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View Full Version : Do all DV camcorders have a TBC?


yg1968
8th March 2003, 15:30
How do you know if a digital camcorder has a Time Based Corrector (TBC)? Is it any digital camcorder that has a passthrough feature? What about digital noise reduction (DNR)? Will any digital camcorder with a passthrough feature have it?

yg1968
8th March 2003, 15:32
I am not sure but think that I may have found the answer:

One advantage of DV camcorders and
VTRs is that they all incorporate time-base correction by their very nature. You
won't need to use a TBC on DV sources to maintain quality, though the proc amp
controls built into some TBCs can be useful for correcting image problems.
See at the bottom of page 37 of this excellent DV guide:
http://www.cilect.org/dvsuccess.pdf

I am not sure but I think that you can conclude from this that all digital camcorders have a device somewhat similar to a TBC that can improve old analog tapes when used with their video in or their passthrough feature (if they have this feature).

Chez_Wimpy
8th March 2003, 21:23
By its nature, the DV deck will have to imploy a TBC-like functionality prior to digitizing an NTSC/PAL signal. However, passing video through a DV deck can still result in dropped frames (and lost a/v sync) if done on-the-fly to an external device (in the case of recording to a computer). In my experience, the TBC of a high end VCR will vastly improve the final DV signal (TBC in the VCR insures no dropped frames will be introduced during the digitizing process). As well, DNR in my VCR no longer necessitates temporal cleanup in the avisynth encoding phase (I did an a-b comparison, and the result of fluxsmooth applied to a DV source matched almost exactly to an unfiltered DV rip using the DNR from a Panasonic AG-1980 VCR). With DNR realtime, time speedups for encoding are substantial.

All of the experiments I have done involved real time DV passthrough -> firewire capture. It seems possible that (DV) taping a signal might introduce fewer problems and eliminate dropped frames.

yg1968
26th March 2003, 23:43
Upon reflection, my understanding now is that miniDV camcorders do not have a TBC because there is no need for a TBC for DV. However, using the passthrough feature also seems to help old vhs tapes.

On a related note, the Sony TRV-350 (a digital 8 camcorder) seems to have a TBC and DNR. I am not sure but I get the feeling that this feature only gets used on Hi-8 tapes (and not with the passthrough feature). I believe that all digital 8 camcorders (with a passthrough feature) have a TBC. However, the TBC is probably not very useful if you don't have Hi 8 tapes. The Sony Canadian website gives more details (than the U.S. website) on the features of the Sony TRV-350:

http://www.sonystyle.ca

Here is a useful link that explains the basics:

http://www.dvdrhelp.com/dvanalog

yg1968
14th August 2003, 20:03
Can anybody confirm my undestanding that most DV camcorders do not have a TBC?

SomeJoe
15th August 2003, 01:27
A Time Base Corrector is a device/circuit that is used to maintain signal integrity when playing back an NTSC or PAL video signal from a tape where the recording mechanism is analog.

The reason it is used on professional/dubbing VCRs is because of the way analog signals are recorded on analog tape mediums (VHS, Beta, 3/4", Hi8). The rotating helical scan head lays down a single scan line of analog video signal on the tape on each drum rotation. Now, think what might happen if the tape is slightly stretched or the tension on the tape is different on the playback VCR than on the VCR that recorded the signal. The resulting signal that is picked up by the playback heads will be skewed the same way as the physical stretch/tension on the tape, resulting in out-of alignment scanlines and "torn" pictures.

A time base corrector reads in the analog signal coming off the playback heads and uses circuitry to ensure that each scan line is sent out the analog outputs at the exact correct time and that each scan line is the same duration in time as the others. Thus, it completely compensates for tape stretch and tension differences.

Now, any digital video recording medium (DV, Digital8, DVCAM, etc.) does not record analog signals, they record digital data. In this case, if the tape is stretched or is off-tension, the digital data can come off the tape at a slightly different time, but this is irrelevant. The digital circuitry will use error correction algorithms written along with the data to the tape to ensure that the original data is completely recovered. Then, the digital to analog converter sends the reconstructed analog signal out. Thus the analog signal is never affected by tape stretch or tension issues.

So, in short, digital recording decks do NOT have a time base corrector in the traditional sense of the circuit, but the effect of the digital recording mechanism is that the problems that used to require a time base corrector to fix are no longer problems to begin with.

theReal
15th August 2003, 18:32
A Time Base Corrector is a device/circuit that is used to maintain signal integrity when playing back an NTSC or PAL video signal from a tape where the recording mechanism is analog.

A TBC is also useful when the recording mechanism is digital but you're still using an analog component output. For example where I work, we're using DVCAM recorders/players, but these don't have digital outputs (it's a matter of cost - these recorders are very expensive anyways but the models featuring SDI digital outputs are unbelievably expensive...)
So, because of the analog component signals, we are using TBCs for all outgoing video signals

SomeJoe
16th August 2003, 00:58
Originally posted by theReal
So, because of the analog component signals, we are using TBCs for all outgoing video signals

I'll look up the specs of your DVCAM decks if you give me the model numbers, but I'm 99% sure they do not incorporate a TBC as has been referred to in this thread.

It is irrelevant that the outputs from the DVCAM decks are analog. They still don't need a TBC because there is no tape stretch/tension problem that causes a time shift to compensate for. The digital to analog converter inside the deck will not have any timing-based problems generating the scan lines in the analog signal output, so a TBC is not necessary.

Adding an outboard TBC to the analog outputs of a DVCAM deck will gain you nothing. In fact, the additional circuitry may degrade the signal.

yg1968
21st August 2003, 06:02
I understand that DV recorded stuff does not need a TBC. But my question is : does the passthrough feature of the camcorder act like a TBC when it converts analog to DV. For example, if you hook up your VCR to your camcorder, play a VHS tape and send the converted (DV) video to your computer through firewire, will your camcorder act like a TBC in this situation and correct your video?

theReal
21st August 2003, 19:20
I'll look up the specs of your DVCAM decks if you give me the model numbers, but I'm 99% sure they do not incorporate a TBC as has been referred to in this thread.

The DVCAM decks don't have TBCs - but the outgoing on-air signal of the station is passed through a TBC. I don't have the technical background yet to say why but all the technicians here agree that we absolutely need this TBC or otherwise the cable company can't send our program on cable.

SomeJoe
22nd August 2003, 01:07
Originally posted by yg1968
I understand that DV recorded stuff does not need a TBC. But my question is : does the passthrough feature of the camcorder act like a TBC when it converts analog to DV. For example, if you hook up your VCR to your camcorder, play a VHS tape and send the converted (DV) video to your computer through firewire, will your camcorder act like a TBC in this situation and correct your video?

Now that's a good question. I don't know for sure, but my suspicion is "yes and no". The analog to digital converter in the camcorder has to make some assumptions of the input video signal timing. Most probably it will attempt to, as closely as possible, sync its internal oscillator with the input analog signal's sync pulses. This will essentially correct major timing variations between each horizontal scan line. But, since the video tape can stretch and vary in between horizontal sync pulses, there can still be some horizontal time-shift of the analog signal which will be mis-sampled by the DV camera's analog->digital converter.

Originally posted by theReal
The DVCAM decks don't have TBCs - but the outgoing on-air signal of the station is passed through a TBC. I don't have the technical background yet to say why but all the technicians here agree that we absolutely need this TBC or otherwise the cable company can't send our program on cable.

Ah, now I see. I'll make this educated guess: the cable company is requiring you to use the TBC so that your output signal stays in genlock with their receiving station, regardless of the video source you feed to it. It means that you can switch between non-genlocked sources behind the TBC all day, and to the cable company it looks like there was no switching transient.

If you never switched away from a continuous output from a DVCAM deck, it wouldn't be necessary, but of course that's not possible. :)

BlueCup
24th August 2003, 23:13
Originally posted by yg1968
I understand that DV recorded stuff does not need a TBC. But my question is : does the passthrough feature of the camcorder act like a TBC when it converts analog to DV. For example, if you hook up your VCR to your camcorder, play a VHS tape and send the converted (DV) video to your computer through firewire, will your camcorder act like a TBC in this situation and correct your video?

Yes, if your camcorder has TBC. My low end Sony TRV120 has it, you can turn it on or off in one of the option menus.

I've done 4 or 5 VHS tapes that have given me HUGE sync problems when capturing with my MPG2 card, but doing the pass thru then encoding later on to MPG2 works perfect. I have just one problem with a certain video tape which I'll probably make a thread about in a day or so, if I can't find out the problem (video related, not audio).

yg1968
29th August 2003, 14:40
I could be wrong but I don't believe that the TBC in your digital 8 camcorder works on anything but Hi8 or 8mm analog tapes. However, I think that the passthrough kind of acts like a TBC in that the analog to digital converter (or passthrough feature) stabilizes the video.