[sdiy] just when you think DIY spirit is fading out

[Gordonjcp]

On Wed, Oct 20, 2021 at 10:19:33AM +0200, cheater cheater via Synth-diy wrote:
> I've been thinking for some time about the question of what Commodore
> could have released when the C128 came out but if it didn't suck, and
> if they used all the possible crazy tricks we've learned since then,
> just for a multimedia machine. For example, a VHS interface would have
> been pretty cool: a sort of "datassette" but based on VHS. Could carry
> data, audio, and video. VHS AV could be comped onto the graphics

Aha, but that's been done, kind of.  The Domesday Project did that with BBC Master computers and LV-ROM players.  I have one of the LV disks.  It's got chunks of data read over the SCSI interface into the computer and then it stored images from around the country as single frames of video.  Because it used Constant Angular Velocity, it could just spin the disk and hold the laser in place to play out the same frame over and over.  You can clearly see the line sync pulses as "spokes" with a pair of "fat spokes" where the field sync pulses are.

https://i.redd.it/x52gjjn7dv201.jpg <- you can see them here, along with a few scratches :-/

> output in order to make interesting graphics. No need for a genlock -
> if the vhs datassette uses the same clock as the computer. Make the

You actually would still use a genlock because you'd be reliant on timing from the sync pulses off tape.  You'd be toiling to get the servos to track an incoming clock well enough to lock to an external device.  VHS players spin the head at 1500rpm (or 1800rpm in the US, I guess) and write one complete frame - two fields - per revolution.  You've got two heads so just as the first field has finished the heads switch over and the second field plays out.  That's a fair chunk of aluminium spinning at a fair speed, and it won't play nice if you want to force it to change speed.  The frame sync pulses come off a linear track like the (lo-fi) audio track, and its timing is controlled by how quickly the capstan hauls tape through - and that's got a bloody great big flywheel too.

The clever part in the Amiga was that it was relatively easy to strip off sync pulses from incoming video and time the video chipset to them.  Off tape directly it was always a bit wibbly-wobbly but cameras tended to generate solid sync.  The term "genlock" of course comes from "GENerator LOCKed" and applies to doing stuff like having a master sync pulse genny that you lock all your cameras to.  Then mixing video sources becomes a considerably easier task because you can guarantee that all the lines and fields will be bang on exact the same across all devices.

If you really wanted a nice solid signal with stuff genlocked onto it you'd use a sync pulse genny feeding both your "genlock" on the Amiga, and a thing called a Timebase Corrector which sounds like it ought to be mounted in an old UK police box but was in fact a spectactularly heavy rack of equipment full of RAM chips and AD/DA converters.  This would grab a frame of video at whatever speed the tape was playing it out, and then play back a cleaned-up digital copy locked precisely to your incoming sync.  If you wanted to mix video off tape then you could do this and just as with all your cameras it would pull it all magically into line.

We've got a lovely old digital vision mixer that work that hasn't been used in years that is basically an 8U rack with about 400 BNCs on the back for various sources and formats, two TBC/framegrabbers, and a bunch of analogue switches.  Bring up an input and it'll route it to the appropriate TBC card, and then you can mix from one to the other.  It even does star wipes.  Magic.

> architecture less crappy (there's a lot of stuff in a C64 and C128
> that makes them unintentionally slow); the Motorola 68k was already
> available for a while and it wasn't very expensive; a good video chip.
> This sounds like an Amiga, sort of, but I think the C64 philosophy
> would have been very interesting to see executed in this form.

Use a 65C816 instead in your C128, and give it high-res graphics with 12-bit colour, and supplement the SID with a DOC chip...

... oh wait, that's pretty much an Apple IIgs, isn't it...?

-- 
Gordonjcp