Announcement

**VIKINGMAN** · 25 October 1999, 15:45

It is true what you say Patrick - The AutoTracking (of most vcr's today) and the TBC is two completely different things and they work both as you describe them!

On another note about TBC - They ALWAYS work during playback and NEVER during recording. So, if you were to copy an analog source (VHS or Video8) to Digital8 (or DV for that matter), you'd have to have a VCR-player, with a build-in TBC to use while playing back to the Camcorder.

It would of course be possible to just make a digital copy of the analog tape, but I don't think the Cam would be able to correct any errors with IT'S build-in TBC afterwards.

Just to clearify things!

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ASUS P2B-S, PII-350 (o/c to 412MHz), 128MB RAM, Cheetah 9.1 GB, Matrox Mill. G200SG, SB 64AWE, Plextor 32x CD-Rom, Sony CDU-924S CD-R, Canon BJC-7000 InkJet and Canon CanoScan 300 Scanner.

**Patrick** · 25 October 1999, 17:11

Hi Vikingman, thanks for backing up what I had to say about auto-tracking and TBCs.

Interestingly enough, TBCs of consumer VCRs and camcorders ARE able to clean up the time base errors of re-recorded footage, as long as the time base errors are within a certain range. I totally agree with you Vikingman, that the playback unit should have a TBC to process the signal BEFORE it's fed into the inputs of a D8 or DV camcorder to maintain the best quality. But sometimes you can get away with it. I have done so myself, and it appears that Keith has also done so in his test which was reported in the other thread that I had referred to earlier.

**Grigory** · 26 October 1999, 06:07

Hi Patric,

I take your explanations with very much interest.
What is not clear for me:
1. Is the original problem related with TBC functioning, i.e. does these frames look "shimmy", or they are totally corrupted frames?
2. Is TBC working on line-by-line only basis?
3. If you play analog 8 mm with D8 camcorder, how does the analog output look? Shimmy lines, but no corrupted frames?

Ok, now about DV TBC.

In DV, the frames are NOT built on line-after-line basis. The image is divided into 8x8 pixel blocks for luma, and in 16x16 pixel blocks for chroma (PAL, in NTSC there are probably 8x32 blocks). Then, DCT transformation is done, then compression, and then the data from macroblock, consisting of a number of elementary blocks (I don't know all details) for say 64x64 rectangle is prepared to be written on tape.
So, every line of output DV frame image has to be extracted from several parts of recorded track, and you can reconstruct this line only after the compressed digital data for nearly all frame is placed into the internal codec buffer. Yes, there may be variations in time, needed to put each kilobyte of DV data from tape into a buffer, but you cannot attribute this kilobyte to a specific line on a frame.
When the buffer data is fully decoded into frame buffer, the codec is able to scan this frame buffer with quartz precision to make conventional analog signal on output. It always MUST be perfect, because each scan line duration is determined by quartz clock!
If you feed the data into firewire, the elementary unit of information is compressed FRAME data. Its data has no "history" of what was the reading speed of each byte from the tape. So, again, for DV, produced by shooting with camcorder, you always have perfect frames even if the tape speed varies seriously within a frame interval.

So, TBC in digital format is not the same as in analog formats. It is just a format internal feature, but not a special optional unit, as for analog devices.

What happens when you feed analog signal from analog camcorder to be recorded on D8 in DV format? Assuming you do not have TBC in playback device, as in the case of VHS, how the timebase errors influence capture device (DV codec)?

DV codec must digitize and store multiple lines or entire field in a local buffer to be able to do DCT on their data.
Digitizing is done via taking amplitude samples at fixed sampling frequency of 13.5 MHz.
The codec has to prepare exactly 720 active samples per line, but the line duration may vary because the TBC is absent.
Capturing analog sugnal, the codec knows only sync pulses and has to put exactly 720 samples per line, even if the interval between hor sync pulses changes for several percent.
As far as I know, even old capture cards used something like TBC to solve this problem. So, per-line TBC is a feature of most analog video capture devices.
There is another problem: vertical sync. For RR_G, it was not adequate, and the card began dropping frames when the vertical sync pulse arrived not in expected time interval. This was common for my VHS-C tapes, and also for live TV signal, when camera switches were not aligned well, as in the case of commercials or news.
BTW, my BT848 card was tolerant to these irreguarities, but RR_G could even start Macrovision protection on some tapes.

What am I speaking about?
I suppose that for some reason, internal analog tape playback mode of D8 camcorder can give more frame rate and/or timebase fluctuations than ordinary 8mm camcorder or VHS VCR. In this case, the signal timings may go out of DV codec tolerance gate, and it begin to produce corrupted frames. In RR_G, the frames were just dropped.
If your observations show that D8 CAN capture the same tape from external playback device, but has problems with its internal source of analog video, then the problem is inside the camcorder.

Summary:
1. I suppose that D8 camcorders DO NOT have any TBC in common understanding. TBC is not required for DV data reading from tape: the nature of DCT compressed digital video format allows frame reconstruction only after entire compressed FRAME data is retrieved, but not individual lines. The data from each line is actually distributed over several data blocks, so lines are not reconstructed sequentially, as in the case of CD audio data or laser disc video.
2. For analog to DV conversion, the digitizer has to implement a kind of timebase correction because of format requirements. And this works, as you confirm. In addition, it has to implement frame rate stabilization for the same reasons.
3. When D8 camcorder reads analog tape, the variations of analog signal may go outside the tolerance limit of DV digitizer. These variations can produce corrupted frames. This does not happen with the same tape played from another analog device. On analog ouput of D8, you see shimmy vertical edges, because there is no ordinary timebase correction for analog signal at all.

What are the reasons of stronger signal variations and what are these variations?

1. Timebase errors: confirmed to be present on analog output for analog tapes, which signal, of course, is a duplicate of what is used for DV digitizing.
2. Frame rate errors: is this confirmed?
3. Tracking errors - you do not confirm them.

However, tracking errors, or signal and sync pulses level variations may be worse in the case of D8, because the heads are obviously of a shape that is better suited for D8 format:
1.narrow gap,
2.designed for narrow track pitch

These two reasons may give less signal level from analog tape, can lead to sync pulses losses, ...

More, D8 format itself is tolerant to timebase errors, as I tried to explain above. What is necessary: to keep the frame rate at constant value. Who cares about fine speed variations within a frame for DV or D8?
So, designing the mechanics and servo control, engineers can pay attention to frame rate stability, but not very much for per-line stability.
Do you see now what is the meaning of phrase in Sony manual?
Maybe, it is saying about absence of TBC, and about higher timebase errors in D8 in comparison with normal Hi8 device? Some other analog signal variations that give DV codec a trouble?

Anyway, TBC unit is incorporated into DV digitizer. Any analog TBC could be only an option, apparently not included.

I am sorry to mix notions of TBC and tracking. I did not know that TBC is responsible only for Hor sync irregularities, but not for frame rate stabilization too.

Relatively slow, but big tape speed variations (in addition to frame rate instability) may cause short tracking errors if the mechanism is unable to follow the required tape speed changes. That was my original idea. Typical tracking mechanics has several seconds time responce, so it cannot follow fast changes well.

Just curious: DV footage is always played at 25.000 fps. You can check this with RR or another card. So, DV data is read with very high frame rate presicion. What happens if you feed the camcorder with old VHS signal? It may have many fluctuations in frame rate, and typically has 24.8x or so frame rate. There are no dropped frames, so the video must be recorded at this variable frame rate, but then played at fixed 25 fps. Am I correct?
So, digitized analog footage is played better than original!
Can anybody check this?

Regards,

Grigory

**Patrick** · 26 October 1999, 13:51

Grigory, it might be easier if I simply state what I've observed, and you can draw your own conclusions. If I try to respond to all the different points that you've made, I'm sure I'll miss something. That and the fact that I just don't know the answers to some of your questions.

A friend of mine shot some footage of a fashion show last year with a Sony Hi8 camcorder. When this footage was played back in his Hi8 camcorder, it looked perfect. Keep in mind now that this camcorder has it's own built in TBC. When this friend of mine bought a Sony TRV110 Digital8 camcorder and played back the same analog footage in this new D8 camcorder, the playback was ALMOST perfect. It was almost perfect except for displaying a "shimmy" in certain areas of the tape. It looked like a time base error to me, so we tried an experiment. I played back this same analog footage in my Sony EV-S3000 NTSC Hi8 VCR with it's TBC turned on, and the picture was perfect. When I turned the TBC of this deck off, the picture was still perfect, except for displaying the "shimmy" in exactly the same areas of the tape as the D8 camcorder had done. As a further experiment, we played back this same footage in the D8 camcorder again, except that this time we ran the signal through an EXTERNAL TBC. The playback was perfect, no "shimmy" anywhere.

My conclusion....During playback of analog 8mm/Hi8 tapes, Digital8 camcorders do NOT make use of any TBC circuitry.

Grigory, I have no way of directly dealing with your questions about "corrupted" frames because I do not have access to a DV capture board. However, in the previous thread that we were involved in, it appears that Keith was able to eliminate "corrupted" frames in his setup by re-recording his analog footage and having it play back as time base corrected digital footage. It's important to note that he did NOT use any form of TBC in the re-recording process. It was ONLY the playback of the newly created digital footage that was being time base corrected.

[This message has been edited by Patrick (edited 26 October 1999).]

**Grigory** · 27 October 1999, 10:43

Patric,

The last question I have:
What happened if your friend recorded analog Hi8 to D8 and then played this DV video?

Were shimmy lines in place, or they were corrected by analog to DV conversion? I could not find these results in your story.

The method of correction of corrupted frames via VHS re-recording may work, but the quality is...

What do you think?

What is even more interesting is:
If I use my D8 as digitizer by recording external VHS source of poor quality, can I expect anything similar to TBC correction, AND, can I get corrupted or dropped frames because the signal stability is also poor?

This was a problem with RR_G.
I'll check this some time and let you know.

Obviously, there is simply NO ordinary TBC inside D8 camcorders. I personally do not think it is very bad: backward compatibility is only compatibility...

Grigory

**Patrick** · 28 October 1999, 01:46

Hi Grigory,

I guess you and I are the only ones still interested in this topic.

Today I did some more tests with two friends who both have D8 camcorders. We played back the analog Hi8 footage which had the "shimmy" in it with one D8 camcorder while we re-recorded it with the second D8 camcorder. We used an S-video connector as we didn't have a firewire cable available. When we played back this re-recorded footage, which was now digital, it had NO shimmy to it.

Again, I have no way of checking for corrupt or dropped frames during capture on a DV board.

I'm not sure what point you were trying to establish with your mention of VHS. If you were asking my opinion about re-recording from Hi8 onto VHS and then back to D8, I'd say that this was the worst possible way to take advantage of the D8's digital TBC circuitry. I don't think I need to mention all the pitfalls of doing it that way. If there is no other option, fine, but it's something you'd only want to do if you were desperate. As I stated in the other thread, the best way to do this is to play back the analog 8mm/Hi8 footage with one D8 camcorder and to re-record the footage with a second D8 camcorder that is connected to the first with firewire.

If your raw footage is VHS to begin with, re-recording it on a D8 (or DV) camcorder will allow you to take advantage of the digital TBC circuitry, but it will only correct an imperfect signal up to a certain point. Footage with huge time base errors will not be totally corrected by the TBC circuitry.

Grigory, your comment about the lack of an analog TBC in D8 camcorders as not being much of a problem is probably influenced somewhat by your lack of any old analog 8mm/Hi8 tapes. I'm sure that there are many people out there who bought D8 specifically because it is backward compatible. When the Sony website states that all D8 camcorders have a TBC, I'm sure that most, if not all people, would have assumed that this TBC worked with ALL footage, be it analog or digital. TBCs make a heck of a difference in image quality, and I believe that Sony has purposely mislead the public to believe that BOTH analog and digital footage will be time base corrected in their Digital8 camcorders.

I'd love to hear a Sony representative deny this.

**IvanP** · 28 October 1999, 08:34

Hi,

Sorry but conclusion is still not clear for me. It's very important to know if I can sale my old Hi8 after buying D8.

SO:

If I'll play back Hi8 or 8mm analog records on D8 and capture as digital (via FireWire) some problems should appear. Am I right?

But I'd like to know what happens if (suppose I'll have dual or analog capture board) I'll capture it via analog output from D8 to analog input of dual board. Captured using DV codec of course. From previous Patric's post I suppose the result will be without "shimmy" objects.

IS THIS STATEMENT CORRECT?

Ivan

**Patrick** · 28 October 1999, 23:14

Ivan, you may be misunderstanding a few things.

You state, "If I'll play back Hi8 or 8mm..... some problems should appear." No, some problems might appear. There's a big difference made in that statement by changing ONE word.

I need to make it clear that the "shimmy" due to a time base error that may occur is NOT necessarily something that happens continuously. It could, but usually it's worse just at the beginning of a recorded scene. That is because at the beginning of each scene in the original recording, the tape transport in the camcorder (or VCR) wasn't quite up to speed when the recording started.

Ivan, you're asking if the last statement you made is correct. It's not. You made no mention of re-recording the analog footage and then playing it back as digital footage. It makes no difference whether you are playing the original analog footage back through S-video or firewire, it is NOT being time base corrected. This means that it is still possible for a "shimmy" to occur. If you haven't already done so, you need to read Keith's posts in the earlier thread that started this discussion.

If you do not notice a "shimmy", and you do not have corrupted frames during DV capture, go ahead and sell your Hi8 equipment. If you do have either problem, use your Hi8 equipment to play back your analog 8mm/Hi8 tapes and re-record this footage on your Digital8 camcorder. Yes, this is a damn nuisance, and that's why I'm so disappointed with Sony over what they've done.

**Grigory** · 29 October 1999, 01:04

Hi Ivan and Patric,

As Russians sometimes say, the problem is so complex that we cannot solve it without a bottle At least three persons are required by a tradition. Otherwise, drinling is only drinking.

OK, thank you for additional information about TBC inside D8 camcorder.

You confirm that while recording analog into D8, internal TBC is working and can correct most of timebase errors.
My conclusion is that TBC is working while digitizing original footage. It cannot do anything else, because otherwize the shimmy effect will be "frozen" into digital frame and cannot be corrected. Remember, each line of DV frame is already 720 pixels, and sampled at 13.5 MHz. There is no way to correct it on playback stage.

So, the signal path is

Hi-8 or D8 camcorder with analog tape
analog signal on its output
digitizer of Sony DV codec on another D8 camcorder
digital timebase corrector inside the DV codec
DV compressor itself
D8 tape

When you play digital footage later, it has already TBC corrected frames, and the image is actually better than original analog played without TBC.

Of course, it is not better than original Hi8 played with TBC, internal or external.
Any transformation from analog to digital and back to analog for TV must introduce some distortions, but they are so small in case of DV that typically you cannot detect the difference.

Sony mentions such conversion as a way to extend the life of your analog tapes.

I can only confirm that even ordinary 8 mm tapes recorded in D8 format have no signal irregularities and have no single frame dropout over full 64 minutes. My understanding is that D8 format has actually better storage reliability on the same media, because the digital storage technology uses 8 Mhz analog bandwidth media (8mm tape) to store only 25 mbps digital stream. The relationship between these digits is like for old good 9600
modem over noisy phone line. You can receive data at such speed without problems, but a sound quality will be very far from Hi-Fi.
So, D8 format seems to be very reliable storage solution, and you can simply copy all your analog tapes from Hi8 to D8 and sell Hi8 camcorder.
This takes some time, but ensures maximum quality of video. You have to take into account that any analog recording is played best on a device that was used for recording. So, copying Hi-8 into DV seems to be better than using "compatible" device.

Patric,
We were discussing TBC, but I could not find in original post an indication that corrupted DV frames on DV output of D8 camcorder playing analog tape are actually related to the TBC.

I could not find an indication that DV stream from analog tape playback has no TBC. This would come to contradiction with what you observed while recording DV from analog inputs. Playing analog tape, the camcorder probably takes it own analog output signal and uses it to produce DV stream. So, there is no difference for the codec: is analog signal coming from inside or from outside. Both should be first corrected, and then compressed in DV.

I'll try to check this with 8 mm tape taken from my father.

I am still thinking that with particular part of particular tape, and with particular Hi8 and D8 units, analog signal produced inside D8 camcorder while playing analog tape can be bad for DV codec (for some reason), and forces it to make distorted frame.
The analog output signal may have some irregularities, but they are somewhat corrected via re-recording to VHS and back. The signal from VHS tape, being of worse S/N ratio, resolution, ... may become better for DV digitizing, because of recovered sync pulses or something else.

Conclusion as it seems for me:

a) Digital recording is perfect, both from firewire and from analog inputs.
b) analog tapes playback may have worse quality in comparison with ordinary analog camcorders. The quality drop is in the absence of TBC on analog path, and, in some unhappy situations, may result in corrupted frames on DV digital output.

So, the possible solution for all people with many analog tapes is either to leave Hi-8 camcorder, or to convert analog to D8 before selling analog device.
However, this may be necessary only for problematic tapes, but not for all.
It is a very ordinary case of incompatibility between different analog recording and analog playback devices, not very rare for pure analog VCR's too.

I think also, that using D8 codec for DV conversion is the best possible solution for any analog tape. The quality is much better than for RR_G or similar cards, and the digital data is recorded on a tape, but not on a disk. I am planning to do so with my VHS_C collection.

What is still unclear:
If one feed bad analog signal to D8 input, how bad can it be to give codec a trouble?
The codec is basically very similar to what is used in all capture cards. My experiments with original VHS_C footage gave:
-While signal irregularities often force RR_G to drop frames, this never happened with DV codec.

However, I did not try really bad tapes. An example of bad tape is one that contains a short piece of paused playback signal from 4 head VHS VCR, recorded on another VCR.
This piece looks acceptable on TV, but makes RR_G codec crazy. It drops a lot of frames and then activates macrovision protection.

I have a plan to convert my old linear edited tapes to digital media, so the question is not pure experimentation for me.

Any other ideas?

Grigory.

**IvanP** · 29 October 1999, 03:27

Grigory & Patrick, thanks for your explanations.

But it's still not clear for me why the path D8(AnalogOut)->XYboard(AnalogIn) deosn't solve possible TBC problem of analog records. Consider XYboard as some dual board with digital/analog I/O and using DV codec (future RT2000, DC1000 or DV500). I think that such a board does the same job as D8 does while recording. I've concluded this from Patrick's post:

[/i]"...
Today I did some more tests with two friends who both have D8 camcorders. We played back the analog Hi8 footage which had the "shimmy" in it with one D8 camcorder while we re-recorded it with the second D8 camcorder. We used an S-video connector as we didn't have a firewire cable available. When we played back this re-recorded footage, which was now digital, it had NO shimmy to it.
...
"[/i]

But also I have to agree with Grigory that tapes are better played back on device that was used for recording i.e. original Hi8 camcorder. Also recording back from PC->D8 digital in is one copy more. However if DV copying is loseless (I hope), this solution takes more time too...But in the case of absence of original recording device or any Hi8/8mm device...?

...And maybe I'll not have any such problem.

Ivan

[This message has been edited by IvanP (edited 30 October 1999).]

Announcement

Is the internal TBC of Digital8 camcorders utilized during analog footage playback?

Is the internal TBC of Digital8 camcorders utilized during analog footage playback?

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