Method for automatically searching a frequency range for signal channels in a receiver for digitally modulated signals, and receiver for applying such a method

A method for automatically searching a frequency range for signal channels in a receiver for digitally modulated signals is described, each channel having a center frequency, a symbol rate, and a modulation type. The method comprises the steps of scanning the frequency range in first frequency steps corresponding with just less than half the minimum expected symbol rate of the channels, measuring the RF level at each frequency step and comparing the measured RF value with a threshold value, and, if the measured RF value is below the threshold value, continuing said scanning and measuring steps until the frequency range is fully scanned, and, if the measured RF value is above the threshold value, assuming the corresponding frequency as an indication of a digitally modulated signal channel and scanning a frequency area in second frequency steps to determine the edge frequencies of the channel, said second frequency steps being much smaller than said first frequency steps, determining the center frequency of the channel from said edge frequencies, and continuing said scanning and measuring steps for further signal channels until the frequency range is fully scanned.

BACKGROUND OF THE INVENTION 
The present invention relates generally to an automatic channel scanning 
method and more specifically to a method for automatically searching a 
frequency range for signal channels in a receiver for digitally modulated 
signals, and to a receiver for applying such a method. 
In a receiver for digitally modulated signals it is desirable to have a 
fast automatic method for finding the signal channels in a frequency 
range. However, to receive a signal, the center frequency of the channel, 
the symbol rate and the modulation type have to be known. In a 
conventional search method the receiver would try every possible 
combination of these values. The number of combinations is however immense 
and therefore this conventional procedure would take an unacceptably long 
time. 
The present invention aims to provide an improved search method wherein the 
search time is considerably reduced. 
SUMMARY OF THE INVENTION 
According to the invention a method of the above-mentioned type is 
provided, said method comprising the steps of scanning the frequency range 
in first frequency steps corresponding with just less than half the 
minimum symbol rate of the channels that can be expected, measuring the RF 
level at each frequency step and comparing the measured RF value with a 
threshold value, and, if the measured RF value is below the threshold 
value, continuing said scanning and measuring steps until the frequency 
range is fully scanned, and, if the measured RF value is above the 
threshold value, assuming the corresponding frequency as an indication of 
a digitally modulated signal channel and scanning a frequency area in 
second frequency steps to determine the edge frequencies of the channel, 
said second frequency steps being much smaller than said first frequency 
steps, determining the center frequency of the channel from said edge 
frequencies, and continuing said scanning and measuring steps for further 
signal channels until the frequency range is fully scanned. 
In this manner a method for automatically searching a frequency range for 
signal channels is obtained, wherein the frequency range can be scanned in 
a fast manner and wherein the step size of the first frequency steps is 
chosen in such a manner that on the one hand the steps are as great as 
possible and on the other hand a channel will never be skipped only when a 
channel is found the second frequency steps are used having such a step 
size that the bandwith of the channel can be determined with sufficient 
accuracy. 
The invention further provides a receiver for signals digitally modulated 
on a center frequency of a channel, the receiver according to the 
invention comprising a tuner, a demodulator for demodulating digitally 
modulated signals, and a control unit for controlling the tuner and the 
demodulator, wherein a narrowband RF level detector is provided receiving 
an output signal from the tuner and providing a RF level indication to the 
control unit, wherein said control unit is adapted to control the tuner 
for scanning a frequency range in first and second frequency steps, said 
first frequency steps corresponding with just less than half the minimum 
symbol rate of the channels that can be expected and said second frequency 
steps being much smaller than said first frequency steps, said control 
unit being adapted to control the tuner first to scan a frequency range at 
said first frequency steps until the output of the RF level detector 
provides an indication of a digitally modulated channel at a frequency and 
then to scan a frequency area around this frequency at said second 
frequency steps to determine the edge frequencies of the channel, wherein 
the control unit determines the center frequency from the edge frequencies 
found. 
The invention will be further explained by reference to the drawing in 
which an embodiment of the receiver according to the invention is 
schematically shown.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Just by way of example and without any intention to limit the scope of the 
invention it is assumed that the receiver shown is part of a so-called 
integrated receiver decoder receiving DVB (Digital Video Broadcasting) 
signals from a cable network or a satellite antenna. The receiver shown 
comprises a RF tuner 1 receiving DVB signals at its input. These signals 
are digitally modulated signals. The receiver further comprises a down 
converter 2 with oscillator 3 for converting the output signal of the 
tuner 1 to a baseband for further processing by an analog/digital 
converter 4 and a digital demodulator 5. A microprocessor 6 is provided as 
a micro controller for controlling the tuner 1 and demodulator 5. Further 
a memory 7 is provided for storing tuning information for the different 
channels present in the frequency range of DVB signals received. To recive 
a specific signal channel in the correct manner, the microprocessor 6 has 
to know the center frequency of the channel, the symbol rate and the 
modulation type. For DVB signals the frequency range is 47-862 MHz, the 
symbol rate is 1-7 MS/s and the modulation type can for example be QPSK, 
16, 32, 64, 128 or 256 QAM. Although in theory it would be possible to 
manually enter this data for all channels available, this would be very 
cumbersome for the consumers. In view of the immense number of 
combinations, a straight forward search method would take an unacceptably 
long time. 
The present invention provides a method for automatically searching a 
frequency range which is applied in the receiver shown and which 
considerably reduces the required search time. For applying the method 
described, the receiver is provided with a narrowband RF filter 8 
receiving the output signal of the tuner 1 after being converted to the 
baseband by the converter 2. The filter 8 is connected to a RF level 
detector 9 and the output of the detector 9 is provided to the 
microprocessor 6. 
When the receiver is installed for the first time or connected to a new 
cable network or channels have been changed in the cable network, new 
tuning information has to be stored in the memory 7. In some cases, it is 
possible for the consumer to enter the required tuning data for a home 
channel whereafter, when the receiver is tuned to this channel, the tuning 
data for the other channels in the network can be found in the so-called 
network information table (NIT). However, DVB channels in a cable network 
that are transmodulated from satellite only have a valid NIT if the 
satellite delivery system descriptors are replaced by cable delivery 
systems descriptors. Such a replacement of system descriptors is not 
always carried out. Therefore the receiver should be able to obtain the 
required tuning data by scanning the frequency range in a fast and 
efficient manner. 
For this purpose the microprocessor 6 starts scanning the frequency range 
in first frequency steps corresponding with just less than half the 
minimum symbol rate of the channels that can be expected. In the present 
example with a symbol rate of 1-7 MS/s, this means scanning the frequency 
range from 47 MHz to 862 MHz in the first frequency steps of 500 kHz. At 
each frequency step the microprocessor 6 measures the RF value at the 
output of level detector 9 and if the measured RF value is higher than a 
predetermined threshold value and preferably is within a small range of 
for example 6 dB of the previous RF value, the approximate position of the 
center frequency of a channel is found. This means that the frequency 
corresponding with the measured RF value is assumed to be an indication of 
a digitally modulated signal. 
Then the microprocessor 6 switches to scanning the frequency area around 
the assumed center frequency in second frequency steps suitable for 
finding tuning, i.e. steps much smaller than the first frequency steps, 
for example steps of 62,5 kHz. In this manner the microprocessor 6 
determines the frequency at both edges of the channel. When these edge 
frequencies f.sub.1 and f.sub.2 have been found, the actual center 
frequency can be estimated by calculating (f.sub.1 +f.sub.2 )/2. Further 
the microprocessor 6 can estimate the symbol rate from the bandwidth of 
the channel as the symbol rate is related to the bandwidth according to 
the formula: 
EQU bandwidth (f.sub.2 -f.sub.1)=symbol rate*(1+.alpha.) 
wherein .alpha. is the roll-off factor of the filter 8, which is normally 
15%. 
Thereafter the microprocessor 6 controls demodulator 5 to try to lock on 
the channel found by trying only different modulation types and perhaps a 
few symbol rates around the value estimated from the bandwidth of the 
signal. As soon as the demodulator 5 provides an indication to the 
microprocessor 6 that it has locked on the channel found, the 
corresponding information can be stored in the memory 7 for future use. 
Then the microprocessor continues to scan the remaining frequency range at 
the first frequency steps for further channels. Of course, scanning the 
frequency range at the first frequency steps is also continued as long as 
the RF level measured at each frequency step is below the threshold value. 
From the foregoing it will be clear that the invention provides a fast and 
efficient scanning method and receiver for applying this method, wherein 
the frequency range of interest is first scanned at frequency steps which 
are as large as possible while guaranteeing that a channel will not be 
skipped. Only when a channel indication is found a fine scanning takes 
place to determine the bandwith of the channel and thereby the symbol rate 
and then a modulation type is determined by trial and error. 
It will be understood that the receiver described and shown in the drawing 
is just given by way of example only without limiting the scope of the 
invention to this type of receiver. It is for example possible to apply 
the invention in a receiver wherein a down converter and oscillator are 
not used. 
The invention is not restricted to the above descripted embodiment which 
can be varied in a number of ways within the scope of the claims.