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The Mahler Project – The ZX Spectrum Orchestra [video]

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Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#11

Post by mattwestcott »

FinnDS wrote: post #3

This is great! Didn't know what the ZX Spectrum was, and this was a nice little video. One interesting thing is, to me at least - a musician, is that the Spectrums are not necessarily in tune; the first one playing the melody seems sharper than all the others. I wonder if the devs were aware of this, and tried fixing it?..

That's probably down to the 128K Spectrums having a slightly faster clock speed than the original 16/48K model (3.5469MHz versus 3.5 exactly). I knew this would cause the timings to drift (hence the need for the RasPi to provide a time source) but I overlooked the effect this would have on pitch...

(On the other hand - since the BEEP routine was being called from BASIC, adding in a pitch adjustment calculation may well have slowed things down enough to throw the timing off again :-) )

Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#13

Post by ghostberry »

Marazan wrote: post #10
Arnt wrote: post #4

Earlier quoted context omitted.

I doubt that's fixable. The Spectrum's crystal was chosen for low price, not for closely matching performance (even day-to-day on the same host), so matching would require finding the hosts' CPU frequencies at that moment and compensating for the differences. Possible using the Spectronet, hardly easy. The Spectrum was a great machine. The kind where the only way to get amazing results was amazing programming. There…

Another nice thing to say was that the CPU was clocked balzingly fast for the time period. Also it has a real actual genuine soul connected to a higher plane of existence that you communed with every time you used it.

Clock speed does not equal performance.

For instance, the Z80's fastest instructions took four cycles, where the 6502's fastest instructions took two cycles.

The Z80's slowest instructions took 23 cycles, and the 6502's slowest took seven cycles.

And even that comparison is over simplified.

Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#14

Post by to3m »

Marazan wrote: post #10
Arnt wrote: post #4

Earlier quoted context omitted.

I doubt that's fixable. The Spectrum's crystal was chosen for low price, not for closely matching performance (even day-to-day on the same host), so matching would require finding the hosts' CPU frequencies at that moment and compensating for the differences. Possible using the Spectronet, hardly easy. The Spectrum was a great machine. The kind where the only way to get amazing results was amazing programming. There…

Another nice thing to say was that the CPU was clocked balzingly fast for the time period. Also it has a real actual genuine soul connected to a higher plane of existence that you communed with every time you used it.

The Spectrum was underclocked - the Z80A can run at 4MHz. Like many machines of the period, the 3.5MHz comes from the display timings (or very much appears to). There's a good diagram here: http://www.zxdesign.info/vidparam.shtml. Summary: 2 pixels/cycle makes 224 cycles per line; (224 cycles/line) * (312.5 lines/PAL frame) * (50 PAL frames/sec) = 3,500,000 (cycles/sec).

(I don't know if there was a 7MHz clock that was divided by 2, or what, or if the thing could genuinely produce 2 pixels per cycle.)

4MHz was somewhat high compared to the 6502-based systems that made up some of its competition, but for many operations the Z80 doesn't run terribly efficiently. Opcode fetch takes 4 cycles and memory read/write takes 3 cycles, making the memory bandwidth of a 4MHz Z80 about that of a slighty-more-than-1MHz 6502. (The 6502 would read or write 1 byte every cycle.) This downplays the value of the Z80's register set and 16-bit operations, though; 6502 zero page is a bit of a poor substitute in many cases, because you lose a cycle from having to read the address byte.

(Compare INC $xx - 5 x 6502 cycles, 2 x opcode, 1 x read, 1 x dummy, 1 x write - with - with INC r - 4 x Z80 cycles, all opcode fetch and then it's done. On the other hand, ADC A,n takes the Z80 8 cycles, because it has to read 2 opcode bytes - something the 6502 will get through in 2.)

Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#15

Post by Marazan »

to3m wrote: post #14
Marazan wrote: post #10

Earlier quoted context omitted.

Another nice thing to say was that the CPU was clocked balzingly fast for the time period. Also it has a real actual genuine soul connected to a higher plane of existence that you communed with every time you used it.

The Spectrum was underclocked - the Z80A can run at 4MHz. Like many machines of the period, the 3.5MHz comes from the display timings (or very much appears to). There's a good diagram here: http://www.zxdesign.info/vidparam.shtml . Summary: 2 pixels/cycle makes 224 cycles per line; (224 cycles/line) * (312.5 lines/PAL frame) * (50 PAL frames/sec) = 3,500,000 (cycles/sec). (I don't know if there was a 7MHz clock that…

True, but you've glossed over the soul that 6502-based computers didn't possess.

Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#17

Post by FreeFull »

Marazan wrote: post #15
to3m wrote: post #14

Earlier quoted context omitted.

The Spectrum was underclocked - the Z80A can run at 4MHz. Like many machines of the period, the 3.5MHz comes from the display timings (or very much appears to). There's a good diagram here: http://www.zxdesign.info/vidparam.shtml . Summary: 2 pixels/cycle makes 224 cycles per line; (224 cycles/line) * (312.5 lines/PAL frame) * (50 PAL frames/sec) = 3,500,000 (cycles/sec). (I don't know if there was a 7MHz clock that…

True, but you've glossed over the soul that 6502-based computers didn't possess.

The C64 does have a soul in its SID chip.

Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#18

Post by to3m »

Marazan wrote: post #15
to3m wrote: post #14

Earlier quoted context omitted.

The Spectrum was underclocked - the Z80A can run at 4MHz. Like many machines of the period, the 3.5MHz comes from the display timings (or very much appears to). There's a good diagram here: http://www.zxdesign.info/vidparam.shtml . Summary: 2 pixels/cycle makes 224 cycles per line; (224 cycles/line) * (312.5 lines/PAL frame) * (50 PAL frames/sec) = 3,500,000 (cycles/sec). (I don't know if there was a 7MHz clock that…

True, but you've glossed over the soul that 6502-based computers didn't possess.

I suppose I'm just by nature not a very spiritual person.

I've spent a lot more time using the 6502, but I like both. They are represent two very different styles. The Z80 is a lot more practical in many respects, with its 16-bit registers and relocatable stack pointer. Some 6502-style register-indexed indirect addressing modes wouldn't have gone amiss, though...

Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#19

Post by tragomaskhalos »

ptaipale wrote: post #8
kranner wrote: post #6

Earlier quoted context omitted.

> the only nice thing I can say about the Spectrum... A restart was always a button-press away if you messed something up. (At least the Spectrum+ had a hard reset button on the side). Not to forget that the OS and BASIC environment were in ROM and took a fraction of a second to boot.

In the original Spectrum, there was no reset button. Nor was there a power switch. So if your machine code got stuck, you'd remove the power plug from the jack and put it back in, to get that power-on self test black screen and then a BASIC prompt. And, eventually, the jack would wear out for those of us who wound up with more bugs than was good for the power jack. Some people actually had to replace the power jack b…

The Z80 has a non-maskable interrupt that jumps to a fixed location in the ROM, so anyone handy with a soldering iron could have wired up a little push-button to the NMI pin to trigger it ... in the Specrum ROM this was supposed to then read an address from a fixed area in the system variables area of RAM and jump to it, except that due to a bug you could only jump to 0, ie a full reset. This always seemed a terrible shame.

Re: The Mahler Project – The ZX Spectrum Orchestra [video]

#20

Post by ptaipale »

tragomaskhalos wrote: post #19
ptaipale wrote: post #8

Earlier quoted context omitted.

In the original Spectrum, there was no reset button. Nor was there a power switch. So if your machine code got stuck, you'd remove the power plug from the jack and put it back in, to get that power-on self test black screen and then a BASIC prompt. And, eventually, the jack would wear out for those of us who wound up with more bugs than was good for the power jack. Some people actually had to replace the power jack b…

The Z80 has a non-maskable interrupt that jumps to a fixed location in the ROM, so anyone handy with a soldering iron could have wired up a little push-button to the NMI pin to trigger it ... in the Specrum ROM this was supposed to then read an address from a fixed area in the system variables area of RAM and jump to it, except that due to a bug you could only jump to 0, ie a full reset. This always seemed a terrible…

Yes, I recall looking at that code in the ROM disassembly and trying to figure out that what the heck is this supposed to mean.
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