Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!uunet!mcsun!ukc!harrier.ukc.ac.uk!mike.ukc.ac.uk!mre From: mre@ukc.ac.uk (M.R.Ellis) Newsgroups: comp.sys.acorn Subject: Re: BBC Microcomputer Message-ID: <1594@mike.ukc.ac.uk> Date: 24 May 91 01:07:01 GMT References: <2784@lee.SEAS.UCLA.EDU> <1991May20.034649.1@vax1.tcd.ie> <4458@stl.stc.co.uk> Reply-To: mre@ukc.ac.uk (M.R.Ellis) Organization: Computing Lab, University of Kent at Canterbury, UK. Lines: 87 In artcile <4458@stl.stc.co.uk>, ajdh@stl.stc.co.uk (Andrew Hurley) writes: >In the referenced article hughesmp@vax1.tcd.ie writes: >>In article <2784@lee.SEAS.UCLA.EDU>, cccph@jessica.cs.ucla.edu >> (Charles Hobbs) writes: >>> >>> The machine has been modified >>> for 120 VAC, but not for NTSC (some screen lines go off the top of >>> the screen, and I can't get any color, just shades of gray). The reason some lines appear to 'go off the top of the screen' is that your television/monitor is expecting 272.5 lines between field sync pulses. The unmodified BBC micro gives 312.5 lines between field syncs - this is because the UK television standard is 625 lines instead of the US standard of 525 lines. Fortunately your monitor can lock to the slightly different signal it is being fed. I am not aware of any way to correct this 'error' in the BBC micro output. >>The grey (if I remember correctly) is adjustable using a link... The link is L39 on a BBC micro and L26 on the BBC B+. If the required link is not fitted to your machine, a capacitor (470pF) should be connected between the emitter of a BC239 and base of a BC309 in the video circuit...on the B+ these transistors are Q9 and Q8 respectively, I can't read the identifiers on my other schematics!!! >This is true, however, the composite video output is tuned to PAL, not >NTSC. Unless the machine has also been modified for NTSC the colour >encoding will be for PAL. > >If my understanding of PAL vs NTSC is correct the upshot of all this is >that a colour PAL signal will come out as a mono picture on an NTSC set or >perhaps worse - ie completely wrong colours due to completely different >techniques of colour coding. This is correct. >PAL codes colour by luminance (mono picture) and chrominance (a combination >of -blue and -red signals) the combination of which allows the calculation >of red, green and blue. I beleive NTSC is rather different and for all I >know the carrier frequencies are also different. Both PAL and NTSC encode the signal using the luma and chroma signals. The chroma signal is phase modulated onto a subcarrier and mixed onto the luma signal. In the standard PAL system (system-I) the subcarrier frequency is approximately 4.43 MHz while the NTSC carrier is 3.58 MHz. The phase of one of the two colour difference signals is reversed on alternate lines in the PAL method of coding, hence the name! The subcarrier frequency of the BBC micro output can be changed by changing the 17.73 MHz crystal X2 (issue 7 BBC Micro and BBC model B+) for a 14.32 MHz crystal. The phase alternation of the output should be inhibited by disconnecting pin 12 of IC48 (BBC issue 7) and connecting it to 0V or breaking the existing link S28 and making the alternative link S28 on a model B+ (S28 is a three pad link with two pads linked by a copper track on the PCB...the header may or may not be connected depending on the age of the computer). >Maybe someone makes a simple converter to produce NTSC coded PAL frequency >signals from PAL ?? I've never heard of one at a reasonable cost...if they do exist they probably decode the incoming signal to YUV (Americans may refer to similar signals as YIQ) and re-encode these. The cheapest way is probably to strip the incoming signal right down to the RGB level!! >You are almost certainly better off trying to get an RGB to NTSC converter >which will plug into the BBCs RGB port so avoiding problems with >PAL colour coding. MOST RGB to NTSC coders will accept the BBC micro output so long as they are fed with the syncs from the BBC micro. PLEASE NOTE THAT :- a) I have NOT tried the modifications I have suggested...they come from theory and circuit diagrams and a small ammount of video experience. b) The pin numbers and component identities are taken from the only schematics I have...namely a BBC B (believed to be issue 7) and a BBC B+ (only one issue released, I believe). c) If anyone (at Acorn??) can confirm/deny the advice given above, I shall be most grateful to them for correcting any errors!! -- Michael Ellis - Darwin College, The University of Kent, Canterbury, England.