Date   : Thu, 08 Jan 2015 08:32:57 +0000
From   : mfirth@... (Michael Firth)
Subject: Fwd: Compromises

> 
> 
Resending to list from the address I'm subscribed from!
> 
> 
>>> On 8 Jan 2015,@00:56, Daniel Beardsmore <public@...> wrote:
>>> 
>>>> On 2015-01-08 00:33, J.G.Harston wrote:
>>>> Daniel Beardsmore wrote:
>>>> Level 1) MOS: four bits per pixel, for a total of sixteen
>>>> distinguishable colours, with no interest in what they look like
>>>> Level 2) Hardware: the DAC generates RGB colour data
>>> 
>>> What DAC? The video system outputs digital colour levels, not analogue. 
>>> Three colour bits, each colour component either on or off, nothing in 
>>> between.
>> 
>> I don't have the will to drag out the details from you one little tiny
snark at a time. I concede defeat to your condescension.
>> 
> It would probably be worth reading up on how the BBC Micro video design
actually works before making sweeping statements about how you think it should
work.
> 
> If you have a look at:
> 
> http://bbc.nvg.org/doc/A%20Hardware%20Guide%20for%20the%20BBC%20Microcomputer/bbc_hw_03.htm#3.6
> 
> and, more particularly the next section:
> 
> http://bbc.nvg.org/doc/A%20Hardware%20Guide%20for%20the%20BBC%20Microcomputer/bbc_hw_03.htm#3.7
> 
> You'll see there is indeed no DAC stage in the BBC, hence "digital TTL
RGB". The digital signals output from the ULA become the colours that are
output with no extra stage (indeed, they are connected to the RGB socket
virtually directly)
> 
> You are correct that it would have been possible to design this differently,
to have analogue RGB levels, but presumably the circuit was designed this
way for a reason - cost and simplicity being two likely ones.
> 
> The ULAs of the time also had limited capacity, so presumably the palette
lookup (the VDU19 system that allows colour remapping) and flashing were
the best that could be fitted in to the ULA they chose to use.
> It is also worth remembering that Acorn had problems with supply and overheating
on the early Video ULA versions, suggesting they were pushing the chip close
to it's limit already.
> 
> As a related issue, a lot of the RGB monitors that the BBC were used with 
(in particular Microvitec CUB monitors) assumed a digital level on the R, 
G and B signals, so weren't capable of displaying other shades of colour 
(later models of CUB did have internal jumpers that would allow switching
to analogue levels) Presumably this was to give a consistent display across
all the component tolerances in the monitor's and BBC's hardware.
> 
> Regards
> 
> Michael
> 
> 
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