head is perfectly parallel to that of the head which carried out the writing of the tracks. IF the angle of the play-back head differs from that of the recording head, it occurs a weakening of the high frequencies which is all the more important as the angle of difference of azimuth is large. To cure this problem, the azimuté recording is used. The tracks are written with heads whose air-gap presents a certain angle of inclination compared to the perpendicular of the axis of the track. And this angle is alternatively opposed of a track to the other. Thus, if a play-back head overflows a little on the track close to that which it is supposed lira, the interfering signal that it will recover is very attenuated since the angle of azimuth will not correspond. In the format DIGITAL Betacam this system of azimuté recording is used to carry out a recording of high density. The angle of azimuth is roughly of 15 degrees between two adjacent tracks.

In format DIGITAL Betacam, the video signals and audio (4 channels) for each screen are recorded according to 6 helicoid tracks. In parallel, the format DIGITAL Betacam uses three longitudinal tracks just like its fellow-members Betacam and Betacam SP: a track of control (CTL), a track of Time-Code (LTC) as well as a track of audio location (CUE). The track of the audio channel 1 traditional is removed in order to leave more place to the helicoid tracks.

Organization of the data inside the tracks

The data of the first screen are recorded on first half of the six tracks and the data of the second screen on second half of the six tracks. Each track is built so as to be able to receive four audio sectors in the center and two video sectors at the ends. The video data of a screen are divided into 12 sectors and the audio data in 6 sectors per channel.

For each audio channel, the various sectors are laid out so as to avoid the errors during the reading. In order to delimit each sector, of the data are added at the beginning and the end of the sector. In addition, of the reference marks of assembly are inserted between each sectors in order to be able to assemble the tracks independently. Lastly, a new method of alignment was set up to work the traditional process parallel to using the signals of the longitudinal tracks of control, in order to ensure a correct alignment on a narrow track, especially at the time of the repetitive operations of assembly at the same point. Its principle consists in using two pilot signals registered voters on the helicoid tracks between the audio sectors and the video sectors (one being in low frequency: 400 Khz and the other high frequency one: 4 MHz).

Longitudinal tracks

In the format DIGITAL Betacam, a track CTL, a track of Time-Code and a track of audio location are located in length of the band.

For the systems with 625 lines, the signal of CTL is a tension of the rectangular type, given rhythm with 50 Hz, whose ascending face determines the beginning of each screen. Each crenel of the signal of CTL does not have a constant duration. It varies in length. The crenel corresponding to the first screen of a sequence 8 screens has one duration equal to 65% of the period of the signal, and that corresponding to the first screen of a sequence 4 screens has one duration equal to 35% of the period of the signal. These variations make it possible to quickly locate the first screens of sequences 4 and 8 screens.

The signal of Time-Code corresponds to the conventional signal of standard EBU. An image is coded on 80 bits. The data corresponding to the number of the image, of the second, of the minute and the hour are coded in two-phase mark and recorded for each image. The principle of the Two-phase Mark is the following: one “0” causes a transition and a maintenance of the level for all the period of clock, while one “1” involves a transition and a change of level with half of the half-period of clock.

The track of audio location is used for the assembly primarily, in order to locate a sound sequence more easily. It is recorded just like the longitudinal audio tracks of the formats Betacam SP.

Treatment of the numeric signal

The interface of entry

The numerical interface series in components, in conformity with standards SMPTE 259M/EBU T.3267/CCIR 656-III, accepts the video signal in component as well as the numerical signals abfg fgd udio 4 channels on a single coaxial cable BNC. The analogical signal in components and the composite signal (with the BKDW-506) are digitized in parallel data, to standard CCIR 601. The audio data of numerical interface AES/EBU or the data of the analogical entry can be selected for the recording. They are converted into data series.

The video interface

The video data are treated according to names 4: 2: 2 (CCIR-601). They are then multiplexed in the order “Cb, Y, Cr, Y, Cb, Y, Cr, Y,…”. The reference of times for the operation of analog-to-digital Conversion is given by the face before timing pulses line. This face is spread out over 4 words and is added before and after the numerical active line. It determines the beginning of the active line (SAV) and the end of the numerical active line (EAV).

The audio interface

In the video tape recorders DIGITAL Betacam, the Quantification of the sound is linear and is done on 20 bits. Two audio channels are multiplexed and stored in a “image” of which the duration corresponds to one period of sampling. Each image is divided into two under-images. The data of the first audio channel are stored in the under-image has and those of the second audio channel in the under-image B. Each under-image is coded on 32 bits and contains at the same time the audio data but also of other auxiliary data.

Encoding by reduction of binary debit

The flow of the video data is reduced approximately of half by the system of recording per coefficient. The numerical video data are initially converted into spaces of frequencies by method DCT (Discrete Cosine Transform). These spaces frequencies are then balanced according to coefficients DCT which correspond to the characteristics of the human vision. The data of a screen are then compressed. Then, of the shortened codes are allotted to the data which frequently appear. It results from it a compression about 2:1 with certainly a relative loss from useful data but the interest of this code lies in the fact that the video tape recorder remains able to go up except for the image.

Treatment of the audio signal

The audio processor carries out the automatic control of profit, the play-back as well as the addition of certain data.

Encoding ECC (Error Coding Correction)

The format DIGITAL Betacam uses a type of correction of error to two codes : the internal code ECC and external code ECC, using both the code Reed Solomon.

In this type of coding, the values has, B, C, D of the data, the sum S of these data as well as the sum Q of the data multiplied by known coefficients are coded. For example:

A=100, B=200, C=300, D=400
S=A+B+C+D=1000
Q=Ax1 + Bx2 + Cx3 + Dx4 = 3000

With the reception, the values has, B, C, D, S and Q is read. For example:

A=100, B=300 (error), C=300, D=400
S=1000
Q=3000

The circuit of correction recomputes the theoretical sums S'=1100 and Q' =3200 then the differences D Q=Q' - Q=200 and D S=S' - S=100

The report/ratio D S/D Q=2 gives the coefficient of the value on which the error occurred. In fact, it is about the value B. When one adds data of synchronization to a block of internal code of 180 bytes, this block is called block of synchronization. It is the basic unit of the video and audio sectors. A video sector consists of 126 blocks of synchronization, a video track of 2 video sectors, a video screen of 12 video sectors.

An audio sector, consists to him of 6 blocks of synchronization. For each channel, an audio screen consists of 6 sectors, i.e. 36 blocks of synchronization. Thus two blocks ECC constitute an audio screen of a channel.

The structure of the block of code interns which constitutes a block ECC is thus common to the video and the audio. On the other hand the structure of the external block of code differs.

Encoding of channel

The format DIGITAL Betacam adopts the system of encoding of channel S-NRZI or scrambled NRZI (Scrambled Non Return to Zero Inverted) which are higher in its characteristics of noise. The characteristic of coding NRZI is that one “1” determines a transition in the middle of the half-period from clock, one “0” does not have any effect. So if the polarity of the signal is reversed, the coded data remain the same ones, moreover, this type of coding makes it possible to transmit with the data, the signal clock.

Treatment of the signal of reading

The numerical data of reading are equalized by automatic circuits of equalization and the errors are corrected by the internal and external codes ECC, which can correct the majority of the data affected by noises and dropouts in the signal reproduced. The irremediable data are corrected by the circuits of dissimulation of errors.

Interface of exit

The video data in components are converted into data multiplexed series and with the audio data, then left to the format the numerical interface series.

For the analogical exits, the video data in components undergo a numerical/analogical conversion into analogical signal components but are also encodées numerically composite, then converted into analogical composite signal.

For the audio exits, numerical interface AES/EBU and the analogical data are available.

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