The importance of blackburst sync in audio post?

[IO Composer]

Hi.
I'm wondering about how important is blackburst sync in an audio post studio? I've typically just used my 96IO interface as my word clock master, but a lab guy who shot an optical track from one of my DAT masters said that I had some erratic drift that was probably caused by a bad sync source. Never happened to me before, but he seemed emphatic that I need to have a separate blackburst generator controlling everything in my studio. Can anyone give me their opinions on this?
Thanks,
Jamey

[vze24x2x]

If you're posting audio for video, you need to resolve everything to Black burst.

[Jsalam112]

Where can an inexpensive black burst box be purchased?

[cmaynes]

Hi Jamey-


If you are running with tape transports, I would say they need to be black sync'd, however If you are syncing to a QT movie, I would say it is less important- The sync of the 96 i/o is at least as reliable as a black generator. If there was a problem with the sync drifting it would be more likely an issue of it being 29.97 timecode vs 30fps and perhaps the DAT or DA88 not being reliable timing wise. You could perhaps use AES from your 96 i/o to word sync your machines.


charles maynes

[Jsalam112]

Do I need a black burst box?

I scored a film several years ago. All audio is in Pro Tools. Back then I used a MTP AV to lock to the beta SP. No problems.
Video sat for four years. Editing was done at various times with different Pro Tools systems that locked to beta and Avid machines. It landed back in my lap recently. I am updating audio to the edited film. I've been locking to a beta SP machine with my HD3 Accel system, 192 io and a sync io. Everything locks as it did four years ago. The final mix is to be delivered on a DA-88.
I have not used a black burst box to date.

Is it really necessary at this point?

[Chief Technician]

In a digital audio studio, if your digital audio devices are not locked to the same clock, you are susceptible to problems with your audio such as clicks, pops, spikes, etc. This is why some device in the studio must be the master clock. Often non-post studios use a word clock generator to act as the master clock and route it to a distribution amp and then to each individual device with a word clock input. Devices that do not have a word clock input derive their clock from the embedded data in a digital input such as AES/EBU.

In a video machine room, if your VTRs are not locked to the same reference, your picture will drift vertically. When all of the VTRs in your machine room are locked to black burst, you do not have this problem and your dubs/clones are fine barring other problems beyond sync. If you do not have the VTRs locked with blackburst, the color framing bit in the SMPTE timecode becomes meaningless.

When using digital audio gear in conjuction with video equipment, everything must have a house sync reference. Digital audio clocks reference to 60Hz, and video clocks reference to 59.94Hz. So how does one overcome this .06Hz difference? By using equipment that can genlock to the house video sync (blackburst). Just as a timecode generator can lineup timecode words with the start of a video frame (the process called color framing mentioned above), audio word clock generators can derive their signals from video sync signals. Some machines, mainly timecode DAT recorders, can lock to either an audio word clock or a video black burst signal. If a digital audio device is not clocked by video blackburst or a word clock that is genlocked to video blackburst, jitter and another anomalies will result.

When using a device that genlocks digital audio gear to VTRs, careful attention must be paid to sample rates. Since audio clocks reference 60Hz and video blackburst references 59.94Hz, the same sample rate for both devices may not be appropriate (because of the reference frequency difference). To compensate for this many audio word clocks have pull up and pull down sample rates. These are rates that are +/- .1% of the 44.1kHz and 48kHz sampling rates. The rates are 44.056kHz, 44.144kHz, 47.952kHz, and 48.048kHz. A recording at 48.048kHz, when referenced to video black burst, will consequently be pulled down to 48kHz (because of the change in the reference). This will then allow digital transfers via AES/EBU to digital VTRs.

When using SMPTE timecode with NTSC color video, you will want the SMPTE timecode generator to be locked to blackburst, otherwise the color framing bit is useless and the transition to the next frame in the count does not occur at the same point in time as the transition to the next frame of picture. This is important if you need to make dubs/clones.

You could resolve your audio gear to the SMPTE timecode on your video master, but that signal won't be as solid as a blackburst generator (plus your reference is gone when the VTR is not playing at normal speed).

[Don Barto]

So Cheif, when I chase timecode from a black referenced digibeta machine with my LE system and layback to tape on tracks 1 & 2, I reference my Mbox to the digital audio outputs of tracks 3 & 4. Assuming the signal from tracks 3 & 4 is good, doesn't that in effect lock my Mbox to house black, and put me at precisely the correct sample rate? And similarly. when I dub from digibeta, I reference Mbox to the digital input signal -- the black referenced digibeta. Won't this result in Pro Tools recording at precisely 48Khz?

[Jsalam112]

Chief

I believe we will be taking the SMPTE from the Beta SP and send it to the Sync io box to run Pro Tools. From the Sync io SMPTE will be sent to a Digital Timepiece for DA-88 TDIF needs. The DA-88 tape will be brought to the authoring house and there it will be locked to the (DVD) film project.
SMPTE has been (reliably) generatorated by Beta SP machines with different sync boxes and different Pro Tools systems over the years. The original code seems to working fine.


I guess my questions are:
1.- Isn't the sync io box a reliable SMPTE/Word clock generator?
2.- At this stage (dumping mix to DA-88) is the Black Burst essential or am I screwed?
3.- If I need a Black Burst Box at what stage is it essential with most of the above scenario already haven taken place?

[dr sound]

Jamey,
I think the "Chief" in his reply said it all. All the pro rooms I know have a blackburst sync generator with everything tied to it. I may have one you can checkout. Give me a call next week.

[Chief Technician]

So Cheif, when I chase timecode from a black referenced digibeta machine with my LE system and layback to tape on tracks 1 & 2, I reference my Mbox to the digital audio outputs of tracks 3 & 4. Assuming the signal from tracks 3 & 4 is good, doesn't that in effect lock my Mbox to house black, and put me at precisely the correct sample rate? And similarly. when I dub from digibeta, I reference Mbox to the digital input signal -- the black referenced digibeta. Won't this result in Pro Tools recording at precisely 48Khz?

In theory, that is correct, and I have no reason to think it would not work. Keep in mind that this is a daisy-chained topology (as opposed to using a distribution amp). WIth an LE system I believe this is the best you can do to lock to blackburst (indirectly, remember your locking to the AES/EBU from the deck, which is locked to the blackburst).