Re: Apple Graphics Tablet

[Michael J. Mahon <mjmahon@aol.com>]

Alex Freed <alex_news@alexfreed.com> wrote:
> On 6/19/2012 4:46 PM, Michael J. Mahon wrote:
>> 
>> I don't think that was the earliest design.
>> 
>> The RCA 630TS, the reference design for the first commercial set, used a
>> PLL. Of course, based on pulling a crystal oscillator, as were all color TV
>> sets.
> 
> May well be the case. I just clearly remember that there were "ringing"
> designs and was able to fine one reference on the 'net
> 
> http://www.hawestv.com/mtv_color/colrtel_burstBlock.htm
> 
> --------------------
> Crystal ringer. To generate the reference subcarrier, most TV receivers
> use a CW (continuous wave) oscillator. Not Col-R-Tel. Instead, the burst
> signal, multiplied at Col-R-Tel's burst amplifier, shock excites a 3.58 MHz crystal.
> 
> During installation, you tie the converter burst amplifier to the
> horizontal blanking signal. The orange wire makes this connection. When
> horizontal blanking occurs, it gates on the burst amplifier. The rest of
> the time, the burst amplifier remains off. (In the Col-R-Tel burst gate
> circuit, "off" actually means saturated.) When the burst amplifier gates
> off, the crystal keeps ringing. In fact, the crystal continues to ring
> over the entire next line period.
> ----------------------
> 
> That page describes a converter of a black and white TV to color, but the
> same principle was used in a number of inexpensive sets.
> 
>> (However, I still don't recall one without *two* PLLs--one for horizontal
>> sync (and burst gating) and another for color subcarrier recovery.)
> 
> Actually I saw designs where there was no PLL for horizontal sync either.
> Instead a free-running oscillator was tuned a bit lower than the proper
> frequency but was tripped early by the sync pulses. Like a pulse from a
> floppy resetting the Woz machine instead of using a PLL for a data
> separator as in early expensive disk controllers.
> 
> -Alex.

Wow--there goes the noise immunity!

It's bad enough that vertical sync worked like this, but its trigger had a
lot of integration, which made it more immune to pulse interference. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon