Youve worked day and night for the past two weeks, composing amusic score for a film project that could really make your career.Youve paid meticulous attention to scene transitions and soundeffects while reviewing your VHS work tape, and now your music fits thepicture with absolute precision.
But the director has unexpectedly appeared at the editing facilityto see the finished music synchronized to film. Suddenly, your musiccues are a little off the mark and the climatic explosion is twoseconds early. Everyone is glaring at you. You have blown it. Big time.And just before you run screaming out of the editing suite, you awakenand realize it was all just a bad dream. You dont even begin theproject until next week.
Unfortunately, the dream could be a prophecy. At some point in yourcareer, a synchronization issue may arise that could determine theultimate success or failure of a project. Luckily, a completeunderstanding of time code can prevent this nightmare from becoming areality. Time code can be extremely confusingand manymanufacturers still dont fully get it, eitherso letsclear the air on some common misunderstandings about synchronizationand, in particular, SMPTE time code.
A BRIEF HISTORY OF TIME CODE
Time code was originally developed by the U.S. military to synchronizemissile test firings in the early 1950s. A system was designed thatdivided each second into segments or framestypically 10, 30, 60,or more per second. The code was transmitted in the form of amodulating audio tone (similar to the sound of a modem) to deviceswhich could interpret the incoming data and trigger simultaneouslaunches.
Eventually, a 30 frames per second (fps) time-code standard emerged.This standard was based on the television-field rate, which was halfthe stableand commonly availableU.S. wall current of 60 HzAC. In 1981, the Society of Motion Picture and Television Engineers, orSMPTE, officially defined and adopted the 30 fps standard as a methodof synchronizing various film, television, and audio elements. A 1986revision to the standard, now commonly referred to as SMPTE, refinedthe code so that each of the 30 frames includes a digital wordcontaining 80 bits of data that allow information such as tapelocation, reel number, and session dates to be stored and recalled.
Time code provides us with the vital location and timing informationof a program, and it is expressed as hours, minutes, seconds, frames,and sometimes, subframes. (Occasionally, subframes will be displayed ina computer sequencer, but video and television are synchronized only tothe nearest frame.) A typical time-code location appears as 01:17:45:12(one hour, seventeen minutes, forty-five seconds, and twelve frames).If subframes are indicated, they are designated by a period and twodigit number following the frames (01:17:45:12.67 or twelve frames andsixty-seven bits).
Time code is transformed into different forms for differentapplications, but each form adheres to the same basic rules of countingtime. Therefore, each form is compatible with the others forsynchronization purposes as long as the proper translation hardware isconnected. Longitudinal Time Code (LTC) is the code that a MIDIinterface or time-code generator spits out of its SMPTE output jack. Itis characterized by an audible, warbling tone. LTC is used when youwant to stripe a tape with time code and then feed it back into yourMIDI interface for locking your sequencer to an audio or videorecorder. You can also use LTC to print time code on the linear audioportion of a videotape. However, most broadcast devices and digitalmultitracks have a dedicated address track that outputs LTC and savesyou from sacrificing an audio track for time code.
If you are dealing strictly with audio recording and will not besynching to video, you will probably only need to know about LTC andMIDI Time Code (MTC). MTC is generated from a MIDI device using aselected master time base (internal reference, incoming SMPTE code,etc.) that is transformed into timing data that can be transmittedthrough a MIDI cable. (MIDI cannot transmit audio signals, so LTC isnot suitable for MIDI applications.) This data carries basically allthe same relevant information about the current hour, minute, second,and frame location that LTC does, only in a form that is recognizableto a MIDI device.
A third form of code is Vertical Interval Time Code (VITC). Thisform of time code is actually buried into the image frames of avideotape (see the sidebar "Video Basics). VITC must be recorded ontothe videotape dub simultaneously with the video image. Unlike LTC,which can be laid to an audio track at any time, VITC cannot be dubbedin at a later date.
However, integrating time code into the video signal has distinctadvantages over LTC, because VITC does not require that you use anaudio track on your recorder or VTR to store the code. Also, VITC canshowthrough a window burn time-code display recorded directlyonto the video tapethe exact time-code location of a single videoframe. This means that you can still read location numbers when thevideo is paused or when you're slowing scrolling through a scene.Because VITC is perfectly synchronized to video images, it is thepreferred source of code when scoring to picture, even if it is oftenconverted to LTC and MTC for the practical use of the composer or sounddesigner.
A lot of confusion with time-code originates from a lack ofunderstanding of this fundamental rule: time-code rate and type areindependent of each other. Learn this, and dont ever forget it.Go back, and read it again. You must know the difference betweentime-code rates and types and how to effectively communicate thisinformation when working with others.
The rate of time code refers to the speed of the code or how manyframes pass within a second of real time. There are only four rates(speeds) of time code: 30, 29.97, 25, and 24 fps. Most music-onlyproductionsand some film-scoring projectsin the UnitedStates and Japan employ 30 fps code.
Way back in the days of black-and-white broadcasts, North Americantelevision ran at exactly 30 fps. After color TV was introduced in the1950s, however, 30 fps was replaced by the slightly slower-running29.97 fps rate. At the risk of oversimplification, this 0.1 percentspeed modification allowed for easier color synching of new programsand the reproduction of older black-and-white programs with existinghardware. Today, all television and videotape equipment runs at 29.97fps without exception.
The speed of time code in Europe is exactly 25 fps (called EBU timecode after the European Broadcasters). This standard is becauseEuropean video and television also runs at exactly 25 fps, making theirbroadcast time calculations based on time-code information veryeasy.
The speed of motion picture film is 24 frames per second, and mostfilm shot at 24 or 30 fps is necessarily "pulled down," or slowed, to23.976 or 29.97 fps when transferred to videotape. (Most film composerswork from a videotape running at 29.97.) The only exception to this isif you happen to be one of the few composers who still scores to theactual film stock.
If you work primarily with NTSC video, the standard in North Americaand Japan, there are two practical time-code rates: 30 fps and 29.97fps. So whats the difference again? Remember, 29.97 code runsthrough frames at a slightly slower speed than 30 code. What does thathave to do with time-code type? Well, one of the advantages of havingan exact number of frames pass within a second, as with 30 fps or 25fps code, is the ability to use the SMPTE counter as a measure ofactual running time (also referred to as wall-clock time).
For example, at a rate of 30 fps, if a program starts at 01:00:00:00(one hour) and ends at 01:01:00:00 (one hour, one minute) on the SMPTEdisplay, the duration of the program is also exactly one minute long bythe clock on the wall. However, with a time-code speed of 29.97 fps,when the elapsed wall-clock time is exactly one minute, the SMPTEdisplay will read 00:00:59:28 (59 seconds and 28 frames). And after onehour of real time, the SMPTE display will only read 00:59:56:12, or 108frames short of one complete hour. Obviously, a 29.97 fps rate can beproblematic for broadcasters, who require exact program lengthinformation for precision timing of television commercials that arebilled out by the second.
To reconcile the discrepancy between 30 fps and 29.97 fps in regardto wall-clock time, drop-frame code (or compensated-mode time code) wasdeveloped. Basically, drop-frame code recounts frames to allow the29.97 rate to "make up" the 108 frames it falls behind every hour ofreal time. This feat is accomplished by skipping the two framesnumbered :00 and :01 at every one minute interval, except the intervalthat falls every ten minutes. (No actual video frames are omitted, onlythe numbers used to identify each frame are altered.)
Using a 29.97 drop-frame rate, simply referred to as "29 drop" or"29d," means that the SMPTE display will be a near-accurate runningrecord of the actual elapsed time. In other words, the time-codedisplay now closely matches the wall clock, and the broadcasterwont have to refund advertising money because the last twoseconds of an automobile commercial was clipped off just before the sixoclock news. The most that the 29.97d SMPTE display will ever beoff from wall-clock time at any given point in a program is two frames,or 66.73 milliseconds. However, continuously running drop-frame codedrifts two frames per day, and broadcast facilities must recalibrateevery few days to maintain accuracy.
Now, heres where it starts to get a little tricky, so listenup. Technically, 30 fps code can also exist as two types: 30 nondrop(30nd) and 30 drop (30d). The easiest time code to work with is 30nd;traditionally designated as 30 fps, with no additional letterdesignation. With 30 fps, the SMPTE display reflects wall-clock time,making it an industry-preferred choice for music production. However,since television and video equipment do not run at 30 fps, a music bedfor video created at this speed will not be in sync with the picture.Sorry.
However, if you are working on a music-only project and will notneed your track to synchronize with video at a later date, you areclear to use good old 30 fps code. Your sequencer will happily chasealong to whatever code you feed it, and the SMPTE display will neatlymatch wall-clock time. Also, MIDI-event editing is easier because youdont have to take into consideration that certain frame numbersare missing in the event list, as with 29.97d code.
A rare and mysterious code is true 30d, running at exactly 30 framesevery second, and employing the same frame number skipping scheme as29.97d. You'll probably never encounter this type of code, as it haslittle use in the post-production or music worlds.
A nagging problem within the time-code community is the unintentionalmisnaming of the various codes. For instance, the term "30 drop," orsimply "drop," is sometimes used when someone is actually referring to29.97d. Erroneous time-code designations are ocassionally propagated bymanufacturers who fail to fully understand the distinctionsthemselvesor at least dont always make the differencesclear in their devices and manuals. The MTP Console software for Markof the Unicorn's MIDI Time Piece II, for example, offers four choicesfor time-code generation: 30, Drop, 25, and 24. It is not made clearwhat "Drop" is in this context. A call to MOTU verified that it is29.97d code and also revealed there is no choice between drop andnondrop generation at 29.97 fps.
The manual for Roland's DM-800, although correctly listing the fouravailable code versions of 30 fps and 29.97 fps (drop and nondrop),incorrectly names both 29.97 types as "30DF" or "30ND," which is thesame identification given to the 30 fps codes. In sort, four differenttime-code versions are sharing two names. However, Roland does statethat the 30DF version of 30 fps is "usually generated only by mistake."Thanks for the clarificationand manufacturers wonder why usersare confused!
To confuse matters even further, Emagics Logic offers theseemingly useless and problematic option of 30d code. After gettingconflicting stories from Emagic, I was finally told that Logic'sEuropean programmers, being more familiar with EBU code than SMPTE,simply made all combinations of time-code rate and type available.
In addition, Logic offers an auto-detection mode that attempts toidentify the rate and type of incoming time code. This is a featurethat should not be trusted. On one occasion, Logic told me thatincoming LTC from a videotape (29.97 fps without question) was runningat 30 fps. Wrong. The potential for incorrect readings is great,especially if the master device is an analog recorder (almost allanalog recorders suffer from transport speed inconsistencies).Fortunately, Logic's auto-detect feature can be deactivated.
Sometimes, a timing problem can be traced to the fact that all SMPTEgenerators are not created equal. Some MIDI interfaces, for example,should not be relied as stable sources of time code. Theyre justnot designed for it. Put your trust in video-referenced devices fromcompanies such as Adams-Smith, Timeline, and Horita, or the codegenerated from MDM address tracks.
You should also be aware that there can be subtle differencesbetween the internal clock speeds of time-code generators andcomputers. A case in point: if you have ever worked with drum loops,you know how tedious it can be to get a loop to retrigger smoothly andmusically so that the loop grooves with the time center of thesequencer. Unfortunately, if you attempt to lock your sequence toquestionable time code (from an unstable generator, playback from ananalog recorder, etc.) your loop may cease to retrigger correctly. Thisfrustrating glitch occurs because the new master time base is not thesame as the internal clock of the computer that you were referenced towhen you fiddled with your drum loops.
Could the internal clock of the computer be slightly unstable?Thats possible, too. I had to make an approximate 0.4 bpm changein my sequence tempo to get a loop to play correctly if I first tweakedout the loop in my computer and later locked it to time code generatedby my MTP II.
But dont let these horror stories frighten you. Here are someguidelines that can sort out most of the potential code problems you'llencounter.
For starters, always use the originating time code as the mastercode for every device you want to synchronize. Remember the drum-loopproblem? The work around for that situation is to tweak your loop whilethe computer is chasing the time code from the recorder, VTR, orsynchonizer that will be your master time base, instead of relying onthe internal clock of the computer. This is called resolving to thecommon reference.
When a device is resolved, it will speed up or slow down to followexactly the time code being referenced. It doesnt matter if youare trying to get a loop to groove to tape or match a music score tothe designated hit points on a video dub. If you resolve your devicesto the same master time-code source, you will eliminate mostsynchronization problems.
For example, if you stripe a 24-track, analog tape with 30nd codefrom an Opcode Studio 4 interface, always use that code for the rest ofthe project. It doesnt really matter whether the MIDI interfacegenerated code a little fast or slow or if the multitrack recorder wasrunning a bit faster than normal. The point is, this code is now themaster reference and every other device must be resolved to it. Aslaving device will chase the code exactly as it was originallyrecorded, minor imperfections and all and will always be in sync withyour master.
If you are going to score music to a video, you can use the mastertime code provided on the videotape, which is called the house sync, asyour time base. Be sure to let the video-editing house know what typeof video format you work with and any preferences you may haveregarding time-code generation. I typically ask for a Hi-fi VHS tapewith time code on both the linear tracks and the left channel of theHi-fi tracks. (Any production audio is recorded onto the right channelof the Hi-fi tracks.) Of course, the type of code (drop or nondrop)being used for the project is usually decided by the director oreditor. You already know what the rate will be, right? Ready for a popquiz?
Once again, it is critical that you stick with the common referenceto ensure your music will always be in sync with the picture.Lets say you are given a videotape with 29.97d code on it and areasked to compose a 45 minute score. You set your sequencer to 29.97d,view the burn-in window on the videotape, and note all your hit points.You create a beautiful score, using the time code on the videotape todrive your sequencer while you watch the picture. All your hits line upand everything is wonderful. Now its time to record the score totape.
If you mix your score to a time-code DAT machine, you can generateyour own code to the multitrack master (or use the address track of anMDM), record the score, and mixthe DAT machine will chase thecode from the multitrack. The DAT master can then be easily resolved tothe house sync of the picture, and your music will be locked (orgenlocked) to each exact frame of the videotape
However, if you mix to a non-time-code DAT, you must resolve themultitrack to video-referenced time code when you mix. This isnecessary because your DAT mix will be freewheeling when laid-backagainst the picture in the post-production studio. But, as long as youare resolved, there will be little noticeable drift between score andpicture once you start everything from the right spot (a job for theediting facility). Yet keep in mind that you are running wild. It isonly the grace of the stable transports of the videotape players, MDMs,and DAT machines that keeps the sound of the slamming door in the rightspot as the hero exits to catch the bad guy.
Time code generated by an MDM address track (like an ADAT with a BRCor DA-88 with a sync card) is rock-solid stable because these arevideo-referenced machines by design. You can use the internal time codefrom the MDM address track, record your score, and mix it tonon-time-code DAT with excellent results.
A solid, clean source of time code is essential for keepingeverything in sync throughout a project. If you wish to transfer housesync from the videotapes linear track to an analog multitrack, itis likely the code will need to be reshaped. This process ensures thatthe code will be in the best possible condition when re-recorded, andcan be read properly later on down the line. Most MIDI interfaces canregenerate house sync from a video tape to provide clean code for yourrecorder.
So, what happens if you suspect the time code has gone bad halfwaythrough your project? Should you just restripe the tape with fresh codeand find new offsets? No! Dont ever restripe tape once you havebegun a project. That is, unless you never want to be in sync with itagain.
When code is not being read properly, it is often due to playbackproblems with the tape deck rather than a problem with the originalrecording of the code. Checking cable connections and playback levels,cleaning the heads, or adjusting the tape bias usually solves theproblem. Occasionally, a touch of equalization can render the codereadable again. However, if equalization seems to be the only fix, itis a good idea to go ahead and regenerate fresh code to a new track. Byregenerating the code, all the original timing information will remainintact and match the tracks you have already committed to tape.
If you are using an MDM and you experience a lot of dropouts,digitally clone the tape as soon as possible. During cloning, the errorcorrection process should restore the code to its original condition.If drop-outs persist, try playing the tape in another machine. Swappingdecks often buys you that one "perfect" play back that will produce anuncompromised clone.
LOCK IT UP
Dealing with time code can be frustrating and confusing, but it isalso an immensely helpful ally. The more you understand it, the betterit can serve you. If you have questions, dont hesitate to asksome accessible audio engineers or video editors for advice andinsight. They are usually happy to offer assistance, as long as theyknow you have a grasp of the basics. So spread the knowledge around.Heres to living in sync!
Composer/Producer Rob Shrock's original score for TexasInstruments recently won the Telecommunicators Corporate FilmImage award. However, his greatest joy is being in sync with his wife,Lori. Special thanks to David Crigger and John Johns.
Wait a minute, how can you have time code mixed in with a videopicture? Actually, it's not hard at all. You see, a video imageconsists of individual frames, or pictures, strung together and playedsequentially at a specific rate. An NTSC video frame consists of 525horizontal lines of colored dots created by an electron beam, orcathode ray, scanning across a picture tube. It takes two scans tocreate a complete frame. The beams draws the odd-numbered lines first,and then momentarily blanks out while it moves into position to begindrawing the even-numbered lines. Each of these half-images is called afield.
The time span between the drawing of each field, as the beam isblanked out and moving back into position, is called the verticalblanking interval. This interval is used to store timing messages thatkeep the two interlaced video fields in sync. It also serves as a placeto store the VITC (Vertical Interval Time Code) form of time code.Thus, VITC is buried between the two fields of the video frames andremains perfectly in sync with the picture.
So confused about time-code issues that you can't even cruise throughthe terminology? Then use this handy glossary to help bolster yourpowers of comprehension.
Absolute timing reference The master time code used to indicatetape speed and position as well as to synchronize various recorders(see house sync).
Black burst A black video signal used as a house syncsource.
Burn-in window A visual display of the current time-codelocation derived from VITC or LTC and embedded into the video image.The window typically appears as a black box in the lower portion of thevideo screen.
Chase The act of a recorder or sequencer followingor"chasing"time-code data to sync with a master machine.
Common reference The master time base; usually a video housesync generator or black burst.
Drop-frame A term for the method of "recounting" individualvideo frames that allows the elapsed time of the SMPTE display toapproximately match elapsed real time. This process, also calledcompensated-mode time code, was developed to reconcile the speeddifference between 30 fps and 29.97 fps time code, as related to actual"wall clock" time.
Frame In video or film, "frame" refers to one of the individualstill images played back sequentially between 24 and approximately 30times per second. In time-code parlance, a frame is a digital wordinterval corresponding to a single film or video image.
Frame rate The speed at which the time code runs (30 fps, 29.97fps, 25 fps, or 24 fps).
Freewheeling Used to describe a device running wild aftersynchronization is lost. This situation is usually caused by a largedropout in time code.
Genlocking The procedure of directly locking time code to theactual picture frames of a videotape for perfect synchronization.
House sync The master time-base reference, usually generated bya video facility, that is used by all slaving devices as the commontime-code source. The master time base can also be called the commonreference.
Jam synching The act of regenerating fresh time code from asynchronizer/generator when the original incoming code experiences adropout or other damage. The new code is created independently of theoriginal code and is not synchronous.
Nondrop frame Also called uncompensated-mode time code, thisprocess numbers each video frame sequentially. However, unlikedrop-frame code, the frames are not recounted to match wall clocktime.
Regenerate The act of taking incoming code and creating newtime code based exactly on the timing information of the previous code.This procedure is essential when additional code is needed to match thecommon reference, such as when bouncing code to a new track.
Reshape The act of regenerating time code to regain thesquare-wave shape necessary for proper translation of the timinginformation. Also called refreshing, the procedure is often necessaryto compensate for signal deterioration when time code is re-recorded inthe analog domain.
Resolve The act of controlling the speed of a slave device bycontinuously comparing and matching the time-code output to the commonreference.
Stripe The act of recording Longitudinal Time Code (LTC) ontoan individual track of a tape recorder from beginning to end.
Video-referenced When a device is synchronized to the videoframe rate.
Wild sync When two devices are running independently at thesame time but are not dependently locked together or synchronized.
SMPTE DOS AND DONTS
- Do use the shortest, most stable signal path to and from thetime-code source when generating or reading code.
- Do use an outside track for recording time code to analog tape and,if possible, leave an empty adjacent "guard" track between the code andyour music tracks.
- Do keep the time-code recording level just hot enough to provide astable sync source without bleeding onto the other tracks (Between -10and -3 on the VU meter is typically a good range for analogrecorders).
- Do stripe a tape in its entirety; do not interrupt theprocess.
- Do resolve all devices to video-referenced time code when scoring topicture.
- Do provide yourself at least ten seconds of preroll before yourprogram starts (fifteen seconds is even better).
- Do notate your SMPTE rate, type, and offset in your sequence, andclearly label all tape boxes and track sheets accordingly.
- Do regenerate code when making analog transfers. In fact, alwaysregenerate code instead of restriping.
- Don't change code settings once youve begun a project. Stickwith one setting from beginning to end.
- Don't restripe code, unless you really know what you aredoing.
- Don't freewheel or jam sync unless it is the only alternative.
- Don't refer to code simply as drop or nondrop. Such designations canlead to confusion, due to the fact that 30 fps and 29.97 fps code canexist in both forms, and all types are different.
SMPTE Made Simple: A Time Code Tutor
By TimeLine Vista, Inc.
tel. (619) 727-3300
fax (619) 727-3620
Time Code Handbook
By Cipher Digital
tel. (800) 331-9066
fax (310) 695-3622
Synchronization from Reel To Reel
By Jeff Rona