As I describe in my TVRO page, this
invention was entirely my own work but John Ma, the project director, is
listed first on all patents.
| United States Patent |
4,783,848 |
|
Ma
, et al.
|
November 8, 1988
|
TVRO receiver system for locating audio subcarriers
Abstract
An improved satellite receiver system is provided which automatically
locates audio signals among the various audio subcarriers received by an
earth station; manual scanning of transmissions received from different
transponders in order to locate audio signals embedded therein is
eliminated. The improved receiver system does not require any input data
characterizing the frequencies or bandwidth of the incoming audio signals
and locates desired audio subcarriers in response to a command signal,
without need for any manual intervention after generation of the command
signal.
| Inventors: |
Ma; John Y. (Milpitas, CA), McCracken; David H. (San Jose, CA) |
| Assignee: |
Capetronic (BSR) Ltd.
(Kowloon,
HK)
|
| Family ID:
|
26293320
|
|---|
| Appl. No.:
|
06/789,424 |
|---|
| Filed:
|
October 21, 1985 |
|---|
| Current U.S. Class: |
455/182.2 ; 455/183.1; 455/207; 455/264; 455/315 |
| Current International Class: |
H03J 7/18 (20060101); H03J 1/00 (20060101); H04B 011/16 (); H04B 001/26 () |
| Current CPC Class: |
H03J 1/0033 (20130101); H03J 7/18 (20130101) |
| Field of Search: |
455/182,183,192,200,260,264,265,207,208,209,315,131 358/195.1 381/98
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
NEC Corporation, "Bipolar Analog Integrated Circuit" brochure, No. IC-1312A Nov. 1984.. |
Primary Examiner: Griffin; Robert L.
Assistant Examiner: Kuntz; Curtis
Attorney, Agent or Firm: Rudisill; Stephen G.
Claims
What is claimed is:
1.
An audio tuning system for receiving and tuning audio subcarrier
signals in a TVRO system receiving RF signals from communication
satellites, converting the RF signals to
an IF frequency range, and demodulating the IF signals, said audio
tuning system comprising,
a super heterodyne circuit producing an output signal, said
circuit including a voltage-controlled oscillator VCO, and mixer for
combining the demodulated IF signals with the output of said VCO to
convert the frequency of the demodulated IF
signals to desired frequencies in an audio IF frequency range,
a detector for detecting an audio signal at a prescribed audio
IF frequency in the output of said super heterodyne circuit, said
detector including automatic frequency control (AFC) means responsive to
the audio IF output of said mixer for
producing an error voltage representing any difference between the
actual and desired frequencies of said audio IF output,
means for adjusting the controlling input voltage to said VCO in
response to said error voltage to eliminate said difference between the
actual and desired frequencies of said audio IF output by adjusting the
frequency of the output of said VCO,
means for generating an "audio seek" command signal, and means
responsive to said "audio seek" command signal for
artificially adjusting the controlling input voltage to said VCO in a preselected direction independently of said error voltage,
monitoring the resulting error voltage produced by said AFC
means in response to said artificial adjustment of said controlling
input voltage,
continuing to artificially adjust the controlling input voltage
to said VCO in said preselected direction in response to monotonic
changes in the resulting error voltage, and
terminating the artificial adjustments of the controlling input
voltage to said VCO in response to a non-monotonic change in the
resulting error voltage, whereby the normal operation of said AFC means
causes the heterodyne circuit to lock on to a
new audio signal to produce the desired audio IF output.
2. The audio tuning system of claim 1 wherein said AFC means is
a phase locked loop including a second voltage controlled oscillator
operating at a fixed frequency, and a phase detector receiving the
output of said second voltage controlled
oscillator and the audio IF output of said mixer and producing said
error voltage in response to a difference between said outputs received
by said phase detector.
3. The audio tuning system of claim 1, wherein said means
responsive to said "audio seek" command comprises a microprocessor
receiving said error voltage and programmed to artificially increase the
controlling input voltage to said VCO by a
preselected increment in response to said "audio seek" command.
4. The audio tuning system of claim 3 wherein said
miciroprocessor is also programmed to continue to artificially increase
the controlling intput voltage to said VCO as long as the resulting
error voltage changes monotonically.
5. The audio tuning system of claim 3 wherein said means for
adjusting the controlling input voltage to said VCO in response to said
error voltage also comprises said microprocessor.
6. The audio tuning system of claim 1 wherein said artificial
adjustments of the controlling input voltage to said VCO are effected in
steps, each of which changes the output frequency of said VCO by about
20 KHz.
7. The audio tuning system of claim 1 which includes means for
limiting the continuing adjustments of said controlling input voltage in
each direction so that the seeking of a new audio signal is confined to
a selected range.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to receivers for TVRO
earth stations which receive audio and video signals from a plurality of
orbiting earth satellites. More particularly, the invention relates to
a TVRO receiver system for locating the
subcarrier frequencies for multiple audio signals included in the
transmissions received from satellites having multiple transponders.
As used herein, the term audio "subcarrier" refers not only to
audio carrier frequencies which are secondary to a main video carrier
handled by the same transponder, but also to audio carrier frequencies
used for FM transmissions in the absence
of video carriers.
In satellite communication systems, a transmitting earth station
generates a modulated carrier in the form of electromagnetic waves up
to a satellite, forming an "uplink." The incident electromagnetic waves
are collected by the satellite,
processed electronically to reformat the modulated carrier in some way,
and retransmitted to receiving earth stations, forming "downlinks." The
earth stations in these systems basically consist of a transmitting
and/or receiving power station functioning
in conjunction with an antenna subsystem and form strategic parts of the
satellite communication system.
A TVRO earth station typically comprises a receiving antenna
such as a paraboloidal dish, a low noise block converter (or a low noise
amplifier (LNA) and a down converter) located at an outdoor antenna
site, and a receiver located near an indoor
television set. The down converter and the receiver are usually
connected by a coaxial cable.
A single transponder in a satellite can carry a color television
channel, including both the video and audio information, and also
several auxiliary services such as radio stations, newservice feeds,
special news teletypewriter channels,
high-speed stock market and commodity exchange data feeds, and/or
teletext data services. A transponder normally has a usable modulating
signal bandwidth of 8 to 10 MHz, and the video information normally
occupies the band up to about 4.2 MHz. The
audio portion of the television channel is placed on an FM subcarrier in
the 5.8 to 7.4 MHz range (usually either 6.2 or 6.8 MHz), which leaves
available all the other FM subcarrier frequencies located above 4.2 MHz.
For example, in several of the
satellites presently orbiting the earth, transponder owners feed
separate audio subcarriers at 5.8, 6.2, 6.8 and 7.4 MHz. Other
transponders on the same satellites carry music services at 5.58 and
5.76 MHz.
A satellite transponder may also be used to carry multiple
narrow-band audio signals in place of a wide-band video signal. These
audio signals may be interspersed with other types of auxiliary
services, and thus the exact frequency of the audio
subcarriers can vary widely among the large number of transponders
presently in orbit, and new subcarriers can become available at any time
as more subcarrier services are squeezed onto existing transponders and
as additional satellites are placed in
orbit. Moreover, there is no fixed relationship among the numerous
audio signals themselves.
Current satellite receivers often include tunable audio
subcarrier detectors for locating the various "hidden" audio
subcarriers. Some of these receivers include a tuning knob for
selecting the 6.2-MHz or 6.8-MHz subcarriers because these are
two of the most commonly used frequencies. Other receivers have a
tuning knob that permits the user to manually scan the FM baseband
spectrum to locate the various audio subcarriers being received from
each transponder; this scanning and tuning
procedure must, of course, be repeated for each different transponder,
which can become a tedious process when searching for the narrow-band
audio signals transmitted by a number of different transponders.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide an
improved satellite receiver system which automatically locates audio
subcarriers among the numerous signals received by an earth station. In
this connection, a related object of the
invention is to provide such an improved receiver system which is
particularly useful in TVRO earth stations because it eliminates the
need for the user to manually scan the transmissions received from each
different transponder to locate the audio
signals embedded therein.
It is another important object of this invention to provide an
improved satellite receiver system which locates the audio subcarriers
quickly and reliably in response to a command signal, without any
further manual intervention after the command
signal is generated.
A further object of the invention is to provide an improved
satellite receiver system which is simple and inexpensive to implement
in an otherwise standard satellite receiver, and which does not
significantly affect the overall cost of the
receiver system.
Yet another object of this invention is to provide an improved
satellite receiver system which does not require any input data or other
intelligence concerning the frequencies or bandwidths of the audio
signals.
Other objects and advantages of the invention will be apparent
from the following detailed description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and other objects and advantages thereof may best
be understood by referring to the following description in conjunction
with the accompanying drawings, in which:
FIG. 1 is a simplified block diagram of a conventional TVRO earth station;
FIG. 2 is a block diagram of a preferred embodiment of the audio
tuner included in the TVRO earth station of FIG. 1 and embodying the
present invention;
FIG. 3 is a flow chart of a software program for controlling the
programmable microprocessor in the tuner of FIG. 2 to execute a "main
process" subroutine;
FIG. 4 is a flow chart of a software program for controlling the
programmable microprocessor in the tuner of FIG. 2 to execute an "AFC"
subroutine;
FIG. 5 is a flow chart of a software program for controlling the
programmable microprocessor in the tuner of FIG. 2 to execute a
"left-channel AFC" subroutine; and
FIGS. 6(a) and 6(b) is a flow chart of a software program for
controlling the programmable microprocessor in the tuner of FIG. 2 to
execute an "audio seek" subroutine.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although the invention will be described in connection with
certain preferred embodiments, it will be understood that it is not
intended to limit the invention to those particular embodiments. On the
contrary, it is intended to cover all
alternatives, modifications and equivalent arrangements as may be
included within the spirit and scope of the invention as defined by the
appended claims.
Referring now to the drawings, in FIG. 1 there is shown a
functional block diagram of a TVRO earth station for the reception of
satellite signals. The system includes an antenna 11, which is
typically a paraboloidal dish equipped with a low
noise block (LNB) converter and related accessories and positioning
mechanisms, for capturing signals transmitted from orbiting satellites;
and a receiver system including a tuner 12, a demodulator 13, a video
processing and amplification section 14, and
an audio tuner 15.
The antenna 11 receives signals transmitted from the satellite
in the four-GHz frequency band (3.7 to 4.2 GHz); and this entire block
of frequencies is converted to a lst IF frequency range of 950 to 1450
MHz by the block converter located at the
antenna site. The 1st IF signals are then sent via coaxial cable to the
tuner 12 which selects a particular channel for viewing and converts
the signals in that particular channel to a 2nd IF frequency range. The
2nd IF frequency range is preferably
high enough to permit the 2nd IF VCO frequencies to be above the 1st IF
block of frequencies, to prevent the VCO from interfering with the
desired signals. For a 1st IF frequency range of 950 to 1450 MHz, this
means that the center frequency of the
second IF frequency range must be at least 500 MHz. A particularly
preferred 2nd IF center frequency in the system of the present invention
is 612 MHz
In the demodulator 13, the 2nd IF signal is passed through an
amplifier and a filter and on to a conventional video detector which
demodulates the frequency-modulated signal to the baseband of the
original video signal (e.g., 0 to 10 MHz),
producing a composite video signal output. The filter preferably has a
pass band that is only about 22 MHz wide; a pass band of this width
passes the essential video and audio information while rejecting
unwanted noise received by the antenna on the
edges of the selected channel.
The output of the demodulator comprises the baseband signals
which range from DC to about 8.5 MHz; this includes video information
from about 15 KHz to 4.2 MHz, and subcarriers from about 4.5 to 8.5 MHz.
The audio tuner 15, which is shown in more detail in FIG. 2,
receives the baseband signals from the demodulator 14, which include the
subcarriers above 4.5 MHz containing audio and other information.
These baseband signals are initially passed
through a bandpass filter 19 having a pass band of 4.5 to 8.5 MHz. The
filter output is then fed to a super-heterodyne circuit including a
voltage-controlled oscillator 20 (referred to hereinafter as the "audio
VCO") receiving a controlling DC input
voltage on line 21, and a mixer 22 for combining the output of the audio
VCO with the incoming baseband signals to increase the frequency of the
incoming signals to a desired intermediate frequency (audio IF) range.
The output of the mixer 22 is passed
through an amplifier 22a and a filter 22b to a discriminator or detector
23 which is tuned to one particular audio IF frequency.
If the audio VCO produces an output frequency 10.7 MHz above the
frequency of the desired audio signal, the frequency of the desired
signal will be raised to an IF center frequency of 10.7 MHz at the
output of the mixer 22. The detector 23 will
then produce the desired audio output if it is tuned to the IF center
frequency of 10.7 MHz, which is a typical IF center frequency for FM
receivers.
In the particular embodiment illustrated, the detector 23 is in
the form of a phase locked loop (PLL) comprising a phase detector 24
which receives both the output from the mixer 22 and the output of a
second VCO 25 operating at a fixed frequency
determined by a resonant tank 26. In the particular example described
above, this fixed frequency is 10.7 MHz. The pulsating DC voltage
output of the phase detector 24 is filtered to a smooth DC by a low-pass
loop filter 27 to produce an audio output.
The loop filter is also connected to a deviation detector 28 which
produces a DC error voltage EV which is proportional to any deviation of
the frequency of the IF output of the mixer 22 from the fixed-frequency
output of the VCO 25. This error voltage
EV is normally used to adjust the input voltage to the audio VCO 20 to
return the output frequency of the VCO 20 to the desired value.
The entire detector 23 is conventional and is included, for
example, in the integrated circuit uPC1211V made by NEC Corporation. In
this particular circuit, the error voltage EV is identified as the "AFC
output" because it is the signal that is
conventionally used for "automatic frequency control" by applying it to a
varactor diode included in the tuning circuit of the audio VCO 20.
The system illustrated in FIG. 2 is duplicated to permi
simultaneous reception of stereo broadcasts. The audio outputs of the
two parallel systems are then feed to a conventional stereo processor.
Such a processor is capable of processing
either monaural signals or any of the three types of stereo signals
currently used for stereo broadcasts via satellite, namely:
(1) The "Matrix Method", which uses two separate subcarriers,
one for the left-plus-right audio signal and the other for the
left-minus-right audio signal.
(2) "Multiplex Stereo", which uses an FM subcarrier for a
left-plus-right audio signal, a double sideband suppressed AM subcarrier
for a left-minus-right signal, and a synchronizing signal for a stereo
demodulator reference.
(3) The "Discrete Method", which uses one subcarrier for the right channel, and a second subcarrier for the left channel.
In order to automatically locate the various subcarriers, the
receiver system of the present invention includes means for generating
an "Audio Seek" command signal, and means responsive to the "Audio Seek"
command signal for (1) artificially
adjusting the controlling input voltage CV to the audio VCO in a
preselected direction independently of the error voltage EV, (2)
monitoring the resulting error voltage EV produced by the detector 23 in
response to the artificial adjustment of the
controlling input voltage CV, (3) continuing to artificially adjust the
voltage CV in the same preselected direction in response to monotonic
changes in the resulting error voltage EV, and (4) terminating the
artificial adjustments of the voltage CV in
response to a non-monotonic change in the resulting error voltage EV,
whereby the normal operation of the detector 23 causes the super
heterodyne circuit to lock onto a new subcarrier to produce the desired
audio output. Thus, in the illustrative
embodiment of FIG. 2, the error voltage EV from the detector 23 is
passed through a filter-amplifier 29 and an analog-to-digital converter
(ADC) 30 to a microprocessor 31. As will be described in detail below,
this microprocessor 31 not only adjusts the
controlling input voltage to the audio VCO 20 for the normal AFC
function, but also responds to an "Audio Seek" command signal (initiated
by the user) to artificially adjust the controlling input voltage to
the audio VCO in a preselected direction
independently of the error voltage EV. These latter adjustments of the
input voltage to the audio VCO are referred to herein as "artificial"
adjustments because they are arbitrary adjustments which, unlike the
normal AFC adjustments, have no
proportional relationship to the error voltage EV.
The "Audio Seek" command signals are generated by a pair of
pushbutton-actuated switches, with the pushbuttons located on the front
panel of the receiver where they are accessible to the user. One of
these switches commands an "up" seek via
input line 32 (searching for the next higher subcarrier frequency), and
the other switch commands a "down" seek via input line 33 (searching the
next lower subcarrier frequency). In addition to the "Audio Seek"
signals, the microprocessor receives two
other user-generated command signals, for stereo transmissions; one of
these signals is supplied to the microprocessor 31 via input line 34 to
indicate that the signal being sought is for the "left" stereo channel,
and the other signal, supplied via
input line 35, indicates it is for the "right" channel.
Following each artifical adjustment of the audio VCO input
voltage, the microprocessor 31 monitors the resulting error voltage EV
produced in response to the artifical adjustment, and continues to
artificially adjust the controlling input voltage
to the IF VCO, in the same direction, in response to monotonic changes
in the resulting error voltage EV. These artificial adjustments are
terminated when the detector has either located a new audio subcarrier
or reached the end of the audio subcarrier
frequency band. As long as the error voltage EV changes monotonically
in response to the artificial adjustments of the voltage CV, the
detector is still trying to lock onto the previous subcarrier, i.e., a
new subcarrier has not started to influence the
detector. On the other hand, when the error voltage EV exhibits a
non-monotonic change, i.e., goes through an inflection point, this is an
indication that the detector has sensed, and is trying to lock onto, a
new subcarrier. At this point, the
artificial adjustments of the voltage CV are terminated, and the
detector is allowed to lock onto the new subcarrier via normal AFC
operation.
To determine whether the error voltages EV produced in response
to successive artificial adjustments of CV are monotonic, the magnitude
of EV is compared with a reference value of EV representing a centered
or locked condition. This reference
value will be referred to herein as "VCENTER" and is the actual value of
EV when the detector 23 is locked onto a subcarrier, i.e., when the
detector is receiving an input signal with a carrier frequency 10.7 MHz
below the output frequency of the audio
VCO 20. As long as the detector is trying to remain locked onto a given
subcarrier frequency, the error voltage EV produced in response to the
artificial adjustments of the control voltage CV will have a constant
polarity, because the conventional AFC
system will continuously try to maintain the output of the VCO 20 at a
frequency 10.7 MHz above the subcarrier frequency. However, when the
frequency of the VCO 20 has been artificially moved closer to a
different subcarrier, the error voltage EV
changes polarity because the AFC system will now try to lock onto the
new subcarrier frequency by shifting the output of the VCO 20 to a
frequency 10.7 MHz above the new subcarrier frequency.
For example, assuming the system is in an "up" seek mode, i.e.,
seeking the next higher audio subcarrier, the controlling input voltage
CV to the audio VCO 20 will be artificially increased to increase the
output frequency of the VCO 20. As long
as the detector 23 continues to attempt to lock onto the original
subcarrier frequency, the detector produces a voltage EV smaller than
the reference value VCENTER in an effort to reduce the output frequency
of the VCO 20. When the VCO frequency has
been artificially increased to a point where it is closer to a new
subcarrier than the original subcarrier, the detector produces a voltage
EV larger than the reference value VCENTER in an effort to increase the
output frequency of the VCO 20 because now
the "lock" frequency of the VCO 20 is above, rather than below, the
actual frequency of the VCO 20.
When the system is in a "down" seek mode, i.e., seeking the next
lower audio subcarrier, the controlling input voltage CV to the audio
VCO 20 will be artificially decreased to decrease the output frequency
of the VCO 20. As long as the detector
23 continues to attempt to lock onto th original subcarrier frequency,
the detector produces a voltage EV larger than the reference value
VCENTER in an effort to increase the output frequency of the VCO 20.
When the VCO frequency has been artificially
decreased to a point where it is closer to a new subcarrier than the
original subcarrier, the detector produces an error voltage EV smaller
than the reference value VCENTER in an effort to decrease the output
frequency of the VCO 20 because now the
"lock" frequency of the VCO 20 is below, rather than above, the actual
frequency of the VCO 20.
In the illustrative system, these functions are all carried out
by the microprocessor 31 in response to the error voltage EV received
via the ADC 30 and the aforementioned ser-initiated command signals,
which are received by the microprocessor 31
via input lines 32-35. The resulting digital output signals from the
microprocessor are passed through a digital-to-analog converter (DAC) 36
which furnishes corresponding DC voltages to the audio VCO 20 via input
line 21. Any suitable microprocessor
may be used in the illustrative system, but it is preferred to use the
MC6803 microprocessor made by Motorola, Inc.
Flow charts of exemplary software programs for controlling the
microprocessor 31 to carry out the functions described above are
contained in FIGS. 3 through 6. Referring to those figures, the main
process illustrated in FIG. 3 is entered at step
100 which determines whether or not any user-generated input signals are
present; these are the signals generated in response to manual
actuation of switches on the front panel of the TVRO receiver. If a
negative answer is produced at step 100, the
system proceeds to step 101 which determines whether it is time to
execute a standard AFC operation. Because the AFC function is
controlled by the microprocessor 31 in this system, and this
microprocessor is also used for a number of other functions,
the AFC function is not carried out continuously but rather at
preselected time intervals .DELTA.T. Of course, these time intervals
are so short that the net effect is the same as a normal continuous
analog AFC function performed without the use of a
microprocessor. A negative answer at step 101 returns the system to the
main program, while an affirmative answer advances the system to step
102 where the AFC subroutine illustrated in FIG. 4 is executed.
The subroutine of FIG. 4 is entered at step 200 where a pair of
"seek status" flags, to be described below, are read. These flags
indicate whether a seek process is already in progress, as determined at
step 201 of the subroutine of FIG. 4. If
the answer is affirmative, the system advances to step 202 to determine
whether it is the "left channel" audio signal that is being sought,
which again is indicated by the "seek status" flags read at step 200.
An affirmative answer at step 202 advances
the system to step 203 where a left-channel AFC process is executed by
the subroutine illustrated in FIG. 5, which will be described below. A
negative response at step 202 indicates that the seek process in
progress is not for the left channel, and thus
the system is advanced to step 204 where a right-channel AFC process is
executed; this process is not illustrated in the flow charts because it
is identical to the process illustrated in FIG. 5 for left-channel
centering, except that all the signals
involved are right-channel signals rather than left-channel signals.
The subroutine then returns to the main program at step 205.
If step 201 determines that a seek process is not is progress,
the subroutine of FIG. 4 advances to step 206 to execute the
left-channel centering process illustrated in FIG. 5, i.e., the same
process executed at step 203; it is the left channel
that is used for monaural transmissions, and thus step 206 ensures that
the AFC function is carred out when a monaural transmission is being
received and when an "Audio Seek" process is not being executed. Upon
completion of the left-channel AFC process
illustrated in FIG. 5, the system returns to the subroutine of FIG. 4
where step 207 determines whether the receiver is in a stereo mode. The
data read at this step can either be stored in memory or generated by a
user input. If the answer at step 107
is negative, the subroutine returns to the main program at step 205. If
the answer at step 207 is affirmative, this subroutine executes a
right-channel AFC process at step 208, and then returns to the main
program at step 205.
FIG. 5 illustrates the left-channel AFC process, by which the
microprocessor achieves the same result as a normal AFC circuit. As
indicated above, there is a counterpart subroutine, not illustrated in
the drawings, for a right-channel AFC
process which is identical to the left-channel AFC process, except that
the right-channel signals CVR and EVR are used in place of the
left-channel signals CVL and EVL
The left-channel AFC process illustrated in FIG. 5 is entered at
step 300, where the left-channel error voltage EVL is read. Step 301
then determines whether the current value of EVL is greater than the
"locked" value VCENTER, i.e., the value of
the error voltage when the frequency of the IF output of the mixer 22 is
exactly 10.7 MHz. An affirmative answer at step 301 indicates that the
output frequency of the left-channel audio VCO 20 is too high and,
therefore, that the controlling input
voltage CVL to the left-channel VCO 20 should be reduced.
In order to reduce CVL, the system proceeds to step 302 where
the current value of the voltage CVL, designated CVL.sub.old, is
retrieved from memory, and then to step 303 where a desired new value
for the voltage CVL, designated CVL.sub.new, is
computed as the current value CVL.sub.old minus the value (EVL-VCENTER).
Step 304 then determines whether the new value CVL is below a value
MINIMUM representing the lower end of the tuning range, below which no
audio subcarriers will be found (e.g.,
4.5 MHz). If the answer at step 304 is affirmative, the value of
CVL.sub.new is set equal to MINIMUM at step 305, and the subroutine
advances to step 306, which is also reached directly by a negative
response at step 304. Step 306 updates the current
value of CVL by setting CVL.sub.old equal to CVL.sub.new. This updated
value of CVL is then sent to the DAC 33 for the left-channel audio VCO
20 at step 307, after which the subroutine returns to the main program
at step 308.
The specific audio frequency to which the receiver is tuned is
always displayed on the front panel of the receiver. Thus, whenever the
microprocessor determines that one of the audio VCO's is at the end of
the tuning range, as at step 304
described above, the frequency display will remain constant and thereby
indicate to the user that the system is at the end of the tuning range.
Thus, there is no point in continuing to adjust the frequency any
farther in that direction.
Returning to step 301 of the subroutine of FIG. 5, a negative
answer at this step advances the system to step 309 which determines
whether the current voltage EVL is less than the value VCENTER.
Negative answers at both steps 301 and 309
indicate that the audio IF frequency is exactly where it should be and
no AFC correction is required, and thus the subroutine returns
immediately to the main process at step 308. An affirmative answer at
step 309, on the other hand, advances the system
to step 310 where the current value CVL is read from the memory.
The subroutine of FIG. 5 then advances to step 311 where a
desired new value CVL.sub.new is computed as the current value
CVL.sub.old plus the value (VCENTER - EVL). Step 312 determines whether
the new value CVL.sub.new is above a value MAXIMUM
representing the upper end of the tuning range, above which no audio
subcarriers will be found (e.g., 8.5 MHz). If the answer at step 312 is
affirmative, the value of CVL.sub.new is set equal to MAXIMUM at step
313, and the subroutine advances to step
314, which is also reached directly by a negative response at step 312.
Step 314 updates the current value of CVL by setting CVL.sub.old equal
to CVL.sub.new. This updated value of CVL is then sent to the DAC 33
for the left channel VCO 20 at step 307,
after which the subroutine returns to the main process at step 308.
Returning to the main process illustrated in FIG. 3, if an input
signal has been generated by the user, step 100 produces an affirmative
response which advances the system to step 103 to determine whether the
active key sensed at step 100 is an
"audio channel" key that is used to select a left or right stereo
channel. If the answer is "yes", the system proceeds to step 104 to
determine whether the left or right channel has been selected. If it is
the left channel, a "left-seek-in-process"
flag is set at step 105. If the response at step 104 is negative,
indicating that a right channel seek has been selected, the system
proceeds to step 106 to set a "right-seek-in-progress" flag.
If the active key sensed at step 100 is not an "audio channel"
key, step 103 produces a negative response which advances the system to
step 107 to determine whether an "Audio Seek" has been requested. If
the answer is negative, the system
returns to the beginning of the main process at step 100. An
affirmative response at step 107 indicates that an "Audio Seek" command
signal is present, and the system proceeds to step 108, where an "audio
seek" process is executed by the subroutine of
FIG. 6. This subroutine is entered at step 400 where the "seek status"
flags set at steps 105 and 106 are read. Step 401 then determines
whether the "left-seek-in-progress" flag has been set. If it is the
left channel flag that has been set, the
answer at step 401 is "yes" and the system advances to step 402 which
determines whether the direction of the requested seek is toward higher
or lower frequencies, i.e., "up" or "down". If an "up" seek has been
requested, step 402 produces an
affirmative response which advances the system to step 403 where a new
value CVL.sub.new for the control voltage for the left-channel VCO 20 is
set at a value equal to the sum of the previous value CVL.sub.old plus a
constant value VSTEP. This value
VSTEP preferably represents a control voltage change sufficient to shift
the output frequency of the VCO 20 by about 20 KHz. A frequency shift
of 20 KHz assures that a narrow subcarrier, e.g. 40-KHz wide, will not
be skipped over during the seeking
process.
From step 403, the subroutine of FIG. 6 advances to step 404 to
determine whether the value CVL.sub.new computed at step 403 is above
the value MAXIMUM representing the upper end of the tuning range. If
the answer is affirmative, the subroutine
immediately returns to the main program because there is no point in
seeking any higher subcarriers. A negative response at step 404
advances the system to step 405 where the value of CVL is updated by
setting CVL.sub.old equal to CVL.sub.new. This
updated value of CVL is then supplied to the DAC 33 at step 406 to
effect a step change in the voltage input to the left-channel VCO 20.
After the step change has been effected in the VCO input
voltage, the subroutine pauses for a suitable delay interval which
allows time for the detector 23 to react to the resultant change in the
output frequency of the VCO 20. This delay
interval is represented by step 407 in the flow chart of FIG. 6.
Following the delay, the left-channel error voltage EVL is read at step
408, and then step 409 determines whether the current value of EVL is
above the value VCENTER. An affirmative
response at step 409 indicates that the error voltage EVL is still
trying to return the system to the original subcarrier (i.e., the
subcarrier to which the system was tuned before the step change in CVL),
and thus the subroutine returns to step 403 to
effect another step change in CVL. A negative response at step 409
indicates that a new subcarrier has been located because the error
voltage EVL indicates that the input voltage to the VCO 20 should be
shifted in the opposite direction. Thus, the
subroutine returns to the main process at step 410 to allow the normal
AFC function to take over and lock the receiver onto the new subcarrier
frequency.
Returning to step 402 of the subroutine of FIG. 6, it will be
recalled that this step determined whether an "up" seek was requested by
the user. If the answer at step is negative, the subroutine proceeds
to step 411 rather than 403 to execute a
"down" seek. Step 411 sets CVL to a new value CVL.sub.new equal to the
sum of the previous value CVL.sub.old minus the constant VSTEP. Step
412 then determines whether the value CVL.sub.new computed at step 411
is below the value MINIMUM representing
the lower end of the tuning range. If the answer is affirmative, the
subroutine immediately returns to the main process because no additional
subcarriers will be found at lower frequencies. A negative response at
step 412 advances the system to step
414 where the value of CVL is updated by setting CVL.sub.old equal to
CVL.sub.new. This updated value of CVL is supplied to the DAC 33 at
step 415 to effect a step change in the voltage input to the
left-channel VCO 20.
After the step change has been effected in the VCO input
voltage, the same delay described above is produced at step 416 to allow
time for the detector 23 to react to the resulting change in the VCO
output frequency. Following this delay
interval, the left-channel error voltage EVL is read at step 417, and
then step 418 determines whether the current value of EVL is below the
value of VCENTER. An affirmative response at step 418 indicates that
the error voltage EVL is still trying to
return the system to the original subcarrier, and thus the subroutine
returns to step 411 to effect another change in CVL. A negative
response at step 418 indicates that a new subcarrier has been located
because the error voltage EVL indicates that the
input voltage to the IF VCO should be shifted in the opposite direction.
Thus, the subroutine returns to the main process at step 410 to allow
the normal AFC function to lock the receiver onto the new subcarrier
frequency.
When step 401 yields a negative answer, the subroutine of FIG. 6
advance to step 419 to determine whether the "right-seek-in-progess"
flag has been set. If the answer is "no", it means that a channel has
not been selected, and the system returns
to the main process at step 410. If step 419 yields a "yes", the
subroutine proceeds through steps 420-427 or 428-434, depending upon
whether an "up" or "down" right-channel seek has been requested. As can
be seen in FIG. 6, these steps are the same as
the corresponding left channel steps 402-409 or 411-418 except that the
right-channel signals EVR and CVR are substituted for the left-channel
signals EVL and CVL.
As can be seen from the foregoing detailed description, the
present invention provides an improved satellite receiver system which
automatically locates audio signals among the numerous signals received
by an earth station. This improved
receiver system is particularly useful in TVRO earth stations because it
eliminates the need for the user to manually scan the transmissions
received from each different transponder to locate the audio signals
embedded therein. This receiver locates the
audio subcarriers quickly and reliably in response to a command signal,
without any further manual intervention after the command signal is
generated. The overall cost of the receiver system is not significantly
effected, because the features of the
invention are simple and inexpensive to implement in an otherwise
standard satellite receiver. Finally, the receiver system does not
require any input data or other intelligence concerning the frequencies
or bandwidths of the audio signals.
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