4 ms·

The first few lines mentioned "acoustic delay lines" which piqued my interest. Wikipedia has a page on this old technology: http://en.wikipedia.org/wiki/Delay_l

The first few lines mentioned "acoustic delay lines" which piqued my interest. Wikipedia has a page on this old technology: http://en.wikipedia.org/wiki/Delay_line_memory#Acoustic_delay_lines http://en.wikipedia.org/wiki/Delay_line_memory#Acoustic_dela... It was a pretty amazing hack, before magnetic memory cores. Because sound moved at a slow rate through a medium like mercury, an acoustic wave (that is, a sound) could be applied to one side of a volume of mercury and be expected to arrive at the other end after a predictable, useful delay. So what would be done is that a column of mercury with transducers on both ends would function as speakers and microphones, which in an acoustic medium are the equivalent of read and write heads! The system memory would be a collection of these columns, each I guess storing one bit. The memory would of course have to be refreshed: when the signal arrived at the other end, it would be fed back into the column, assuming I suppose that there wasn't a new signal waiting to be written to that bit instead. The article mentions that this was not randomly accessible memory, but rather serially accessible. From that and other bits of information, I gather that the device would visit each bit in sequence, according to some clock, and produce a signal on the read line corresponding to the value in that bit. You had to wait for the memory device to read out the particular bit you were waiting for. Does anyone know if this a correct understanding of how this kind of storage worked? What a cool way to store bits!

fuzzix 14y ago
An aside, I believe the early computer witnessed by Lawrence Waterhouse in Cryptonomicon used this mercury column system, it was described as visibly moving up and down inside the columns. I hadn't heard of anything like it at the time, it was an intriguing idea - I wasn't even sure Stephenson hadn't made the whole thing up.
- phireal 14y ago
  I think the version in Cryptonomicon was based on actual sound waves (in tubes filled with air) rather than in mercury as the book mentions to noise coming from his laboratory.
geon 14y ago
> Because sound moved at a slow rate through a medium like mercury Actually, it moves pretty fast through mercury; 1450 m/s. > each I guess storing one bit. (...) Does anyone know if this a correct understanding of how this kind of storage worked? From the Wikipedia article: > Typically many pulses would be "in flight" through the delay, and the computer would count the pulses by comparing to a master clock to find the particular bit it was looking for. > EDSAC, designed to be the first stored-program digital computer, began operation with 512 35-bit words of memory, stored in 32 delay lines holding 576 bits each > The average access time was about 222 microseconds Hmm... That would be a memory loop time of 444 us. At a speed of 1450 m/s, you need a delay line at least 64.38 cm long. And you'd need an impressive 1.3 Mb/s data transmission rate. Another interesting fact: > Since the speed of sound changes with temperature (because of the change in density with temperature) the tubes were heated in large ovens to keep them at a precise temperature. Other systems instead adjusted the computer clock rate according to the ambient temperature to achieve the same effect.
- coroxout 14y ago
  For an idea of scale, this page includes a photo of Maurice Wilkes next to the mercury delay lines of EDSAC: http://amturing.acm.org/info/wilkes_1001395.cfm http://amturing.acm.org/info/wilkes_1001395.cfm The tubes are 5 feet long (the whole tank is about 6' including the transducers at each end). I think that tank holds 16 tubes, so I'm guessing from the figures in geon's post that there must have been another tank too.
dcminter 14y ago
The Hodges biography of Turing has lots of in-passing mention of fascinating technology like this. I think my favourite was the use of a CRT as a memory array (by picking up the charge on the fluorescent screen and feeding it back to the electron gun to refresh it!) which suggested to Turing the idea of using light to stimulate the feedback cycle and thus writing directly to memory with a very real "light pen"! I'm probably borking up the details there but my point that the biog is great stands. I guess there are readers of HN who never encountered the later "light pens." A photodiode picks up the raster on a CRT based monitor and with appropriate timing logic uses this to decide where to draw pixels. I had a cheap light pen on an 80s microcomputer before I ever got my hands on a mouse. Ah, happy (but often frustrating) days...
- ZoFreX 14y ago
  > A photodiode picks up the raster on a CRT based monitor and with appropriate timing logic uses this to decide where to draw pixels Incidentally this is how "light guns" worked, and also why they sadly do not work on LCD or plasma screens.
  - bonyt 14y ago
    Once I made a program that was like duck hunt, except you held a webcam at the screen. When you fired, the screen flashed black with a patch of white where the duck was, the camera took a snapshot and looked for how close to the center of the webcam's frame the white was to decide if you had hit it or not. It worked surprisingly well.
    - ZoFreX 14y ago
      Awesome! I think some light gun games used this method too :)

fuzzix 14y ago

An aside, I believe the early computer witnessed by Lawrence Waterhouse in Cryptonomicon used this mercury column system, it was described as visibly moving up and down inside the columns. I hadn't heard of anything like it at the time, it was an intriguing idea - I wasn't even sure Stephenson hadn't made the whole thing up.

phireal 14y ago
I think the version in Cryptonomicon was based on actual sound waves (in tubes filled with air) rather than in mercury as the book mentions to noise coming from his laboratory.

geon 14y ago

> Because sound moved at a slow rate through a medium like mercury Actually, it moves pretty fast through mercury; 1450 m/s. > each I guess storing one bit. (...) Does anyone know if this a correct understanding of how this kind of storage worked? From the Wikipedia article: > Typically many pulses would be "in flight" through the delay, and the computer would count the pulses by comparing to a master clock to find the particular bit it was looking for. > EDSAC, designed to be the first stored-program digital computer, began operation with 512 35-bit words of memory, stored in 32 delay lines holding 576 bits each > The average access time was about 222 microseconds Hmm... That would be a memory loop time of 444 us. At a speed of 1450 m/s, you need a delay line at least 64.38 cm long. And you'd need an impressive 1.3 Mb/s data transmission rate. Another interesting fact: > Since the speed of sound changes with temperature (because of the change in density with temperature) the tubes were heated in large ovens to keep them at a precise temperature. Other systems instead adjusted the computer clock rate according to the ambient temperature to achieve the same effect.

coroxout 14y ago
For an idea of scale, this page includes a photo of Maurice Wilkes next to the mercury delay lines of EDSAC: http://amturing.acm.org/info/wilkes_1001395.cfm http://amturing.acm.org/info/wilkes_1001395.cfm The tubes are 5 feet long (the whole tank is about 6' including the transducers at each end). I think that tank holds 16 tubes, so I'm guessing from the figures in geon's post that there must have been another tank too.

dcminter 14y ago

The Hodges biography of Turing has lots of in-passing mention of fascinating technology like this. I think my favourite was the use of a CRT as a memory array (by picking up the charge on the fluorescent screen and feeding it back to the electron gun to refresh it!) which suggested to Turing the idea of using light to stimulate the feedback cycle and thus writing directly to memory with a very real "light pen"! I'm probably borking up the details there but my point that the biog is great stands. I guess there are readers of HN who never encountered the later "light pens." A photodiode picks up the raster on a CRT based monitor and with appropriate timing logic uses this to decide where to draw pixels. I had a cheap light pen on an 80s microcomputer before I ever got my hands on a mouse. Ah, happy (but often frustrating) days...

ZoFreX 14y ago
> A photodiode picks up the raster on a CRT based monitor and with appropriate timing logic uses this to decide where to draw pixels Incidentally this is how "light guns" worked, and also why they sadly do not work on LCD or plasma screens.
- bonyt 14y ago
  Once I made a program that was like duck hunt, except you held a webcam at the screen. When you fired, the screen flashed black with a patch of white where the duck was, the camera took a snapshot and looked for how close to the center of the webcam's frame the white was to decide if you had hit it or not. It worked surprisingly well.
  - ZoFreX 14y ago
    Awesome! I think some light gun games used this method too :)