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yz

Aqua Marine

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Feb 1 11 11:03 AM

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So...

What does that do exactly? Each button selects a ratio, but with all of them in, what happens to the ratio and other stuff?

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ssltech

Aqua Marine

Posts: 4,044 Member Since:22/01/2011

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Feb 1 11 12:51 PM

This is another long answer, I'm afraid. -I might have to do it in stages.

Part one
(In which we meet our hero, and a plot is revealed!)

The Circuit

Lets refer to a schematic. There are lots of different versions out there, but let's use this one because it's fairly clear,and quite easy to read:

open it in a second window, or print it out, if that helps you to refer to it and follow the explanation which I'll attempt.

So... Look at the ratio switches S1 to S4. -They are two-pole switches each has an 'A' pole and a 'B' pole.

The way that ratio is altered uses two resistive 'ladders'. One ladder uses the 'A' poles of each switch, and the other ladder uses the 'B' poles.

The two ladders perform two tasks: One alters the 'enthusiasm' with which the unit initiates its 'squashing' behaviour, and the second one controls the unit's 'sensitivity'.

Sensitivity Training

Let's look at the easy ladder first. -This is comprised of resistors R26, R25, R24, R2 and R23... and it uses the 'B' poles of the four switches; S1b, S2b etc.

This controls the units sensitivity. -One function of its being there is that it helps to make switching between ratios less bothersome, and keep the unit operating at a reasonable region of the gain structure... Here's what I mean:

Once the signal is nicely above the threshold where limiting action takes place, the higher the ratio you select, the more the needle will dive back to the left, and the lower the output. -In a unit with a variable threshold control, this can be compensated for by just raising the threshold slightly... but the 1176 doesn't allow you to do that because the threshold (in each ratio setting) is fixed, so rather than having you constantly fiddle with the input and the output (resulting in an overly-complicated dance of knob-twiddling, which can also mess up any carefully-arranged gain structure you might have had) the approach is to change the signal sensitivity of the DETECTOR; which is the part of any dynamics device (whether a gate, a limiter, dynamic filter or a noise reduction circuit) which determines how loud the signal is at any given instant.

In the 1176 the detector is called the 'Gain Reduction Control Amplifier' and it's down at the lower half of the page. -It sniffs its input as a 'split' from where the signal is fed to the final output stage, RIGHT before the "output" control so that that knob doesn't affect it.

The audio signal enters the chain of resistors at R26, and trickles down all the way to the far end, R23, where it it connects to ground, and therefore there is no longer any signal. -The resistor values are all approximately in the 50k ohm order of magnitude, so let's make it easier to count on our thumbs and treat the overall chain as being about 250k.

The four switch wipers connect to different 'rungs' on the ladder, and the signal level at each 'rung' is proportional to the resistance ratio above and below it. -Since we're treating all resistor values as equal, you can see that the 'top rung' has 20% of the resistance above it, and 80% of the resistance below it, so the signal level passed on will be about 80%. The next rung down has 40% above and 60% below, so the signal level is passed on at about 60% of the original level... the remaining rungs obviously pass 40% and 20% correspondingly.

In this manner, as a different limiting ratio is selected, (as determined by the ladder at the other poles of the switch), the unit it made progressively more or less 'sensitive' to the signal level.

[Oh, by the way... Ignore the "T&C" ("test and calibrate") resistor which is shown as a box... it's shown because if NO ratio is selected, allowing the input to 'float' might pick up some stray signal, so the provision was made to ensure that the input was never left COMPLETELY unterminated.]

The actual values (56k, 68k,47k and so forth) would likely have been tweaked by empirical 'what feels most useful' means.

Now... while we're looking at this 'sensitivity' ladder, let's consider what happens when we press MORE than one switch in. -Let's concentrate on S4b -which in our simplified model is the '80% tap'- and S3b -the '60% tap'. -If we close BOTH switches, you can see that we provide a short-circuit path around the resistor in the middle (R24)... effectively removing it from having any effect on the circuit... we've 'built a bypass' around that part of the ladder. Now instead of having FIVE 50k rungs in a 250k ladder, we've got FOUR 50k rungs in a 200k ladder.

The signal percentage which is therefore passed on to the detector in this example will be 75% of the original signal value.

If we close S4b and S2b.... we bypass both R25 AND R24. Now we have THREE 50k rungs in a 150k ladder, and the 'tap' at 66%.

Pressing S4b and S1b... we bypass R25, R24 and R2... completely removing them from having any effect on the circuit... now we have TWO rungs left in the ladder, and we're taking out tap from the mid-point between the two rungs. -In our simplified model, this would give us a 50% signal level.

Consider this about this section of the 'all-buttons-in' trick: when more that one button is pressed, the buttons in between have NO effect whatsoever... Looking at the part which we've looked at so far; if we 'build a bypass' from S1b to S3b, then S2b no longer has ANY influence on the circuit. Likewise, if we press in S1b and S4b, then both S2b AND S3b are bypassed; "Buttons 1 and 4" is the same as "all-in".

Okay... that's all can really do  for now. -If this is helpful and you can follow my description of how the schematic is working this far, I'll address the second half (S1a through S4a) when I next get chance to sit down and type a good long attempted explanation.

There's actually a SECOND aspect to what happens as a result of the 'sensitivity switching', but I'll come back to that later.

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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yz

Aqua Marine

Posts: 2,828 Member Since:26/01/2011

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Feb 1 11 3:01 PM

Ok, so if I got it right, the unit is 'more sensitive' for lower compression ratios and 'less sensitive' for higher ones, or conversely, the threshold is lower for lower CRs and higher for higher CRs.

Right?

And ONE part of the "All Buttons In" thing is that the effective threshold ends up being somewhere near the middle-of-the-road.

Also, there's at least THREE parts to it:

- the threshold, which you explained, set by the B poles of the switches;
- the ratio itself (I suppose...) which is set by the A poles of the switches;
- something else related to the GR control amp yet to be disclosed;
- did I read the schemo correctly and saw that the A poles also have an influence in the time constants? It's been a while... :blush:

Hey, great info so far, can't wait for the next chapter!

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ssltech

Aqua Marine

Posts: 4,044 Member Since:22/01/2011

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Feb 1 11 3:28 PM

Okay... Part Two.
(In which our hero loses his way, and settles down with a sandwich.)

So.... we've looked at the first ladder and how it affects the signal level being fed INTO the detector stage.

Now... let's get a little bit theoretical so that we make sure that we all see HOW the limiter is forced to be more or less 'savage' once the signal crosses the threshold of limiting, and into the realm of gain reduction.

Let's consider a nice, predictable, repetitive signal to which we want to apply some limiting. To keep things easily describable, we're going to consider a 'pulsing' sine-wave. -The frequency is not particularly important, but let's just say that it's 1kHz and be done with it.

Let's define the 'pulsing' so that we can all visualize the waveform. -This particular signal alternates between one level a 6dB louder level. Now if we input a 3 volt peak-to-peak sine wave, the PEAK level is 1.5Volts away from silence (1.5V up, 1.5V down) and doubles in the 'loud' bursts to 3V peak.

Now, let's say that our unit has a comparison threshold set JUST ABOVE the 'quiet' parts; at exactly 1.6 Volts. Even the very tallest parts of the quieter sections will sit just below the threoshold, but the (6dB boosted) LOUDER parts will cause the peaks of "the loud bits" to exceed the threshold by 1.4 Volts, (since they peak at 3V).

So -in this example- a 6dB 'surge' in a signal sitting just below the threshold produces 1.4V.

Now let's move BOOST the signal level using simple amplification. Let's double its size, so that the peaks of the quiet parts are now 3V from ground, and the peaks of the loud parts are now 6V from ground. We'll also raise the DC "comparison" threshold to 3.2 Volts.

In this second example, the quieter peaks (at 3.0V) do not cross the (3.2V) threshold, but now the SAME 'surge' in level (remember, the actual dB increase of the test signal has NOT been changed) produces peaks which -at 6.0V- exceed the threshold by 2.8V.

So... the same amount of volume change in the test signal (6dB both times, remember) produces MORE voltage at the output of the detector (which simply 'reports' when the signal exceeds the threshold voltage... and by how many volts) by a significantly larger amount.

Bearing that in mind for a moment, let's just make sure that we understand how the limiter controls he signal level. In simplest terms, we have a potential divider just like our 'ladder', formed right at the front end of the signal path. It's formed by R6 (where the signal from the unbalancing stage first comes in) and the FET Q1, which goes to ground. the 'tap' at the middle is where the 'sniffed' signal is passed on towards the output. If the FET Q1 has -for easy reckoning- a momentary value of 27k -which is exactly equal toR6- then the ongoing signal level is halved. Just like the earlier 'ladder' example, the signal level which is 'passed onwards' can be shown as the proportion of the FET's conduction resistance to the total resistance of the chain (R6 plus the FET).

So... we can REDUCE the signal level by shunting more of it to ground with the FET, but no matter what we do, we can never INCREASE it. -No matter HOW high the resistance of the FET, it can never exceed the total resistance of the 'chain', because the chain resistance is equal to the FET's resistance plus 27k.

Okay, so the amount of gain reduction is increased by an increasing voltage on the Fet's third terminal... -the Gate; the one with the arrow. In simple terms, more volts equals more gain reduction.

Since the same 'dB jump' in test signal level produces a greater amount of gain-reduction if we re-jig the signal gain and the 'voltage comparison' threshold, hopefully that helps show that the effective RATIO is therefore increased. (higher ratios produce greater gain reduction for the same increase in incoming signal level).

So, now that the THEORETICAL mechanism by which we want to change the effective ratio has been described, we can look back and -hopefully- see how the first ladder which we examined comes to play its role in the overall plan. -Already we've got preset GAIN CHANGE for each of our ratios. Now... to complete our functional ratio change, we need to alter the threshold point at which the detector reports that the signal is now crossing into 'gain change territory'.

So... looking back at our well-thumbed schematic, the circuit needs to be sensitive to both positive and negative going peaks, not just 'look at one peak and assume that the other half of the wave is the same'. -Human voice, strings, woodwind and brass instruments are all examples of HIGHLY asymmetric waveforms.

The detector ('gain reduction amplifier') does this by creating TWO outputs, in opposing polarity.  One polarity output appears at the emitter of Q13. The inverted waveform appears at the emitter of Q15.

Now, if a peak appears at EITHER of these points which exceeds a comparison voltage, we want that 'amount' by which the waveform passes that threshold, and forward it to the gain reduction control point.

To do this for us, the two diodes (CR3 and CR4) take the AC-only parts of the waveforms coming from the two emitters, (the DC part being blocked by the two capacitors, C19 and C20), and ONLY pass any parts of the signal which exceed zero volts...since -being diodes- they don't conduct backwards.

Now of course, diodes would always conduct 'positive' voltages 'forward'... but taking a look at the circuit, point number 21 is 'dragged' NEGATIVE, via two resistors R74 and R75, which take a negative voltage from the wipers of another 'switched-resistor ladder' (the one which we haven't looked at yet), and use it to hold the diodes OUT of conduction until the 'high spots' of the signal get 'tall enough' to push the diodes ABOVE the zero volt line, at which point they will start to conduct, and the point where they join feeds the FET and starts to push it into gain reduction.

One or two small details I'd like to touch on: The time constant circuit (attack and release) is in between the 'output' of the diodes and the gate (voltage control input point) of the FET. -This makes sense, since it's where we "slow down" how quickly the FET reacts... -Simple enough, -yes? -the FINAL detail is that the voltage control doesn't ever go ALL THE WAY down to zero. -Right next to the time constant (attack/release) controls is one more adjustable resistance labeled "Q bias", which is there to hold the FET control voltage SLIGHTLY away from zero volts, so that it is always VERY slightly reducing the signal, even at "zero GR". This is a calibration thing, and is there to make sure that there's no 'dead spot' before the gain reduction starts... a bit like making sure that the clutch in a car is always 'slightly engaged' to ensure that the car moves IMMEDIATELY that the brakes are released... -Sorry, it's the best analogy I can think of right now. -If the clutch weren't slightly engaged, there'd be a  'hesitation' after the brakes are released and before drive begins... just like if the FET wasn't slightly biased into conduction, there'd be an analogous 'hesitation' between the detector starting to conduct and the FET starting to reduce gain... the 'gap' in the clutch in this analogy is what I'm trying to use to explain the FET's 'pinch-off' voltage, which varies slightly device-to-device and batch-to-batch.

So, if that explanation of HOW it is meant to work hasn't baffled everyone, in the next section I'll try to show what the effect of pressing multiple switches in might be, and why you shouldn't always put too much trust in gain reduction meters!

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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ssltech

Aqua Marine

Posts: 4,044 Member Since:22/01/2011

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Feb 1 11 4:47 PM

Right...

Part Three:
(in which a dramatic confrontation is reached, and our hero overcomes all opposition to win the day!)

So... having outlined what happens to the first 'ladder' when we subject it to the abuse of "having its buttons pushed" in a manner which God never intended, let's consider what happens when the second ladder is subjected to the same treatment.

So this time, the ladder is designed to determine what 'tap' voltage is fed to 'tug' the diodes out of conduction far enough to resist waveform peaks for a greater or lesser range of volts.

The 'ladder' is comprised of five resistors: R45 is connected to a zener-regulated negative 10Volts DC rail. the path trickles down through R45, R44, R43, R42 and R87, where -instead of connectong to zero Volts ground, it connects to "almost ground"... which is in fact the wiper of a 2k ohm trim pot in parallel with a 3.9k resistor (for a parallel resistance of about 1.5k), fed from negative ten volts via R82, a 10k resistor. -This gives the trim pot wiper an potential range from zero volts (if the wiper is turned all the way to the 'ground' and of the trim pot) to almost two volts. -This voltage (usually somewhere just below a volt, but depending on the pinch-off voltage characteristic of the signal FET) is resistively derived and NOT actively regulated... take note.

So, when all buttons are out, or no more than ONE button is pushed in, none of the resistances in the 'ladder' are bypassed, and the "tugging' effect that the ladder has toward the negative 10V rail is equal to the total resistance... which is just shy of 5k ohms.

However, if we start 'bypassing' any of the resistors in that ladder, the resistance of the total chain is reduced. -In fact, if we push all 4 buttons in, we bypass R42, R43 and R44 completely, and the bias point is now 'tugged' towards the negative 10V rail with a resistance of less than 2.5k. This 'pulls' the bias point low, and -to return to our 'clutch-and-brake' analogy- changes how much 'clutch drag' happens, by altering the voltage which is present on the wiper of the bias pot.

Of course, it's also changed the proportional division of the threshold 'ladder', and had a corresponding effect on the threshold bias voltage applied to bias the diodes.

Now... one final thing. -The gain reduction metering.

Remember when I mentioned that -in a properly calibrated unit- you can't cause the signal level to EVER increase (other than overcoming the slight -usually 0.5dB- on-bias of the FET)? -well, that's perfectly true. -but we haven't considered the SECOND FET in the 1176.  -Every version since the blue-stripes has two FETs; one in the signal path, one NOT in the signal path.

The SECOND of the two FETs is used in the gain-reduction metering circuit. -The job of the GR meter is to show how much GR is being applied at any given instant. It does this by making a bridge -like a wheatstone bridge- with the second FET as one 'limb' of the bridge. Changes in resistance of this FET cause the output voltage at the end of the bridge circuit to rise and fall... falling as the GR voltage tells the FET to conduct, and rising again as the GR voltage diminishes.

In the Gain reduction DISPLAY circuit, it IS possible for the bias voltage to shift and make the needle read ABOVE zero VU... even though the signal path FET (which is used in a purely attenuation-only 'shunt' configuration) couldn't actually do this.

As a result, when the modified resistance chain tugs at the voltage which forms the bias point at the base of the time constant resistor/capacitor network, it can tug the control voltage's 'quiescent' bias position away from where it is set in calibration. -It also pulls the gain reduction display away from correctly tracking what's actually happening in terms of relative gain change.

The net result of all this is that -over the years- I've heard some outrageously inaccurate descriptions of what's going on. "It makes the signal like 3dB louder" is one. -Not only wrong, but easily demonstrated to be wrong. -On any 1176, you can test the tracking between GR metering and actual gain reduction by using a sine wave. -Set the unit so that it's indicating about 3dB of gain reduction, and then switch "GR Off" on the attack knob. -Now, switch output metering to "output +4" and adjust the OUTPUT control until the needle points to zero VU. -Now, switchign the GR on again, you'll see exactly how much the signal is REALLY being reduced. -If it matches your gain reduction display... excellent. -Repeat the test at  5dB... 7dB... 10dB... and so on. -Several (but not all) revisions had a 'tracking' adjustment. Later revisions did... earlier ones didn't.

Now for the "all buttons in' GR metering test- With the ratio set to 4:1, wind the output level full on, and feed a very quiet sine wave to the input. Switch the meter to read 'output +4dB'  Creep the input level up until the unit reads zero VU. the unit should not be compressing. -Now, if you switch between GR and output, the needle should indicate the same poisition. -If you've trimmed your meter tracking, you'll know that your needle should at least give you a reasonable (if not perfect) representation of what's happening. -So, if the "it makes it louder" claim was true, the "Output +4dB" metering would also increase along with the GR indication moving to the right when you press the buttons in...

Try it. -It doesn't. -It shouldn't be able to increase ANY more than a half-dB, if your unit is calibrated correctly.

However... The 'needle in the red' which has almost become the 'public face' of the unit when it's operated in this condition somehow speaks to us in an instinctive level, and everyone feels just a little bit 'naughty' when they do this. -We KNOW it wasn't "meant" to be used this way. We KNOW that a VU needle isn't supposed to sit in the red all the time, and yet here we are, breaking all sorts of rules... -It make us FEEL like we're somehow 'cheating' and getting away with it!

So...

The ratio is modified, the 'neat' onset of compression is messed up (by upsetting the quiescent bias of the FET control voltage) and the gain reduction metering is forced to lie to us, in a way which makes us FEEL better.

At last out hero's true nature is revealed. It appears that he has feet of clay, but we don't love him any less... Speaking for myself, at least!


-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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ssltech

Aqua Marine

Posts: 4,044 Member Since:22/01/2011

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Feb 3 11 8:14 AM


Keith, you are on quite a roll!

-doc


Aw shucks, Doc!


-Welcome to the forum, by the way.


I am dumbfounded! Keith, that was fantastic! Brilliant!
I have the notion you could go on about most anything this way.

-tidewater

I'm flattered... Being in a place where I spend a lot of the time being asked questions by non-technical people tends to force me to TRY and come up with analogies, comparisons or explanations which can help people have 'lightbulb moments'.  Sometimes, by looking at things which I half understand and being forced to try and invent a 'mental model' to explain it to others, I end up seeing more of the picture, and understand it better myself!Anyhow... Now that we've been through the idea of changing the 'DC threshold' (the 'comparison point' where the diodes start conducting) and also changing the 'signal sensitivity' to affect an overall hange of compression 'enthusiasm', if anyone feels inclined, they can look at the Fairchild 660/670 schematic, and see that there are TWO ways to adjust the 'compression onset'; one for "DC Threshold" which moves the comparison point, (just like in the 1176) and the second labeled 'AC Threshold', which increases the signal (since the signal is -by definition- Alternating Current) -again, much like the 1176.

Whereas the 1176 has these two functions inter-linked by being altered simultaneously on the same set of push-buttons, the Fairchild has the 'AC threshold' as a front panel knob, and the 'DC threshold' as a trim pot. -Having a look at the operation instructions for the Fairchild, you can see that adjusting the 'DC threshold' changes the effective ratio of the unit... while forcing the 'AC threshold' front panel knob to be 'turned up' to make the signal 'rise' to meet the raised comparison threshold.

Link: Fairchild 670 schematic

Looking at the first channel, The 'AC threshold' (the attenuator feeding the 'Gain Reduction Control Amplifier') is R115, [between the third and fourth transformers reading left to right] which corresponds to the resistor ladder on switch poles 'B'  in the 1176, and the 'DC threshold' (which biases the control voltage to a range where the unit has to 'push' to a higher signal level in order to begin gain reduction) is set by R142, [over on the right].

Larry is a man who has made FAR more of a study of the Fairchild operation than I have, so I'll defer to him for any corrections to any assertions I've made here!

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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kenfavata

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Posts: 425 Member Since:22/01/2011

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Feb 3 11 9:06 PM

I'm skeptical.
I mean, has anyone actually seen an electron?

...Tx for this.

An interesting point to note is that all this understanding should apply to certain popular SW emulations - since they mimic the HW units behavior. 

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LeforaGuest

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Feb 3 11 11:07 PM

My God! You are right.. Keith could be lying. He has way more description than there is stuff inside an 1176. Frankly, I don't see how all that could be going on in there. Now I wonder what extra-governmental agencies are involved. Do they represent the reptilian alien overlords, or the greys. He could even be one.. he exhibits technical knowledge beyond my comprehension... at least the ability to effect ampitude across a time domain! Amplitude travel!

That freaks me out. I wonder if he can fix a flying saucer? He hints at it in his signature.

(midi theramin)

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larrchild

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Feb 3 11 11:35 PM


Larry is a man who has made FAR more of a study of the Fairchild operation than I have, so I'll defer to him for any corrections to any assertions I've made here!


None to find.
You're on wry, too.

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ssltech

Aqua Marine

Posts: 4,044 Member Since:22/01/2011

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Feb 4 11 7:15 AM


You're on wry, too.

-larrchild


It's hard to tune a salad on wry.

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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larrchild

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Feb 4 11 7:19 AM

It appears that Keith has studied this 1176 thing.
Keith, what's the time?
Keith: " Well, first you need some springs and a reciprocating pendulum.. "


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ssltech

Aqua Marine

Posts: 4,044 Member Since:22/01/2011

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Feb 4 11 8:10 AM

Keith, what's the time?
Keith: " Well, first you need some springs and a reciprocating pendulum.. "

-larrchild

Bloody hell, Larry... -do you have some sort of tele-monitoring thing on me???

-Only this morning, someone caught sight of my watch and said 'Crap... is THAT the time?' -to which I answered 'No, -time is an abstract concept... that is a wristwatch!'


-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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