Introduction to the Basic Principles
of the SMPTE/ EBU - Time Code
This introduction to the basic principles of the SMPTE/EBU - Time Code does not intend to replace the specification sheet issued by EBU and SMPTE. Instead this introduction attempts to explain the construction and meaning of the term "Time Code" by means of practical examples.
Our introduction was designed for "novice" users and will put the many terms connected with Time Code, such as color . framing, userbits, etc. into context.
For all professionals, the extracts from the specification sheets such as code structure and signal forms, may provide a supplementation to their knowledge.
The first introduction was written in 1984. A long time ago. Today's studio's technology is far more advanced and many of the video formats and Time Code products listed in the following text have long since been outdated. But the LTC and VITC Time Code's are still in use and the norm has only been modified in the non critical areas. Therefore we think, it could be still helpful for novices to read this "outdated" introduction, in order to understand the modern system as a whole.
We would be pleased to receive any suggestions or criticism from you, and hope that this introduction to Time Code will facilitate your initial understanding of this technology.
ALPERMANN + VELTE
Electronic Engineering GmbH
The history of the development of Time Code
The development of the first video - recorder by the Ampex company in 1956 marked the beginning of the video recording on magnetic tape. It was also the start of many problems which are connected with the editing and reworking of video - films. For automatic editing a process had to be developed, by means of which the exact tape position of the beginning and end could be located. Until this time only films had been worked with. The guide - holes in the film could be counted exactly, so that the location of cuts could be clearly defined.
Initially, a mechanical tape counter was also used for video - tapes, which operated by counting the conveyance of the driver shaft. The disadvantages of this process are well - kown.
The exact location of the cut is a random matter, due to the mechanical deficencies, e.g. tape - slip.
On Insertion into the video - recorder, the tape must first of all be rewound to the start, so that the counter can be adjusted. The counter is then normally set to zero.
In 1963 the first pulses were recorded on the audio - track of video - tape (CTL - pulses), thus providing the tape with "electronical guide holes". As a result of this, the mechanical counters could be exchanged for electronical counters. However, this process also proved inexact: The counting process becomes inexact as a result of drop - outs or bad contact between head and tape at the beginning and during rewinding of the tape.
To adjust the counter, the reference point (zero - point) must first of all be located.
These problems are still well - known today, because the majority of Umatic editing systems and almost all VHS editing systems still work with this process.
The above problems were then eradicated by a system which was instroduced in 1967 and standardised in the SMPTE and EBU standard in 1972. In this process a picture number is recorded parallel to every video - picture.
SMPTE and EBU, LTC and VITC
EBU is an abbreviation for the "European Broadcast Union" and SMPTE is the abbreviation for the American organisation "Society of Motion Picture and Television Engineers". Due to the differing television standards in America and Europe, NTSC and PAL, some aspects of the Time Code are designed differently. SMPTE has designed a Time Code - standard for NTSC, and the EBU has adopted this standard. In the Federal Republic of Germany and most European countries an industrial standard exists for the Time Code, which is identical to the EBU standard.
Until a few years ago the Time Code was recorded parallel to the video - picture on an audio - track or a special audio - track for the Time Code, the Time Code track. This method of recording is termed Longitudinal Time Code, or, in its abbreviated form: LTC. Several problems concerning the LTC led to the development of the "Vertical Interval Time Code", abbreviated: VITC. VITC records the code into one of the data lines of the video - signal.
What is Time Code?
First of all Time Code means that Time Code - data is recorded parallel to every picture. When the video - tape is played, this data can be read and the appropriate picture number can be displayed. Each picture is clearly addressed and can be located exactly. The information which is located under each picture has four blocks:
The information for each frame has four parts:
- The time in hours, minutes, seconds and frames
- The userbits
- The controlbits
- The synchronisation word
The time is, in original sense, the picture number. Depending on the standard being used, in one second 24 (film), 25 (PAL - video) pictures must be counted. In the frames, therefore, the particular picture within the second is stated.
There are simple reasons for selecting the tape number as a time with 8 positions instead of using a simple number with 4 positions. The address of the picture is able to give actual time, which is important when recordings are to be document chronologically, as would be the case, for example, when a football match is recorded.
Furthermore, the duration of a particular scene or of a program can be established by means of simple calculations with the Time Code values. Individuals takes on a tape can be distinguished from another by means of a simple trick, which involves stating the take number in the hours (scene 7 begins with the time 07:00:00:00)
In addition to the time, the userbits are also recorded for each picture in the Time Code. The userbits are comprised of 32 bits, by means of which an 8 - figure number or four ASCII signs can be displayed, The operator is free to decide which numbers are used and what they are to signify. The date can be inserts here, or a production number or similar, as desired:
With the ASCII character set is described, which includes all the characters of a standard typewriter keyboard, i.e. letters, numbers and symbols. Four such characters can be recorded under each picture. In principle, entire texts can be fed into the Time Code in this manner by computer. The word "timecode - Inf" would then fill three pictures:
The prevent that ASCII characters are read as numbers and vice versa the Time Code has two controlbits (bit 27 and bit 43) which state how the userbits are to be interpreted. The userbits make the use of Time Code interesting in areas where more than just an "electronic tape - counter" is required.
The identification of recordings with coded numbers is important in archives, e.g. when particular pictures are be called up by EDP:
In the case of video - oriented film editing, video - copies are made of the film negative. The particular film and roll within the film contained on a video - tape can be stated in the userbits.
Tapetime and realtime
Realtime refers to the actual time of day. Tapetime is related to the length of a tape. A tape which is 15 minutes long e.g. has a tapetime of 10:00:00:00 for the beginning and 10:15:00:00 for the end, irrespective of how much time is taken for the recording.
An editing system can, from time to time, react unfavourable to Time Code jumps- i.e. a discontinuous Time Code, such as occurs, for example, when the first and final five minutes of a football match are recorded and the time information of the Time Code contains the actual time. The Problem arises from the fact that the editing system uses the time information of the Time Code as a tape counter.
Let us presume that the first scene starts at 15:00:00:00 and the end of the second scene is at the 16:30:00:00. On the tape the two points are 10 minutes or 15000 frames apart, if the 5 - minute scenes are recorded consecutively. A cut with these two points is logically calculated by an editing system a length of 1,5 hours or 135000 frames. Not every editing system is then able to correct properly. A second time - information unit can be inserted into the userbits. If the actual time is programmed into the userbits and the time of the Time Code contains tape time, then it can be still be calculated from the Time Code when, e.g., the goal was scored, and problems with the editing system are therefore avoided in spite of the use of actual time.
Two of the controlbits are used to identify user information. A further bit contains a drop - frame code. This code is used only for NTSC - video. The frame - rate of the NTSC - video is precisely 29.97 pictures/sec., the Time Code, however, counts 30 frames. A video - controlled Time Code - generator would, after some time, no longer run synchronously with the actual time. It is now possible to jump frame numbers at certain intervals - according to standards - for correction purposes. When this process is employed in NTSC, the drop frame mode has to be set in the Time Code.
The fourth controlbit contains the colour - frame code. Further bits are available in the Time Code, which at present are not allocated any function, thus allowing scope for the continued development of the Time Code.
The color - frame code in the Time Code is a bit, which is set when the Time Code is colour - connected. With PAL - video there is a distinction between 4 - field and 8 - field colour - connection (field = half - picture).
The 4 - field sequence is related to the burst - phase position. With PAL, the burst - phase position is turned through 180 degrees from line to line (PAL = Phase alternating line). With an even number of lines the burst - phase position is equal. The burst - phase position is unequal after the 625 lines of the picture. After two pictures (four half - pictures), l.e. the equal number of 1250 lines, it is equal. The film can be cut at the point without loss of colour.
The 8 - field sequence is related to the phase of the chrominance carrier. The phase is the same every four lines; related to a clear editing point every four pictures or eight half - pictures.
The colour - frame bit (colour - frame flag) indicates only that the Time Code is being colour - connected. The position of the picture in the 4 - field or 8 - field or 8 - field sequence is displayed in the numbers referring to the time. Specific bits of the time numbers must bear particular relation to one another. A detailed description of this coding would go beyond the scope of this instroduction. The information is not, however, visible in the Time Code, and is therefore only relevant in connection with automatic editing systems.
Colorframing in practice
Our portable Time Code - generator AV TC 20, does not posses colorframing and with our studio - Time Code - generator colorframing is an option. There are good reasons for this.
The most important pre - requisite for colorframing is that the video - channel of the recorder is sufficiently high quality to ensure that a consistent, decodable relationship between picture information and color information is maintained after recording has been carried out. With all VHS recorders and with most of the Umatic - recorders, however, this is not, with the 8 - field sequence at least, the case; colorframing is therefore pointless. In the case of Umatic, the four - field sequence at least is usually possible.
The second prerequisit is that the editing systems relate the colorframing to the Time Code. No VHS editing unit does this and only very few Umatic editing units. It is typical for Umatic editing units and those which operate with Time Code, that the correct color entry is balanced directly among recorde and player.
Thirdly, finished productions are often sent through a timebase corrector, to eliminate small faults relating to color and synchronisation. The timebase corrector regenerates the chrominance carrier and the synchronizing signal of the video. Behind the timebase corrector there is no longer any relationship to the chrominance carrier of the original recording. Any colorframing of the Time Code which were carried out previously were pointless!
The result can also be expressed on positive terms. Behind the timebase corrector, colorframing is worthwhile, if recording is subsequently carried out on a recorder of appropriately high quality and the tape is forwarded for further editing work with an appropriato editing unit. And: colorframing is always useful for 1 - and 2 - inch machines, when the appropriate editing unit is able to process the colorframing of the Time Code.
Video synchronisation is essential for clear allocation of Time Code - information to a picture. Each frame of the Time Code commences with the vertical blanking interval of the first half - picture and is the same length as the picture. A Time Code - frame is then located under every picture! The standards specify a tolerance field for the Time Code - frame related to the blanking interval:
LTC - Format
One Time Code - frame contains 80 bits. The picture shows the organization of these bits and the informations stored.
Please click graphic for a better image
16 bits of the Time Code - frame are allocated for the synchronisation word. The synchronisation word an arbitrary bit - combination, which is selected, so that the same combination cannot occur at any other position in the code.
When reading the Time Code, the synchronisation word informs the Time Code - reader that a complete unit of information has been read and can be processed or displayed. Furthermore, the running direction of the tape can be established by means of the synchronisation word.
Modulation and Recording
When recording the Time Code it is "pushed" bit by onto the audio - track. The bits are modulated so that the information can be read when the tape is played back at varying speed ("Bi - phase - mark" - modulation). A polarity - change takes place at the beginning of each bit. When a bit concerned is set,(H - level, L - value), a further polarity - change is effected in the middle of the bit - transfer.
In the diagram of the Ltc - format, bit 59 is termed a non - allocated bit. The German Industrial standards suggest employing this bit as a phase - bit. This is intended to ensure that the first edge of a new Time Code - frame is always rising. Without this phase - bit, the edge is sometimes rising and sometimes falling, depending on the "chance" - value resulting from the modulation. For correction purposes, bit 59 is set at H - or L - level, as appropriate.
The advantage of the phase - bit is, that the synchronisation of the Time Code with a video - signal or a second Time Code can be easily observed with the aid of an oscilloscope.
In practise there is a snag here: Some manufacturers of Time Code - equipment have assumed that this bit is not allocated and have therefore used bit 59 for their own purposes or have at least ignored it. Consequently the Time Code generated with the phase bit cannot be read with such equipment or only very inadequately.
Although it should not be possible in accordance with the specifications, in practise the competibility of the Time Code might become obsolete. For this reason Time Code - generators from Alpermann + Velte do not allocate bit 59; the function can, however, be added at any time on customer´s request.
On account of their steep edges, digital signals are not particular suitable for recording on audio - tracks. The consequences of the steep edges - cross talking and overriding of the audio - channel with high frequencies - do not occur so blatantly when a normal signal is used:
With the PAL - system (25 frames / second), one video - picture is forty milliseconds long. In this period eighty bits are transmitted, one bit, therefore, takes 0.5 milliseconds. This represents a bit - frequency of 2 kilohertz. If all bits were set (H - level), then recording would be carried out at 2 kilohertz, as the polarity is changed once again in the middle of each bit. This is the theoretical upper - limit of the recording frequency. The Time Code audio - channel must be able to process an almost rectangular signal with a maximum frequency of 2 khz.
On small snag connected with the LTC comes in the form of the so - called decoding delay, i.e. the slip of one frame between the generator - time and reader - time. At the beginning of frame five, for example, the appropriate number is displayed, and then, during the next forty mmilliseconds (duration of frame), the coded information is recorded. The next frame is now in position, and the number of the sixth frame is displayed.
When reading, however, all information under frame five must be read, before the reader can display the number of frame five. At this point, the generator is already displaying the number of the sixth frame. When generator and reader are to display the same time, as the recommended by EBU and SMPTE, then the reader must add one frame to the value read out.
With assemble - edits, al signals are re - recorded; if it was started in this manner, jumps in the Time Code would be unavoidable. The consequences of such jumps have already been described in the section "Actual time".
To avoid this, the Time Code reader reads in the Time Code during the pre - roll phase and passes these values on to the generator.
If the tape runs past the cut, the frames are counted on appropriately. In this manner, the Time Code is continuously led over the assemble cut. This process is called jam - sync.
A similar process is employed to copy Time Code. The Time Code of the machine being played is continuously read and the value is transmitted to the generator. The generator passes these values on the recording machine. The signal form on the Time Code is thus regenerated. This is termed continuous jam - sync (cont. jam).
Why is VITC - Code necessary?
The LTC - Time Code has two fundamental problems. First of all, the code cannot be read for stills and at low tape speeds. Secondly, is almost impossible to copy the code. Even on the signal is so distorted, that it is impossible to read it without errors.
These problems are unknown to the VITC. The code be read from stills because it is an integral part of the video - signal, and whenever the video - copy is good, then the VITC I.D. also cleanly copied.
The time information and data information is transmitted in the VITC - code in the same way as in the LTC - code. The VITC - code can also contains the recorded control - bits as used with LTC and the statements on colour - connection also apply here. This is were the similarities between LTC and VITC cease, however.
The VITC information is set directly into one line of the video. After the vertical blanking interval, the video has an area of empty lines, which for some time have been used as carriers for additional information. Video text, for example, can also be transmitted in these lines. As these additional uses were developed only gradually, a specific line could not be allocated to the VITC. The standard states that one of the lines from line 6 to line 22 is to contain the VITC - generator; therefore, the generator has to be told in which line the VITC is to be set.
The VITC information is set once in each half - picture. A new control - bit distinguishes between the first and second field. Editing is thereby made possible to half - picture accuracy. A new and fundamental aspect of VITC is an eight - bit wide information unit, which enables faults to be detected during read - out of the Time Code. This 8 - bit is generated from all other bits of a VITC - frame in accordance with a stipulated process, the CRS - check. During read - out, all bits read out are similarly send to the check. The eight CRS - bits generated by the generator. In this way, faults can be detected during the read - out with more than 95 % certainty.
The VITC - Code Format
One VITC - frame has 90 bits. 18 of these bits are synchronisation bits. 2 bits stipulated in the value are each followed by 8 bits of numbers information. The coding is shown by the following diagram:
Please click graphic for a better image
VITC - Recording
With a VITC - system, the voltage level of a bit in the video - signal corresponds with the digital information. The voltage changes only when the digital information also changes. This process is called NRZ - format (non - return to zero level).
The VITC - standard states the position of the information in one line and stipulates the signal form:
Time Code for Film and Sound
Time Code is, of cause, also employed for film and sound recordings. Unfortunately, in the case of films, it is not easy to apply and read out the Time Code. Various processes have been developed for this, all which contain advantages and disadvantages, but up to now no process has managed to become established successfully. Therefore, we cannot go into this subject in more detail here.
For audio - tape machines, of course LTC is the only possibility. VITC can only be employed when digital audio - recording is used with video - machines. Colour - frame coding has no use here and video - synchronisation of the Time Code is only possible with parallel recording of picture and sound.
Normally therefore, the Time Code - generator must be controlled by an built-in time reference, in the form either of a quartz pulse or the mains frequency. Nevertheless, the EBU - SMPTE - standard is used for the sound, although the frames cannot be counted for the sound. The Time Code used for the video and sound alike is of extreme importance for a further main application of the Time Code.
Synchronisation of Audio - and Video Tape Machines
The audio - channels of video - tape - machines generally do not meet the requirements of professional sound technology. For this reason it is standard practise to record the sound belonging to the video on an audio - tape machine. When the recordings are subsequently worked on, the video - machine and the audio - machine must run parallel. Frame - accurate synchronisation of sound and picture can be achieved by comparing the Time Code - information of both (or several) machines. The position of the synchronisation words and of the individual bits of the Time Code to one another allow for a further degree of control - accuracy, so that machines are perfectly able to run exactly parallel to one another within approx. 50 microseconds.
VITC or LTC?
It would appear logical to employ LTC for audio - tape machines and VITC for video - machines. Video machines will, however, continue to be operated with LTC for a long time to come; among the reasons for this the fact that the use of both codes make neccessary the purchase of double the amount of equipment (LTC - and VITC - generator) and the fact that most equipment, in particular the synchronizers at present operate only with the LTC. In general, the VITC - equipment is considerably more expensive than the LTC - equipment. For these reasons, LTC is recorded in practice, even when VITC is being used.
Alpermann + Velte: Tomorrow´s Technology for Today´s Time Code
AV is a group of engineers, physicists and technicians. They form an innovative team, combining creative powers and state of the art technology for a development line which points the way for others. AV develops and produces complex Time Code systems at todays high quality level requirements.
Moreover, AV sets the pattern for technological evolution. The latest CAD/CAM facilities and costly neasursing and analyzing equipment help in developing chips, microprocessors and boards which keep open options for meeting future challenges.
© Alpermann+Velte Electronic Engineering, 3.1.1984 - 2003, Reproduction of parts or all of this document a admissible only after prior consent of the publisher and with direct quotation of sources.