Line CN for analog signals
Line CN for analog signals
With so much information and promotion discussed concerning the new Digital Terrestrial Television (DTT), its COFDM modulation, the incipient enhancements it will soon bring, such as the High-Definition (HDTV), or the interactive services, eccetera, we tend to forget that the analog signals will co-exist in our day to day work for quite some time.
What? Are we not switching off all analog TV signals in Europe, depending on what country, around 2012?
Well, yes we are, but it does not erradicate a lot of real scenarios where that digital signal is transmodulated at the headend and then distributed in an analog format throughout the building.
Transmodulators are electronic devices that demodulate an input signal, extract its services and contents and remodulate the information into an output signal of a different modulation format.
For example, the DVBS or DVBS2 to QAM transmodulators take advantage of a coax distribution format in larger installations to bring satellite services to the final users at their office/home.
Another important example which is vastly used is the transmodulator to PAL/SECAM analog format. We are able to bring all the new services brought to us by either the terrestrial or satellite content providers without the need to invest in any new decoders or new television sets. The QPSK to PAL/SECAM selects a video and audio service in any input transponder and the channel is transmodulated to analog in the desired frequency for its distribution.
Fig. 1 – Example of analog PAL channel spectrum. SPAN 100 MHz.
Because this digital to analog transmodulation method is so frequently used, the professional installer must ensure that his measuring analyser is equipped to thoroughly measure both digital and analog types of signals in all their modulation formats.
Traditionally the analog distributions have been measured using Level, CN and VA parameters, which most field spectrum analysers in the market can give you somewhat accurately.
Well, you may be in for a surprise, but Level, CN and VA alone won´t do it for you anymore.
Fig. 2 – Analog capture with good (green) 80,5dBuV of Level
There is a potential problem lurking... There are many instances when a signal with excellent level, CN and VA readings could have horrible quality at the receiving end (fig.3)
Fig.3 – Defective quality with good Level, CN and VA readings
So, why is this happening? In this particular case exposed, the degradation is caused by a tone interfering with the channel. The higher the amplitude of the interfering tone, the higher the interference, which causes a greater degradation of the quality of the image at the far end. Neither Level, CN, nor VA could ever detect that problem because they simply are not affected by it.
So, what do we do about it? We need a numerical measurement that objectively characterises the quality of the image –for example a measure in dB- thus eliminating the always subjective human factor and giving the installer a clear comparison of tweeks and changes as we calibrate the signal in search of the perfect image.
We need the LINE CN !!
The Line CN measures the signal to noise ratio on a specific video line, instead of measuring the carrier as the standar CN does. The PAL system has 625 lines, where line 5 or 6 are traditionally “empty of content” and used for TEST purposes. By synchronising to these video lines the field spectrum analyser can give an exact idea of the quaility of the video. No more doubts in your analog installations.
Figures 4, 5 and 6 illustrate very clearly various scenarios with different Line CN levels. It is necessary to obtain at least 40 dBs of Line CN to ensure the maximum potential of the quality of the analog video signal throughout time.
Fig. 4 – Defective image. Good Level, CN and VA, but Line CN only 24dB (red)
Fig. 5 – Marginal image, with good Level, CN and VA, but Line CN of 35dB (yellow)
Fig. 6 – Perfect analog installation with excellent Level, CN, VA and Line CN (green)
Only the Line CN is able to idenfity the problem and the H45 field spectrum analyser represents it graphically and numerically, so that the installer can pursue the needed adjustments mathematically.
The Line CN is part of a complete package of measurements that the H45 provides to characterise any analog TV channel or distribution network to its fullest potential of quality. Amongst them we can see the real Impulse Synch, or even view the graphical representation of any video line. You can zoom on any channel for detailed analysis or scroll along the different lines.
The Digital Processing engine that Televes has incorporated into the H45 line of field spectrum analysers allows the meter to capture up to 20MHz of radioelectric spectrum in less than 10ms, giving us unprecedented analysis of the digital signals available within it (COFDM, QPSK, 8PSK, QAM... even future ones like COFDM-2), without forgetting the need of analog signals and distribution networks.