Analog TV transmitters’ parameters measure

By Mikhail Leiderman

Review contents:

Measure seven times Analog TV transmitters’ parameters measure Are You Producing Legal and Valid Video? “Beta TV Com” measuring equipment Measuring devices for terrestrial and cable television from Broadcast Arsenal Group of companies Measuring TV equipment from IMOS Company TV signals level measurers Measuring tools of Planar Company RAP-TV digital measuring system Kathrein TV signals measuring tools Leader Electronics measuring equipment for broadcast sphere Promax measuring equipment Recom-10 TV signal meter Sunrise Telecom’s measuring equipment for cable TV network analysis New devices from Tektronix Equipment from Wohler Company

Matter of qualitative and mobile measure of analog TV transmitters’ parameters will remain actual issue in Russia 10…15 years as minimum. It can be said for sure keeping in mind both the speed of move toward digital TV in Europe and those difficulties which will appear at total replace of TV equipment in Russia.

TV transmitters’ parameters and their impact to the quality of picture received

According to Russian specefications (GOST 20532-83, GOST P 50890-96, RTE 95/98) following parameters of video channel for TV transmitters must be measured:

Fig. 1. Analog TV transmitters parameters classification

Besides, according to RTE 95/98 in the process of broadcasting TV programs continuous monitoring of following parameters must be performed:

Transmitted radio signal’s parameters influence to not only quality of picture received but also to electromagnetic interoperability. Briefly this influence is described in chart.

Transmitted radio signal influence
Parameter	Result of parameter’s deviation
Power ratio of video and audio careers	Intermodulation products’ level increase when audio career exceeds maximum level
Modulation coefficient	Non-linear distortion increase when the coefficient exceeds normal value; S/N ratio decrease when the coefficient is under normal value
Intermodulation products’ level	Well known ripple on a screen reacting onto audio level; cross talk to neighbor channels
Side bands characteristics	Linear distortions increase, cross talk to neighbor channels
Instability of career frequency	In range of ±50 kHz practically doesn’t influence to a picture of modern TV sets; serves as secondary indicator of transmitter’s frequency synthesizer quality

Linear distortions are caused by frequency and phase response non-linearity of transmission path and they don’t lead to new harmonics appearance and don’t depend on signal level.

Influence of linear distortions
Parameter	Result of parameter’s deviation
Transition characteristic	Black and bright (sometimes chrominance) edges near to brightness changes, smeared vertical lines
Defect of line frequency pulses flat part	Left to right brightness change on long objects
Defect of field frequency pulses flat part	Top to bottom brightness change on long objects
Luminance and chrominance signals amplification difference	Chrominance details lost. No visible influence to color reproduction (SECAM), decrease of chroma saturation (PAL, NTSC)
De-synchronization of chroma and luma signals	Coloration of objects’ edges on a picture, “fringe”
Total frequency response of video channel	Very different distortions including ones described above (all linear distortions are caused by distortions of frequency and phase responses

Non-linear distortions appear due to non-ideal both static and dynamic amplitude characteristics of transmitter’s circuits (transfer functions of these circuits depend on signal level).

Non-linear distortions influence
Parameter	Result of parameter’s deviation
Luma channel’s non-linearity	Details lost on dark areas and glares
Chroma channel’s non-linearity	Mostly doesn’t affects (SECAM), insufficient color saturation (PAL, NTSC)
Differential amplification	Mostly doesn’t affects (SECAM), color saturation change upon brightness change (PAL, NTSC)
Differential phase	Color edge on sharp bright transitions (SECAM), color tone change (PAL, NTSC)

Noises and artifacts influence
Parameter	Result of parameter’s deviation
Luma channel S/N ratio	“Snow”, chaotic color and b&w dots
Luma to interference ratio	Slowly moving horizontal bars

Equipment for TV transmitters’ parameters measure

Standard techniques for parameters measure described in official manuals presume the use of testing TV demodulator and universal measuring devices. In recent time two totally new classes of equipment has appeared: video analyzers and RF analyzers. Use of computers and modern digital processing methods within these devices allows measure using the same techniques but with higher precision and pretty less time.

Comparison given below is equally correct both for planned (during broadcast breaks) measures and exploitation monitoring during broadcast process. Additional circuits needed for on-air monitoring are colored. It is also presumed that test signals generators support insertion of control lines into full color TV signal.

Measures using traditional devices set

The measures are conducted according to the following structure scheme.

Fig. 3. Structure measuring scheme using ’classic’ devices set

Signal from test signals generator is fed to transmitter’s modulation input. A transmitter’s output signal goes to measuring demodulator via routing splitter. Am operator monitors demodulated signal on the waveform monitor and calculates parameters’ values using appropriate techniques.

For measuring following parameters additional devices should be connected to routing splitter output:

• Career instability — frequency measuring device;
• Side bands characteristics — side band analyzer;
• Intermodulation products level — selective voltmeter or specter analyzer.

To measure a differential phase a specialized differential phase distortions measuring device should be connecter to demodulator’s output.

Main components of measure errors:

• Non-ideal output signal of test signals generator;
• Operator’s mistake of waveform monitor reading interpreting;
• Distortions caused by testing demodulator.

Modern measuring demodulators are very sophisticated and expensive devices but nevertheless the distortions caused by them are comparative to precision error limits of parameters measured. Lets explain it using little example. Most of modulators perform synchronous detection of a signal passed through filter which has frequency response (in ideal) looks like shown on a figure.

Fig. 4. Ideal frequency response of measuring demodulator’s filter

Hardware realization of such filter having minimal pulsation of response in bandwidth up to Fvc + 6 MHz (Fvi is video career frequency) and at the same time providing sufficient (40…50 dB) audio career attenuation is very complex task. Even famous industry leader Rohde&Schwarz Company provides response non-stability of its demodulators in ±0,25 dB range.

Measure using video analyzer

Main difference of measures using video analyzer in comparison with traditional technique is that the demodulated signal is being digitized and its samples are being transferred to computer which performs all routine operator’ work: readings accept, results calculations, edge limits monitoring. An operator works with convenient graphical interface: he just selects appropriate parameter and immediately sees on the screen its value, waveforms, measuring errors control results. The advantages of such approach are obvious:

• Increase of measures precision due to avoiding operator’ mistakes;
• Sufficient time saving for measures;
• Ability of automated measures with log creation and printing;
• Many additional services and functions.

Besides, a test signals generator within video analyzer is just A-to-D converter which test signals samples created by computer are downloaded to. So, there is a possibility to compensate an error of generator itself: either by pre-calibration and pre-distortions insert of by generator’ output signal digitizing (transmitter’s modulation input) during measures and appropriate results correction.

Fig. 5. Structure measuring scheme using video analyzer

Measures error is defined by testing demodulator. To illustrate it one can note that modern video analyzers can provide their own response instability at ±0,05 dB level or better in 0…6 MHz range while the most perfect demodulators provide total response instability at ±0,25 dB level.

Still for measure of side bands characteristics and career frequency instability additional devices must be connected.

Measures using RF analyzer

RF analyzer fully realizes all advantages of video analyzer described above. But in contrast to video analyzer it digitizes not demodulated signal but intermediate frequency signal (modern ADCs allow IF digitizing directly or by quadrature converters). Video signal demodulation (including Nyquist filtering and audio rejection) is being performed by computer’ software application which allows made distortion caused by virtual demodulator very minor. For example there is no problem in realization of audio rejection filter with response equal to Butterworth filter of 250th degree and with absolutely linear phase response.

Fig. 6. Structure measuring scheme using radio frequency analyzer

Tuner is an electronic device and definitely causes distortions but in comparison with hardware demodulator it has the following advantages:

• Tuner doesn’t have a need of using high figure of merit resonance circuits (for instance filters like audio rejection one);
• Tuner is a part of RF analyzer and sufficient deal of distortions caused by it can be compensated by software tools during calibration process.

All transmitted radio signal parameters including side bands characteristic and career frequency instability are measured by RF analyzer directly. And for modulation coefficient measure “0 definition” is not needed. It became available because the digitized RF signal (IF signal) is available for processing.