Naked Science Forum
Non Life Sciences => Technology => Topic started by: syhprum on 22/12/2013 10:11:09
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When TV restarted in the UK in the period between 1946 and 1952 a lot of people were prepared to watch 11cm high pictures nominally of 377 lines but as few domestic receivers interlaced properly actually 183 lines.
I wondered what could have been done if modern data compression had been available.
I often watch 360 line broadcasts of F1 races on the internet with a data rate of 400Kb/s and a 9/16 format but would still find them watchable at 180 lines and the early 5/4 format.
My calculations show that a bandwidth of 9kHz would have sufficed for this and the whole country could have been covered by the two high power 200kHz stations.
Colour would have been considered miraculous!
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Am I the only one who worked with domestic TV in the 1940,s ?
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What was the frame rate on these early TV transmissions? 183 lines, 50 times per second in the UK?
With a roughly square monochrome picture, this would present an image of 183 lines x 183 pixels x 8 bits = 270kbits, or a raw data rate of 13Mbps.
I would be surprised if this could be carried in 9kHz AM bandwidth. Assuming there is not too much long-distance interference at 200kHz, you might be able to fit 8 bits/Hz; with some error-correction, that leaves about 50kbps for the video information.
Even with modern data compression, this seems like a bit of a stretch...
But another barrier would be memory technology: 34 kBytes of random-access memory, read out continuously at 13 Mbps was far beyond the technology of the time - this would require about 300,000 vacuum tubes, each drawing at least 1 W of power, or enough to power a fair-sized town of the day. With an average lifetime of 20,000 hours, the memory bank would fail after about 4 minutes.
...And that doesn't include the processing power necessary to calculate error correcting codes and discrete cosine transforms!
The early TV transmissions were a great accomplishment - but so are the smartphones that we take for granted today...
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In September 2001 I was able to watch a live TV broadcast via a 56Kb/s modem fed from a nominal 4kHz telephone line of the historic attack on the USA by the Saudi militants when they destroyed the twin towers.
Of course the hardware would have been virtually impossible to implement with valve technology.
The picture format was 5 by 4 and I was assuming it was displayed on a 7" CRT as was common at the time the video was meant to display 3MHz lines on the test card but to cope with the poor signal strength this was commonly reduced to 1.5MHz.
Attempt's were made transmit live TV to the USA via the newly opened TAT1 cable but the owners would not co operate as the cable was too valuable for speech channels
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Well current HD television (1080 lines by 1920 pixels) has an uncompressed data rate of 1.48Gbits/sec - there are better 'versions' of HD (same resolution, different colour space) used in aquisition/post production, but the digital signal that hits the "statistical multiplexer" before the signal is transmitted (terrestrial, satellite or cable are much the same) is 1.48Gbits/sec.
So - the data rate of a modern HD encoder/multiplexer takes the data rate down to around 10Mbits/sec (less than 1% of the uncompressed signal) thanks to the wonders of MPEG4 and the slightly better H.264 codecs (or AVC, depending on your manufacturer). So - a 405-line signal (the kind of B&W tele we're talking about) which had an analogue bandwidth of around 3Mhz could be reasonably expected to have a digital data rate of 8Mbits/sec (Nyquist and all that) although I've never seen a 405-line encoder!
So - it's monochrome (so you can loose a half of the data rate of a 4:2:2 equivalent colour data stream) you could sensibly wind up with sub 40kbits/sec for an H.264 encoded 405-line television signal.
Figuring out the UHF bandwidth required is a tad more complex; every 36Mbit/sec multiplex transmitted from Crystal Palace lives in the same channel spacing as an old 6Mhz analogue channel; it's all done with QAM & OFDM (Quadrature Amplitude Modulation and Orthogonal frequency-division multiplexing) which is a subject in itself. Suffice to say we can fit four HD channels into one MUX. Using the same numbers you could fit a thousand such 405-line B&W into a modern DVB-T2 transmission mutliplex.