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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Record entire MUX through a video tape



 
 
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  #1  
Old July 4th 03, 08:24 PM posted to uk.tech.digital-tv
Tony Stanley
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Posts: 35
Default Record entire MUX through a video tape

"Martin Underwood" wrote in message
...
"John Porcella" wrote in message
...

"Mark Johnson" wrote in message
...
If I were to tune my VCR into the frequency of a digital station, and

record
the channel, would I be able to play back the garbage into the digital

stb
aerial,


I don't know how it would be possible to use your VCR to become a
transmitter to your set top box.


Sounds easy enough to transmit to the set top box: set the RF output

channel
of the VCR to the frequency that the multiplex is originally broadcast on.
So if Mux 1 is broadcast on UHF channel 50 (for example), record channel

50
on the VCR and then make the VCR play back channel 50 into the digital box
as if the signal was coming off-air.

Two problems. Firstly the VCR almost certainly wouldn't be able to record
the bit stream faithfully, particularly as the VCR is geared to record a
proper PAL signal synchronised at 50 Hz / 625 lines, whereas the digital
stream would look like low-level noise (it's specifically designed to look
like noise so as to have minimal effect - no regular patterning - if
received as co-channel to an analogue broadcast). Secondly you'd need to
take great care that when your VCR rebroadcasted the signal on Channel 50
(for example) you didn't interfere with the real broadcasts: connecting

the
VCR's output to the TV aerial (accidentally!!!) would be very antisocial!

Nice bit of lateral thinking, though.




More problems than that unfortunately.

-The tuner would be designed for an AM video signal chopped up with a Nicam
and Audio carriers, totaling 8 MHz, where as the VCR takes the 5MHz Video
signal and records it. The QAM multiplex is 8MHz so you miss out the end
bits.
-The Tuner would be designed for much larger analogue signals and the gain
profiles and S/N would probably not allow the signal through.
-VCRs wouldn't be able to lock to the QAM signal and could not co-ordinate
the head with the tracks. VCR's will not quite often not record anything if
they can detect Pal video sync.
-QAM signals have continous information flow, VCR rely on the fact that
analogue is fully real time and has gaps between the pictures to do head
switching.
-Even if you could persuade a VCR to record with external sync circuitry and
get the signal to it, even S-VHS would only record 5MHz or so, a normal VHS
cuts it down to about 1MHz, nevermind the bits miss out during sync.

So in short no, not a mission.

The way to do this is to use D-VHS which have only seen a few products in
the states for recording HDTV. Even this I am not sure if it can manage the
bit rate for transport stream (seem to remember 14Mb/s). Transport streams
take up a hugh amount of data, for instance a CD would be filled in about 4
minutes.

HDD is the only way I am aware of doing this now.

--
Tony Stanley ++Always Learning++



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  #2  
Old July 5th 03, 01:42 AM posted to uk.tech.digital-tv
Wrightsaerials
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Posts: 93
Default Record entire MUX through a video tape

From: "Tony Stanley"

This is the wonder of the internet. Someone poses a ridiculous question, a few
people have a laugh, then out of the blue someone actually makes a few sensible
suggestions that might even lead to something.
Bill
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