Suggested Reception Values

The actual method of Digital Broadcasting is specified by a set of standards produced by the Digital Video Broadcasting (DVB) Group, a European consortium for broadcasting transmissions. The DVB standards define such things as the type of modulation, error correction and guard bands between channels. They are known as DVB-T and DVB-S for terrestrial and satellite broadcast respectively.

Implications of recent changes is that room will be available for some DTT high definition channels, UK viewers will be able to access HDTV defined by a new set of standards called DVB-T2. Not all channels will change and it is expected that DVB-T2 will exist alongside DVB-T for some years. The new DVB-S2 standards have already been implemented by BSkyB (currently offering more than 30 HD Programmes)

Aerial Reception
It is widely expected that the channels will be reallocated between the DTT multiplexes, leaving multiplex B clear for HD channels using the DVB-T2 standards.

Of course, anyone wanting these HD channels may need a new set top box and meter.

The analogue TV channels broadcast from any transmitter in the UK were carefully chosen to minimise any interference between them. They were also grouped so TV aerials would need to operate over relatively small band of frequencies. Aerial manufacturers therefore make aerials to cover a single channel, a group of channels or the whole UHF Band.

If the level of the signal from the aerial is not sufficient, the performance of the whole system will never be perfect. It is therefore necessary to use an aerial with sufficient gain, depending on the signal strength at the reception site.

The following conclusions can be drawn for different types of aerials across the UHF band

• The gain tends to increase for higher channels
• The biggest aerial has only 4dB more gain than a 10 element Yagi aerials
• A grouped aerial has typically 4 dB more gain than the equivalent wideband version

In the world of telecommunications, noise is degrading interference, picked up by the aerial from space, the inherent structure of matter, nearby objects, light dimmers and electrical appliances. The effect of noise can sometimes be minimised by using a filter to reduce the noise, or by using a grouped aerial over a wideband version.

Amplifier noise figures are quoted in dB and, the lower the noise figure, the better. If the analogue signal is allowed to fall to too low a level, the received picture and/or the sound quality will be degraded by the noise level. The lower the signal level, the more likely it is to happen. If the received signal level is then amplified, this will not help because the noise is boosted as well.

The ratio between the level of an analogue signal and the level of noise in the same frequency band is known as the signal to noise (S/N) ratio. This is quoted in dB. The lowest S/N ratio anywhere on the system limits the overall performance of the entire system.

If the signal level is too high it can overload the electronic circuits in amplifiers and receivers, causing distortion. Therefore, for each part of the receptions system there is a window of operation between these two levels.

The window of operation to achieve satisfactory results for domestic analogue systems is as follows, in dBμV

The ratio between a digital carrier wave and the level of noise is known as the carrier to noise (C/N) ratio.

Digital pictures have two states; they are either perfect quality or non-existent. Digital signal quality is always defined by the number of errors occurring in the data received. If errors occur in the signal reaching the video decoder in the TV receiver, square blocks will appear randomly over the picture, usually an area of high action or detail. This can be due to a poor C/N ratio.

The fact that digital pictures disappear suddenly is referred to the "cliff effect". The installer needs to know just how close to the edge of the cliff they are. It may be that the system is right on the edge, yet giving perfect pictures. It is impossible to tell just by looking at the pictures and, the next time it rains (or whichever reason may change the signal level) the picture will freeze.

A terrestrial digital TV receiver will function with an input level of between 45 and 70 dBμV. At the lower level there is a risk of interference from impulse noise from, for example, a nearby fridge or central heating switch, causing the picture to block or freeze. The recommend minimum level is therefore 50 dBμV.

It is recommended to measure the C/N on each multiplex in addition to the signal level. An agreeable limit of C/N on domestic DTT for a 64QAM Multiplex:

Whatever the signal level and C/N ratio, the sole factor that determines the performance of DTT reception systems is the number of errors reaching the digital receiver. This is measured as BER (Bit Error Rate). The lower the BER the better.

Digital receivers will operate satisfactorily if the BER before error correction is equal to or better than 2E-4 (2 errors in every 10,000 bits of data)

Satellite Signal Measurements
It is important enough that the signal level at the receiver input is high enough to ensure that the noise introduced by the receiver does not significantly affect the overall C/N ratio, but low enough to ensure that the receiver is not overloaded. A poor C/N ratio results in a black and white horizontal flecks called sparklies on analogue pictures accompanied by clicks on the sound and data errors in the teletext, and intermittent blocking or complete loss of picture on digital receivers.

The LNB has a gain of 50-60dB, which is usually high enough to compensate for losses of up to 70 metres of downlead cable plus and outlet plate and flylead. For longer lengths of cable an in-line IF amplifier may be required. This is normally powered by the LNB voltage what is supplied by the satellite receiver.

The required signal levels and C/N ratio at the receiver input under clear sky conditions are as follows:

*In practice, the signal level can fall to 47 as long as the C/N ratio is achieved
**For and FEC of ¾; 2Db less for an FEC of 2/3

As with terrestrial TV, the overall performance of a satellite TVRO system is determined by the number of errors reaching the digital receiver and whether the receiver can cope with them, dependant on 2 factors

• The C/N ratio at the receiver input
• The amount of error correction contained in the received signal i.e. the FEC parameters

This Installation guide was kindly reproduced with extracts taken from the book by R A Calaz; Digital TV, Satellite & Multimedia ISBN 978-0-9562748-0-9. The entire book is highly recommended as many of the installation and troubleshooting solutions have not been mentioned.

http://www.amazon.co.uk/Digital-TV-Satellite-Multimedia-Calaz/dp/0956274803/ref=sr_1_1?s=books&ie=UTF8&qid=1324468067&sr=1-1