May 282011
 

TELEVISION TRANSMISSION

In TV broadcast both the sound signal and the video signal are to be conveyed to the viewer using radio frequency.  These two signals have very distinct features. The audio signal is a symmetrical signal without continuous current but the frequency does not exceed 20 kHz. The video signal consists of a logical component, the sync and the field sync and an analogue part according to the line picture scanning.  This unsymmetrical signal thus has a continuous component.  The frequency bandwidth also extends from 0 to 5 MHz. The two signals modulate the carrier waves whose frequencies and types of modulations are as per established standards.  These modulated carriers are further amplified and then diplexed for transmission on the same line and antenna.  This technique is used with High Power TV Transmitters.  However for LPTs i.e. transmitters operating at sync peak power less than 1 kW, both the signals (video and audio) are modulated separately (In most of the present day TV transmitters the picture signal is amplitude modulated while the audio signal is frequency modulated) but amplified jointly using common vision and aural amplifiers.  Both of these systems have merits and demerits.  In the first case (separate amplification) special group delay equalisation circuit is needed because of errors caused by diplexer while in the second case inter modulation products are more prominent and special filters for suppressing them are required.  Hence technique of joint amplification is suitable only for LPTs and not for HPTs.

Though frequency modulation has certain advantages over amplitude modulation, its use for picture transmission is not permitted due to large bandwidth requirements, which is not possible due to very limited channel space available in VHF/UHF bands.  Secondly as the power of the carrier and side band components go on varying with modulation in the case of FM, the signal with frequency modulation after reflection from nearby structures at the receiving end will cause variable multiple ghosts, which will be very disturbing.  Hence use of FM for terrestrial transmission of picture signal is not permitted.

Thus amplitude modulation is invariably used for picture transmission while frequency modulation is generally used for sound transmission due to its inherent advantages over amplitude modulation.  As the picture signal is unsymmetrical, two types of modulation is possible.

i)  Positive modulation

Wherein the increase in picture brightness causes increase in the amplitude of the modulation envelope.

 ii) Negative modulation

The increase in picture brightness causes reduction in carrier amplitude i.e. the carrier amplitude will be maximum corresponding to sync tip and minimum corresponding to peak white.

In television though positive modulation was adopted in initial stages, negative modulation is generally adopted (PAL’B uses negative modulation) now a days, as there are certain advantages over positive modulation.

 Advantages of Negative Modulation

 i)Impulse noise peaks appear only in black region in negative modulation.  This black noise is less objectionable compared to noise in white picture region.

ii) Best linearity can be maintained for picture region and any non-linearity affects only sync which can be corrected easily.

iii) The efficiency of the transmitter is better as the peak power is radiated during sync duration only (which is about 12% of total line duration).

iv) The peak level representing the blanking or sync level may be maintained constant, thereby providing a reference for AGC in the receivers.

v)  In negative modulation, the peak power is radiated during the sync-tip.  As such even in case of fringe area reception, picture locking is ensured, and derivation of inter carrier is also ensured.

VESTIGIAL SIDE BAND TRANSMISSION

Another feature of present day TV Transmitters is vestigial side band transmission. If normal amplitude modulation technique is used for picture transmission, the minimum transmission channel bandwidth should be around 11 MHz taking into account the space for sound carrier and a small guard band of around 0.25 MHz.  Using such large transmission BW will limit the number of channels in the spectrum allotted for TV transmission.  To accommodate large number of channels in the allotted spectrum, reduction in transmission BW was considered necessary.  The transmission BW could be reduced to around 5.75 MHz by using single side band (SSB) AM technique, because in principle one side band of the double side band (DSB) AM could be suppressed, since the two side bands have the same signal content.

It was not considered feasible to suppress one complete side band due to difficulties in ideal filter design in the case of TV signal as most of the energy is contained in lower frequencies and these frequencies contain the most important information of the picture.  If these frequencies are removed, it causes objectionable phase distortion at these frequencies which will affect picture quality.  Thus as a compromise only a part of lower side band is suppressed while taking full advantage of the fact that:

 i) Visual disturbance due to phase errors are severe and unacceptable where large picture areas are concerned (i.e. at LF) but

ii) Phase errors become difficult to see on small details (i.e. in HF region) in the picture.  Thus low modulating frequencies must minimize phase distortion where as high frequencies are tolerant of phase distortions as they are very difficult to see.

The radiated signal thus contains full upper side band together with carrier and the vestige (remaining part) of the partially suppressed LSB.  The lower side band contains frequencies up to 0.75 MHz with a slope of 0.5 MHz so that the final cut off is at 1.25 MHz.

 RECEPTION OF VESTIGIAL SIDE BAND SIGNALS

Corresponding to the VSB characteristics used in transmission an amplitude versus frequency response results.  When the radiated signal is demodulated with an idealized detector, the response is not flat.  The resulting signal amplitude during the double sideband portion of VSB is exactly twice the amplitude during the SSB portion.

 This characteristic is shown in Fig.

 

 
Fig- Response for VSB reception

 

In order to equalize the amplitude, the receiver response is designed to have an attenuation characteristics over the double side band region appropriate to compensate for the two to one relationship.

This attenuation characteristic, the so-called nyquist slope, is assumed to be in the form of a linear slope over the + 750 kHz (DSB region) with the visual carrier located at the mid point (-6 dB point) relative to SSB portion of the band.  Such a characteristic exactly compensates the amplitude response non-symmetry due to VSB.  Fig..

 

 

 

Typical receiver characteristics

 

 

 

Modern practice for purposes of circuit and IF filter design simplification, also provides an attenuation of the upper end of the channel such that colour sub carrier is also attenuated by 6 dB as shown in Fig.

Typical receiver IF characteristics

TV receivers have Nyquist characteristics for reception which introduces group delay errors in the low frequency region.  Notch filters are used in receivers as aural traps in the vision IF and Video amplifier stages.  These filters introduce GD errors in the high frequency region of the video band.  These GD errors are pre-corrected in the TV transmitters (using RX pre corrector) so that economical receiver filter design is possible.  The group delays of the RX and TX with pre-correction are shown in Fig.

 

 

Group delay curves

 

 

Depth of Modulation

Care must be taken to avoid over-modulation at peak-Luminance signal values to avoid picture distortions and interruptions in vision carrier. The peak white levels when over modulated tend to reduce the vision carrier power or even cause momentary interruptions of vision carrier.  These periodic interruptions due to accidental over modulation result in interruptions of the sound carrier in inter carrier receiver systems which produces undesired sound buzz in the receiver output.

 Therefore, to prevent this effect, the maximum depth of modulation of the visual carrier by peak white signal values is specified as being 87.5%.  This 12.5% residual carrier (white level) is required because of the inter-carrier sound method used in TV receiver .

Carrier signal and modulation envelop

The depth of modulation is set by using a ramp signal or step signal as given in the manual.  It should be 87.5% for 100% modulation (i.e. m = 1).

Inter Carrier

 The TV receivers incorporate inter carrier principle.  According to our system, the inter-carrier i.e. the difference between the vision transmitter frequency and sound transmitter frequency is 5.5 MHz.  Hence it is to be ensured that even when the modulating video signal is at white peak, 12.5% of residual carrier is left so that sound can be extracted even at the peak white level, where the carrier power is minimum.

Power Output

 The peak power radiated during the sync. tip or sometimes the carrier power corresponding to black level is designated as the vision transmitter power.  This power is measured by using a thruline power meter after isolating the aural carrier.  The power read on thruline meter is multiplied by a factor of 1.68 to get the peak power (vision) radiated.  As transmitter output is connected to an antenna, having a finite gain, the effective radiated power (ERP) is obtained by multiplying the peak power by the antenna gain (w.r.t a half wave dipole).  Hence a 100 W LPT using transmitting antenna having a gain of 3 dB w.r.t a half wave dipole will have an ERP of 200 W or 53 dBm or 23 dBW.

 In TV broadcasting, the sound signal is transmitted by frequency modulating the RF sound carrier in accordance with the standards. The sound carrier is 5.5 MHz above the associated vision carrier. The maximum frequency deviation is + 50 kHz which is defined as 100 per cent modulation in PAL-B system.  In the case of NTSC, the maximum deviation permissible is + 25 kHz.

Standards

The characteristics of the TV signal in sections 1 and 2 refer to CCIR B/G standards.  Various other standards are given in Table .

 Table 1

 

Vision/sound carrier spacing
channel width

Frequency Range

Vision sound carrier spacing

5.5 MHz

Channel width

7 MHz (B) VHF

8 MHz (G) UHF

Sound Modulation

FM

FM deviation (maximum)

+ 50 kHz

 

Standards

American

European PAL

No. of lines per frame

525

625

No. of frames per second

30

25

Field Frequency Hz

60

50

Line Frequency Hz

15750

15625

Channel  MHz

6

7

Video BW, MHz

4.2

5

Colour subcarrier, MHz

3.58

4.43

Sound System

FM

FM

Maximum sound deviation kHz

25

50

Intercarrier frequency MHz

4.5

5.5

 

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