Science Questions

How long does it take for radio to reach me?

Sun, 16th Dec 2012

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Question

Ross asked:

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, it's certainly better than revision.

Ross (Zoology student, Dundee University).

Answer

John -   It does depend a lot on the transmission chain that's involved.  It depends whether the radio station itself is digital, has a digital sound desk, digital processing amplifiers, and digital transmission lines to the transmitter site.  All of these things can add a bit in terms of delay.

Typically, I would say that youíll be talking about less than 50 milliseconds in total for your average radio station.  Maybe less than that if you have a completely analogue radio station feeding digital circuits.  If itís an analogue radio station, feeding analogue lines to the transmitter which of course is analogue, with an analogue audio processor, the delay is virtually zero.

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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, it's certainly better than revision.

Ross C

What do you think? Ross C, Fri, 31st Oct 2008

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. 

 

Pumblechook, Fri, 31st Oct 2008

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. techmind, Sat, 1st Nov 2008

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. lyner, Sun, 2nd Nov 2008

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