Digital delay
I recently invested in a set-top box to receive my TV signals in digital format. While setting it up I noticed my analogue TV in the next room was showing the same channel, but the action was 2 or 3 seconds ahead. Why is a digital signal delayed? And what are the implications of attempting to set one鈥檚 watch by the time signal?
鈥� Digital TV boxes buffer the signal they receive in order to decode and process it, and produce a smooth picture. The digital processing unit within the set-top box is simply taking a while to decode the signal.
Analogue and digital signals are transmitted at the same time. It is the set-top box that is causing the delay. If you really want to set your watch by the digital signal you could calculate the time difference between your analogue and digital TV. Failing that you鈥檙e going to be about 2 seconds late for all your meetings.
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Phil Taylor
Colchester, Essex, UK
鈥� Rather than having to process data continuously (and instantaneously) as analogue systems do, digital equipment can store a block of data in memory, and then process it.
This opens the door to a wide range of signal-processing techniques that are not possible in an analogue system, such as compression and decompression, error detection and correction, and sophisticated filtering.
There are trade-offs between the amount of processing that can be done and the time required to do it. In broadcast applications, where the channel is one-way, the processing delay is considered unimportant. Instead, emphasis is placed on getting the best sound and picture quality, and the most efficient use of transmission bandwidth. Wristwatches, I fear, are near the bottom of the agenda.
Ben Jones
Digital systems design engineer
By email, no address supplied
鈥� TVs in the UK display 25 frames per second. Analogue TV signals encode a separate image for each frame. Digital TV is different. Instead of a new image, a map is transmitted. The map tells the decoder where to move segments of the previous frame to create the next frame and then adds any new elements to the frame. In that way static images don鈥檛 have to be encoded in every frame and simple moving images can be encoded in a smaller bandwidth.
For instance, on analogue TV, a white square moving up by five pixels would need the same amount of broadcast 鈥渟pace鈥� as a complete change of image. With digital TV, you would only need to tell the decoder the coordinates of the pixel you want to move and the vector describing the new position. This takes up less space than sending a completely new image, but because you don鈥檛 get a complete picture with each frame the digital signal takes time to decode.
As for the clocks: the ones in the corner on the TV picture are accurate on analogue TV but not on digital. However, teletext and programme guides on digital TV do take the delay into account and are accurate.
Dean Whittaker
University of Bath, Somerset, UK
鈥� The delay in digital TV also occurs on digital radio, and not all manufacturers have the same delay time. If you have many analogue and digital radios about the house all tuned to the same station the echo effect can be unbearable.
Diane Whitehead
Littlehampton, West Sussex, UK
鈥� Only radio clocks and GPS time signals are reliable. Digital radio and TV have long and indeterminate delays, and even the telephone talking clock is unreliable because of the packet-switching used by the phone network.
Chris Woolf
Broadcast Engineering Systems
Liskeard, Cornwall, UK
Whisking disaster
For years, whenever my family have come to visit, I have made meringues, which involves whisking egg whites until they are thick. I have always used free-range eggs, but recently I bought organic free-range ones, and no matter how much I whisked these whites they would not thicken. Why should organic eggs behave in this way? Is something missing from the birds鈥� organic diet that prevents the whites of their eggs thickening?
鈥� Your correspondent may have jumped to an unwarranted conclusion from a single instance. I regularly whisk organic free-range egg whites without any problem, and bearing in mind that in culinary history all eggs could once have been so described, it seems unlikely the egg is the problem.
J. Oldaker
Nuneaton, Warwickshire, UK
鈥� A good, stiff meringue froth demands a complex interconnection of suitably distorted protein molecules. Anything that interferes with the interlinking of the molecules leaves the whites an unappetising slush. Oil is the usual culprit. Use clean, dry utensils, free of detergent. Before the whites have formed a stiff froth, the merest drop of cooking oil, cream or oily yolk getting into the whites can ruin the meringue.
Jon Richfield
Somerset West, South Africa
鈥� The eggs from our pet chickens, while not certified organic, are as close as we can get, and make wonderful meringues. The problem, I suspect, is freshness. Whites from eggs less than 5 or 6 days old will not whip up. Supermarket eggs can be up to 2 weeks old, but the organic ones were probably newly laid.
This raises the question of what changes eggs undergo as they age to allow them to make successful meringues.
Phil Baker
Uxbridge, Middlesex, UK
鈥� The eggs may have been too young. I assume that the protein molecules develop cross-linkages as the egg ages, enabling the albumen to contain the air bubbles when whipped.
Lorna English
By email, no address supplied
This week鈥檚 questions
Worried sick
When I鈥檓 nervous I always feel physically sick. What causes this, and does it have an evolutionary advantage?
Frances Harkin
Melbourne, Australia
Pre-inventing the wheel
Wheels are a pretty effective method of getting around. Is there any reason why they never evolved in nature?
Tyrone Peeler
London, UK
Escalating concerns
I have noticed that when I travel on an escalator, the handrail always moves at a different speed to the stairs. You would expect it to move at the same speed but it never does. Why not?
Bernd Haupt
Nuremburg, Germany