Impedances and transmission line effects...

[ppppenguin]

Wavelength of PAL subcarrier is about 68m. If you meerely remove the receive end termination on a video cable carrying PAL what happens depends on the sending end termination. If the sending end has good return loss (= accurate 75R match) all that happens is the amplitude at the receive end doubles. The reflection is absorbed at the sending end.

These days it's probably easier to sabotage SDI in this way, but it won't look so nice on a 'scope.

[Cardigan]

Wavelength of PAL subcarrier is about 68m.

In free air: but the propagation velocity in a typical coaxial cable is about 2/3rds of the speed of light, so λ = 45m in a piece of cable.

[ppppenguin]

Cardigan is quite right. For most solid dielectric cable, as used for video, the velocity factor is about 0.67. For semi-airspaced cable as used for RF and also these days for digital video the velocity factor is commonly around 0.8 to 0.85.

[pwdrive]

Some stuff on transmission lines at audio frequency and why:

http://audiosystemsgroup.com/TransLines-LowFreq.pdf

Lawrence.

[ppppenguin]

Interesting artcicle. I hadn't realised that there was a sharp transition in the characteristic impedance in the middle of the audio band. I knew that the impedance went very high as you approached DC and that mattered for long lines. The author says that you need to account for transmission line effects at between 1/10 and 1/20 x wavelength. Below that you can treat the cable as lumped R and C.

[pwdrive]

Yes, it's all down to the time period versus transit time...I know that 'cos I'm in the middle of watching a video about it on Utube as coincidence would have it.

Lawrence.

[Gryphon]

2MHz corresponds to a wavelength of 150m in free space, so rather less than that in screened cable.  Someone upthread said that you need to start thinking transmission line theory if the line length approaches one-tenth of a wavelength (not the figure I would use myself, but never mind).  Mmm, 150/10 = 15.

Well if Trevor's tie-line gives him grief at 2MHz, I've no doubt that Trevor will know full well that he needs to terminate it. Certainly, we've established that it's absolutely fine for audio...

So if not 1/10, then what's your preference, and why? Some say 1/4. Interested in your thoughts...

If Murphy v310 does want to terminate his line correctly, it will help him enormously to know what the Zo is.  Which rather, I think, brings us back to where we started.

I'm going to give a politician's answer to your question, which means I'm going to duck it, but unlike most contemporary politicians, I'm going to have a stab at justifying the duck.

I, like, I imagine, many of my contemporaries, left University with the idea that lines of less than lambda/4 were easy, because you could treat them more-or-less like hook-up wire off the reel.  That little misconception didn't long survive encounters with real lines of all sorts, on behalf of my then employer.  So the answer I would now give is that such "rules-of-thumb" aren't helpful, because they can be misleading, and that the answer is what lawyers call "case-specific": in other words, it depends on what you are trying to do, how well you need to do it, and the unintended consequences of not doing it well enough.

As a generality, the problem is easier with low-level signals than with lots of power to transmit.  Power, especially r.f. power, is expensive and awkward to get rid of, so you want as much as possible going where it is intended.  Narrow-band applications are easier to solve than wideband: in the former case, tuned stubs and other resonant solutions are available. Nor is this an analogue-only problem: reflections on circuits carrying digital signals are a major cause of timing jitter.  This can be a serious problem in, for example, backplanes in equipment carrying high-speed digital signals, where the tracks act as a parallel line, spaced by a mixture of air and the backplane dielectric.

To show how easy it is to fall over in real life, if you don't think carefully enough about what's going on, consider for a moment the situation shown in the attached drawing. S and R are two centres heavily interconnected by lines. Some of these are co-axial tubes with interstitial pairs, which are routed directly between S and R.  We have tried to allocate the interstitial pairs to music circuits, but there are not enough of them, and so some musics are in multipair cables.  These do not run directly between S and R but via a third point which we will call L/SHE. Ignore for a moment the limb shown between L/SHE and E.

The cable we are interested in is the 50-pair shown in the drawing, with distances marked. Most of the pairs are used either for control lines or for voice telephony, but about 10 are equalised for use as music circuits. A user department complains that one of these musics is no longer serviceable as such, as it has a wideband notch in the frequency response, about 20dB deep and centred on 4.5kHz.  Investigation shows that all the other musics in this cable are like that as well.

Liaison with our friends at L/SHE reveals that, in preparation for some work on site at E, the run between L/SHE and E has been added to the line, by jumpering it on the frame at L/SHE, overnight. (S and E are in different buildings on the same site, about 200m apart)  The instructions to the frame technician were to jumper only the pairs in use for telephony, but he has misunderstood and jumpered all 50.  The line between L/SHE and E is not electrically terminated at E but the pair ends are left open-circuit on the frame there.

This is audio, right? No need to treat it as a transmission line over these distances, right? Lambda for 4.5kHz is about 67km in free space. 1km is lambda/67.  Yet the initial suspicion that the line between L/SHE and E is the cause of the problem is entirely confirmed by asking the Exchange Engineer at L/SHE to insert a plug in the break-jack which has been provided there.

What is going on?  Well, as described upthread, the characteristic impedance of the pairs is probably somewhere in the 110 - 140 ohm range.  They are, however, used with a sending chain at S which assumes that the Zo is 600 ohms, and terminated in 600 ohms at R.  So, even before we add the stub, there are probably standing waves on the line: however,  their effects  have been allowed for when the line equaliser was built.

Adding the stub does two things: it produces an impedance discontinuity at L/SHE, and it adds further standing waves due to the open circuit at E from which, clearly, reflection is total.  The saving grace is the line loss: the d.c. resistance is of the order of 170 ohms/km with the loss rising with frequency.

[pwdrive]

Not sure if I've read it right, ignoring for the moment the line L/SHE to E, are you saying that the line ran from S (the sending end) to R (the receiving end) and that there was no load at L/SHE and that there was a termination load at R of 600 Ohms impedance at R and that the source impedance at S was 600 Ohms?

Back to L/SHE to E.........L/SHE to E, is that length the stub you referred to?

Equalized for music...Anything reactive about that so far as the line is concerned?

Lawrence.

[Gryphon]

Not sure if I've read it right, ignoring for the moment the line L/SHE to E, are you saying that the line ran from S (the sending end) to R (the receiving end) and that there was no  load at L/SHE and that there was a termination load at R of 600 Ohms impedance at R and that the source impedance at S was 600 Ohms?

Back to L/SHE to E.........L/SHE to E, is that length the stub you referred to?

Equalized for music...Anything reactive about that so far as the line is concerned?

Lawrence.

S is the ending end: sending end impedance 600 ohms.  R is the receiving end, terminating impedance 600 ohms.

Nothing at L/SHE except a frame to join the cables S -> L/SHE and L/SHE -> R.  L/SHE -> E is the stub, added overnight by accident.

There is an equaliser at R which contains reactive components, but such that there is no in-band reactive component to the impedance "seen" by the line from L/SHE.

If you can explain the observed effects without invoking transmission line concepts, I'd be fascinated to know. Much cleverer people than me have tried unsuccessfully (doesn't mean it can't be done, of course).

[Mark Hennessy]

If Murphy v310 does want to terminate his line correctly, it will help him enormously to know what the Zo is.  Which rather, I think, brings us back to where we started.

Well no. We started with the OP wondering about the basics of using short coax cables with audio signals. Where really, loading effect is the big "trap for young players", as David Jones would put it. Transmission line effects not an issue.

Trevor's coax came up later. But measuring Zo is a trivial piece of work; hardly worth debating...

I'm going to give a politician's answer to your question, which means I'm going to duck it, but unlike most contemporary politicians, I'm going to have a stab at justifying the duck...

Obviously, in engineering disciplines the answer is usually "it depends". But rules of thumb are good to get us in the ballpark before we quantify more precisely, or to sanity-check when we have. Rules of thumbs are just that; everyone (hopefully!) knows that they normally need to be qualified in reality.

So perhaps your point is that not everyone appreciates the meaning of "rule of thumb"? Perhaps you're frustrated with folk investing too much in rules of thumb rather than actually "doing the math"? If so, I can empathise with that!

To show how easy it is to fall over in real life, if you don't think carefully enough about what's going on, consider for a moment the situation shown in the attached drawing. S and R are two centres heavily interconnected by lines. Some of these are co-axial tubes with interstitial pairs, which are routed directly between S and R...

Fortunately, it's rare to send analogue signals any distance these days.

The idea that a stub causes problems is no surprise, and the fact it can happen in the low-km range is no surprise for an audio signal either.

In practice today you're much more likely to find that a stub causes problems for AES3 audio. It's why Guillmetti patch panels carrying AES should use special normalising plugs that blank the upper row of "listens", preventing someone from creating a stub. But a scary number of patch panels are populated with standard normalling plugs, and it's just luck that more services aren't taken off air.