Digitally Extended Low Threshold
2.2 ; 1.4 oder 0.5dB
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.
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
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
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.
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 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 processing
The 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
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
For the most popular low threshold receivers we have modification
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
Many satellite receivers have also one (or more) external video input
which is elegant way to connect DIGITEX.
IF Input: 480MHz(479.5MHz)/75W
Optional IF (on request): 130/140MHz
Input Signal Range (adjustable):
-50 - -30dBm
TV Standards: PAL/SECAM with
also on request: NTSC with 525
IF Bandwidth (adjustable): 8
Threshold Values According to
Video Output (AGC): 50Hz - 5Mhz
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.
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