Coaxial vs CAT 5

It is best to use CAT5 for both audio and video because the coax cable is designed for broadband video (TV). One thing you should look for in terms of quality of the signal is a balun (balanced-to-unbalanced transformer) on both ends. I have never seen one for SCART though. If your equipment has either S-Video or RCA outputs, use those instead. For short distances you could even go with unbalanced outputs. That is, use simple direct - connection jacks. Besides, check with your equipment manuals - some of the outputs may already be balanced, so you can save some money by not buying one of the two baluns that are required to complete one circuit. one circuit represents one channel of audio or one baseband video signal.

One thing that will cause problems, is the DC component of a composite video signal. Transformers don't pass DC.

isn't anyone concerned with high-end rolloff for audio on UTP due to the capacitance ?

For speakers UTP ad too much resistance.

info_at_cabling-design_dot snipped-for-privacy@foo.com (Dmitri(Cabling-Design.com)) writes:

All TV video signals are originally designed to be transfered through

75 ohm coax cable. That's the what the video professionals, TV broadcasters, majority of CCTV installations etc. use. There are various kinds of 75 ohm coaxial cables available (like there are varioust types of 50 ohm ciaxila cables as well), thicker, thinner, higher loss, lower loss, optimized for high frequency use, designed for lower frequency use in mind, cables that do well all frequencies well from DC to highest frequencies. Propably most common type of cable used to carry video signals is RG-59 B/U type 75 ohm coaxial cable. Many other 75 ohm cioaxial cables work very all also. Generally if you take practically any 75 ohm coaxial cable, evenones designed for antenna installations only in mind, those carry baseband video signal at least somehow acceptably.. There are some data of some vieo coaxial cables at Nowadays it has also been possible to tranfer vidoe over other kind of wire types, for example though CAT5 twisted pair wiring with pretty good quality and quite cheaply. Because the video is originally designed for 75 ohm coaxial cable, you need to use a balun adapter between your video equipment and the twisted pair wiring. This balun adapter adapts the unbalanced video signal to balaced signal that can travel through twísted pair wiring well and maybe also adapt the 75 ohm impedance to 100 ohm wiring (there are many adapters that do not adapt the impedance, they just rely that the 75 ohm - 100 ohm mismatch does not cause too severe problems to picture quality at normal uses, and usually this is true).

You need to use baluns for both audio and video signal if we are talking about the audio and video signals that ae used in consumer audio equipment, because both of those signal interfaces normally unbalanced on those equipment.

Video and audio signals need different kind of balun transformer designs to work well. There are commercial adapters available that have baluns for both audio and vidoe signals built inside on compact unit. It is also possible to build such adapters yourself. I have done that. For audio signals the balun is notnign else than just 1:1 audio isolation transformer. For video signals either a current mode balun (common mode coil) or 75ohm-100ohm video isolation + impedance adaptation transformer (possibly with DC blocking capacitor in input) is used. Components are available for those tasks.

I have never seen blauns with SCART interface either. Generally with SCART interface you need to take an adapter that for example adapts the SCART connector signals to one video connector (S-video connector or composite video RCA) and stereo audio signals (RCA connectors). Then you just wire those to your adapter that adapts those signals to CAT5 wiring.

Very short distances whee cables are away from noise sources direct connection to jacks might work somehow.. With shielded CAT5 wiring usually somewhat better than with more common unshielded CAT5 wiring. In most practical real world systems this kind of direct connection leads to a system that picks lots of humming noise.. The unshielded CAT5 cabling resistance against outside noise lies with the combination of use of twisted pair wires and use of balanced signal sources+receivers. The twists and balanced signal source make the systme so that the noise picked by the wires is picked in such way that it gets canceled in the balanced receiver. If you connect this wires to unbalanced signal source and unbalanced signal receiver, those properties do not work, and the noise that gets picked up the wiring gets added to the noise (the twisted pair signal carrying wire picks less noise compared to untwisted wire, but considerably more noise than a properly shielded signal wire would pick in the same environment).

That's one option if you have equipment that use this kind of interface. balanced audio signals are seen on many professional audio equipment and some expensive home hifi equipment. I have seen balanced video signal interfaces built in only on some CCTV cameras and related CCTV equipment... Practically other video equipment I have seen have used "normal" unbalanced video interface (connectors being usually RCA, BNC, SCART or S-video minidin).

The standard audio interconnect cables have HIGHER capacity (30 pf/ft) than CAT5E UTP (15 pf/ft), so this is not a problem at all.

You normally don't drive a speaker via UTP (although possible with multiple pairs in parallel). You'd normally send a line level signal via UTP, which works just perfect as long as it's balanced. Even unbalanced on short distances is OK.

LOL! Do you think we have Golden Ears? Odd "capacitance" that doesn't affect 100 MHz would somehow disturb 10kHz.

This may be true for some low ohm speakers, but the solution is usually just to boost the volume.

FWIW, I'm using about 50' of 24ga Cat5 to wire the satellite speakers on a 5.1 audio system. Maybe a bit softer (~3dB lower volume) than 20ga.

-- Robert

Or you could modulate the A/V outputs of your PC onto a TV channel, feed it from the PC to the distribution center, and send it via the CATV distribution system to every outlet in the house. That way, at any TV in the house, you can tune to channel 85 (or whatever channel you modulate the PC to) and there it is. At your entertainment center, tune the VCR to channel 85 to listen to your PC through your entertainment system.

CIAO!

Ed

Ro Catha> I have Cat 5 and Coaxial cabling installed in my house.

Actually, at least for long distances, cable characteristics are important at audio frequencies. For example the cable used to run telephone service to your home, has a significant rolloff within the voice band. The phone companies use loading coils to flatten the response within the desired frequency range.

A normal transformer with isolated primary and secondary does not pass DC. That's true. There are video applications where the DC matters and there are video applications where the signal path does not need to pass DC through. For example very many video transmission paths (fore example when carryuing composie video signal) are AC coupled originally. There is usually is usually a capcitor in the video output circuits and video input circuits. This makes the system AC coupled. The missing video DC level is restored on the video signa reception with a black level clamp circuit that sets the DC level of black picture to right DC level. Many video transmission paths do not need to pass DC. And for those video isolation transformers that apass frequencies from 50 Hz to highest video frequency work well.

For the applications where there is need to pass DC there are balun designs that can pass DC. Those are current mode baluns (=common mode coil) that do blun conversion but can't provide you galvanic isolation (between in and out) and can't easily provide impdance matching (input impedance = output impedance).

I have used both types of baluns for video applications.

The capacitance does affect 100 MHz signals on the cable, and it will affect the 10 kHz audio signal as well. The capacitance is there. For audio frequency signals the capacitance is there to affect the high frequency roll-off, but also the signal source impedance affect how much this rolloff there is. For high freuqency applications the CAT5 wiring is always terminated to 100 ohms signal and source (=low impedancde) and the cablign works a a transmission line (it nis no longer modelled as just capacitance, but the combination iof cable impedance and capacitance form a system where the effect of just pure capacitance quite much dissapears).

A cable in audio applications for carrying microphone and line level signals can be modeled as a low-pas filter. A first-order high-cut (or low-pass) filter is formed by an output's source impedance and the capacitance of the cable. The frequency at which a filter attenuates 3 dB is called its "corner frequency". With short cables (low capacitance) and low output impedances, the corner frequency typically occurs well above the audio band. With longer cables and higher output impedances,the corner frequency drops and may drop into the audio band. The formula for the corner frequency is:

F = 1 / (2 * PI * R * C)

where F is the corner frequency in Hz, PI is 3.14, R is the source's output impedance in Ohms, and C is the cable's total capacitance in Farads. A first-order filter has a slope of 6 dB per octave. This means that beyond the corner frequency, the response will drop 6 dB for each doubling of frequency. Generally it doesn't seem likely that you would get detectable loss even at 20kHz unless you have one or more of these conditions: unusually high source impedance (many kilo-ohms), unusually high capacitance cable or unusually long cable length (tens of meters).

Typical transmission characteristics of CAT5 wiring: DC Resistance: 8.99 Ohms/100metres Capacitance: 13.5-17 pF/feet (45-57 pf/meter)

The capacitance of a normal shielded audio interconnection cables is typically 2-3 times higher than the capacitance of the CAT5 wiring! One meter of typical shielded audio interconnection cable (RCA cable) has typically capacitance of around 100 picofarads.

So for a 100m Cat5 cable, at 8 Ohms source impedence, the corner frequency is 3.9 MHz . I don't hear up that high. We're talking about speakers on Cat5. A phono cartridge might have some trouble :)

Good to know. So capacitance is a non-issue for using Cat5 in audio.

-- Robert

Load coils don't go on lines until 20kft (6km). At that distance, I don't think the Cat0 line would have bandwidth (S/N=1) over 100 kHz.

I thought the load coils were to retain signal strength (volume), not frequency response.

-- Robert

Without loading, the frequency response follows an inverse exponetial curve. With loading, it's essentially flat over the desired range, then drops like a rock.

Loading coils have been eariler used to extend the range of a local loop for voice applications. Load coils are inserted at specific intervals along the loop (3..6 Kfeet distance). Load coils are inductors that are added in series with the phone line. They compensate for the parallel capacitance of the line, which attenuated the higher voice frequencies more than lower frequencies. By adding inductance (load coils) periodically into the cable facility, the capacitive effect can be cancelled, thus causing the attenuation across the voice band to be equal. Load coils benefit the frequencies in the high end of the voice spectrum at the expense of the frequencies above 4 kHz. Load coils are are often found at loops extending farther than 12,000 ft. A typical load coil is 88mh coil (type H88), which will cancel 6000 ft of typical telephone cable capacity. They are typically installed at 6000 feet spacing. Load coils benefit the normal telephone operation on normal line, but do not allow modern broadbans services on lines with load coils. Since ADSL and ISDN depend on frequencies much higher than 4 kHz, they will not work a coil loaded line, because those highe frequencies cannot pass through the coils properly. New digital telephone services require 'unloaded' copper pairs. For example all load coils must be removed for any DSL or ISDN operation.

By the way where do you see 8 ohms source impedance on practical audio systems ? I have not seen that much. The output impedance of a typical hifi amplifier built using transistors, fets, ICs etc.. have output impedance that is considerably lower than 8 ohms. The ideal amplifier has zero output impedance (=pure voltage source). Practical audio amplifiers typically have effective output impedance considerably less than one ohm. So in normal hifi system speaker output the source impedance is less than one ohm and the load impedance is tha 8 ohms. With some tube amplifiers situations can be somewhat different, those have typically considerably higher output impedance than modern amplifiers built using other techniques. So the 8 ohms source impedance assumptation might hold for some tube amplifiers, and for other practical amplifiers the source impedance is considerably lower than this 8 ohms.

With normal line level signals the source impedances are typically in 30 ohms to 5 kohms range depeding on the equipment used. Professional audio equipment have typically the source impedance in 30 ohms to 600 ohms range. Normal hifi equipment typically have source impedance from few hundred ohms to few kilo-ohms..

I was using 8 ohms as an upper bound to give the lowest corner freq. Since impedence is less, cutoff goes even higher!

-- Robert

I don't think that's what he was saying. He said that it depends on the source impedance, length of cable, etc. For instance, the usual method of running a PA system to a speaker dozens of yards away is to use thin wire, could be cat5. So the PA puts out 70.7V line voltage. The impedance is higher than 8 ohms, maybe 500 or 600 ohms. The matching transformer at the speaker drops it down to 8 ohms. So the impedance of the source isn't just 8 ohms, it's higher. And the length of a long cable could affect the speaker's output, especially if you have a tweeter for those freqs of 10k and above.

Another example is telephone line. You could run a POTS set over hundreds of feet, maybe up to a few thousand, without any noticeable effect at the higher freqs, 3 to 4kHz. But you get up to 5k or 10k feet, and then you have to put loading coils on the lines to maintain the quality of the telephone audio.

But what I've been talking about is that longer run of several hundred yards. If instead I use a tube preamp to drive an amplifier, the output impedance might be more than 10kohms. In that case, with the high Z, a dozen yards (36 feet) of any cable, shielded, coax, cat5, or twisted pair, might cause noticeable attenuation of the high freqs. That's why the microphones for consumer use might be high Z, and will work fine with a ten foot cord. But you put an extension cable on them and you find them to be muffled. So with pro equipment, the mics are low Z, and can have dozens of yards of extension cable without problem.

So this audio roadie guy gets an idea that he can make an adapter from XLR to RJ-45, and run his microphones over his home netowrk. It should work okay with low Z mics with balanced line, but the consumer mics would need a transformer to drop the Z down to 150 ohms, and to balanced line. Then at the other end, another to put it back to high Z unbalanced line to go into his preamplifier. Putting the high Z unbalanced mic signal directly over the cat5 would probably get him nothing but HUMMMMMMMMM...

The roloff isn't limited to the voice band as you imply. It affects any freq, but the longer the line, the lower the freq, until it's affecting the voice freqs at thousands of feet.