Naked Science Forum
Non Life Sciences => Technology => Topic started by: Ross C on 31/10/2008 23:10:04
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Ross C asked the Naked Scientists:
Dear Dr Chris,
After a day of boredom I worked out that it would theoretically take 0.002 seconds for your show to reach me in Dundee (ignoring the time for your voice to reach the microphone and the time for the sound wave to reach my ears were ignored). However, I was wondering how long it would realistically take if electrical processes such as modulation, demodulation were to be taken into consideration (as well as other factors such as possible non-straight propogation) etc.
Love the show (http://www.thenakedscientists.com/HTML/podcasts/), it's certainly better than revision.
Ross C
What do you think?
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Radio Waves travel 100km in 0.00033 secs. Pretty fast, but digital process that are used now can introduce delays of up to a few seconds. Even analogue transmissions can be delayed slightly because digital systems are used to get the programme to the transmitter. Try listening to the same radio stations by different means at the same time such as radio 4 which is on Freeview, AM, FM, Satellite and DAB. (Some of the delay on the satellite signal will be the time for the signal to do a round trip og 72,000 km). You will find that some will be out of step with others.
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Analog modulation such as amplitude-modulation and frequency-modulation are effectively instantaneous. You might argue that there's a system delay of one or two cycles of the carrier, but that's not really worth worrying about.
Digital radio systems (such as DAB, Freeview...) incur two sources of delay: firstly the audio is analysed in 40-100ms blocks for audio-compression (mp3-like) this gives a pipeline (end-to-end) delay of typically four times this block-length (record-encode-transmit/receive-decode-play). MPEG digital television has even greater delays as the picture-compression happens over many frames - so its blocks may be as much as half a second or so (the sound is delayed to -hopefully- time with the picture). Secondly, many digital broadcast systems use COFDM modulation which again codes the data into discrete blocks (known as "symbols") which are typically a few 100 microseconds, incurring a further system delay of about 4 times this much.
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One of the biggest delay factors in analog sound broadcasting will be in the audio filtering. For AM with a top cut of about 5kHz this would be about 0.2ms. There will be a similar delay in the IF filters in the receiver.