OT Trying to copy a VHS tape in NTSC format (UK)

I have. Let's say I have a 5000 foot piece of coax and a 5 foot piece. One is used to 450 MHZ, and the other at 11 GHz. The change is not the same.

You need a change per distance, and frequency, because each type of cable has different loss characteristics.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

It happens to buried cable, as well. I've had crews out on Christmas day, after a 70 degree drop in under 12 hours. My workers had to use propane torches to thaw their hands while digging through frozen dirt & rock to make repairs.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you\'re crazy.

In message , Michael A. Terrell writes

OK.

At 450MHz and temperature T1 centigrade, your 5000 foot of coax will have an attenuation of A1 dB.

[The value of A1 obviously depends on the characteristics of that particular type of coax, and on its length. For a different frequency / temperature / length / type of coax, the value of A1 will be different.]

At temperature T2, the change of attenuation will be approximately (T2-T1) x 0.02 x A1 dB.

At 11GHz and temperature T1, your 5 foot of coax will have an attenuation of A2 dB.

[Again, for a different frequency / temperature / length / type of coax, different value of A1.]

At temperature T2, the change of attenuation will be approximately (T2-T1) x 0.02 x A2 dB.

Essentially, you don't have to know anything about the frequency / length / type of coax. Although these do determine the amount of initial attenuation, all you need to know is the actual value of the initial attenuation, and the change of temperature. The higher the initial attenuation (for whatever reason), the greater will be the change of attenuation.

--
Ian

this can be done fairly easily but one need a tv card in a PC like Hauppague. The card and software will produce an AVI file and you can go from there to a dvd or vcd is quite easy

You still need to know the length. I used to have to do the calculations for CATV system design on a four banger calculator, before PCs were common. Levels at the highest & lowest temperature was one limiting factor of the amplifier spacings. Some cables that were almost identical at first glance in the catalogs could be 100 feet difference in maximum length. At that time, pressurized, fused disk was the best .750 cable available, but it was so fragile we wouldn't use it. Then you had to wade through all the various foam types, the DC resistance per

100 feet, maximum certified frequency and a dozen other numbers.

A 5000 foot cable will have twice the change of a 2500 foot cable forthe same temperature change. You still haven't stated if your .02 dB change/degree is per f*ok, per 100 feet, or per mile.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you\'re crazy.

I've done that with a PC (you don't need latest technology, a PC from

2000 onwards may do). You need a TV capture card compatible with the standard you have in your VHS tape. I have an Easy TV that can capture both PAL and NTSC color video. Then a video card with TV output that can output the standard you want to record. I have an ATI Radeon card that can output both PAL and NTSC.

Plug the video to the capture card composite video input and view the signal full screen. Then on the TV card video output you have the signal converted Of course you need a VCR capable to record NTSC in colour (which may be the hard part to get). It may be easier to capture the video in an AVI file and burn it in a DVD.

"Michael A. Terrell" wrote in news:de- dnf1Hb6v9fs_UnZ2dnUVZ snipped-for-privacy@earthlink.com:

Look at the examples he worked for you.

Those units are especially convenient because you do NOT need to go through complex calculations.

The complex calculations have already been done and you are given the final results. All you need to know is the starting temp, the final temp and the loss at the starting temperature. Everything else has already been taken into account.

The 5000 foot cable will start with twice the number of dB loss as the 2500 foot cable. Therefore the change per degree will be twice as much, automatically.

As he said, you do NOT need to know the length or the frequency, all you need to know is the dB loss and the temperature change.

The units are (dB change in loss per degree change in temperature) / (dB loss at T1)

So, if you are given the current dB loss at T1 and multiply that by (dB change per degree change in temperature) / (dB loss at T1)

You get dB change per degree change in temperature. You then multiply that by degrees change and get dB change.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

One would hope, but it is not true. Everyone so far has assumed that coax is a pipe with smooth sides. Put something in and it slows down because it rubs against the sides, but it comes out basicly unchanged with a linear degradation.

The problem is that coax is more like a set of one wavelength pipes with an incomplete one at the end. How this effects the signal is dependent upon the frequency.

If the coax is used for close to one frequency, such as a single channel carrier, the effect will be the same. As the length approaches 1/2 wavelength there will be more antenuation, but since it effects the single carrier nearly the same amount, there will be minmal phase and linerarity distortion.

If the signal were on the other hand a baseband video signal, or a group of channels such as an analog cable tv system with 200 channels ranging from low band VHF to high band UHF and anywhere in between, it becomes more complex.

These days, it's not as much of a problem as digital video is carried over a few analog channels, and they are close in frequency.

It was also far more critical in the early days of color TV since there were no such things as phase locked loops and other methods of decoding a phase modulated signal without compensating for distortion.

Geoff.

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Geoffrey S. Mendelson, Jerusalem, Israel gsm@mendelson.com  N3OWJ/4X1GM

In message , Michael A. Terrell writes

I have! It's 'per dB'.

I'm sure you must have used a similar rule-of-thumb formula yourself (but maybe in Fahrenheit). Maybe you use one which requires you to know the length and attenuation per unit length (which will give you the actual attenuation). It all amounts to the same thing in the end. Somewhere, you need to input numbers for (or which give you) the cable attenuation.

Please read carefully: The formula states that the cable attenuation changes by appx .02dB per dB per degree C. Note the 'PER dB'. That 'dB' the actual cable attenuation. A 5000 foot cable will have twice the attenuation of a 2500 foot cable so, as you rightly say, a 5000 foot cable will have twice the change of a 2500 foot cable for the same temperature change.

To use that formula, the important thing is to know the ATTENUATION of the cable. If you don't know this, you have to find out. You may know this from actual measurement, or by calculation from the spec figures for the loss per unit length at the frequency of interest, and the actual length.

All I can say is "Thank heavens for optical fibre!"

--
Ian

You did pages of calculations to verify the data, because the construction costs were over $18,000 US per mile. You also had to be careful that an amp didn't get placed too close to an intersection, as well as minimize the number of power supplies. The power company assumed that they were all loaded at 95%, and didn't meter them. A rule of thumb design would have to be cleaned up & modified to almost work, where a properly designed coaxial plant simply worked the first time. Finally, I bought a Commodore 64 and wrote my own software that not only did the standard calculations, it did backfeeds. That is, lines run backwards from a bridging amp or line extender's normal physical layout. We were providing +10 dB on every channel to over 10,000 active ports. A well designed system needed little maintenance. A poorly designed system needed five to ten times the crew. Considering the then target lifetime was a 20 year life, most systems weren't paid of for over ten years. If it took much longer, you may never make a profit.

Until some idiot tries to steal copper wire and cuts thousands of fibers, looking for drug or beer money.

BTW, Cable TV around here is 'Fiber Enhanced' which means that the regular services are still on coax, and the internet, telephone and other services are on fiber. Unless you have FIOS, it is still a piece of coax between the CATV plant and your house, apartment, or refrigerator box.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you\'re crazy.

I did read that carefully, and wondered. It looked like a typo, as if you'd accidentally added "per degree".

I've never seen this before, and I suspect most other readers didn't.

Or even worse, cuts the fiber cables under the sea because they happen to drop anchor in a forbidden zone. It has happened twice in 2008 to an Arab owned consortium with cables to Egypt from Europe.

Since here in Israel we are not welcomed by the consortium, we had our own cables routed elsewhere. :-)

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel gsm@mendelson.com  N3OWJ/4X1GM

"William Sommerwerck" wrote in news:givqvq$3n1$ snipped-for-privacy@news.motzarella.org:

Actually, for things like temperature coefficient of resistivity(TCR), such things as 'ohms per ohm per degree' are pretty common.

To be completely clear, TCR should be stated explicitly in units like.

ohms of change in value per ohm of initial resistance at T1 per degree change.

When I used to design and build resistors and capacitors for Sprague in the late 60's and early 70's, the components had spec'd TCR and TCC in such terms.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

I think so. If you are in a metro area with a lot of immigrants.

Out in the boondocks, no.

Dave

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That looks more like the pronounciation of the big slidy downhill ice track.

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