An Introduction to Timecode

[Tekkie]

Hi everybody,

 

This has been nagging at me for ages. I've been trying to learn about smpte timecode since I've worked on a number of shows where timecode has been used to accurately sync lighting to music/video. After hours and hours of looking through forums I've decided to ask the blue room.

 

From all my time trawling the internet I know what timecode is, that it is sent to a lighting desk via and audio signal (I've even downloaded some timecode signal) but do not understand what goes on in a show environment.

 

Say theoretically we are lighting a dance show tour using only recorded music, the lighting has to look the same each time the song is played, the order can change though each night. We want to dedicate each song a 10 minute block of timecode (just to be organised).

 

The sound guys have their laptop or cd player or whatever they use with their songs on. Do they have to record over each song a seperate timecode value (ie. song 1 starts at 00:10:00:00, song 2 starts at 00:20:00:00)? Before each show they learn that night's order of play so the timecode wont necessarily follow chrinologically each night.

 

How come the audience doesnt hear the bleeping timecode sound alongside the music if the timecode is dedicated and part of that specific piece of music? How does the lighting desk know exactly what timecode is output ie. not just start from 00:00:00:00 for each song?

 

I hope I've have made the questions clear. This is only a educational question so there's no hurry to know the answer immediately :) .

 

Thanks!!

[aidso]

From a studio point of view(should be the same for lighting, been awhile since I used time-code lighting so could be rusty). Time-code is generated along side the audio\video. It is not an audio signal. Say the band\DJ have a track 4 minutes long, most people within studios will start time-code at 10:00:00:00 as it gives room to play either side of the time-code as you cant have a negative time-code.

 

As for programming a lighting show on most desks you would program your cues as you normally would. But instead of having them go on the "go" button you would set them to go on time-code "10:**:**:**" so when the desk sees that time-code arrive in it triggers that cue. As for you saying that the order of songs would change I would normal have built several different cue lists, one for each song and then just load what ever song is coming next.

 

Another thing to watch out for with time-code(gets me on a regular basis) is if it is NTSC or Pal as both are out of sync with each other. I.e your desk is set for Pal and you are putting NTSC into it, it will not sync as NTSC has 30fps and Pal is 25fps

 

That's it in a nut shell. Im sure someone will come along with a long winded post that will go into the different types of time-code such as SMPTE\LTC\VITC.

[Bryson]

A dance show using timecode is probably using LTC on a separate track - so it is effectively an audio signal, but it's on a track that never makes it to the speakers! It just gets routed to whatever needs it. (ie: not the FOH!) So if you're going to use a CD, then that's not ideal - you'll have to do the music in Mono. We used to use 2 ADAT machines, or later, a Tascam 24-track Hardrive playback machine. The Timecode just lived on the last track, and everything else was used for music, vocals, backup music (for sick musicians) and so on. It also meant we could still "mix" the show live.

 

Timecode includes an absolute position, so going back and forth shouldn't really matter - however, in the real world you'll need a few seconds of pre-roll to "lock up" the timecode with the console. Also, while shutlling back and forth in the material, you should make the console go back to manual mode while you mess about - otherwise it may trigger unusual cues you aren't expecting - then re "lock" timecode once you're ready to go. This may be a touch too time-consuming and error-prone to do multiple times per show. We used to do it once in one of our shows and it would occasionally not work in time and I would end up being a human timecode interpreter, hitting go when I saw the right code on the timecode clock!

 

See also the Wikipedia Article on Linear Timecode which explains how, exactly, it gives the absolute time, rather than relative time.

[mac.calder]

The sound guys have their laptop or cd player or whatever they use with their songs on. Do they have to record over each song a seperate timecode value (ie. song 1 starts at 00:10:00:00, song 2 starts at 00:20:00:00)? Before each show they learn that night's order of play so the timecode wont necessarily follow chrinologically each night.

 

How come the audience doesnt hear the bleeping timecode sound alongside the music if the timecode is dedicated and part of that specific piece of music? How does the lighting desk know exactly what timecode is output ie. not just start from 00:00:00:00 for each song?

 

When you use SMPTE timecode with a PC or CD player, what you need to to is make sure you have a spare channel - on a CD, that would mean using a mono mix on the left channel and placing SMPTE on the right, on a PC that may mean using a multi-track playback application with a multi-channel audio card/interface.

 

Basically when you master your audio tracks, you place a second track along side the actual audio - your timecode track, your show audio is routed to your sound console, however the timecode track is routed to it's own channel - on a CD, left would have audio, right would have SMPTE timecode. Some PC applications will automatically give you timecode, on an output you define in the settings, based on the timeline, if you are planning to run your show from the PC, you can just space your songs out so that each song is spaced out according to your SMPTE plan and use jump markers for playback - it saves having to create a SMPTE track for each song.

 

When it comes to show time, the SMPTE channel does not get played through the PA, it gets run to the lighting desks SMPTE in (sometimes labeled LTC).

 

There are a number of tricks to working with timecode when you need to jump about - you need to have states between songs that look identical and you need to have a few seconds of pre-roll - that is, silence at the start of the track with only timecode running. This allows the desk to "see" some timecode and observe that it was not just a glitch. It will then jump forward to where it now belongs and playback of the next song can commence. On some desks you will need to make sure that it follows timecode in both directions - ie that it will allow you to jump backwards and forwards, not just forwards.

 

It is very simple, but can quickly become very complex - and could be better explained with some screenshots - so I will try and post some later today.

 

--damn it - pipped to the post :)

[Dmills]

Book for you to look up: "Control systems for live entertainment", Huntingdon, John.

 

Covers timecode, MMC, MSC and a lot of the other approaches.

 

Regards, Dan.

[dbuckley]

The sound guys have their laptop or cd player or whatever they use with their songs on. Do they have to record over each song a seperate timecode value (ie. song 1 starts at 00:10:00:00, song 2 starts at 00:20:00:00)? Before each show they learn that night's order of play so the timecode wont necessarily follow chrinologically each night.

There needs to be a timecode data alongside the audio track. so as the audio track plays, the timecode "track" stays with the music.

 

At this point you need to know there are multiple forms of "timecode" which differ both by how they are (or are not) stored, and the electrical interface used to get timecode between bits of equipment. But each format eventually delivers the same information.

 

LTC: (Quoting the Wiki) Linear Time Code (pronounced litsy, like ditzy) is one of the two "proper" timecode formats, and is an audio signal often recorded on a track of a multi-track tape. As an audio signal it can be carried across and by normal audio pathways, including walkie talkies.

 

VTC: Used by the video boys, simplifying, it means that the timecode is "in" a video signal, and can be extracted by suitable equipment.

 

MTC: MIDI time code, which is timecode transmitted along a MIDI cable. This is the sort you encounter most often in show control and theatrical situations.

 

So, timecode is recorded alongside a "track" (audio or video) either as an audio signal on a seperate track (so for stereo using LTC you need three tracks, on video you can hijack one of the audio tracks) OR hidden in a video signal (how is unimportant, and differs by analogue or digital video anyway) OR generated by the gubbins in a digital audio recorder, so it doesn't need a track, and is never audio, it just appears by magic.

 

In a show control context, if you're lucky, the master device will have a MIDI output socket on it and MTC will just tumble out of that with no further ado. If you're unlucky (or up for adventure!) you'll need a box to convert from one format to another in real time.

 

The reality is that most stereo PC based playback solutions wont generate any form of timecode. There is software that does, but its rare. Thus multi-track playback is a good way to get a LTC track. RSD do a number of products, including their Sound Server which actually generates LTC on the fly. But in most contexts, MTC is more useful than LTC, as lighting desks mostly want MTC.

 

All (ok, there may be exceptions) digital multitracks will do MTC though, and an easy solution is any digital audio recorder, control it by MIDI to select the song to play (usually a MIDI program select message will do) and then use the MMC start command to get playback under way. The MTC offset is an attribute stored and recalled with the song. This is by far the least hassle way of having a master timecode system. You can low-tech this and use the front panel controls on the recorder to select the sond and start playback.

 

As noted above, timecode is an absolute value, and once the source rolls, given a short while, any chasing device can determine where it needs to be and locate itself to there. "Chase" is an old-fashioned term, because back in the day of reel-to-reel tape, if you had a chasing tape recorder it would, if required, rocket through thousands of feet of tape at high speed to get to where it needed to be... In todays non-linear world, the chasing devices will be in time in well less than a second.

 

How come the audience doesnt hear the bleeping timecode sound alongside the music if the timecode is dedicated and part of that specific piece of music?

Because with the LTC, timecode isn't in the music, its alongside the music.

 

How does the lighting desk know exactly what timecode is output ie. not just start from 00:00:00:00 for each song?

As noted above, timecode is absolute. Now to actually assemble the current time takes several frames of data, because timecode doesn't have the absolute value of every frame, the time is spread out over several frames, but given a few frames the chasers can get into lockstep.

 

Note also the warning above about different timecode framerates, TC can be 24, 25, 29.97 (a/k/a drop frame) or 30 fps (frames per second) and for most applications it doesn't matter a jot which you use, as long as all the equipment is set to the same! If they are not the same then ugly things will happen.

[kitlane]

VTC: Used by the video boys, simplifying, it means that the timecode is "in" a video signal, and can be extracted by suitable equipment.

 

Sorry to be pedantic, but that should be VITC - Vertical Interval Time Code, which describes where the timecode signal is placed.

[dbuckley]

Thats funny - I know that, yet still screwed up. Well, to err is human, I guess :)

 

VTC would not be pronounced vit-zee...

[benash]

Bravo to all the folks who've posted in this thread. I only had a very basic understanding of timecode and you've all done a great job of explaining it.

[djandydee]

One other wee thing about VITC and LTC.

When used in an edit suite environment where frame accurate insert editing is required the Video Tape machines use both VITC and LTC

- LTC is useless when the tape is stopped as it is played off a linear track than requires movement. LTC however is good up to FF or RW speeds as it is based around a 4KHz audio signal which can still be read at 150KHz.

- VITC is useless at anything above x2 play as it is based on out of vision data on line 19 and 21 which becomes unstable above this speed. It is however OK when stopped.

 

LTC is fed on a balanced audio circuit but is quite high level (I have just measured +4dBu) and being a fairly square wave audio signal can "get onto" poorly screened microphone circuits. We always use seperate cabling, krone blocks and jackfields for LTC from analogue audio although AES dosen't seem to care.