"Outline specification for the BBC MICROCOMPUTER system"

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby 1024MAK » Thu Oct 13, 2016 11:46 am

Although this is an interesting discussion, no one has considered that the cost of the teletext hardware was in part determined by the costs of including teletext in teletext equipped televisions.

You should also consider that a ROM based character generator could have been used in the BBC Micro in place of the teletext character generator chip. This could have also had a small 1k byte RAM footprint.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Thu Oct 13, 2016 11:50 am

As I recall, around 1987-88 a teletext television was priced about £25 extra compared to an ordinary one. But then, as a premium feature, it would have more than the usual profit margin on it.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Coeus » Thu Oct 13, 2016 1:07 pm

BigEd wrote:As I recall, around 1987-88 a teletext television was priced about £25 extra compared to an ordinary one. But then, as a premium feature, it would have more than the usual profit margin on it.


And not all of the cost would have gone on that character generator ROM either as it would be necessary to recover the digital data from the TV signal and provide some RAM to store it in. I think Phillips/Mullard made a suite of chips for teletext of which the SAA5050 was just one.

So with the prospect of providing higher quality text output than the available bit mapped resolution would permit in less memory with an almost certainly cheap chip that was already available in volume for the TV market and which had the added advantage of satisfying the BBCs desire for a teletext display why wouldn't they choose to implement it as they did?

As I mentioned in the comment about the the RM380Z, using a character generator ROM alongside bit-mapped graphics wasn't a new idea and it was also an idea that persisted for some time afterwards too. I remember some of HPs 16/32 bit machines having a text screen that overlayed the graphics where each could be modified independently. It took quite some time for the resolution of typical bit-mapped graphics to be able to produce reasonably good looking text.

In fact I would argue that the text of typical computer displays remains sub-standard though using a character generator ROM is no longer the solution. The quality of text display on modern smartphones and tablets knocks the quality on a typical PC into a cocked-hat.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Richard Russell » Thu Oct 13, 2016 1:23 pm

paulb wrote:No-one really complains about the default font on the Acorn machines.

That's a sweeping statement! I would suggest that an 8x8 font is only acceptable (if at all) once you've got used to it, and the target market for the BBC Micro was primarily people who had never been exposed to any kind of VDU other than teletext on their TV. Take a simple feature like the slash through the zero, to distinguish it from an O; in those days that was entirely alien to the man-in-the-street and it often had to be explained.

Make it 16K or 20K (or 32K or 40K) if you want a slightly nicer font.

And multiply by four again to get the equivalent resolution that character rounding provides, and you end up with the 160k figure I quoted.

I don't recall any of the competing machines offering the same level of Teletext support

Well of course none of the existing machines, including the Acorn Atom, did. Whoever won the contract would have had to engineer teletext compatibility into whatever machine they ended up delivering.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Thu Oct 13, 2016 1:28 pm

That expectation that the BBC machine should be able to mix character mode and bitmap mode supports the idea that bitmapped fonts (at "medium resolution") were not expected to be the normal fonts in use. At that time most computer users (not microcomputer users) would be familiar with using a text mode visual display unit which would offer readable text - usually in monochrome but also usually at 80 characters per line.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Richard Russell » Thu Oct 13, 2016 2:02 pm

BigEd wrote:That expectation that the BBC machine should be able to mix character mode and bitmap mode supports the idea that bitmapped fonts (at "medium resolution") were not expected to be the normal fonts in use. At that time most computer users (not microcomputer users) would be familiar with using a text mode visual display unit which would offer readable text - usually in monochrome but also usually at 80 characters per line.

Indeed. My first 'software based' Videotex emulation was on the VGA card which you could switch to an unusual 9-bits-per-character text mode (i.e. with each character selectable from a set of 512 different symbols that you could program). I worked out that every possible teletext character could be generated that way (96 alphanumeric characters, 96 upper-half D/H alphanumeric, 96 lower-half D/H alphanumeric, 64 contiguous graphics, 64 separated graphics, 16 upper-half D/H contiguous, 16 lower-half D/H contiguous, 16 upper-half D/H separated, 16 lower-half D/H separated), 480 different symbols in all. The remaining 32 symbols were used to contain control-code abbreviations which could be displayed for diagnostic purposes. I remember thinking what a fluke it was that this (only just) fitted into the 512 symbols available.

An approach similar to this could have been used to implement a teletext-compatible display without requiring either a dedicated chip like the SAA5050 or high-resolution bit-mapped graphics. In addition to the 9 bits per character to select the required symbol you would need 3 bits for the foreground colour, 3 bits for the background colour and a 'flashing' bit, making 16 bits per character altogether.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Thu Oct 13, 2016 2:39 pm

(The first machine I owned was a Compukit UK101 - character based output but ugly, low resolution, and monochrome. The worst of all worlds, but affordable, and that was crucial. No point making a machine which isn't affordable!)

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby 1024MAK » Thu Oct 13, 2016 3:05 pm

BigEd wrote:No point making a machine which isn't affordable!)

Depends on who is buying! What you say is correct for most consumer goods, but in other areas, the requirements may be different.

Getting back to the discussion, the BBC Micro would have been a poorer computer without it's teletext mode. What I find surprising is that it (and of course the Master range) and a few others were the only computers that used the SAA5050 chip. If you look at some other computers that used text mode displays, the complex circuitry needed many 74 series logic chips to do the same as the 6845 and SAA5050 combined could do. So where 80 column text was not needed, but a text display was, why did no one else use these two chips together?

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby davidb » Thu Oct 13, 2016 3:11 pm

Richard Russell wrote:An approach similar to this could have been used to implement a teletext-compatible display without requiring either a dedicated chip like the SAA5050 or high-resolution bit-mapped graphics. In addition to the 9 bits per character to select the required symbol you would need 3 bits for the foreground colour, 3 bits for the background colour and a 'flashing' bit, making 16 bits per character altogether.

Or perhaps have a high resolution monochrome mode in which each character cell can have its own foreground and background colours chosen independently from the standard palette, a bit like what the Spectrum has with its attribute system. I wonder if the Spectrum had a decent teletext display program.

I suppose that would have just added complexity to the otherwise uniform use of memory in the regular modes. Probably just easier to use an off-the-shelf component instead, even if it needed some shoehorning into the design - I seem to remember from one of the online interviews with ex-Acorn people that the SAA5050 needed a slightly different clock to the rest of the machine and this involved some stretching of the waveform here and there.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Coeus » Thu Oct 13, 2016 3:49 pm

1024MAK wrote:...If you look at some other computers that used text mode displays, the complex circuitry needed many 74 series logic chips to do the same as the 6845 and SAA5050 combined could do. So where 80 column text was not needed, but a text display was, why did no one else use these two chips together


Did those designs pre-date the availablility of the SAA5050?

Out of interest why the limit of 40 columns?

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby paulb » Thu Oct 13, 2016 3:51 pm

Richard Russell wrote:
paulb wrote:No-one really complains about the default font on the Acorn machines.

That's a sweeping statement! I would suggest that an 8x8 font is only acceptable (if at all) once you've got used to it, and the target market for the BBC Micro was primarily people who had never been exposed to any kind of VDU other than teletext on their TV. Take a simple feature like the slash through the zero, to distinguish it from an O; in those days that was entirely alien to the man-in-the-street and it often had to be explained.


Sure, 8x8 was very basic, although there will have been machines with even lower resolution bitmap fonts, but it turned out to be good enough for most people. I don't remember many reviews complaining about it.

Richard Russell wrote:
Make it 16K or 20K (or 32K or 40K) if you want a slightly nicer font.

And multiply by four again to get the equivalent resolution that character rounding provides, and you end up with the 160k figure I quoted.


True, but again, the Archimedes provided mode 7 with a bitmap mode approach that used only 40K. It didn't have the smooth fonts, which would have been well within the capabilities of the machine, and Acorn could have sidestepped the vertical resolution issue by using anti-aliasing. In other words, they could have used 640x256x4bpp (80K), 320x256x8bpp (80K) or even 640x256x8bpp (160K) and kept within the base monitor capabilities. That they didn't rather indicates that they probably didn't particularly care about the visual quality of Teletext support at that point.

Of course, one could argue that nobody bought an Archimedes to use mode 7, which as others have noted was very useful to be able to cram as much code and data into the original Beeb as possible. But maybe it says something that in 1987, by which time people's expectations of computers had increased since the introduction of the original Beeb, Acorn did the minimum to provide Teletext/Viewdata support. Back in 1980/1981, the general public would have probably been satisfied with such a "low fidelity" implementation, given that those who had encountered computers would have found some of them only supporting upper-case letters and others only offering monochrome displays.

Richard Russell wrote:
I don't recall any of the competing machines offering the same level of Teletext support

Well of course none of the existing machines, including the Acorn Atom, did. Whoever won the contract would have had to engineer teletext compatibility into whatever machine they ended up delivering.


And none of the subsequent competitors, either. One might have expected the C64 to be oblivious to Teletext, being an American machine, but the Amstrad didn't have dedicated Teletext hardware in the base machine. (It did have adapters, though.) Yes, the resulting output isn't very nice, but the adapters could have done what the Morley adapter did on the Electron and couple the capability of obtaining data with the capability of displaying it nicely. (And it appears that one of the Amstrad adapters allows a choice between the equivalents of Beeb mode 2 with all colours and of Beeb mode 1 with four colours.)

Anyway, I was just intrigued by the inclusion of Teletext, the motivations for doing so (whether the BBC genuinely expected users to widely adopt Teletext/Viewdata services), and the consequences for both the customers (choice of displays) and the general platform (what the software ended up using). In effect, I wonder what the omission of the Teletext hardware might have done to the range of Beeb software, whether people would have managed without it (as had to be done on the Electron) or whether they would have chosen to target other systems instead, inflaming the loudening criticism that Acorn's machines generally offered less memory than the competition: something which mode 7 only mitigated for certain kinds of applications.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Thu Oct 13, 2016 4:02 pm

As an indication of the cost of RAM, we see in the outline specification
"at least 16K RAM should be supplied at the outset."
and of course we know that the Beeb was launched with 16k, and with the 16k model expected to be the better seller.

A 1k Teletext mode makes even more sense in this context.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Richard Russell » Thu Oct 13, 2016 5:23 pm

paulb wrote:I don't remember many reviews complaining about it.

I'm guessing these "reviews" were in computing magazines, or at least by people already familiar with computers. The objective of the BBC Computer Literacy Project was to educate people who had never used, and probably never even seen, a computer previously. The requirements for this target audience were quite different, and I think the BBC can take the credit for approaching the specification from that perspective. Speculation about what might or might not have happened had the specification been different is not something I can contribute to.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Thu Oct 13, 2016 5:28 pm

There's a good writeup of the legacy of the computer literacy project by Tilly Blyth of the Science Museum:
The Computer Literacy Project (CLP) had the grand ambition to change the culture of computing in Britain’s homes. The innovative backing by a broadcaster – the BBC – working with an entrepreneurial company – Acorn – led to the creation of an open technology, supported by a range of materials (programmes, courses, publications and software) delivered across a multitude of channels. The BBC Micro was complemented by activities that increased demand for computing generally, by promoting cultural shifts in attitudes towards computing and delivering learning into homes and schools.

(The full doc is a 78-page PDF - you have been warned)
(I'm not sure what the open technology was, unless it was the spec of BBC Basic and the ABI of Acorn's MOS.)

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby jgharston » Thu Oct 13, 2016 6:01 pm

Skipping past a few messages, so I may have missed bits...

Acorn already had experience in using the SAA5050 in the Teletext VDU board for the System, so using a different character generator ROM would mean starting a design from scratch, and also going against the BBC specification. Also, Acorn already had experience in bitmapped graphics display with the bitmapped graphics board for the System. Both the teletext board and the graphics board used the 6845, so that provided prior experience as well. Putting the two systems together programatically selectable is (almost) simply a matter of three extra buffer ICs (plus finangling a 6MHz clock for it).

I also notice some confusion in posts between teletext display and teletext aquision, eg in discussion of "an add-on for the CPC". The costs of making a TV into a teletext TV is not comparable to making a computer have a hardware teletext display, the display is just the display. Fundamentally, just an SAA5050 and 1K of memory. A teletext TV is data seperation, aquisition, decoding, storage, processing...... and then display.

BigEd wrote:(I'm not sure what the open technology was, unless it was the spec of BBC Basic and the ABI of Acorn's MOS.)
Essentially, anybody with a soldering iron and knowledge of digital interfacing could build anything they wanted to and plug it in. Try doing that with an iShiny. I hear you can't even plug headphones into the nowadays.

Code: Select all

$ bbcbasic
PDP11 BBC BASIC IV Version 0.25
(C) Copyright J.G.Harston 1989,2005-2015
>_

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Thu Oct 13, 2016 6:10 pm

jgharston wrote:
BigEd wrote:(I'm not sure what the open technology was, unless it was the spec of BBC Basic and the ABI of Acorn's MOS.)
Essentially, anybody with a soldering iron and knowledge of digital interfacing could build anything they wanted to and plug it in.

Yes, also open for interfaces and hardware expansion - at least in the sense that nothing was protected or needed a license.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby paulb » Thu Oct 13, 2016 6:19 pm

Richard Russell wrote:
paulb wrote:I don't remember many reviews complaining about it.

I'm guessing these "reviews" were in computing magazines, or at least by people already familiar with computers.


Well, yes, where else would these reviews be? It would actually be interesting to see a review from another source if you can track any down. New Scientist may well have had them, but maybe that's too "familiar with computers", too.

Richard Russell wrote:Speculation about what might or might not have happened had the specification been different is not something I can contribute to.


Well, one thing that I was genuinely interested in was the background for the Teletext requirement, which I think got addressed by the comment by Coeus about the RM 380Z and the text/graphics overlaying feature.

Personally, I thought it was instructive to see what other manufacturers did - or, in this case, didn't bother to do - when it came to supporting Teletext in general - not the overlaying feature, obviously, which appeared to evaporate from the specification - but I guess I'm done digging stuff up if it's not interesting to anyone else.

jgharston wrote:I also notice some confusion in posts between teletext display and teletext aquision, eg in discussion of "an add-on for the CPC".


If you're implying that I am confused, being the one who brought up CPC add-ons, then you are mistaken. The point there was that the CPC displays the Teletext pages without dedicated hardware, and for those who were interested in this (which from the "reviews" doesn't sound as if they expected it to be too many people) maybe the crude low-resolution rendering did the job. Of course, had a market for high-resolution rendering of Teletext pages existed, those add-on makers could have done what Morley did when making their add-on for the Electron. In fact, from memory, the Morley add-on was also pitched at the Beeb and actually offered a higher level of support for the Viewdata standards than the Mullard IC in the Beeb, or this was at least a supposed selling point.

Maybe the Morley thing was for Prestel rather than broadcast text services, but since I'm probably already repeating myself and it appears to be a waste of time actually looking this up and referencing it, I guess I'll leave it at that.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Richard Russell » Thu Oct 13, 2016 6:49 pm

paulb wrote:which appeared to evaporate from the specification

Nothing "evaporated" from the specification: "This specification does not set out to be definitive but it represents a desirable set of characteristics". It would have been entirely inappropriate (and impractical) for the BBC to have produced a definitive specification, not least because it could have been seen to favour one manufacturer over another if their existing machine was a better fit.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby jgharston » Fri Oct 14, 2016 2:53 am

paulb wrote:In fact, from memory, the Morley add-on was also pitched at the Beeb and actually offered a higher level of support for the Viewdata standards than the Mullard IC in the Beeb, or this was at least a supposed selling point.

The Morley add-on was a teletext receiver - the teletext display is displayed by the SAA5050 inside the Beeb.

Code: Select all

$ bbcbasic
PDP11 BBC BASIC IV Version 0.25
(C) Copyright J.G.Harston 1989,2005-2015
>_

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Fri Oct 14, 2016 7:11 am

I wonder if we're sometimes talking about Mode 7 text display and sometimes talking about rendering Teletext pages? If there's a difference.

I have a Morley teletext adaptor right here - it was quite expensive, as I recall, and it takes in UHF and captures pages internally, to be downloaded via the User Port. It came with a ROM which quite possibly did some processing on the raw page data, but of course if you displayed a page then the work would be done by the Mode 7 hardware in the Beeb.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Richard Russell » Fri Oct 14, 2016 8:52 am

BigEd wrote:I wonder if we're sometimes talking about Mode 7 text display and sometimes talking about rendering Teletext pages? If there's a difference.

The difference is that some of the functions that would normally have been carried out by the companion address-generator chip (not used in the BBC Micro) have to be done in code. The main one concerns double-height operation: if a row contains one or more D/H characters it determines what is displayed in that row and the next: the data in the next row (if present at all) is ignored. In MODE 7 this doesn't happen (you can for example display the top half of a D/H character in one colour and the bottom half in another, something which is impossible in the Teletext/Videotex standard).

So Teletext or Videotex adaptors like the Morley (or to be precise the accompanying software) need to scan the page for any double-height control characters and if found copy the contents of that row into the next. It's not an onerous task: BASIC code is probably fast enough. The only other slight difference between MODE 7 and Teletext is in the way the Conceal control character is acted upon; in MODE 7 it is ignored, but for compliance with the Videotex specs anything following a Conceal must be hidden (replaced with spaces) until the user activates the 'Reveal' function.

But these are substantially trivial. The 'difficult' aspects of the Videotex/Teletext standard are implemented within the SAA5050.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Fri Oct 14, 2016 9:25 am

Thanks, that's very clear!

I'm certainly interested in further discussion and findings. I had a quick look in the New Scientist, and found this from September 1981 ("Schools' computer sits a public examination"):
BBC Micro, New Scientist, 10 Sep 1981, p664.png


Also, from Jan 1982, some production problems:
BBC Micro, New Scientist, 7 Jan 1982, p4.png

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Richard Russell » Fri Oct 14, 2016 9:38 am

Richard Russell wrote:The 'difficult' aspects of the Videotex/Teletext standard are implemented within the SAA5050.

I had fun doing it with TTL chips five years earlier:
Image

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Fri Oct 14, 2016 10:05 am

Wow - 100 TTL on three boards - very much shows the power of integration. I found a couple of mentions of this decoder (built as a kit):
http://www.radios-tv.co.uk/Pembers/Tele ... l#Decoders
http://www.astra2sat.com/teletext/ceefax-and-oracle/

I did at one point buy a VCR with integrated teletext decoder - it produced the relatively ugly bitmap fonts, but it could record the decoded pages, and indeed could record a program with the visible subtitles from page 888.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby paulb » Fri Oct 14, 2016 10:13 am

jgharston wrote:
paulb wrote:In fact, from memory, the Morley add-on was also pitched at the Beeb and actually offered a higher level of support for the Viewdata standards than the Mullard IC in the Beeb, or this was at least a supposed selling point.

The Morley add-on was a teletext receiver - the teletext display is displayed by the SAA5050 inside the Beeb.


Image

Image

Maybe the "on board Teletext display controller" and "second generation Teletext chip technology" can sink in before the chorus of naysaying resumes. I guess I could try and dig up a "review" from Electron User where they actually mention this, but since nobody will bother to read it, it would be wasted effort.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby 1024MAK » Fri Oct 14, 2016 10:34 am

Morley must have made more than one version then. Only the unit I have (for a BBC Micro) ONLY has a UHF input. There are NO video outputs. The only other connections are to a power plug (that fits in the BBC B external power output socket) and a IDC line socket that fits in the User Port.

So this unit is only a teletext receiver / decoder, not a display generator.

It looks like the Electron version had a teletext receiver / decoder, AND a display generator. Hence the teletext video came from a separate RGB output on the unit.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Fri Oct 14, 2016 11:12 am

Here's the detail - the Beeb one does indeed do a little less and costs rather less too:
Morley's BBC adapter costs £74.45 but you must also buy the software which costs £24.95 on ROM or £23.95 on disc. The optional PSU is £9.95. The Electron RGB adapter is £149.95 and the UHF adapter is £177.90.

(I think that UHF adapter is the whole thing, appears as a £28 add-on in Paul's post.)

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby Richard Russell » Fri Oct 14, 2016 11:43 am

paulb wrote:Maybe the "on board Teletext display controller" and "second generation Teletext chip technology" can sink in before the chorus of naysaying resumes.

As has been pointed out, simply referring to the 'Morley teletext adapter' doesn't make it clear whether you mean the adapter for the BBC Micro, which incorporated a UHF tuner and teletext data-extraction - with the display being handled by MODE 7 - or the adapter for the Electron (shown in your photos) which of course had to have its own display capabilities because the Electron doesn't have MODE 7. I think it was pretty clear in the context of this thread (which is about the BBC Micro specification, not the Electron) that it was the former that was being referring to.

As regards "2nd generation teletext", Level 2 Teletext (which I am assuming is what you mean) did not arrive until 1988. Full-field teletext, as mentioned by Morley in the ad, is simply a higher speed delivery mechanism and doesn't have any implications for the display.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby 1024MAK » Fri Oct 14, 2016 12:02 pm

"second generation Teletext chip technology" most likely just means a later version or type of integrated circuits compared to a lower level of integration in an earlier version or type. The overall functionality may not be significantly different.

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Re: "Outline specification for the BBC MICROCOMPUTER system"

Postby BigEd » Fri Oct 14, 2016 12:08 pm

Here's the full quote from the Morley ad for the Elk adaptor, as posted by Paul:
Inside you will find the latest in second generation Teletext chip technology, which unlike our rivals who are still using chips designed more than eight years ago, will be fully compatible with any future advances in the Teletext system.


Edit: there are photos of Acorn's teletext adapter. Looks like it contains SAA5030 and SAA5020. ("The Teletext chipset ... the SAA5000 series, consisting of the SAA5020 Teletext Timing chain (TIC), or the SAA5025 525-line Teletext Timing chain (TIC), the SAA5030 Teletext Video Processor (VIP), the SAA5040 series of Teletext Acquisition and Control Circuit (TAC), and the SAA5050 Series Teletext Character Generator (TROM)." - ref.)
Last edited by BigEd on Fri Oct 14, 2016 1:01 pm, edited 1 time in total.


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