Contents - Previous - Next
8.2 Television archives
In this section the originating television signal will be referred to as the television standard and the signal recorded on the video tape as the recording format.
1. Video Archive Requirements
The Video Archivist's main aim is to preserve videotape recordings for future use and to make them available on an acceptable format and standard when required. Although this appears simple, it is extremely complex, due to the number of television standards and recording formats. Added to this is the question of the long term stability of the magnetic carrier and the lack of verified reliable information on long term storage conditions for magnetic media.
It is possible to keep the original of each recording and replay it when necessary. This, however, presents many problems. It would be necessary to have working equipment to reliably replay whatever television standard and recording format was used at the time the original recording was made. This implies the need to keep equipment working for every format held. It also implies the ability to display any television standard.
Although this may be the ideal situation, it is not practical. The recording format may no longer be in general use, the equipment no longer made, the expertise to maintain it rare and spare parts unobtainable. It is for these reasons that the task of the Video Archivist is particularly difficult.
It is not necessary for all archives to handle every television standard and every recording format. It is, however, necessary to provide copies on a current recording format and to a television standard acceptable to the user.
2. The Purpose of Archival Re-Recording
Re-recording becomes inevitable in order to overcome the following problems:
2.1 Large quantities of recorded material on redundant formats.
2.2 Equipment and expertise to replay old formats becoming scarce.
2.3 Recordings of obsolete television standards that require standards conversion in order to be reused.
2.4 Proliferation of recording formats that makes it difficult to have a single archive format.
2.5 Although video tapes are improving magnetically and becoming physically more robust, new formulations are required to prove their long term stability.
2.6 The most appropriate long term storage conditions require further definition before developing international standards.
Taking each point in turn, a strategy can be devised to minimize these effects.
2.1 Redundant formats can be transferred to a current format, preferably a robust one in majority use with a proven record of reliability and stability for both the format and the media. As this preferred format will change in time it would be necessary to transfer again at some time in the future.
2.2 The transfer of obsolete formats to current formats could remove the need to maintain old equipment and expertise. There will never be, however, any guarantee that some recordings have not been missed. This will be a perpetual problem which can only be resolved by nominating or establishing one central agency to hold equipment and provide a service to other archives.
2.3 It will be necessary to transfer obsolete television standards to current standards. Today this would be mainly 405 and 819 to 625 lines. In the future it will be the transfer of current standards to high definition. Any attempt to preserve recordings on television standards no longer in use, in order to preserve the architecture of the originating signal, will simply multiply the problems of maintaining obsolete equipment. Fortunately, standards conversion will always be to a higher quality system and, although the signal will not be preserved in its original form, there should be minimal loss of quality, provided that the standards convertor is of a sufficiently high quality.
2.4 Currently there is no ideal archival format available. Ideally a tape, or working copy, taken from any archive should be playable on equipment in general use. However, this may not be the ideal archival format. It may not be the most robust or the best for long term storage. Providing the archival format is signal transparent it could be acceptable to copy to a current format for use. It is apparent that there could be an archival format which is not a current format, providing that it is signal transparent, robust and the tape is capable of a long life.
2.5 Although tapes are improving magnetically and are becoming physically more robust, tape dimensions, recorded track dimensions, and recorded wavelengths are getting smaller. These are conflicting requirements for an archive tape. New magnetic pigments may not be as chemically stable as previous pigments. It will take many years of use to prove their long term stability.
2.6 The best tape storage conditions are only now becoming understood. Evidence shows that the acceptable humidity level has been overestimated. This will mean better environmental control of the storage areas and strict control of the acclimatization of the tapes entering and leaving the controlled storage area is necessary. It would be better not to let the master archival copy of the tape out of the controlled area and to only allow access to the material by the use of copies made in the controlled environment.
The complex circumstances of any video archive means that copying will take place for any or all the reasons listed above. No information carrier has an infinite life. This means that at some time in the future a copy will have to be made. As it will also be necessary to copy as preferred formats change, the opportunity should be taken to update the material accessible from the archive. A video archive will have to be able to make copies in order to ensure that both the master and access copies are on a format and television standard that is acceptable to the user.
There are many television standards in current use. For example:
625 line PAL; 625 SECAM; 525 NTSC and 525 PAL M.
Some standards are no longer used:
405 and 819 line.
Others standards are in development for future use:
High Definition Television (HDTV) in a number of proposed standards.
Each colour television standard can be recorded in its composite form; that is as PAL, SEC or NTSC. It can also be recorded in its component form; that is in the form of separate luminance and chrominance signals, as would be originated before en-coding into the composite signal.
Since the introduction of television recording there have been many recording formats. Some, like the original Quadruplex format, introduced in 1956, are no longer used as an origination format.
There are many reasons for the proliferation of recording formats. Different circumstances require a different record-replay performance and it is the overall performance that governs the cost of the video tape recorder (VTR). Improved technology allows higher density packing of the signal on the tape. Using less tape requires less storage space to store the same programme length.
Television is a time dependent signal. Each frame, field, line and cycle of colour subcarrier occupies a precise period of time. When recorded, this time dependent signal is translated by the scanning video head into a precise space pattern on the tape. The replay process translates space back into time. Any irregularity in physically placing the signal on the tape may invalidate the time relationship of the reconstructed signal. It is this precise placing of the signal on the tape and the parameters of the signal itself that determines the recording format.
All video recording formats, until recently, used analogue signal recording techniques. Analogue recording introduces signal losses, distortions and noise, which is multiplied each time it is re-recorded.
Recently, digital recording techniques have been introduced. This allows the signal to be re-recorded many times without any apparent degradation. Digital recordings are, therefore, said to be transparent; that is identical error free copies of the original can be made each time.
The broadcast formats use more rugged and higher quality equipment and tape in order to ensure a high quality end product. Programme material is often re-recorded many times in the post-production process. "Domestic" formats are usually designed to record and replay, not to re-record over many generations. Their cost is, therefore, reduced, as is the quality of the replayed signal. "Industrial" formats usually fall between broadcast and domestic.
4. Practical Considerations
Not all video archives are going to have the budget, expertise and personnel to cover every television standard and recording format. Each archive will, however, be required to service the television standards and recording formats it holds, as well as being able to provide copies on whatever format is required.
Access to archive material can be for different purposes. For example, access may be required to:
a. Make an accurate copy of the original television programme material. This would be an exact copy to the same format and must be carried out with as little degradation as possible. It may be necessary to replace a tape in storage which is degrading.
b. A copy on a different format or television standard keeping the signal quality as near as possible to the original. The quality required when changing format or standards depends upon the reason for transfer. If it is to provide a master copy the highest quality must be maintained at all times.
c. A viewing copy intended for content viewing only. Copies do not have to be of high quality if for viewing purposes only. It may be satisfactory to use domestic formats. Whenever copying a master tape, however, regardless of the format it is copied to, it is still necessary to take the greatest care.
Video tape is a vulnerable information carrier. Care and handling of magnetic tape is extremely important. When considering the replay of master tapes, it is necessary to take extra precautions. Although the replay machine may be clean and aligned to the manufacturer's specification, it is imperative that the tape is also clean and packed onto the spool correctly. This can be carried out by using an appropriate cleaning machine before the tape is loaded onto the replay machine.
It is advisable to identify the recording format before any attempt to replay it is made. Although most recordings are identified on the spool, some are not. Tape width does not identify the format. Replaying a tape on a machine intended for a different format may cause irreparable damage. Video recording formats are identified by the International Electrotechnical Commission as IEC Standards. There is a publication for each format which specifies the signal parameters and the magnetic track pattern on the tape. It is only by identification of these track dimensions that a format can be identified without replaying the tape. The magnetic pattern on the tape can be made visible by using a suspension of fine iron particles to "develop" the pattern and measuring the result with a microscope. This method causes tape damage and should only be used on tape that is no longer required.
A non-destructive method is to use magneto-optical garnets for tape inspection. This device allows the measurement of track dimensions without damaging the tape.
5. The Re-Recording Facility
Each re-recording facility will be different as it will be organized to service the needs of the individual archive. Recording and replay equipment will be required for each format held in the archive, as well as those formats which are handled from outside the archive and the formats required by users of the archive.
Television video tape recording equipment is complex. Each machine is made to record and replay tapes to a specific television standard and recording format. The manufacturer of the machine provides detailed alignment and maintenance instructions as well as recommended test and alignment equipment to carry out the work. This work can only be carried out by competent personnel. Although simple operations can be carried out by trained operators, it is inevitable that the replay of some archive material will require intimate knowledge and understanding of the format and standard in order to obtain the best results. This could be provided by trained technicians. If, however, it is necessary to maintain the equipment to the required standards, particularly for formats and standards no longer in regular use, qualified engineering expertise will be imperative.
Monitoring will be required for both audio and video. For audio, loudspeaker and signal level monitoring is required, as well as a means of level adjustment and line up signal generation. For video, picture, waveform and vector monitors will be required for each television standard covered. Colour picture monitors are graded according to the quality required. Grade 1 is for use in areas where colour balance and picture geometry is critical; Grade 2 for less critical areas; and Grade 3 is similar to a good quality domestic television receiver.
Television pulse generators are required for each standard used. They usually also generate the basic alignment signals necessary for the alignment of video tape recorders.
VTRs intended for programme making and broadcasting include built in time base correctors (TBCs). These provide accurate time correction of the replayed signal so that it precisely matches the television standard. Domestic and industrial VTRs do not require accurate time correction and do not usually include time base correction. If it is required to copy a non-time base corrected signal onto a broadcast format, it will be necessary to use an external TBC to correct the signal before re-recording.
Television standards converters are in constant development. For commercial reasons they are designed to convert between the 625 and 525 line television standards in current use, with as little loss of picture quality as possible. Equipment to convert between 405 or 819 and 625 lines is extremely rare. If it is required to convert between these standards, it will be necessary to commission the building of suitable equipment, or to have the conversion carried out by a facility that has the equipment.
Conversions between the PAL and SECAM colour systems will require decoders and encoders. Conversion between composite and component systems will require the relevant decoders and encoders capable of translating between the signals. Signal losses and distortion when translating between component and composite, in either direction,' can be disturbing. The equipment used for this purpose must, therefore, be the best possible.
Some recording formats, particularly broadcast formats, use SMPTE/EBU time code to identify each television frame recorded. This digital code is recorded either as part of the video signal or on a separate longitudinal track. Each frame is identified in hours, minutes, seconds and frames. It is used as a cuing signal to locate a particular part of a recording.
A means of distributing signals between the various items of equipment is required; the sophistication depending upon the flexibility and quality required.
Tape cleaning equipment not only cleans the tape, it also packs the tape at a constant controlled tension which is advisable before storage. There are a number of commercial tape cleaners available. Most also provide a tape evaluation facility which is not required in an archive situation. Machines which clean and pack the tape use wiping pads or scraping blades or a combination of both. Although blades can provide a useful cleaning effect, if not correctly maintained they can cause tape damage. Wiping pads which are automatically advanced as they are used are efficient and safe. Cleaners are usually designed to cope with one tape width, or in the case of cassettes, one format. Machines which cope with two tape widths, although available, are a compromise and more complex to use. It is usually preferred to have a cleaner for each tape width.
The operational area must be clean with a comfortable environment. Space and shelving is required for acclimatization and storage of the tapes in use.
6. Tape Storage and Handling
Cleanliness is essential in all video tape operational and storage areas. It is important because minute dust particles and debris can cause loss of the replayed signal by disrupting the intimate contact between the replay head and the tape surface. This signal loss is referred to as dropout. Smaller signal losses in video recording cause irritating short flashes on the picture. As well as dust elimination, it is important that air purity is maintained. The air in many industrial locations contains abrasive particles and acidic water vapour.
Stray magnetic fields are not normally a problem. Although at a distance of 50mm the field is negligible, it is advisable to keep tape away from heavy electrical machinery.
The magnetic properties of videotapes, other than chromium dioxide, are essentially unaffected by temperatures up to 300EC. Storage temperature constraints are due to the tape plastics rather than the magnetics. The video tape base film is dimensionally stable to over 80EC. The binders used to hold the magnetic coating to the base, although they have excellent mechanical properties, are susceptible to breakdown in conditions of high temperature and humidity. The chemical reaction is known as hydrolysis. Low temperatures can cause the tape lubricators to migrate to the tape surface. Cleaning removes the lubricator without any signal loss.
Rapid changes in temperature are undesirable as stress and contraction may be introduced into the tape pack due to thermal expansion.
Tape manufacturers publish recommended storage conditions. Some have recently proposed a reduction of relative humidity from 50%, +/-10% to between 30% and 40%. The operational environment will be different from the ideal storage conditions and, therefore, tape moved into the operational area must be acclimatized before use, preferably for 24 hours.
Video tape is provided on reels or in cassettes. Cassettes protect the tape better than reels. Handling video tape requires purpose built shelving designed to hold the boxes firmly in a way that keeps the reel supported vertically by the centre hub. Table space to safely open the boxes should be provided near the machine to be used.
7. Technical Equipment Review
The technical equipment required by a Video Archive can be considered in 15 separate groups.
7.1 Reference Signal Generation
A reference television synchronizing pulse generator (ZPG) for each television standard used. This SPG provides a stable timing and colour subcarrier signal as a reference to VTRs, other equipment and test signal generators. It usually provides a complete set of reference signals, mixed syncs, mixed blanking, field and line trigger, burst gate, PAL ident and colour subcarrier as well as black and burst and colour bars. Although current equipment requires a composite reference (black and burst or colour bars) older VTRs and equipment may require separate syncs, blanking, colour reference signals etc. Normally SPGs have only one output for each signal. If more than one output is required, it will be necessary to use distribution amplifiers to provide an output for each piece of equipment.
7.2 Test Signal Generation
Test signal generators for each television standard used will be required. They provide signals to ensure the correct operation of the VTR and other equipment. Although test signals are a maintenance aid, some provide valuable alignment signals when recorded at the start of a tape. The complexity of test signals will depend upon the requirements of the archive. Those working in a broadcast quality environment will need composite signals to measure and adjust colour differential phase and gain, luminance chrominance gain and delay, luminance linearity and pulse and bar response. Separate generators, generating luminance and colour separation signals, will be required if component systems are used.
7.3 Video Tape Recorders
Video tape recorders are required to replay the tapes held in the archive. Machines are also required to make copies on whatever format is preferred.
7.3.1 Replay VTRs
The archive may hold recordings made on many formats and many standards. Although some early quadrupled machines were switch-able between 405, 625 and 525 line standards, high band and lowband, later quadrupled and other formats were not. It is, therefore, unfortunate but necessary to have dedicated machines for each format and standard. Machines for obsolete formats are no longer manufactured, spare parts are rare and the ability and skill to maintain them scarce. If it is necessary to handle these obsolete formats, it is recommended that more than one machine is kept, preferably all of the same make and model, as this will considerably reduce the difficulties of maintenance.
7.3.2 Record VTRs
Record formats will depend upon the individual requirements of each archive. Viewing copies can be made on readily available domestic formats, it removes the need to use valuable master tapes for viewing. Each archive will have a preferred archive format and television standard. The choice will depend upon individual requirements of the archive. Some will choose to copy individual redundant formats and standards onto the preferred one. As technology progresses it may be necessary to abandon the preferred format for one more suitable.
Users of the archive will require copies on their own preferred format. As most archives prefer not to allow external use of master tapes it will be necessary to have machines to make suitable transfers.
7.4 Time Base correction
In order to re-record non-broadcast recordings onto broadcast formats, time base correction is necessary. In some cases it will be necessary to use time base correctors (TBCs) capable of a wide correction range, in excess of one television frame. The signal performance of the TBC will depend upon the requirements.
The manufacturer of the VTR provides a recommended list of equipment and spare parts required to keep the machines in satisfactory working order. Spare parts for older equipment may not be readily available. If it is intended to service obsolete formats it will be necessary to hold a reservoir of major spare parts. This will be mainly mechanical items, reel motors, scanners, pinch rollers etc. as well as video and audio head assemblies. As electronic components become obsolete, a competent engineer can usually find a replacement which might, however, require some re-design of the circuit. For redundant formats it is best to have more than one machine as described in 7.3.
Test signal generators are covered in 7.2. other electronic measurement, alignment and servicing equipment requirements, for both audio and video, can be obtained from the equipment manufacturer's maintenance and alignment information.
7.6 Signal Distribution
A means of distributing audio and video signals throughout the facility, which will include the distribution of television pulses, test and alignment signals as well as signals between VTRs, standards converters and colour trans-coding equipment. The complexity of each facility will be different. In the simplest case equipment can be plugged up as required. More complex facilities will require video distribution amplifiers and audio and video jackfields to route signals as required. The most complex facility may need remote switching of signals between equipment. This would require audio and video switching matrices.
7.7.1 Audio monitoring
Some VTRs provide audio level and loudspeaker monitoring; some do not. It is advisable to provide good quality level and loudspeaker monitoring separate from the VTR.
7.7.2 Video Monitoring
Picture and waveform monitors for each television standard as well as vector monitors for each colour system are required. Picture, waveform and vector monitors that are switchable between 625 PAL and 525 NTSC are available. Monitoring of other standards will require dedicated equipment.
7.8 Standards Conversion
Equipment for standards conversion between 625 PAL, 625 SECAM and 525 NTSC, in any direction is available. Converters between other standards have been made but are no longer in production and are not generally available. If it is necessary to convert from an obsolete standard, converters will probably have to be designed and built or the conversion done by a facility with the necessary equipment.
7.9 Colour Trans-coding
Although standards converters will trans-code between PAL and NTSC systems, to trans-code between PAL and SECAM will require suitable decoders and encoders.
7.10 Composite and Component Signals
Most component recorders are able to take in composite signals, as they have internal decoders. They also contain encoders to provide composite outputs. If it is intended to use component VTRs in a composite environment and they do not have built in decoders and encoders, they will have to be provided externally.
As well as the alignment signals at the start of each recording, it is advisable to have an identification before the programme material. This usually consists of a video identification of the programme and a clock indicating the time before the start. The "VD clock" is programmed from a keyboard to provide what ever information is required.
7.12 Time Code
If it is required to use time code to locate items in a recording, time code readers will be required in order to display the time. The ability to search to a pre-set cue point is built into some VTRs. If it is necessary to transfer time code when copying tapes, it is advisable, in order to preserve its integrity, to re-shape the code before re-recording. If it is necessary to record new time code a generator is required.
7.13 Tape Cleaning
Tape cleaning machines also provide controlled tension tape packing. There are some machines that also provide tape evaluation. This is not required for archive use. The method of tape cleaning varies from machine to machine, the choice depends upon the requirements. Machines are usually dedicated to a particular tape width or cassette format.
7.14 Format Identification
To measure track patterns requires a microscope, a measurement bed and a method of developing the magnetic tracks. Although a suspension of fine iron powder can be used, the use of non-destructive magneto-optical garnets is preferred.
7.15 Tape Maintenance
Some tape will be damaged and mechanical tape splices could dry out and part. A means of removing damaged tape and re-making old splices should be provided. Two inch quadrupled was the only format to be mechanically spliced and splicing blocks do exist. For other formats, splicing blocks made for one inch and half inch audio tape can be used.
When handling video tape great care should be taken. It is advisable to wear cotton gloves as used for film handling.
The above covers the typical equipment requirements of a video archive. Archives will have to determine their own individual requirements, depending upon the standards and formats serviced. The installation of the equipment will depend upon the flexibility demanded by the archive, the work load and the need to cope with rarely used standards and formats.
Contents - Previous - Next