Thu Dec 11 21:56:19 CET 2008

demos for dorkbot

I'd like to do the synth + the TV.

For the TV I have 13.5 Mhz xtal + now there's the bitbanged monitor
freeing one of the serial ports for raw binary output.  I don't
remember which chip can do this though..

The serial port seems to be this one: in 18f2550 datasheet, section
20.3 synchronous master mode.

The 13.5 Mhz is standard video pixel clock.  Can probably use one of
the usb devices they have higher clock ratings.

So... A better TV out circuit.  What i tried before is to use the 75
ohm impedance as a voltage divider, to not have to use a 75 Ohm output
impedance.  This is a dirty trick yeah..

So, to do it properly, generating the 4 levels (0V : 0.33V : 0.66V :
1V) requires a buffer amp to isolate the load impedance from the
resistor divider.  This frees resistor values a bit, but requires a
rail-to-rail amp if fed from a single supply.  For demo purpose it's
probably best to stick with the two simple resistors.

Asked Bert: To do it properly, provide double levels + add a unit
buffer opamp with a 75 ohm resistor in series (this assumes line is
properly terminated), maybe also add an output transistor to deliver
the current.

Non-terminated: it's possible to get the line levels always correct if
the buffer has zero output impedance.