Digital time lag, time signals and delays on TV and radio

When you watch digital TV, or listen to DAB radio, you may notice a time lag. Why is this? And how can you set your clock or watch accurately with the digital delay? We explain the reasons and some workarounds.

 

The problem

Since the 1920's, in the UK, we've been getting our over-the-air entertainment using "analogue", but now we're living in a digital world.

On the plus side, Digital TV and radio gives us loads more channels of TV and radio, and loads more choice.

On the down side, with digital TV and radio, we lose a few things too - Ceefax, VideoPlus, and the ability to set our clocks from the BBC.

 

Why is there a digital delay?

If you sit an analogue FM radio and a DAB digital radio next to each other, both tuned to the same radio station, you'll notice a one or two second time difference. Similarly, watch the news on Sky satellite in one room, with analogue TV on in another room, and you'll notice the delay.

Why is this? The main cause of the delay is down to the way that TV and radio channels are combined together (multiplexed) and compressed. The digital technology is used to squeeze loads of channels onto a single frequency. Your digital TV or radio receiver is receiving the encoded and compressed signals, and using an onboard microprocessor to decode the incoming data and convert to video & audio. This encoding and decoding process takes a little processing time, hence the delay.

Satellite TV? If you get your TV via Satellite, there's also another type of delay. The round-trip to the orbiting satellite and back to the ground. For the technically-timed, the typical delay is 900ms and 2200ms, depending on the data encoding/decoding used, and allowing for the 72,000 kilometre journey.

DAB Radio? DAB is encoded using a system called MUSICAM before it is transmitted, and your radio decodes this for playback. If you're technically-minded, the detailed explanation is that the time interleaver introduces a 384 microsecond delay and the audio coder/decoder introduces a delay of several tens of milliseconds. You can read more on this subject in a PDF we found here

 

So, how can I set my watch if there's a delay?

There are several ways to get to an accurate time:

• Use an FM radio. Radio 4 broadcast a set of "pips" at the top of most hours, which can be used to set your clock or watch. The FM GMT pips aren't delayed, and are accurate. You may have heard that there are plans to phase out FM radio, but BBC Radio 4 will still be on FM until at least the end of 2015.
• Get yourself a radio-controlled clock. These are available from around £10. They work by getting the time from a built-in radio that's tuned to an analogue transmitter that broadcasts a time code. These radio controlled clocks are amazingly accurate, and commonly available.
  Pictured to the right is the Acctim Radio Controlled Wall Clock, available for under £10 from Argos online
  More on Radio controlled clocks, including availability on our Radio Controlled time page.
• Phone BT's speaking clock - Dial 123 from a BT landline phone, and set your watch at the third stroke
• Time over Internet - You can set up some computer equipment to pick up the time accurately by setting the kit to update the time from something called an NTP server, and getting time over the Internet

 

Useful Links:

• Radio controlled clocks and watches
• FM Radio Switchover Information