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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 10th Jan 2012, 3:26 pm   #1
radioman
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Default UHF Signal generation with Teletext Clock

Hi,

I own a couple of video/hard disk recorders (with analogue tuners) which no longer automatically set their clocks since analogue switch-off.
Given the number of power cuts we've suffered recently plus the fact that the on-board clocks don't keep very good time, is there any easy way of generating a signal to do this ?

Andy
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Old 10th Jan 2012, 5:19 pm   #2
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Default Re: UHF Signal generation with Teletext Clock

How about a small UPS to maintain power to them during power cuts?
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Old 10th Jan 2012, 11:18 pm   #3
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Default Re: UHF Signal generation with Teletext Clock

I think Andy is saying the clocks don't have good time keeping now the signal is not present, hence the need for generation of a permanent signal as a sync.

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Old 11th Jan 2012, 12:05 am   #4
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Default Re: UHF Signal generation with Teletext Clock

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Originally Posted by radioman View Post

......is there any easy way of generating a signal to do this ?
I’m afraid that the simple answer to your question is, probably, no. It’s certainly possible, but not within the level of effort that you’d want to put into it for just a couple of recorder clocks.

Basically, what you need to do is to create the bit stream that comprises Packet 30/8/1. This comprises one Teletext data line which doesn’t form part of any other particular page, so has the advantage that it can be broadcast on it’s own if necessary (as in this case).

Most of the data in the packet can be repeat generated as required, as long as the time code bits are updated by connection to a real-time clock feed. Normally the data is regenerated and rebroadcast on the turn of every second of the clock code.

Once you have this data assembled into a bit stream you will need to feed it to an off the shelf Teletext encoder chip (probably complete overkill for this application) or come up with a simpler DIY method of doing the analogue encoding. The DIY method would entail an NRZ encoder followed by a Raised Cosine Filter to limit the bandwidth to under 5MHz and reduce inter-symbol interference.

Then you need a syncronised blank raster to insert the analogue signal into. Feed the whole lot into a UHF modulator, on a channel that your recorders are tuned in to, and Bob’s your uncle!

As I said, a lot of work to save the hassle of manual clock setting. The only other option may be picking up a piece of suitable redundant studio broadcast equipment, but a lot (most?) of it seems to work with SDI rather than composite video.
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Old 11th Jan 2012, 10:16 am   #5
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Default Re: UHF Signal generation with Teletext Clock

Thanks for the replies and especially Parabola for your detailed explanation of what is required. I had a feeling there might not be a simple way to do this !
Oh well, I'm sure I'll get quicker at re-setting the recorder clocks the more often I have to do it.

Regards,

Andy
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Old 14th Jan 2012, 8:35 am   #6
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Default Re: UHF Signal generation with Teletext Clock

Build a 60kHz MSF receiver to get the time. Use a PIC micro to decode its output and generate a Teletext line with the right sequence of 0s and 1s, and spit out a whole picture with just this line repeated all the way down and proper syncs. Now you need to modulate it onto a carrier. You'll get a picture that looks like a bar code, but it will have the time code in it and that's all that matters.

If you use an audio-capable modulator, the device will have another use: playing sound through the speakers of old TV sets.
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Old 14th Jan 2012, 10:54 am   #7
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Default Re: UHF Signal generation with Teletext Clock

Andy.
Why not turn your aerial round to Divis then you can do it the old fashioned way with a "real" analogue signal!

I do it here in Killie.

PS Just a thought the RT Russell test card generator produces 3 pages of text, would there be any way of feeding in clock info?
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Old 15th Jan 2012, 11:09 am   #8
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Default Re: UHF Signal generation with Teletext Clock

AJS, that is exactly along the lines of what I was thinking about !
- might give it some more thought now.

Trevor, I'm pretty sure I can't receive Divis here, but that would only be a short term solution anyway.

Keep the ideas coming.

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Andy
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Old 15th Jan 2012, 5:22 pm   #9
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Default Re: UHF Signal generation with Teletext Clock

Quote:
Originally Posted by murphyv310 View Post
Just a thought the RT Russell test card generator produces 3 pages of text, would there be any way of feeding in clock info?
a) I don't think it is possible to include anything as dynamic as clock info. The contents of the TCG are static - programmed in advance from a PC.

b) As far as I recall, the time info. used by the likes of VCRs etc was taken from a separate data packet (known as the TSDP?) rather than from the teletext page headers.

If someone could come up with a gadget to obtain the time from a reliable and available source and insert the appropriate data packet into a video stream, I wouild be interested at the right price. I always have to remember to reset the (now free-running) timer on my DVD recorder before doing a timed recording.
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Old 15th Jan 2012, 5:45 pm   #10
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Default Re: UHF Signal generation with Teletext Clock

Forget using Richard Russell's generator for this task.

TSDP is Packet 30, as I previously mentioned. See here for for info - http://pdc.ro.nu/packet30.html

If you use a PIC for this, I think you're still going to need to implement some sensible analogue waveform shaping filter, as mentioned above, or it's never going to make it through the receiver's IF strip, or maybe not even the modulator, properly. It's the rasied cosine waveform shape that produces the familar teletext 'eye' that the decoder eye height set up routines always refer to.

If you performed the frame generation and data assembly in software on a PC, then the clock is already available (and accurate if synced to a time server), all you then need to do is get it out into the outside world. Maybe a USB interface chip connected to a DAC + filter, or if you've got a PC old enough to have a parallel port then that may be another option.
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Old 15th Jan 2012, 8:25 pm   #11
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Default Re: UHF Signal generation with Teletext Clock

Hi.
All of the above is out of my league these days, my thought about the RT was of the the top of my head, so i'll leave it to those in the know.
Something buildable though would be interseting as I have two Panasonic analogue DVD recorders that are regularly turned off.
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Old 22nd Jan 2012, 3:28 am   #12
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Default Re: UHF Signal generation with Teletext Clock

I was just having a browse through the NZ Freeview technical specs (as you do...) and receivers have to output Teletext in the VBI when they receive any digital service that includes Teletext.

Quote:
The minimum requirement for this service for STBs' and PVRs shall deliver the Teletext and line 21 subtitling information via the VBI to the Teletext decoder of an equipped Television.
(see page 100)

So in theory you could try a NZ FreeviewHD (terrestrial) STB - provided someone is broadcasting Teletext on DVB-T on ch25-62 and the box supports MPEG2 (I suspect most won't, as we have never broadcast it so there's no MPEG2 requirement, everything here is H.264) and it has a UHF output - of course any UHF out on a NZ box will result in no audio for you...
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Old 30th Jan 2012, 8:57 pm   #13
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Default Re: UHF Signal generation with Teletext Clock

For anyone interested , here’s a little windows application that displays the dynamic data, as the relevant decimal byte values, that you would need to insert into, an otherwise static, Packet 30 (TSDP) data line generated by some other software or hardware.

Download link - www.vintagepixels.co.uk/imogen/TSDP_v1-0.zip

Generated data is based on information provided from previous links in this thread. Because computer clocks (should) adjust for GMT/BST automatically, Byte 9 becomes irrelevant and can remain statically set for a zero offset.
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Old 31st Jan 2012, 3:12 pm   #14
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Default Re: UHF Signal generation with Teletext Clock

I'm afraid this is a bit 'over my head'..

However, I thought I'd try an experiment using hardware I've already got.
I noticed that clock data must be being transmitted from the Astra2 satellites since the time is displayed on my DVB satellite receiver when in standby.
I found out after much searching that some channels ( e.g. Channel 4 - Sky 104 and the Travel Channel - Sky 251) are still transmitting Teletext data which the receiver is sending out on the UHF output. Plugging the TV in and tuning to this channel gave me hope that the needed clock info. might be present but despite the time being displayed 'top right' as expected it didn't work, so I assume this is a different data-packet to what my video needs.
So it looks as though the satellite receiver can :-
1. Receive clock data from the signal - i.e. it "knows" the time
2. Insert Teletext data into the VBI
3. Combine this data with the video signal
4. Modulate this onto a UHF channel

BUT, it doesn't appear to re-send the clock data where my recorders are looking for it.


Oh well, it was worth a try.

Andy
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Old 31st Jan 2012, 5:24 pm   #15
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Default Re: UHF Signal generation with Teletext Clock

Quote:
Originally Posted by radioman View Post
....it didn't work, so I assume this is a different data-packet to what my video needs.
Yes indeed. The time information on the top right of the page is sent as part of the page header row (row/packet 0) and is just an ascii string that is correct when the header row is broadcast. The implementation of the 'rolling header' system means it can appear to operate like a real-time changing clock.

Quote:
Originally Posted by radioman View Post
Oh well, it was worth a try.
It certainly was, but probably not too surprising that it wouldn't play ball. For it to work the DVB box would have to generate and broadcast it's own packet 30 based on the DVB time info, but what about the EBU allocated broadcaster tag that is also contained in packet 30? It would no longer be a source from a 'real' broadcaster. I suspect that, and probably the fact that the box designers didn't see any real need or cost vs selling point proposition to do it, is why you won't have any joy going down that route

Last edited by Parabola; 31st Jan 2012 at 5:33 pm.
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Old 18th Mar 2012, 3:31 pm   #16
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Default Re: UHF Signal generation with Teletext Clock

If you have a Sky satellite box for 28.2degrees east and use it for Freesat and tune into RTE1 or 2 you will get Aertel (Irish teletext). No picture or sound but the teletext works.
The Sky box has a RF modulator so it will lock the Tv or VCR to the correct time!
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Old 19th Apr 2012, 11:47 am   #17
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Default Re: UHF Signal generation with Teletext Clock

Quote:
Originally Posted by Parabola View Post
... The DIY method would entail an NRZ encoder followed by a Raised Cosine Filter to limit the bandwidth to under 5MHz and reduce inter-symbol interference.
If anyone can find it, there was an article in Mullard Technical Communications during the early days of Teletext (mid 70s) which described a teletext test generator, mostly based on standard TTL logic.

IIRC it was primarily intended for eye height measurements, so the output waveform would need to be good!

It produced sync and framing code but I can't remember exactly what happened after that. Probably just pseudo random data rather than anything displayable as teletext but it could form the ideal basis for a suitable generator.

The Raised Cosine Filter, IIRC, was based on a single inductor.

If anybody can find the article and scan it, it could prove very useful.
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