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

[William Sommerwerck]

I knew an old ATT guy who worked in both cross country coaxial

& microwave video feeds. He told me how much of a pain it was to
equalize a section of cross county coax used for network TV feeds,
and keep it that way as the temperature changed.

Thanks for the confirmation. I didn't realize coax was so
temperature-sensitive.

 

[Ian Jackson]

Thanks for the confirmation. I didn't realize coax was so
temperature-sensitive.

Its attenuation changes by appx .02dB per dB per degree C.

 

[Ian Jackson]

At what distance? At what frequency?

Any and any.
Think about it.

 

[Ian Jackson]

You should see what it does to solid aluminum jacketed coax during a
sudden temperature drop. The shield can shrink three inches in a 500
foot run, and either pull the jacket out of the connector, or rip the
aluminum. It is called a suckout in CATV slang.

In the USA, where it is common practice to strap the coax outdoors on a
messenger wire stretched between the utility poles, the coax can indeed
undergo horrendous changes of temperature (both on a daily and a
seasonal basis). To allow for the physical expansion and contraction
which occurs, it is pretty well mandatory to have an expansion loop near
each pole. This prevents the cable being wrenched out of the connectors
at low temperatures. While the line guys may call this a 'suckout', a
suckout normally refers to an unexpected notch in the frequency
spectrum. Of course, this might be a direct result of the jacket having
been partially or completely pulled out, leaving the inner as the only
connection.

 

[Ian Jackson]

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.

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.

 

[Leo Tick]

I need to copy a PAL-VHS tape in NTSC format for someone in the US. I
have a Sony VCR which can read tapes in NTSC format and I want to dub
this tape from a second VCR. My first attempt produced a tape which
can be played whether the Sony is in NTSC mode or PAL mode. Have I
succeeded or am I wasting my time?

I appreciate this is off topic here; perhaps someone could point me to
a more appropriate newsgroup if there is no simple answer?

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

 

[Jeroni Paul]

I need to copy a PAL-VHS tape in NTSC format for someone in the US.  I
have a Sony VCR which can read tapes in NTSC format and I want to dub
this tape from a second VCR.  My first attempt produced a tape which
can be played whether the Sony is in NTSC mode or PAL mode.  Have I
succeeded or am I wasting my time?  

I appreciate this is off topic here; perhaps someone could point me to
a more appropriate newsgroup if there is no simple answer?

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.

 

[bz]

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.

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 fook, per 100 feet, or per mile.

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.




--
bz 73 de N5BZ k

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

[email protected] remove ch100-5 to avoid spam trap

 

[Geoffrey S. Mendelson]

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.

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.

 

[Ian Jackson]

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.

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 fook, per 100 feet, or per mile.

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!"

 

[William Sommerwerck]

The formula states that the cable attenuation changes by

approximately .02dB per dB per degree C. Note the "PER dB".
That "dB" the actual cable attenuation.

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.

 

[Geoffrey S. Mendelson]

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

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.

 

[bz]

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.

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.

[email protected] remove ch100-5 to avoid spam trap