Maximum length of AV cables?

[rodgreen]

Hi all from down under ...

Is there a maximum recommended length for AV (RCA Video) cables before loss of signal quality?

I want to hook up my Marvel to our TV/Video and need about 10 metre of cabling for the job.

Regards

Rod
Newcastle, Australia

[This_Idiot]

10m is a lot, but I don't think it should kill your chances.

eg. My prime input device for reviewing is my VCR. This goes through a switch box (so I can route it to different machines) via a 2m link, and then another 3m to my "usual" test machine (ie a total of 5m plus the box). By using uni-directional gold patch audio leads from Tandy (Radio-shack), I don't get any discernable signal loss

[rodgreen]

Thanks "Idiot". I'm not sure if I can buy them already made that long, or if I'll have to make them myself. The audio is not a problem (basically just speaker wire!) but the video should be shielded I guess.

I'll wait and see if anyone out there has tried this themselves.

Cheers, Rod.

[Patrick]

Unless you want hum, and/or static, and/or taxi calls coming over your sound system, I'd advise you to use shielded audio cables.

[Rick6612]

Hi Rod,

For longer cable runs, you can buy standard coax cable (TV cable) and little adapters that screw into the ends of the coax cable and convert the ends RCA (phono) jacks. I have a 100' (33 meters?) coax with these RCA jack converters on the end that I've used to connect my video camera to my computer from a long way away and it seemed to work pretty well.

Rick

[rodgreen]

Thanks Rick & Patrick - I take you point with shielded audio cables (although maybe I can pick up the neighbuors dirty phone calls?)

The coax sounds good, but I need to have 2 stereo connections (with both left and right channels) and two video connections, plus an antenna connection. Sounds fairly bulky with coax unless I group it together in a plastic "hose". I guess this will stop interferece.

[Spielberg]

Rodgreen,

I connected my Marvel G400 to my VCR with an RCA-cable from 10 meters. My VCR is connected to my TV via a short Coax cable.

The quality is fine; when I use the Marvel TV tuner and watch this picture via TV-out on my TV, I can't tell the difference between this and watching TV directly.

So cables of 10 meter will probably work fine, unless you have high quality demands ...

Spielberg

[rodgreen]

Thanks Brian .... I understand (most) of what you are saying so excuse my ignorance when I ask ... what are BNC connectors?

Rod

[MultimediaMan]

Brian,

Your suggestion to use a three wire cable for Audio inputs is an excellent one. I've been doing this one for years in my car audio installs (all of my RCAs share a common ground). Precision Power and Xtant both recommend simply using a good quality RCA barrel and a quality wire twisted into pairs for RCA patch cabling. I can attest to its effectiveness (and economy).

Shielding, for audio cables (15Hz to 30kHz @ 0.1 - 16VAC), is a dubious feature (Unless you are using 24 AWG or thinner wire, which makes for an excellent AM/FM antenna). Most shielded audio cables that have the shield grounded at either end of the signal path show more susceptibility to noise. Square wire and other esoteric wire core geometries for low level analog audio signals is overkill, and can cause more problems than it solves.

I've seen S-Video cables as long as 75' with no loss in signal quality - your results may vary depending on your gear.

Best,
Matt



------------------
My (Current) Primary system:

Abit BE6 (PL BIOS)
P3 450
384MB PC-100 SDRAM (Non-ECC)
Matrox Marvel G200-TV AGP (NTSC) v2.6 BIOS
3Com 905B-TX NIC
SBlive (W/ OD I/O Card, Liveware 3.0 + Update)
ADS Cadet Radio Data AM/FM Card (8 Bit ISA)
3x WD AC28400 EIDE HDDs (ATA66 Enabled)
Creative DVD5241E EIDE 5x DVD-ROM
M/S Windows 98SE (English)
DirectX 7a

My (Current) Secondary system

Data General/ ALR 2650 Mobo (v6.004 BIOS)
2x P2 266 (SMP)
128 MB PC-100 SDRAM (ECC)
3 x Number9 Revolution 3D 8 MB PCI Adapters
Adaptec 6901 10BT NIC
Onboard Adaptec AIC7895
Onboard Intel 82558 NIC
Onboard Cirrus Logic SVGA adapter (Disabled)
Creative ModemBlaster 5630 Data/Voice/Fax Modem (PCI)
Creative SBLive! (Full Version)
2x WD 4.5 GB Enterprise SCSI3 HDD
Panasonic LK-MW602 CD-R
Nakamichi MBR 7.4
2 EIDE HDD Swap bays
Windows 98SE/NT Server Dual boot
DirectX 7a/SP-6

Tertiary system:
Dell Inspiron 7100 Laptop

[Brian Ellis]

May I go a little technical here?

For audio, any good screened (shielded) wire is just fine and I recommend you use a three-core wire, one for each signal and the other for the earth (ground) return: the shielding should not be used as the return and should be connected to the earth return ONLY at the source end. This way you will get minimal extarneous signals.

Video is a horse of a different colour. We are dealing with bandwidths of nearly three orders of magnitude higher here and the only effective way of handling this is to use a matched characteristic impedance transmission line (also for networking). Coax cable usually has a nominal characteristic impedance of either 50 or 72 - 75 ohms, depending on its geometry and dielectric. The one used for BNC-terminated thin-wire networking is 50 ohms. The important thing is that the source impedance, the cable characteristic impedance, and the load impedance must all be the same value. If not, reflections will occur in lines over a few metres long, and the signal level will drop. Also, you will lose some of the HF quality, showing a a slight "blurring" of the signal. Reflections will show as a series of diminishing "echoes" to the right of a sharp change of luminance in the TV image. The longer the cable, the wider will be the distance between the echoes. If everything is perfectly matched, then even 100 m of coax should not degrade the signal, other than a very small attenuation (no blurring, no echoes). So much for the theory, what about the practice? Quite frankly, I haven't got a clue what consumer TV impedances are. My camera specs say 75 ohm terminated, so I would guess that this may be standard. 75 ohm is also used for monitor RGB connections. In this case, use 75 ohm characteristic impedance coax, not the 50 ohm cable used for networking. The usual RCA/Cinch connectors are really an abortion and probably do not match 75 ohms, but this would probably be of small consequence. To perfect the job, you could consider changing them to the professional BNC connectors (!!!Warning!!! These must also be the 75 ohm type, as the ones used in networking are 50 ohm).

So, what would be the effect of mismatching the cable impedance? Up to a few metres, I don't think you would notice the difference. Above that, you would certainly start to get visible image degradation.

Sorry for these long techy details, but impedance matching is always a problem.

------------------
Brian (the terrible)

[MarkD]

Heheh... so far for composite video-out... and what about S-video connections ?

[Brian Ellis]

Rod

BNC? Can't remember what the letters stand for, except that the B is bayonet and the C is connector. These are the standard small (low-power) coax connectors used throughout the electronics industry and are made by numerous companies. The insulation is PTFE (Teflon), the mating parts are thick gold on nickel plated and the general quality is very good. They are actually also available in double-screened versions for the nec plus ultra. They conform to various standards, such as IEC 16958, CECC 22120, BS 9210 and MIL-C-39012. They are available for crimping, soldering or "twist-on" the more popular coax cables, such as RG58 (50 ohms) and RG59 (75 ohms). They are not cheap and you may have to pay more than a buck each, up to $5, depending on the type you choose. The characteristic impedance of this sug and plocketry holds well, right up into the GHz range.

Other than BNC, there is a series of plastic (polycarbonate) video connectors called RP5. These are of poorer materials than BNC items, there is less choice of configurations and they are more expensive, to boot, a single plug costing $4.50 (presumably because they are made in smaller quantities than the BNC which is a truly universal pro system).

I've also been looking up cables. I see that some manufacturers are offering special 75 ohm video coax cables. The only merit I can see for them, over general-purpose coax cables, is that they are 50% more expensive The RG59 type, for example, is a good GP 75 ohm coax at about $50 for a 100 m roll, with an attenuation of < 1,3 dB/10 m up to 100 MHz. A similar video cable is about $80/100 m with a slightly lower attenuation, but only measured up to 10 MHz. Composite cables are available for video/audio/data. For example, a Scart cable has 6 75 ohm video channels, 4 screened audio channels and 3 screened data channels at c. $220 for a 25 m roll. RGB 75 ohm cables cost $250 per 100 m roll.

The catalogue also has some made-up audio-video cables of various types, but these are limited to a max of 4 m so are probably not impedance-matched (including composite).

Screening (shielding) is of five types. The worst one is metallised plastic foil; then comes alu foil (both these types are difficult to make a reliable contact to); the next best is a spiral of laid thin copper wires; after this is a double spiral ditto, each in opposite directions, but the undoubted best is a woven copper wire tress. The latter is recommended for all audio cables, even if it costs a tad more. All coax cables, as far as I know, use the woven type. If you do as I say earlier for audi cables, using the woven tress, you should have no problems. Some composite cables use the woven tress for individual screening and one of the other methods for the overall, external screen. This is valid.

While on the subject of cables, there are some VERY expensive cables sold for hi-fi apps, especially speaker cables. These offer no advantages over cheap cables. I remember one of the hi-fi mags, a few years ago, comparing a so-called mono-crystalline solid silver speaker cable, and other specialised speaker cables and ordinary wire. They found the best performance (measured by instruments and with auditive tests) was given by an ordinary 15 amp power cable at less than 1/100th price of the most expensive one. Do not be misled into thinking that because there is a lot of blurb and the connectors have 0,1 micrometres of gold plating, they will perform better than much cheaper ones: they won't.

FWIW

------------------
Brian (the terrible)

[MultimediaMan]

BNC = Bayonet Nut Connector, referring to how the connector mates much like a bayonet to a rifle.

[rodgreen]

Thanks Brian, Matt and Multimedia Man!

I'll see what's available in Oz ... noting that the highest price doesn't always mean the best product!

Regards

Rod