Fri Apr 06, 2012 11:11 am
2 Parts to your 'genlocking-the Pi' problem:
1/ Can it be genlocked sufficiently stably by receiving an external GPI (or 2 GPIs)….
(If not, then the frame-store method is required)
2/ Can the subcarrier-burst (and therefore chroma modulation) EITHER be turned off (allowing grey-scale keying with monochrome timing requirements) OR can the subcarrier be phase-locked to within the required few degrees of subcarrier (at 4.43MHz) in order to be able to mix with the colour contained in the signal.
You say that you do not want to buy 'an expensive mixer'; but that is simply the cost required to do the job properly. Whilst you may perceive the 'colour' of video to be low resolution, to mix it in the encoded domain requires timing accuracy to a few nanoseconds…..
Answer to (1)? Assuming you have created the circuitry to detect vertical and hoirizontal and generate the interrupts:
The GPI's might be able to lock vertical sync and then horizontal to sufficient accuracy for monochrome-timing, to (mono-)genlock the Pi GPU output …. but we are still awaiting public information on the video capabilities in this area – only crude 'TV Standard' and Active Area ('-/overscan') blanking have known options in the config file. [I don't think this is an area where 'computer' people have the same 'interest/priority' as 'video' people - they were not planning on designing a TV station]
Answer to (2)? – Either Monochrome or Colour-locked Output Capability?
However, I foresee NO PROSPECT of being able to control the phase of the subcarrier generated, to within 2-3ns**: this is not being sold as a 'video' product. Also, as a 'caption generator' or slide/movie replay source, the timing delay caused by going through an external frame-delay for synchronisation is not a problem -and there are plenty of these devices available – Eg: see the Datavideo range for a complete range of solutions! **Hence also the desire to be able to switch off colour encoding -for improved mono displays – especially on small portable screens ]
PAL colour coding has taken the baseband chroma information, and inserted it onto a carrier at 4.43MHz (PAL50), and since the PHASE conveys the colour information, the 2 signals must be mixed synchronously to avoid loss of colour …. and that means within 2-3ns (1 cycle, or the whole 360degree colour spectrum in 225ns ). This requires dedicated timing circuitry which is unlikely to be 'available' in the Pi – and we do not know the accuracy of the subcarrier being generated either! (Some Japanese computers or even cameras omitted the 25Hz offset many years ago)
THEREFORE the only option to you is going to be via the 'External Framestore Synchroniser' approach, as others before you, have taken to using. Hopefully you can now understand why this is the case
I don't want to buy an expensive mixer, otherwise I wouldn't have asked you.
But unfortunately for you, the technical requirements are clear ; the framestore solution will be your most cost-effective option – whether this is incorporated within the mixer or externally genlocking its inputs.
'expensive' is simply a subjective valuatio -some
may consider all video to be expensive
And referring back to your 1st post: there is no S-Video output. It is HDMI OR Composite