SMPTE QuickGuide

Time Address

SMPTE Time Code provides a unique address for each frame of a video signal.   This address is an eight digit number, based on the 24 hour clock and the video frame rate,  representing Hours, Minutes, Seconds and Frames in the following format:  HH:MM:SS:FF. The values range from 00:00:00:00, to the largest number supported by this format; 23:59:59:29, or, no more than 23 hours, no minutes or seconds greater than 59, and no frames above the highest allowed by the rate being used (29 in this case for 30 frames/sec).  This format represents actual clock time, the duration of scene or program material and makes time calculations easy and direct.

Frame rate

The Frame is the smallest unit of measure within SMPTE Time Code and is a direct reference to the individual "picture" of film or video. The rate is the number of times per second pictures are displayed to provide motion.  There are four standard frame rates (frames/sec) that apply to SMPTE: 24, 25, 30, and 30 "Drop Frame".

24 Frame	Frame rate based on U.S. standard motion picture film.
25 Frame	Frame rate based on European motion picture film and video, also known as SMPTE EBU (PAL/SECAM color and b&w).
30 Frame (also called "30 Frame Non-drop")	Frame rate based on U.S. NTSC black & white video.
30 Drop Frame	Frame rate based on U.S. color video.

The Frames figure advances one count for every frame of film or video, allowing the user to time events down to 1/24th, 1/25th, or 1/30th of a second. Unless you have an application that specifically calls out one of the above frame rates, it doesn'tmatter which you use, as long as you use it consistently.  Using more than one frame rate is allowed but it adds complexity and difficulty.  Most SMPTE applications outside of broadcast video use the 30 frame non-drop rate because it matches real time.

Sync, Framing, and User data

Additional information is encoded into SMPTE Time Code to provide extended functionality.  This includes Sync data, Framing data, and User information, all usually transparent to the average SMPTE user.  Each of these is described in greater detail in the technical segment of this guide but I have included a brief description below for reference.

Sync Data

The sync data identifies the end of each frame of Time Code and notes whether the videotape is moving forward or in reverse.

Frame Data

The frame data indicates whether Drop Frame Time Code is being used and/or if Color Framing is active.  

User Information

The User Information allows the user to encode additional desired coding data.

Two versions of Time Code

There are two versions of SMPTE Time Code. The first version, and by far the most common, is Longitudinal Time Code (LTC).  This Time Code combines all of the format information described above into a digital signal transmitted serially by a SMPTE source.  The signal characteristics are compatible with standard audio channels so that they can be recorded onto audio tape tracks, or to an audio track on video tape. The second version of SMPTE is called Vertical Interval Time Code (VITC or "Vit -see") and was designed to be encoded directly into the video signal, frame by frame.  As the name implies, this Time Code is located in the video signal during the vertical blanking interval. Both versions of SMPTE share much of the same formatting but there are enough differences to make them incompatible with each other.   With either Time Code type, some kind of generator incrementally produces a complete frame address as each frame advances.

ETC products support LTC only. Applications using VITC must use an LTC translator.

Transmission and Distribution

American National Standard ANSI SMPTE 12M-1986 provides the industry specification for SMPTE LTC and VITC distribution.  VITC, being encoded directly into the video signal, simply depends on video distribution. Unfortunately, the standard says nothing about the transmission medium or distribution of LTC other than to say "...to be recorded on a longitudinal track of video and audio magnetic tape....” implying that the signal is what is commonly referred to as "Standard Audio".  The problem is that no one agrees what "Standard Audio" is.  In most cases, SMPTE can be handled just as though it were any other audio signal, however, the following points should be considered when planning an application: The SMPTE source will determine how devices are interconnected and whether or not additional equipment will be needed just to clean up the signal.  Most equipment that directly generates Time Code will produce a strong and very clean signal.  When viewed with an oscilloscope, the waveforms are square, with little noise.  Most tape sources produce a signal that is somewhat slewed and distorted.  This is a normal characteristic of tape and when viewed with an oscilloscope, the waveform is much more rounded and often more noisy.  Either signal will give adequate performance.  Some Time Code readers prefer the cleaner signal over the noisy one, while others can't handle the sharp edges of the digital signal.  The length and type of interconnect affects this as well.  In most cases this equipment will be "plug and play" but in a few, some trial and error will be necessary to get everything working together.  Various pieces of equipment are available to deal with these circumstances.  The interconnect cabling for SMPTE can be either Unbalanced (single ended) or Balanced (differential).  It doesn't matter which is used, or even if it is mixed in a given installation, as long as proper termination and impedance matching is observed. Two-conductor, shielded audio cable is most commonly used for distribution. 

SMPTE is a relatively high level signal and can crosstalk to adjacent, high gain microphone cables. Because of the nature of the signal, audio compression/expansion, equalization, noise canceling and automatic gain control should be defeated.   These features, which improve the sound quality, virtually destroy the SMPTE signal.   The typical output will be +4 to +8 dBm, with a source impedance of 50 to 600 ohms.  This is roughly a 1.2 to 2 volt, peak-to-peak signal.

When recording Time Code to tape, use the outside tracks where possible.  These values have been recommended by Cipher Digital for the various tape formats:

Tape Format	Where to Record	SMPTE Best Levels
1"	3rd audio or cue track	-10 to -5 VU
3/4"	Outside Track-Audio 1 or the Time Code Track.	-5 to 0 VU
2"	Cue track, also called Auxiliary or Time Code  Track	+3 to +5 VU
ATR	Edge Track	-10 to -5 VU
VTR	Audio Channel 2	+1 to +3 VU

Time Code Signal Conditioning

If the SMPTE Time Code signal begins to degrade, a number of processes can be employed to correct the problem.

Reshaping

Reshaping Time Code is the process of filtering the incoming signal and restoring its electrical properties.  It cannot restore any missing data.

Refreshing

Refreshing Time Code is the process of reading and locking onto the code and then generating a reconstruction of the original signal from it.  If parts of the source code are missing, they will be recreated but if the input signal goes away, the output time code will stop.

Jam Synchronizing

Jam Synchronizing is similar to Refreshing except that if the incoming signal has problems or goes away all together, the jam sync will continue to send out Time Code based on the original input.

Various signal conditioning devices are available to accomplish any one or any combination of the above processes.

Key points thus far:

• Time Code provides a "Frame accurate" means of synchronizing Video and/or Audio equipment with other production devices.
• It is based on a 24 hour clock providing an Hours, Minutes, Seconds, and Frames format with frame timing to 1/24th, 1/25th, or 1/30th of a second.
• Longitudinal Time Code (LTC) is intended for audio and video applications and Vertical Interval Time Code (VITC) is intended for video only.
• Additional control data and user information is encoded into the Time Code signal.
• Transmission and distribution of the signal is through audio equipment for LTC and video equipment for VITC.
• Signal conditioning may be required depending on the installation.
• Various conditioning techniques and devices are available.

SMPTE LTC

ETC products that incorporate SMPTE read Longitudinal Time Code (LTC).  This version of Time Code was chosen because it is most prevalent in the industry and because it provides the user with the greatest system flexibility and ease of installation.   If you have an application that uses VITC, translation to LTC is required.   ETC's implementation supports only a single SMPTE LTC input.   No SMPTE outputs or pass-through connections are supported.  The 24,  25, and 30 (non-drop) frame rates are directly supported, however, due to the synchronization methods employed, virtually any frame rate between 24 and 30 frames/sec. including 30 Drop-Frame can be used.   The only timing issue to keep in mind is that, if the external signal drops out and the internal clock is enabled, the internal clock will run at one of these three user-selectable frame rates.   It is also recommended that the internal clock frame rate be set as close to the rate of the external source as possible so that re-synchronization errors don't occur.

The Hardware

In most cases the hardware required for SMPTE is optional and should be ordered as a separate item, either with the product initially or possibly as a field upgrade option.  This usually involves the installation of some hardware within the given product.   The SMPTE input itself consists of some signal conditioning circuitry and a single integrated circuit that handles all of the SMPTE overhead.   One or more adjustment devices may be incorporated in the circuitry to provide a means of "tuning" the input for a given application.  The input itself is typically a single three pin female XLR connector that supports either balanced or unbalanced signal cabling.

The Software

The software allows you to set up SMPTE on the given product, and provides a display mode for programming SMPTE times and events.  The SMPTE event programming display provides full creation and editing capability for writing and testing SMPTE events. SMPTE optional settings are accessed through a system Setup display and provide a way to enable SMPTE, configure such things as Frame rate, and chose whether a Cue List or Event List is displayed. We shall now examine the common features of the event programming displays.

Events

An event is some action such as a "Go", "Sub Bump", "Macro", or equivalent command programmed to occur at some SMPTE time.  The format of an event varies from product to product but usually consists of an Event Number, a SMPTE Time, a method of running cues in faders, a method of activating macros or submasters, and some method of labeling the event.  The following example shows how this was implemented in the Expression 2 console family. 

Event #	SMPTE Time	A/B	Rate	C/D	Rate	Bump	Rate	Macro	Label
23	01:23:10:29	533	200	421	1500	5 On			Rain Effect

This example shows a single row out of an event list depicting event 23.  This event occurs at SMPTE time one hour, twenty-three minutes, ten seconds and twenty-nine frames, and starts cue 533 in the A/B fader at rate 200, cue 421 in the C/D fader at rate 1500, activates submaster 5 at its recorded rate, plays no macro, and is called "Rain Effect". 

The Event List is constructed of many such events.    They are usually organized in ascending order by SMPTE time, with event numbers arbitrarily assigned in reference to the total number of events.  Events can be complex, containing an entry for each attribute, or simple with just one attribute.  The editing features typical of ETC products allow the user to create and edit SMPTE events individually or in groups.  All standard editing features (such as insert, delete, and copy) are provided.  One point to keep in mind; do not rely on the event number in the list to identify a particular event, as this number will shift as events are added or deleted. Rely only on the SMPTE time address for event identification. 

SMPTE Learn

Some ETC products incorporate a feature called "SMPTE Learn".  When engaged, SMPTE Learn takes whatever console operations occur (i.e. Go presses, Sub Bumps, Macro commands, etc.) and automatically programs events using the SMPTE time available when the event occurred.   This results in a "real time" event program that you can then fine tune or edit.

Starting SMPTE

If an ETC product is set up for SMPTE, it will usually be ready to receive Time Code as soon as it completes its power up sequence.  If no Time Code is present, some form of "Waiting for Input" message will be displayed and the product will monitor the input until it does see SMPTE.  If time code is already running, the product will synchronize to the incoming signal and begin executing its SMPTE events.  The Time Code address received by the product is usually displayed on the main system screen as well as on the SMPTE edit display.

Manual Control

Manual control is provided to allow the user to test events independent of the Time Code source.  A "Manual" button toggles between Manual and normal mode, interrupting SMPTE. A "Step" button allows you to step from event to event at whatever pace you wish. The SMPTE source is still running so the event list may jump to whatever the valid time is when returned to normal mode.  If the internal clock is in control, Manual mode resets it to zero.  The "Pause" key allows you to stop SMPTE as in Manual mode, but does not reset the internal clock.  Be careful if using this feature during a production.  It allows you to go into manual mode and manipulate events, but unpredictable behavior, such as re-run or missed events, could result depending on the SMPTE time at the point of return to normal mode.

Internal Clock

An internal SMPTE Clock is included in the SMPTE option in order to synchronize the incoming signal with the operation of the given product.  This internal clock drives the SMPTE Clock display and is used to keep track of event timing if the external signal loops back, jumps, drops out momentarily or goes away altogether. This is similar to Jam Synchronization but is strictly an internal function and does not provide Jam Sync to other devices.  The internal clock can be enabled and disabled, preset to a value, started, stopped or set up to loop. The characters of the SMPTE Clock display change color to indicate whether it is using internal (Red) or external (Green) time code.

If the internal clock is disabled, the product waits until it recognizes valid SMPTE and then spends about one second synchronizing to it.  The SMPTE times recognized are displayed and corresponding events are executed.  If the signal goes away, the SMPTE clock stops at the last valid frame.  Whenever the signal restarts, loops back, or jumps ahead, the product again needs to re-synchronize.   If the internal clock is enabled, the product starts the internal clock when it finds valid SMPTE, usually within a few frames.  If the external signal goes away, the internal clock takes over and continues "free-wheeling" until the signal comes back or it is stopped manually.  If left running, the internal clock runs to 24 hours, loops back to zero, and continues counting.  Any events will be re-executed when the internal SMPTE time comes around again.

The rules and logic for internal/external synchronization are discussed in the technical section.

Loops

SMPTE shows are often set up to play from one SMPTE time to another, and then loop back to the beginning and start over.  This is very common in theme parks where a short demonstration or show is replayed many times an hour.  There is usually a SMPTE source of some kind that plays these times as a continuous loop.   Since our products re-synchronize to whatever the SMPTE source is, they simply follow the time code and loop back with the signal.  The internal clock should be enabled for these applications so that time isn't wasted re-synchronizing after the loop, but some method of telling the internal clock about the loop is needed so it doesn't continue past the loop time if the external signal goes away.   The SMPTE Reset Loop feature takes care of this by programming parameters for First and Last valid frames. The Last valid frame is where it loops from and the First valid frame is where it loops to.   Whenever the internal clock is enabled and in control, the internal clock continues to loop through the show. If the loop times are not programmed (i.e., left at their default values of 00:00:00:00 and 23:59:59:29), the internal clock will continue to 24 hours and then loop to zero as usual.

Setting and Starting the Internal clock

The internal SMPTE clock can be set to any value and then started manually without the presence of a valid input signal.  In most cases, a single soft-key called "Internal Clock" serves to set the SMPTE time, and another softkey enables and disables the internal clock.  The internal clock can be set to any value within the SMPTE range and started from that point when enabled. This feature can be used to test a SMPTE show when no signal is available or even to time a show to 1/30th of a second in applications that don't use SMPTE.   The enable/disable key toggles the clock on or off.   Keep the following two conditions in mind: The power up state of our products when the SMPTE input and the internal clock are enabled is to first wait for input.  In other words, there may be no SMPTE signal and the internal clock is enabled, but the internal clock will not run until the external signal has been recognized once.  The internal SMPTE clock can be manually started from this condition by using the "Internal clock" key to select a starting time.

Key points of this section:

• ETC provides an optional SMPTE LTC reader on many of its products to control their operation.
• The standard frame rates of 24, 25, 30, and 30 "Drop frame" are supported.
• Display modes provide full feature edit capabilities for creating timed events.
• A Learn mode is incorporated to automate the creation of SMPTE events.
• Manual control features are provided to aid editing and testing of SMPTE shows.
• An internal clock aids synchronization with external SMPTE signals and can provide an internal Jam sync capability if the signal is lost. 

SMPTE Synchronization, Drop Out, and Latency

Latency

This is a measure of response time between the receipt of a given SMPTE frame and the output from the cue which starts at that frame.   The delay is due to decoding time of the SMPTE frame, internal execution delays, and the need to transmit the dimmer outputs.  In most cases this will be three to five frames.    The response times of the dimmer and lamp filaments are not included in this figure.

Synchronization and Drop out

The following eight rules describe the logic used by ETC for SMPTE Synchronization and Drop out. 

Startup conditions

Rule 1 - Regardless of whether it is enabled or not, the internal clock is set to zero after system boot and after exiting Manual mode in the SMPTE display.

Rule 2 - If the internal clock is disabled, synchronization when the SMPTE signal is started takes 30 to 60 frames.

Rule 3 - If the internal clock is enabled, synchronization occurs within two or three frames if the SMPTE signal starts at zero, otherwise it takes 30 to 60 frames.

Drop out

Rule 4 - If the external source is in sync but the next frame doesn't show up within two frames, the product switches to the internal clock.  If the external source is restored at roughly the same SMPTE time, it resyncs in two to three frames. Otherwise, re-sync occurs per Rule 6. This is most common if an input cable is accidentally unplugged or an audio channel shut off.  When restored, the SMPTE time is essentially the same so a re-sync can occur rapidly.

Sync

Rule 5 - If the same frame is received continuously, the product switches to the internal clock.  If the external source is restored at roughly the same SMPTE time, it re-syncs in two to three frames.  Otherwise, re-sync occurs per Rule 6.

Rule 6 - Valid frames received slightly out of time but within two seconds of the last valid time are considered in sync.  The time received is processed and sent on to the display.    This rule allows for alternating and descending SMPTE time.  For example, the current time is 01:15:10:22; the next frame received is 01:15:09:05.  This falls within the two second window of this rule and is processed accordingly.  If the signal continuously alternated between these two times or continued to descend within two seconds of the last time, it would still pass this rule.

Rule 7 - If the frame goes outside of the two second tolerance, the internal clock takes over and the next 30 consecutive external frames are monitored for the following:

• If within 30 frames the time reverts back to where it was, the internal clock will switch back to the external source.  This happens immediately and is transparent to the user.
• If 30 frames of new times are received, then the internal clock resets to this new time and returns to the external source.  During these 30 frames, the internal clock is running and any events programmed for those times will execute.  Also, any events programmed to occur during the first 30 of the new frames will be missed.

Rule 8 - The minimum time between the First and Last frames of the SMPTE reset time is five seconds.

A word about Drop Frame and Color Framing

Drop Frame

US NTSC color video runs at 29.97 frames/sec.  If synchronized with program material at 30 frames/sec, there is an extra .03 frames every second, adding up to 3.6 seconds every hour or 108 extra frames.  Drop Frame Time Code was developed to lose these extra frames every hour without causing re-synchronization problems.  The solution was to drop two frames each minute except for every tenth minute (minute 00, 10, 20, 30, 40, and 50).  An example would be 01:22:59:29 advances to 01:23:00:02. Codes 01:23:00:00 and 01:23:00:01 are dropped.   This allows the time code to run almost exactly in time with 30 frame non-drop.

Color Framing

Full bandwidth color video has a complex, even/odd, alternating frame characteristic that can be described as "A" or "B" frames.   This Framing sequence must be maintained when the material is being edited; otherwise, noticeable horizontal shifts will occur at the edit points.  For this reason, the Color Frame feature of the Time Code provides a more convenient means for identification.  In short, Time Code ending in even numbers define video frames having "A" characteristics,  and those ending in odd numbers define frames having "B" characteristics (for example 01:22:05:00 = A , 01:22:05:01 = B).    

Additional SMPTE signal information

As briefly stated in section one, the SMPTE signal contains more data than just the time code address.   Each SMPTE frame, or Time Code Word as they are sometimes called, actually contains 80 bits.  This is encoded in a binary BCD format with 32 of these bits used to encode the SMPTE time; four bits for each of the eight digits in the address.  The remaining bits are used for User and Sync bits.

User Bits

An additional 32 bits are assigned as User bits, to encode whatever additional information the user desires.  These are sometimes encoded with information about where the video was shot, the subject matter, the reel number, etc.  Needless to say, special equipment is required to generate this data.  ETC products ignore the user bit portion of the SMPTE signal.

Sync Bits

The Sync bits are comprised of the last 16 bits in the SMPTE word and are not accessible to the user.  The end of each frame, the direction of the tape, and the bit interval of the signal is defined by these bits.    Unassigned Bits within the SMPTE frame address some bits are never used because some numeric values are never exceeded.  If we look at the maximum SMPTE time of 23:59:59:29, we see that this is the case for the Frames and Hours tens place, where 2 is the largest number, and Seconds and Minutes tens place where 5 is the largest number.   These bits are referred to as "Unassigned" except in those cases where a function has been added.

There are two such functions, or bits that are worth mentioning; The "Drop Frame Flag", and the "Color Frame Flag".  These are the two highest order bits of the Frames Tens digit, bit 10 and bit 11.

Drop Frame Flag

This is controlled by Bit 10.  If Drop Frame SMPTE is present, then this bit is set to a binary 1. Otherwise it is set to a binary zero.

Color Frame Flag

This is controlled by Bit 11. If Time Code synchronized to the RS-170A Color framing standard, this bit is set to a binary 1. Otherwise it is set to a binary zero.   In some cases, the equipment that generates SMPTE does not handle these bits correctly.  The problem is usually that they are not initialized by the generating equipment and can therefore change states at random.  Watch for this if you see odd behavior with frame sync.  

ETC Product note: Our Expression 2 console family was originally specified for Non-Drop so these bits were expected to be zero.  If these bits came through set to binary 1, the console interprets illegal values for the Frames tens.  These bits were masked out starting with software version 1.8.

Audio Considerations

With most SMPTE applications depending on audio signals for distribution, the following considerations should help plan or troubleshoot an installation.

Common Interconnection Wiring

SMPTE LTC audio interconnections can be balanced, unbalanced or a combination of both.  Audio equipment varies considerably in this area.  There are four common connector types that you are likely to encounter, RCA, 1/4" pin Tip/Sleeve phone plug (1/4P-T/S), 1/4" pin Tip/Ring/Sleeve phone plug (1/4P-T/R/S), and 3-pin XLR.  The wiring lists below show the most common configurations for the various output/input interconnections.   

Unbalanced line, Single conductor cable

RCA and 1/4P-T/S to same
Tip	Signal wire 1
Sleeve	Shield

Unbalanced line, Dual conductor cable

RCA and 1/4P-T/S to same
Tip	Signal wire 1
Sleeve	Signal wire 2
Shield on one end of cable only

Unbalanced source to Balanced input, Dual conductor cable in all cases

Unbalanced  Output		TO	Balanced  Input
1/4P-T/S			1/4P-T/R/S
Tip	Signal wire 1		Tip	Signal wire 1
Sleeve	Signal wire 2		Ring	Signal wire 2
			Sleeve	Shield
1/4P-T/R/S			1/4P-T/R/S
Tip	Signal wire 1		Tip	Signal wire 1
Ring	Signal wire 2		Ring	Signal wire 2
Sleeve	Signal wire 2		Sleeve	Shield
1/4P-T/S			3P-XLR
Tip	Signal wire 1		Pin 1	Shield
Sleeve	Signal wire 2		Pin 2	Signal wire 1
			Pin 3	Signal wire 2
3P-XLR			3P-XLR
Pin 1	Open		Pin 1	Shield
Pin 2	Signal wire 1		Pin 2	Signal wire 1
Pin 3	Signal wire 2		Pin 3	Signal wire 2

Balanced source to Balanced input, Dual conductor cable

Balanced  Output		TO	Balanced  Input
1/4P-T/R/S			1/4P-T/R/S
Tip	Signal wire 1		Tip	Signal wire 1
Ring	Signal wire 2		Ring	Signal wire 2
Sleeve	Shield		Sleeve	Shield
1/4P-T/R/S			3P-XLR
Tip	Signal wire 1		1	Shield
Ring	Signal wire 2		2	Signal wire 1
Sleeve	Shield		3	Signal wire 2
3P-XLR			3P-XLR
1	Shield		1	Shield
2	Signal wire 1		2	Signal wire 1
3	Signal wire 2		3	Signal wire 2

Balanced source to Unbalanced input, Dual conductor cable.

Balanced  Output		TO	Unbalanced  Input
1/4P-T/R/S			1/4P-T/S or RCA
Tip	Signal wire 1		Tip	Signal wire 1
Ring	Signal wire 2		Sleeve	Signal wire 2
Sleeve	Shield		Sleeve	Shield
3P-XLR			1/4P-T/S or RCA
Pin 1	Shield		Tip	Signal wire 1
Pin 2	Signal wire 1		Sleeve	Signal wire 2
Pin 3	Signal wire 2		Sleeve	Shield

ETC SMPTE Input Pinout

ETC products provide a 3-pin female XLR connector for SMPTE.

Pin 1	Com or Shield
Pin 2	Signal +
Pin 3	Signal –

For balanced operation, connect source directly to input 
For Unbalanced input, connect pins 1 and 2 together in the user XLR 3-pin male plug, and connect "Signal" to pin 3. 

ETC Unbalanced input cable end

Pin 1	Shield
Pin 2	Shield
Pin 3	Signal wire 1

Audio 
The term "Standard Audio" does not provide a specific set of guaranteed parameters for audio signals, but instead gives a sense of the range of possibilities for the signal.  So, rather than state a series of absolutes that your audio must meet, we provide you with the range of input signal our products will accept.

This range is:     0 to +14 dBu    

20K-Ohm input 
As mentioned earlier, any audio effects such as Equalization, Noise Cancellation, etc., should be defeated.  Special attention should be paid to this if the signal runs through an audio console, as many of these effects can easily be applied at the touch of a button. If the signal needs to be boosted, a wideband, line level audio amplifier will work just fine.  Over-driving the signal will cause as many problems with SMPTE as not enough signal.

If amplification does not solve a signal problem, you may need to condition the signal by refreshing, reshaping or Jam synchronization.