2004-08-02

Comms have got slower - The Pips

Back in the years BD (Before Digital) when you heard the time signal on the radio you could be confident that you were hearing it a few milliseconds after it had left the signal generator.

Nowadays you can have several radios going in the same room and they will all put the time signal out at slightly different times:
  • conventional analogue radio
  • DAB (Digital Audio Broadcasting) radio
  • sound only channel on cable TV
  • sound only channel on satellite TV
  • on a computer over the internet - up to 20 seconds delay
And of course if you are using a 'listen again' facility on a radio station on the web, then the time signal will bear no relation to reality.

All the digital transmission methods incorporate data compression. This involves taking a sample of the signal over a time frame, say 50 milliseconds and looking for patterns in it. You cannot send the compressed data until the end of the time frame so there is an inherent delay.

But in fact most of the delay on DAB and cable TV and part of the delay on the other digital methods happens in your receiver and is due to buffering. The packets which make up the program you are hearing are interleaved along the transmission path with packets for other channels and may not arrive at regular intervals. To avoid glitches, the receiver keeps a queue of received packets so that it can process them at regular intervals.

1 Comments:

Blogger kim said...

Oh... you're going to hate this. Some stations now play them out via minidisk at a time to suit the programme....

5 December 2004 at 00:31:00 GMT  

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