DIGITEX & DIGITEX PLUS
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Digitally Extended Low Threshold Demodulator
2.2 ; 1.4 oder 0.5dB Threshold ...
When receiving satellite signals within the center of the footprint of a satellite the quality of picture and sound is no problem. The situation completly changes when receiving signals at the edges of the central footprint or in case of sidelobe reception.
CARRIER-TO-NOISE (C/N)
The quality of picture and sound is determined by the carrier-to-noise (C/N) value of the received signal [dB] at the antenna input of the satellite receiver; refer also to the table below.
For instance a 60cm offset antenna (LNB NF 1.2dB) gives in case of ASTRA reception in Middle Europe 13dB C/N. At the Canary Islands the signal quality rapidly drops down to 4dB C/N when using an 6.0m dish equipped with an ultra low noise LNB with NF of 0.6dB. To improve this value by 3dB the antenna size has to be increased by factor 1.4 which means 8.40m. Precision antennas of this size are hard to find and also very expensive.
Standard satellite receivers are constructed for minimum C/N levels of about 6 to 7dB and are not suited for weak signal reception. On the other hand so-called "4dB Low Threshold-Receivers" with variable bandwidth or user selectable bandwidth steps are offered.
When the receiver`s IF (bandwidth) is narrower than the original transmitted bandwidth, some of the picture "fidelity" is lost. Reducing the bandwidth may reduce the appearent of noise in the picture, but in the process there are now new forms of picture degradation introduced.C/N value vs. picture quality
With this table the input C/N value[dB] could be determined (+/- 1dB) by using a standard satellite receiver with 27MHz IF-bandwidth and 6-7dB threshold.
C/N<8dB: Full quality for consumer applications
C/N 7-8dB: Good picture, but first sparklies appear in bright coloured areas
C/N 6dB: Still good picture, but a few sparklies are always visible
C/N 4-5dB: Sparklie noise increases, butthe picture is still watchable
C/N 3-4dB: Picture rapidly becomes washed out by sparklie noise, but still in colour
C/N 1-2dB: Picture hardly to recognize, colour disappears
C/N 1dB: Noise is so strong that picture details are not recognizable
C/N 0dB: Picture dissapears completly in the noise, only the vertical sync bare is sometimes recognizable
ANY BETTER WAY ?
As a solution for weak signal reception so-called "Low Threshold Receivers" are offered, which work with bandwidth reduction principles. Reducing the bandwidth for instance from 27MHz to 12MHz gives a about 3dB better C/N value.
How do Low Threshold devices work ?
For low cost production most of them use analog principles. Phase-Locked-Loop (PLL)-demodulators are well suited. By reducing the bandwidth the feed-back information of the PLL is slowed down.
In case of weak signal reception short signal interruptions would result in drop-outs/sparklies of the video signal. But if the PLL is reacting slower most of the sparklies would disappear. On the other hand this causes strong noise within saturated colour parts and smearing sharp edges.
In practice a compromise has to be found between noise reduction and the lengths of smearing edges.This principle is a satisfying solution for C/N ratios down to 4dB according to 27MHz IF-bandwidth.
But many of our customers told us that they have much worser C/N ratios in the range from 0 to 4dB. For this range another solution had to be found, which we call the second digital generation.
THE DIGITEX PRINCIPLE ...
The feed-back information of the PLL is composed of two elements instead of only one component:
1.Real-time analog information
2.Digitally delayed picture information.
This results in quicker response of the PLL behaviour and most of the above described problems would disappear.
This principle is neither new nor revolutionary. A British based company already introduced in 1987 a demodulator unit using similar principles. This high-end unit is now out of production. It was constructed for 70MHz input frequency and was also intended for professional applications.
The DIGITEX demodulator is a cost-effective solution for semi-professional applications. This was only possible by using a new generation of complex circuits for picture processingThe unit needs the 480MHz IF-frequency of the satellite receiver. This signal is availible at the tuner`s SAW-filter and could be fed to an additional F-connector at the rear panel.
This requires some technical skills and knowledge about the principal functions of a satellite receiver.
If small smearing edges are acceptable the unit could be operated down to a C/N ratio from 0-1dB where the first drop-outs just occour:
An artificial synchronisation could be additionally activated in case of distorted sync pulses.
IN WHICH WAY IS DIGITEX CONNTECTED TO THE RECEIVER ?
We have already mentioned it - DIGITEX needs a 480MHz IF signal from the satellite receiver`s tuner with constant level.
This constant level is very important especially with very low antenna input levels; otherwise DIGITEX could not develop full performance.
High-end receivers could deliver a constant IF-level at antenna input levels between 30-90dBuV while standard units have only a dynamic range from 50-80dBuV.
Only a few new receivers deliver this signal already at the rear panel.
The other receivers have to be modified with an additional 480MHz output.
For the most popular low threshold receivers we have modification instructions available.
The modification does not affect the receiver`s performance - no matter whether a DIGITEX is connected or not.
The output of DIGITEX is a video signal with with automatic brightness control and also a baseband signal (unfiltered & unclamped) which can be used for connecting an optional SOUND PROCESSOR.
The video signal can be directly connected to the TV-set via the A/V input while the satellite receiver is connected via the TV-set`s antenna input (UHF-modulator output).
Many satellite receivers have also one (or more) external video input which is elegant way to connect DIGITEX.
TECHNICAL DATA
IF Input: 480MHz(479.5MHz)/75W
Optional IF (on request): 130/140MHz
Input Signal Range (adjustable): -50 - -30dBm
TV Standards: PAL/SECAM with 625 lines
also on request: NTSC with 525 lines
IF Bandwidth (adjustable): 8 - 36MHz
Threshold Values According to 27MHz Bandwidth:
DIGITEX <2.2dB
DIGITEX with Option SUPER-FEEDBACK <1.4dB
DIGITEX PLUS <0.5dB
Video Output (AGC): 50Hz - 5Mhz / 1Vss
Baseband Output (AGC): 50Hz - 8MHz / 1Vss
Supply: 12 - 15V DC / max. 400mA
Dimensions: 31 x 113 x 165 mm
Weight: ca. 350g
Specifications are subject to change without notice.
All trademarks accepted.
TEST REPORTS:
Christian Mass: 1.4dB FM-Schwelle kein Traum mehr! - TELE-satellit 12/94
Christian Mass: DX-Corner / Darf`s ein paar dB weniger sein? - TELE-satellit 7-8/96
Paul van Rossum: Verbeter zwaake satellitsignalen RAM No.178 7-8/96
Bob Cooper: Threshold Extension - Does it really work? SatFACTS monthly 2/95
Eric Wiltsher: TESUG Newsletter 2/95