Re: Write Signal on Disk II Question

["Bryan Parkoff" <nospam@nospam.com>]

>>> Yesterday evening I managed to capture first data.
>>> Since the stepper is was not active, I connected my Oscilloscope to
>>> the data output an moved the head by hand until I found a good signal.
>>> This is what I got (I shortened it)
>>> (Garbage) FF FF FF FF D5 AA 96 FF FE AA AA AA AB AB AB DE AA EB FF FF
>>> FF FF FF FF FF FF FF D5 AA AD 96 96 96 (many of them) B4 B4 B4 D2 D2
>>> D2 (many of them)
>
> It's almost impossible to get the sampling rate exactly in sync with the 
> disk bytes that will always have some mechanical jitter. There are two 
> approaches possible:
>
> 1. Brute force. Sample at a much higher rate, say 2 MHz and let the 
> software figure it out.

    I have no idea how narrow or wide between clock bit on depending of 
rotation speed.

For Example:   "_" is zero bit.  "1" is 1 bit.

___1____1________1____1____  280 RPM
_____1______1__________1______1______  300 RPM
_______1________1____________1________1________  320 RPM

    What happen when Disk II Controller attempts to detect read pulse at 2 
MHz.  It detects wide zero bit and it thinks to be zero bit before zero bit 
is shifted into data register.  If you adjust to slow rotation speed, it may 
detects little less wide zero bit so it is able to detect one bit before it 
is shifted into data register.
    Is my concept correct?  It looks like your sampling bit stream may be 
slow to detect correct bit.

> 2. Synchronize to the incoming data. That's what the EDD card is doing. 
> Sort of a digital PLL that is being reset at the data pulse.
>
> Talking about the EDD card. It's quite rare but is very easy to re-create. 
> Either by just copying the design with 8 TTL chips or using a single CPLD 
> to duplicate the functionality. The question is, does anyone still need 
> it? Does the software help capturing protected disks to images? 
> Duplicating protected floppies is probably not in high demand these days.

    It is only a question that I want to know if spiral scheme like Where in 
the World is Carmen Sandiego can be copied using EDD Plus card.  I prefer to 
capture all original bit stream from disk into binary image.  I should not 
make mistake to buy and find this out that it cannot be copied.  I can be 
disappointed.

Bryan Parkoff