video signal on a 500 m coax

[proton]

Hi, I've to send a standard PAL video signal throuh a 500m long coax cable, do you think that the signal at the end will be too much degraded ?
Eventually which solution do you suggest ? I've seen a design where
the signal was modulated on a 70 MHZ carrier, but does it helps ?
thx
proton

 

[Sebi]

Hi,

no problem with 500m, when signal too low, increase the video signal input. Cable loss on 70MHz higher.

 

[Moss]

You need to build simple cable driver because attenuation is significant ( more than 30dB ) with some IC from Linear technology or Analog devices.They have a lot such drivers. Sebi is right attenuation at 70Mhz is higher that is why you don't need to modulate the signal.

 

[Borber]

First you must know the cable specs to use proper cable corection module in your video correcting amplifier. Cable correction must be oposite of attenuation curve of your cable. Building cable correction amplifier is not complicated and if you want sample circuit....
Frequency dependent cable attenuation is not linear with frequency specialy at lower frequencies up to 1MHz. All depends of cable you use.
Generally it is not enough to compensate only gain loss if low linear distortion of PAL signal is wanted. Such correction amplifiers are usually called Video Distribution Amplifiers and are extensively used in TV production chains.
borber

 

[algilsan]

Well, as Borber said, the attenuation of a coax cable is dependent of frecuency, but at this frecuency you can considerer constant attenuation given by manufacture, like x db/m. Since you have 500m you will have a considerable attenuation in db. So you will two make an output amplifier that deliver enough power to the cable, to get at the other end at least 1mVpp.

i hope you be usefull

 

[Sebi]

I think typing failure from agilsan: not 1mV, 1Vpp on output...

 

[algilsan]

Thanks Sebi, you are right, thousand of pardons

 

[exata]

video signal on a 500m coaxial cable

A much better solution is to use a cheap twisted-pair cable and a diferential signal transmitter and receiver. Twisted pairs can be used at distances up to 1Km. To understand what am I talking about take a look at http://www.astel-cctv.com/pro/s5r.html

 

[Borber]

I must repeat that attenuation of video signal is very frequency dependant and you can not consider it as constant for a specified length of cable. Crominance component of video signal is more attenuated as luninance component. So it is neccessary to involve frequency dependant cable attenuation which can be done only by means of cable corrector.
Quallity cable has at 100m length attenuation at 1MHz of about .5dB and 1.2dB at 5MHz. RG59 cable has 0.75dB at 1MHz and 2.1dB at 5MHz for the same length. For 500m you can simply multiply attenuations by 5.
Cables has such properties that slope of attenuation with frequency is changing. At about 1MHz there can be seen a knee in frequency response.
Regards

 

[TurboPC]

Analog Video Signal over 500m?

Even with a Belden 8281 cable which is used inside professional studio, you can't go much over 100 feet without starting to see degradation. Up to 500 feet, you can still equalize the signal at the other end of the cable and produce an acceptable signal. After EQ, you have to restore the DC gain, because even studio cable will give you a significant attenuation. More than 1000 feet? You will have a problem with the group delay of each frequency...

An additive to Borber's comment: Chrominance is around 3.58MHz (NTSC) and 4.43MHz (PAL). Luminance is from DC to 5.5MHz but the broadcasters will limit it to 4.2 MHz in North America. Hence, high frequency luma details can be even more afftected than the chroma.

Anyway! Here's my solution for you: go digital!

Here's the simple solution. Buy some Miranda Picolinks. You will be able to drive at least 200 meters per section (with goooood cable). You need a ASD-272p to convert from analog composite (NTSC or PAL) to serial digital and an SDM-272p to convert from serial digital back to analog composite. Prices are around 500$ to 700$ US (I think).

**broken link removed**
**broken link removed**

If you want S-VHS (S-Video) input/output, look at their 272p family....

Have fun!

 

[TurboPC]

use fiber....

An add-on to may last post... I think someone makes a box that converts 270Mbps serial digital to/from fiber optics. This can give you kilometers of distance... :lol:

 

[Borber]

I agree with TurboPc. Another problem with analogue solution is hum. In case of bad "ground" it can be real problem.
Go digital and use optic-fibre.
BorBer

 

[Borber]

I agree with TurboPc. Another problem with analogue solution is hum. In case of bad "ground" it can be real problem.
Go digital and use optic-fibre.
BorBer