VHS duplication amps

[Jeff Donald]

Dre,

My previous post wasn't directly pointed to Ed but rather Dylan. In his post that started this thread he point out that he is using a mini DV deck for sourcing his dupes. Mini DV does not require a TBC.

The problem with looping through signals with consumer VCRs is not the bandwidth, but the other improvements done to the signal. The HQ spec calls for at least 4 different signal modifications (improvements?), edge detail, noise reduction etc. It is the repeated implementation of the HQ spec that degrades the signal. Signal lose is usually minimal if proper cables are used.

Jeff

[Jeff Donald]

Since I didn't really conclude the above post, the bottom line is don't loop signals through consumer VHS decks. The signal degradation caused by the implementation of the HQ spec is measurable with a waveform monitor/vector scope and visually noticeable.

Jeff

[Andre De Clercq]

As you maybe know, "HQ" for vcr's is for 99% an invention of smart marketing people, on the same level as the "AI based noise reduction", or "fuzzy logic based tape mechanism"... On a wavefrom monitor there will be no differences.

[Jeff Donald]

There are seven different specifications of improvement in the HQ designation. In order for a manufacture to display the HQ mark, four of the seven must be included. Doing this from memory the improvements are detail enhancement, 20% boost of white clip level, dynamic picture enhancement, luminance noise reduction, CCD chrominance, CCD luminance and one I can't remember.

The HQ spec is not some marketing hype. It was developed by Yves Faroudja. He was presented the Lifetime Achievement Award by the Academy of Television Arts and Science. I believe he holds patients on all the HQ circuits.

You do not see the HQ designation on professional duplicators (VTRs). The looping through of the signal would degrade the signal quality. It is a consumer implementation that improves picture quality. But repeated application of the improvements by looping the signal degrades the image noticeably. But it is a cumulative effect, you may not have noticed the lower quality in just one generation. But it is there, and looping just degrades the signal each machine.

Jeff

[Andre De Clercq]

"HQ" is commercial stuff, indeed first licenced as a package deal to JVC beginning 1970. The reason why only seven "improvements" are involved is because these where the only fantasies they could sell to the consumer world at that time...Even if clip levels where taken somewhat higher (risky but alowable thanks to better tapes), even if they could sell some noise reduction tales, even if he packaged some primitive non linear sharpening techniques, it could never attract pro people, and moreover all this "optimisations" do not mean that there is a need for more bandwidth, better linearity... in feedtrough circuits. In this respect, the "HQ" label has no reflevance w.r.t. daisychaining vcr's. Faroudja's Awards (55 patents) relate to much more fundamental an professional video processing techniques.

[Jeff Donald]

I have a feeling we're discussing differences in NTSC and PAL. Faroudja was asked (circa 1984) by JVC (inventor of the VHS format) to come up with improvements that would counter the improvements Sony made to Beta, known as SuperBeta. Farjouda agreed and came up with improvements that include Chroma Noise Reduction, Luminance Noise Reduction, Detail Enhancer, White Clip Enhancer, Dynamic Picture Enhancement and others.

The four primary enhancements are required and specified in the S-VHS designation. Many of the JVC Professional S-VHS VTR's contain all 7 enhancements. If you look on the back of any VHS deck with the HQ and ALL S-VHS decks you'll see a sticker acknowledging the License from Faroudja. I don't think these companies pay millions of dollars in licensing fees for circuitry that doesn't work.

Many VHS HQ decks and ALL S-VHS decks have switches that disable certain HQ circuits if you are editing (copying). Directions for doing so are contained in their instruction manuals. If in doubt read any S-VHS manual about Edit Switches. They turn the circuits on for improved playback to a monitor, but are turned off when copying the signal. Very plain and simple and contained in the manuals.

They are active enhancements applied to the signal. They work or else JVC, Panasonic, Sony and others wouldn't pay millions for the license to use them. Looping through of signals that have the HQ enhancements applied will degrade the signal. If it didn't, why are there switches to turn off the enhancements when editing? Why are companies spending millions on licensing and building VCR's (both pro and consumer) with switches that do nothing?

I was President of a large production and post production company in Cincinnati, Ohio from 1985 thru 1996. One of the divisions of the company did video tape duplication (third largest in the city, over 100 duplicators). I've tested duplication systems based on consumer decks, looping through a common video signal. The source was time base corrected with a Nova 920 TBC. The source deck was a Panasonic AG-7500A. The duplication machines were Sharp consumer VHS decks. Belden 75 ohm cable with Canniare connectors. The output of the 10th deck was connected to a Leader 5870 Waveform Monitor/Vectorscope. The test were performed by my staff broadcast engineer. The conclusion was the looping of the signal was degraded by the consumer VHS decks. The loss of quality was measurable on the Leader 5870. The visual difference from the 1st tape to the 10th was very noticeable and would not have passed any reasonable quality control standards. Now maybe there was increased sunspot activity that day that degraded the signals and the loss of quality had nothing to do with HQ circuitry. But I do know that looping the signal in consumer decks degrades the quality below my standards. I wouldn't give such tapes to any of my clients. But maybe my standards are higher than yours.

Jeff

[Andre De Clercq]

I would like to know your standards, not in fuzzy terms like "degrades..". Try to do do it like pro's do it for buffer amps, in terms of bandwidths, transient properties, S/N, diff amp and diff phase... I am not used (not able) to discuss non artistic matters in artistic terms. On the Faroudja story...I know this man and met him several times starting when is was still with ITT(Oceanic 1970) in Paris and later on as an SMPTE member when he moved to Sunnyvale. Unless you want to teach the forum about his inventions, but I don't need further info which is irelevant to the basic question. I know very well what he did and is still doing, and even today we have professional contacts as a licensee for a number of his patents (pro line doublers...). F.Y.I the way looptrough properties are measured is not by using a VCR as source signal generator...just inform your staff broadcast engineer.

[Jeff Donald]

Dre....

I think you need to understand that the vast majority of the members and guests here are not engineers. They are men and women working in small studios, production companies, students and independents. Many of them own their own business and as such owe everything (house, car, wife, dog etc) to the bank. The majority do not work in a broadcast environment. They are not engineers and many have no formal training in video production or post production. Their world (the one that puts food on the table) is a subjective one. It's not a world of signal to noise ratio, bandwidth, or even 75 ohm termination.

The majority of the tape dupes go to brides, brides mother, art directors and other creative types. Their world is subjective also. If they have a complaint, it's described in very subjective terminology. I've never heard a bride or art director complain about the signal to noise ratio. But they will tell you the video looks fuzzy, heavy, thick, noisy, grainy, light, dark, washed out etc. But never have I heard, Can you boost the signal to noise ratio? or Do you think the termination is set wrong?

My standards, when I post to the threads here are for the large part, subjective. Why? The majority of the community here doesn't own a waveform monitor/vectorscope (non-software) or know how to really read one. Writing in terms that aren't understood is useless. The men and women come here looking for answers in everyday, real world terms. Just like what their clients and customers use. It's not 100% accurate and no one is claiming it is.

My posts are meant to help the community by relaying my real world experiences. My motivation is to help them become better directors, lighting and sound technicians, photographers, editors and business people. My posts reflect the real world experiences I've had in over 20 years in this industry. I live in a subjective world and many of my experiences are subjective.

So, when I tell Ed, Dylan and the others not to loop the signal through on consumer VHS decks it's based on real world experiences. Just the way they make dupes. Not with a signal generator, but a source deck playing a tape and looping the outputs. It's not pure science, nor is it meant to be. That's not our audience.

Our audience for the largest part is not broadcast. It's men and women working day to day, in an industry they love, putting food on the table for their families. Few, if any can afford to loose clients. My advice is meant to minimize the likelihood of that happening. I know from real world, subjective, experience that looping the signal will produce poor quality tapes that their clients will probably complain about. They can avoid this by using a DA. Yes, the DA will cost some money. But not as much as losing a client from poor quality tape duplication.

F.Y.I. I'm the one that told my engineer how I wanted the tests run. Because I wanted to be able to look the client (who commissioned us to build his duplication system) in the eye and tell him his VHS dupes were going to look just fine.

Jeff

[Ed Frazier]

Jeff and dre...., I've been following your discussion with much interest and appreciate the comments you have both made. Since I'm a member of the group Jeff described in his last post, this may be a dumb question, but I'm a little confused.

Jeff, where in the machine is the HQ Spec implemented? If I understand what you have said, these circuits can be disabled on SVHS consumer machines. With HQ off, is the feed through signal not affected by this circuitry and consequently not degraded (at least for that reason). Your argument seems to be that the HQ Spec is the culprit, so turning it off should solve that problem.

My JVC manual describes three settings that I'm assuming are the ones you are talking about:

Picture Control (Auto, Edit, Soft or Sharp) - Adjusts playback picture quality

Digital R3 (On or Off) - Edge correction to luminance signal is performed

Video Stabilizer (On or Off) - Automacically correct vertical vibrations in picture during playback

These decks also have a Digital TBC/NR switch. Is that also part of the HQ Spec?

When making copies from my NLE, I use the following settings:

Picture Control - Edit
Digital R3 - Off
Video Stabilizer - Off
Digital TBC/NR - On

Are these settings giving me the best results, given the limitations of consumer grade equipment?

[Jeff Donald]

Are you using your NLE as the source and then recording and looping your signal through JVC S-VHS decks? How many decks is the signal looped through? It looks like the settings are correct for duping the signal. My concerns are the TBC. Since there is no time base error to correct it is not needed. The noise reduction (if it's active) my have a determental cumulative effect. The first copy or two in the chain may look fine. Copies at the end of the chain by show visual abnormalities. The other possible problem is if the TBC contains a Proc Amp to adjust the color (phase). If someone accidently adjust the Proc Amp on deck 1 the color changes will cascade through the chain.

Jeff

[Andre De Clercq]

Ed,
If you have playback (source) from NLE or DV VCR, or an analog source VCR containing a TBC (never active in record mode!), or playback through a digital mixer or whatever digital interface, there is no need for TBC. Altough there is no need for the timebase compensation part of a TBC, in the above situations, you can of course still put a dedicated TBC unit between yr source player and the recording part (like I do) because a TBC can also correct color shortcommings, match Y/C delays, H-phase, sharpen/unsharp...sometimes interesting, depending on the source material properties/quality. On the setting problem, keep in mind that not a single setting of your recording VCR's has any influence on the properties of the feedthrough signals. So whatever recording VCR setting you apply (TBC,NR,sharpen...) what gets in, gets out, unaffected by yr settings. Only the recorded signals from the individual VCR get the settings.

[Ed Frazier]

Jeff, I use the Matrox RT2500 card which allows for analog in/out. Using all S-VHS connections and quality cables, the Kramer matrix switch mentioned in an earlier post is currently configured as follows:

---IN ------- Matrix ------ OUT

NLE-----> Port #1--> VCR #3--> VCR #4--> Switch Port #4
VCR#1--> Port #2--> VCR #1--> Switch Port #2
VCR#2--> Port #3--> VCR #2--> Switch Port #3
VCR#4--> Port #4--> Monitor(TV)

The only units that are daisy chained are VCR #3 & 4.

Most of the projects that I'm currently working on are finished within a week and most copies (always VHS) are made directly from the Premiere timeline (four at a time) while the files are still on a removable hard drive. I also make one S-VHS master of the edited project that is used to fill additional orders after the files are removed from the system. The reason I don't put the edited master back on MiniDV is because the projects are usually close to two hours in length. I retain the original unedited DV tapes and Premiere project files.

While all copies made directly from the timeline seem identical, even the one from VCR #4, there *is* a noticable difference in those made from the S-VHS master, although I've yet to hear a complaint. Most of my customers drive race cars which might explain a lot.

Since my source is analog (S-VHS from NLE breakout box), is the TBC switch doing anything or should I turn it off?

[Jeff Donald]

Ed,

You can turn the TBC's off it really doesn't matter. The daisy chaining of just two VCRs is not a big deal either. I've started to see the image quality decrease when you get 4 or more chained together.

Jeff

[Andre De Clercq]

If an analog player (yr S VHS) is used as a source you need a TBC, like I wrote in an earlier post (oct 24) to you. So it must be set *on* on yr VCR. Two remarks:
TBC's incorporated in consumer VCR's have limited TBC performance( no field memories). They are good enough for TV playback or monitor playback if set for "fast sync" but sometimes still introduce remaining timebase unstabilities on further recordings.
Even if S-VHS has good horizontal luma resolution (not as good as DV), the chroma processing and resolution for S-VHS is the same as in VHS and about only 30% of what you get out of yr NLE. Extra noise and reduced chroma resolution (color leakage) is what deteriorates the signal.

[Ed Frazier]

Thanks Guys,

I really appreciate all the info. I'll turn the TBC switch off unless the deck is being used as a source.