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Let’s take the example of a single-band UHF antenna. This looks a bit like a fish skeleton. The “bones” are arranged in pairs on opposite sides of a central “spine”. The bones near the front are shorter than those near the back. The critical difference of course is that an antenna is made of metal. The antenna needs to be made of something conductive to receive the electromagnetic wave that we call the TV signal.

When an electromagnetic wave hits the conductor it generates a small voltage. A simple aerial consists of a bar or rod of metal cut in half. A feeder cable is connected across the cut. This simple type of antenna is called a dipole.

If the dipole is half the size of the electromagnetic waves the reception is more efficient. Another way of improving the antenna is to add further rods (the technical phrase is elements) in front of the dipole. Positioned precisely and cut slightly shorter than the dipole they act as “directors” to concentrate the electromagnetic waves on the dipole. Of course they have to be attached to a central “spine” to keep them in place. This bit is called the boom.

To reduce ghosting and interference from transmitters located behind the receive antenna additional elements can be added behind the dipole. They are longer than the dipole and are called reflectors. In the best antennas these aren’t simply single rods but a vertical mesh or array of rods, angled towards the front of the antenna. This type of antenna is called a Yagi, after it’s Japanese inventor.

There are other types of TV antenna. Set-top antennas tend to be simple loops (technically they’re folded dipoles) or rabbit-ears. The latter of course are simple dipoles with the two halves angled towards each other. One type of rooftop antenna you may see is the “bowtie”. This consists of a type of folded dipole in front of a mesh plate. This design isn’t as efficient as a Yagi but does reject interference from the rear pretty well. For this reson it’s a good antenna for use near the transmitting station. It’s possible to mount bowties one above another and connect them together so the outputs electrically add together. This is a good technique where good rear rejection needs to be combined with higher efficiency.

VHF radio and TV antennas are almost always of the Yagi design. The elements are longer and further apart than a UHF Yagi. Sometimes you see dual band antennas with long elements on the same boom as more numerous shorter elements for a higher band.

admin So how does ... work? aerial, antenna, tv, yagi

MP3 compressed sound files are finally becoming popular with online music stores, now that DRM is on the way out. Although MP3 compression is not quite as efficient as AAC and WMA it is good enough. The files are much smaller than WAV files and audio CD data files - around one tenth of the size, depending on data rate. A song in the form of a 4Mb MP3 file would take up 40Mb if it was stored an an uncompressed a WAV file. That means the files are good for sending via broadband and they’re also ideal for systems with limited memory like personal MP3 players.

Digital audio works by passing the audio signal into an analogue to digital converter which takes thousands of measurements of the voltage level every second. The sound is sampled 44,100 times per second for CD audio. Each sample uses 16 bits for each channel so the sound can be accurately reproduced. Which adds up to an awful lot of bits per second.

To reduce the number of bits a second the data has to be compressed. There are lots of different ways of compressing signals. Compression systems fall into two groups – lossy and lossless. Lossless systems allow the original data to be accurately reconstructed. Lossy systems allow higher compression levels but don’t allow the original signal to be faithfully reproduced.

Now what’s all that got to do with MP3? Well MP3 files needed to be small but also to sound reasonably good which sounds pretty impossible. MP3′s design takes advantage of a feature of human psychoacoustics – loud sounds hide quiet ones. In short you don’t have to send the data for any quiet sounds that occur at a similar frequency to loud ones.

MP3 divides the audio frequency range up into lots of different bands for the analysis and compression. The sampling rate can also be reduced to further reduce the bit rate. Both technniques result in some loss of audio quality though.

The most common data rate for MP3 files is 128 kilobits per second, which is roughly equivalent to FM radio quality. Higher bit rates of 190 kilobits and above allow full hi-fi quality.

Currently most MP3 systems use fixed bit rate (FBR) sampling, which means an awful lot of bits wasted during silence and simpler sound sequences. It also means worse quality during complicated sequences. Variable bit rate (VBR) produces better quality sound for a given bit rate but uses more power to decode – a crucial difference for battery-powered devices.

admin So how does ... work? cbr, compression, fbr, mp3