Looking for advice for a new PC....

[bsquared]

Originally Posted by ShiningArcanine

Computer processors are DC circuits.

They most certainly are not!

Transistors in a CPU must talk to each other, and they do so over tiny wires laid out in the silicon. The bits transmitted back and forth are approximations of square waves, limited by the bandwidth of the transistor. These are AC signals.

Originally Posted by ShiningArcanine

transistors "cannot switch AC:"

I don't know what you're talking about here. If you stick an AC signal into a transistor's gate (or base, if you're thinking of BJT's for some reason), you'll get a square wave out if the AC signal swings through the appropriate threshold voltage.

Originally Posted by ShiningArcanine

Also, "All of the electrons in a computer travels at a fixed rate, roughly 0.9c" should have been "All of the electrons in a computer travel at a fixed rate, roughly 0.9c." I apologize for my misuse of the English language.

Again, when you transmit a bunch of bits from A to B, the electrons in the transmission medium barely move. The field's propagate. In motherboards, the speed is closer to 1/2c, unless they are being designed with fairly exotic dielectric materials (read: expensive).

[Uncwilly]

The holes move. Actual movement of indvidual electrons is less important than the speed of the holes.

http://en.wikipedia.org/wiki/Electron_hole

[davieddy]

Originally Posted by ewmayer

...simply do a circular motion as the wave passes and wind up where they started. [This is also what happens for AC power, which is why AC rather than DC is used for large-scale power distribution: Tesla understood this, Edison failed to].

Don't transformers enter the explanation of why AC is used in a
crucial way? Or is your explanation related to this in a way I have
yet to fathom? Power dissipated in the cables is I²R whether it is
AC or DC.

[ShiningArcanine]

Originally Posted by bsquared

They most certainly are not!

Transistors in a CPU must talk to each other, and they do so over tiny wires laid out in the silicon. The bits transmitted back and forth are approximations of square waves, limited by the bandwidth of the transistor. These are AC signals.

I thought that they used voltage thresholds where anything below a value k would be 0 and anything equal to or greater than that value would be 1 and that motherboards have DC-DC circuitry to step down the voltage of the current flowing into a processor to the voltage necessary for the processor to operate. I thought all of this meant that a DC current was running through the processor.

Originally Posted by bsquared

I don't know what you're talking about here. If you stick an AC signal into a transistor's gate (or base, if you're thinking of BJT's for some reason), you'll get a square wave out if the AC signal swings through the appropriate threshold voltage.

If you use an AC signal, can you get the transistor to operate as it is does with a DC signal?

Originally Posted by bsquared

Again, when you transmit a bunch of bits from A to B, the electrons in the transmission medium barely move. The field's propagate. In motherboards, the speed is closer to 1/2c, unless they are being designed with fairly exotic dielectric materials (read: expensive).

I was probably thinking of beta radiation when I said 0.9c. I did not think there was a difference between the speed of beta radiation and the speed of electrons in a wire.

[bsquared]

Originally Posted by ShiningArcanine

I thought that they used voltage thresholds where anything below a value k would be 0 and anything equal to or greater than that value would be 1 and that motherboards have DC-DC circuitry to step down the voltage of the current flowing into a processor to the voltage necessary for the processor to operate. I thought all of this meant that a DC current was running through the processor.

http://en.wikipedia.org/wiki/CMOS
Transistors have threshold voltages, yes. As referenced above, CMOS is the processing technology used in microprocessors. Roughly, a bunch of transistors are connected to positive and negative rails (Vdd and Vss), and as the gate voltages are swung through the transistion voltage, the output swings from one rail to the other. The rails are DC voltages. Each state, 0 or 1, is a DC voltage. But the transistion between states creates an AC signal. There is a transient current pulse with every bit transistion which makes for a very non-DC current profile. This is why your motherboard has hundreds of tiny surface mount capacitors surrounding the CPU. They are there to help alleviate the enormous transient current demand of the CPU due to millions of transistors changing state nearly simultaneously.

Originally Posted by ShiningArcanine

If you use an AC signal, can you get the transistor to operate as it is does with a DC signal?

Sure, there is nothing different. A DC signal is just a slow AC one ;) Seriously though, if you are doing an experiment with a transistor where you dial up the gate voltage with a lab supply, for instance, and watch for a change in output current; that is also an AC signal. Just a very slowly varying one. As the signals get faster and faster, you just need better and better transistors to be able to see the transitions.

Originally Posted by ShiningArcanine

I was probably thinking of beta radiation when I said 0.9c. I did not think there was a difference between the speed of beta radiation and the speed of electrons in a wire.

The speed of the *field* in the wire (not the electrons) is equal to 1/sqrt(epsilon_r * mu_r), where epsilion_r and mu_r are the relative permittivity and permeability of the dielectric material surrounding the transmission line. For basic motherboards, this is usually FR-4 or some variant, which almost always has epsilion_r of about 4. Thus the speed is about 1/sqrt(4) = 1/2 slower than light in a vacuum.

[bsquared]

Originally Posted by davieddy

Don't transformers enter the explanation of why AC is used in a
crucial way? Or is your explanation related to this in a way I have
yet to fathom? Power dissipated in the cables is I²R whether it is
AC or DC.

http://en.wikipedia.org/wiki/Power_distribution

Power is also V²/R.

Transformers are there to increase the voltage at the expense of the current. The net power stays the same, but you don't want large amounts of current sloshing around in the wires because large currents create large magnetic fields and also tend to melt copper. The use of AC is what fundamentally allows the use of transformers.

[davieddy]

Originally Posted by bsquared

http://en.wikipedia.org/wiki/Power_distribution

Power is also V²/R.

Power dissipated in the cables is only V²/R
if you interpret "V" as the potential drop along the cable
V=I*(resistance of cable).

[petrw1]

Originally Posted by IronBits

Wait until spring, or when you are ready to buy and re-ask the question as many things will change between now and then. ;)

Anything big and new and relevant to mention since 4 months ago?

[Batalov]

Originally Posted by axn1

Processor of choice: http://www.tigerdirect.ca/applicatio...=CP2-DUO-Q6600

Q6700 was a ridiculous follow-up to Q6600, but Q9300 looks sweet (value for money). Buy it bare OEM and get a Tuniq Tower cooler or some other nice cooler -
http://www.newegg.com/Product/Produc...82E16819115040
http://www.newegg.com/Product/Produc...82E16835154001
...and get it to run at 3.2-3.6GHz.

I disagree that C2Q's "perform not much better than C2D". They scale decently to 4 threads. {{Rant}} I wish I could say the same about my work comp which is an dual-quad-Xeon: that one scales horribly. Still, I run it on 7 threads (7 different LLs! not 7 threads on 1 LL - that's a complete waste) which is marginally better than 6 or 5. Worse than 8, though. {{/rant}}

At home I run a Q6600 with Corsairs PC2-8500s and all what people said above is true -- you cannot afford to go lower than 8500 memory these days. But I have a complicated relationship with Corsairs - they keep switching numbers and voltages (while selling it as the same part); I bought 2Gb more and they don't quite match -same part number, different voltages! (I need to play some more with BIOS). Maybe you could do better with a G.SKILL or Kingston.

Well, my 2 cents.

[James Heinrich]

Originally Posted by petrw1

Anything big and new and relevant to mention since 4 months ago?

I'd recommend an Intel 9000-series (45nm) quadcore for the CPU (e.g. Q9300 or Q9450). The Q9450 is likely worth the extra ~$60, partly for the extra 166MHz clock speed, but mostly for the doubling of cache from 6MB to 12MB.

As for RAM, you want it as fast as you can afford, at least DDR2-1066. Some nice fast DDR3 would be nice, but it's still pretty expensive compared to DDR2, although prices have fallen significantly in the last few months (4GB of DDR2-1066 is still a fair bit cheaper than 2GB of DDR3-1600).

You can hedge your bets with a board that supports both DDR2 and DDR3 (e.g. Gigabyte GA-EP35C-DS3R) to use DDR2 now with the option to upgrade to DDR3 when prices drop.

[Housemouse]

I am looking at a Dell XPS720.
2Q6600 Quad-Core
8MB L2 cache
2.4GHHz
1066FSB
3GB Dual channel DDR2 SDRAM at 800MHz-4 DIMMs

If I am testing in the 42,000,000 range on all four cores, can I expect to complete 4 numbers per month?