|
|
| Line 1: |
Line 1: |
| '''Output power''' of a [[TV transmitter]] is the [[electric power]] applied to [[antenna (radio)|antenna]] system.
| | НАША КОМАНДА<br><br> |
|
| |
|
| There are two definitions: '''nominal''' (or peak) and '''thermal'''.
| | КОНСТАНТИН<br>дизайнер <br><br>Я был на Вашем месте, я тоже заказывал [http://kks.by/?cat=33 заказать дизайн]. Обращайтесь и я Вас удивлю!<br><br>ЕКАТЕРИНА <br>режиссер <br><br>Я арт-менеджер и мое любимое дело - это фото-режиссура.<br><br><br>РИЧАРД<br>кот <br><br>Я символ студии и я слежу за работой Кати и Кости.<br><br>http://www.kks.by |
| | |
| It must be noted that more or less, without exception Analog Television systems put about 70% to 90% of the transmitters power into the Sync Pulses. Digital television modulation systems are about 30% more efficient than Analog modulation systems overall.
| |
| | |
| == Analog vs Digital ==
| |
| | |
| Analog
| |
| * The large amount of energy that Sync Pulses use is largely independent of the measurement system and efficiency of the analogue TV transmitter (as most transmitters in use average around 75% efficiency).
| |
| * The transmission of FM audio (including Stereo subcarriers) is only overall the 3rd largest consumer of TV transmitter power.
| |
| | |
| Digital
| |
| * DVB like transmission systems, with their groups of related carriers are not quite as energy efficient as 8VSB systems
| |
| * 8VSB transmission systems only provide limited one carrier wave that consumes about 7% of the transmitters energy that under multipath conditions can be lost causing a signal loss event
| |
| | |
| == Power defined in terms of voltage ==
| |
| | |
| The average power for a sinusoidal drive is covered in this
| |
| [http://web.mit.edu/6.763/www/FT03/Lectures/Lecture6.pdf MIT lecture] | |
|
| |
| :<math> P = \frac{1}{T}\int_0^T i(t) \cdot e (t) dt\,\!.</math>
| |
| | |
| For a system where the voltage and the current are in phase, the output power can be given as
| |
| | |
| :<math> P = \frac{1}{T \cdot R}\int_0^T e(t)^2 dt\,\!.</math>
| |
| | |
| In this equation <math> R </math> is the resistance and <math> e (t) </math> is the output voltage
| |
| | |
| == Nominal power of a TV transmitter ==
| |
| | |
| Nominal power of a TV transmitter is given as the power during the [[sync pulse|sync]] interval. (For the sake of simplicity aural power is omitted) Since, the voltage during the sync interval is a fixed value,
| |
| | |
| :<math> P_n = \frac{E_p^2}{2\cdot R}\,\!</math>
| |
| | |
| :<math> P_n = \frac{E^2}{R}\,\!</math>
| |
| | |
| where <math> E </math> is the [[root mean square|rms value]] of the output voltage.
| |
| | |
| To measure the nominal output power, measuring devices with [[time constant]]s much greater than the line time are used. So the measuring equipment's measure only the highest level (sync pulse) of a line waveform which is ''' 100%'''.
| |
| | |
| This power level is the commercial power of the transmitter.
| |
| | |
| == Thermal power ==
| |
| In analogue TV broadcasting, the video signal modulates a carrier by a kind of [[amplitude modulation]] (VSB modulation or C3F). The modulation polarity is negative. That means that the higher the level of the [[video signal]] the lower the power of the [[RF signal]].
| |
| | |
| * The lowest possible modulating signal during the synchrone interval yields ''' 100%''' of the carrier. (The nominal power of the transmitter.) The blanking level ('''300 mV''') yields ''' 73%''' (in an ideally linear transmitter).
| |
| | |
| * Usually the figure 75% is found to be acceptable. The highest modulating signal at white ('''1000 mV''') yields only ''' 10%''' of the carrier. (so called [[residual carrier]]).
| |
| | |
| * Sometimes 12.5% is used as the residual carrier so the output power applied to the antenna system is considerably lower than the nominal power.
| |
| | |
| The thermal power which can be measured by a [[microwave power meter]] depends on the program content as well as the residual carrier and sync depths.
| |
| | |
| == Ratio of thermal power to nominal power ==
| |
| Since the program content is variable, the thermal power varies during the transmission. However, for testing purposes a standard line waveform can be applied to the transmitter.
| |
| | |
| Usually line waveforms corresponding to '''350 mV''' or '''300 mV''' black image (and without field sync) are applied to the input of the transmitter.
| |
| | |
| For [[CCIR System B|System B]], the duration of the black level '''300 mV''' (together with the front and back porches), is '''59.3 μs''' and it corresponds to ''' 73%''' of maximum voltage level. The duration of the sync pulse is '''4.7 μs'''. The total duration of the line is '''64 μs'''.
| |
| | |
| :<math> P_t = \frac{E^2}{64\cdot R}\cdot (4.7\cdot ( 100 %)^2 + 59.3\cdot ( 73 %)^2) \approx 57% \cdot \frac{ E^2 }{R}\,\!</math>
| |
|
| |
| So the maximum thermal power applied to the antenna system is ''' 57%''' of the nominal power, even in the black scene. In normal program content this ratio may be around ''' 25%''' or less.
| |
| | |
| == References ==
| |
| Research papers & lectures
| |
| * This MIT lecture covers the issues of Analog TV transmitter power [http://web.mit.edu/6.763/www/FT03/Lectures/Lecture6.pdf]
| |
| {{-}}
| |
| | |
| {{Analogue TV transmitter topics}}
| |
| | |
| [[Category:Television technology]]
| |
| [[Category:Broadcast transmitters]]
| |
НАША КОМАНДА
КОНСТАНТИН
дизайнер
Я был на Вашем месте, я тоже заказывал заказать дизайн. Обращайтесь и я Вас удивлю!
ЕКАТЕРИНА
режиссер
Я арт-менеджер и мое любимое дело - это фото-режиссура.
РИЧАРД
кот
Я символ студии и я слежу за работой Кати и Кости.
http://www.kks.by