Diversity radio system with RDS

An RDS receiver having dual tuners and dual antennas operates in two distinct modes, a diversity mode and a non-diversity mode. Diversity mode is when both tuners are tuned to a signal with the same program audio content and the audio from both tuners is blended together in a manner to minimize the effects of multipath distortion. In non-diversity mode, a forcing circuit isolates the tuner output signals so that one tuner provides the audio output while the other can be retuned to any other frequency for purposes of gathering RDS data.

BACKGROUND OF THE INVENTION 
The present invention relates in general to RDS radios having dual tuners, 
and more specifically to a dual mode radio for operating in both a 
diversity and a non-diversity mode to minimize the effects of multipath 
distortion while maintaining efficient collection of RDS related 
information, including collecting signal quality data for RDS alternate 
frequencies. 
Space diversity radio receiver systems can be employed to reduce the 
effects of multipath distortion in mobile receivers. Multipath distortion 
is a localized effect resulting from interaction between multiple signals 
from a single transmitter that have traversed different paths to reach a 
receiving antenna. By switching between spaced antennas in a diversity 
radio receiver, specific multipath events can be avoided since the spacing 
of the antennas helps insure that not both of the antennas will experience 
the same multipath event at the same time. 
The prior art has also shown that separate tuners may be connected to each 
antenna and that the tuner output signals can be combined to provide 
improved diversity reception. 
Separate tuners have also been employed in radio data system (RDS) 
receivers. In the case of RDS receivers, however, the tuners 
simultaneously receive at different frequencies. Standard RDS broadcasts 
transmit auxiliary digital data within the radio signal in order to 
achieve various automatic functions of the receiver. The data transmitted 
on a subcarrier includes alternate frequencies (AFs) at which the 
identical audio program can be heard. Thus, the receiver can automatically 
monitor AFs to determine whether a stronger or higher quality signal can 
be received by switching the tuner to a different frequency (e.g., as a 
vehicle moves relative to the broadcast transmitters). In an RDS receiver 
having just a single tuner, AFs can only be checked by briefly switching 
the tuner to an AF to detect its signal strength and then quickly 
returning to the original frequency before any detectable break is heard 
in the reproduction of the original broadcast. In RDS receivers having a 
second tuner, the second tuner is dedicated to collecting information 
about AFs (i.e., is not used for audio reproduction) and can monitor any 
alternate frequencies for as long as desired. 
Based on the AF information which is collected and stored in memory in the 
receiver, the tuner which is reproducing audio signals can be switched to 
the strongest AF whenever the currently received signal becomes degraded. 
However, the response time required to detect signal degradation (such as 
a multipath event) and then to switch to an AF is too slow to prevent 
distortion from being heard. Thus, a dual tuner RDS radio system is needed 
which obtains RDS data gathering while providing improved immunity to 
multipath distortion. 
SUMMARY OF THE INVENTION 
The present invention provides a radio architecture and method of operation 
wherein maximum RDS operational flexibility is achieved in order to 
optimize RDS performance while obtaining immunity to multipath distortion 
using diversity reception. 
In a primary aspect of the invention, a radio receiver for a mobile vehicle 
which is capable of receiving subcarrier data from broadcasts containing 
such data operates in either a diversity mode or a non-diversity mode. A 
first tuner produces a first tuner output signal in response to a 
broadcast signal at a first selected broadcast frequency. A second tuner 
produces a second tuner output signal in response to a broadcast signal at 
a second selected broadcast frequency. A signal mixer produces a mixed 
tuner output signal in response to the first and second tuner output 
signals. The signal mixer proportionally combines the tuner output signals 
when in the diversity mode according to relative measures of signal 
quality. The signal mixer isolates a selected one of the tuner output 
signals from the mixed tuner output when in the non-diversity mode. A data 
demodulator is responsive to either the first tuner output signal, the 
second tuner output signal, or the mixed tuner output signal to recover 
the subcarrier data. A control is coupled to the first and second tuners, 
the signal mixer, and the data demodulator and selects the diversity mode 
or the non-diversity mode in response to the signal quality. The control 
controls the first and second selected broadcast frequencies such that 1) 
the first and second selected broadcast frequencies each provide a user 
selected program when in the diversity mode, or 2) the selected one of the 
first and second tuner signals is searched for alternate frequency 
information and the other one of the first and second tuner output signals 
provides the user selected program when in the non-diversity mode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The RDS radio receiver shown in FIG. 1 operates in a diversity mode or in a 
non-diversity mode. In diversity mode, a pair of tuners are each tuned to 
the same user program (i.e., tuned to the same frequency) and their 
outputs are combined according to their signal strengths and the detection 
of multipath in order to minimize the effects of multipath distortion. In 
non-diversity mode, an audio output is reproduced from one tuner while the 
other tuner is used for scanning alternate frequencies to detect their 
signal quality and other data. 
More specifically, the RDS diversity radio receiver includes a first tuner 
10 with an antenna input 11 and a second tuner 12 with an antenna input 
13. Antenna inputs 11 and 13 are preferably connected to spaced antennas 
14 and 15 which are separated by a distance sufficient to avoid 
simultaneous multipath conditions at each antenna. 
Tuners 10 and 12 are controlled by a microcontroller 16 via a control bus 
19. Tuner 10 generates an FM multiplex tuner output signal designated FM 
MPX 1, a signal strength signal SS1, and a noise detection signal ND1. 
Tuner 12 generates a tuner output signal FM MPX 2, signal strength signal 
SS2, and noise detection signal ND2. Signals SS1, ND1, SS2, and ND2, are 
all connected to a mix control circuit 17, microcontroller 16, and a 
switching and conditioning circuit 18. The FM MPX 1 signal from tuner 10 
is provided through a voltage-controlled amplifier 20 to one input of a 
summer 21. The FM MPX 2 signal from tuner 12 is provided through a 
voltage-controlled amplifier 22 to a second input of summer 21. 
Mix control circuit 17 provides a pair of gain control signals G.sub.1 and 
G.sub.2 through a forcing circuit 23 to amplifiers 20 and 22, 
respectively. Forcing circuit 23 receives a control signal from 
microcontroller 16 vis control bus 19 to select between the diversity and 
non-diversity modes as will described described below. 
The output of summer 21 provides an FM MPX MIX signal to a stereo decoder 
24 and to a switching circuit 25. The output from stereo decoder 24 
provides left and right decoded stereo signals for reproduction. Switching 
circuit 25 also receives as inputs the multiplex signals FM MPX 1 and FM 
MPX 2 from tuners 10 and 12. Switching circuit 25 receives a control 
signal from control bus 19 which selects between the FM MPX 1, FM MPX 2 
and FM MPX MIX signals for input to an RDS demodulator 26. RDS data 
recovered by demodulator 26 is provided to microcontroller 16. 
Tuner output signals FM MPX 1 and FM MPX 2 are also provided to an audio 
correlator 27 which determines whether the audio content is identical on 
the two tuner output signals. Based on a comparison of the two signals, 
correlator 27 provides an indicating signal to microcontroller 16 to 
identify whether the audio content is the same. 
In operation, the radio receiver of FIG. 1 operates in either a diversity 
mode or a non-diversity mode under control of microcontroller 16. When in 
diversity mode, tuners 10 and 12 are preferably tuned to the same 
frequency to receive the same user selected program from both antennas 14 
and 15 so that the received signals can be combined in a manner which 
avoids multipath distortion. As described in co-pending application serial 
number (198-0140), which is incorporated herein by reference, mix control 
circuit 17 is responsive to multipath conditions to provide gain control 
signals G.sub.1 and G.sub.2 to mix the relative contribution of the tuner 
output signals to the final mixed tuner output signal so that the 
proportion of each individual output signal in the mixed signal is 
inversely proportional to the likelihood of multipath in that signal. 
Thus, tuners 10 and 12 each generate a respective signal strength (SS) 
signal and a respective noise detection (ND) signal. The noise detection 
signal may be derived, for example, by bandpass filtering the demodulated 
tuner output signal with a bandpass from about 100 kHz to about 500 kHz 
(which measures noise and distortion from multipath and other noise 
sources). Gain control signal G.sub.1 as supplied to amplifier 20 tends to 
increase with increased signal strength SS1 in tuner 10 and tends to 
decrease with increasing noise content ND1. Likewise, gain control signal 
G2 increases with increasing signal strength SS2 from tuner 12 and 
decreases with increasing noise content ND2. When in the diversity mode, 
forcing circuit 23 directly passes gain control signals G.sub.1 and 
G.sub.2 from mix control circuit 17 to amplifiers 20 and 22. 
Also when in diversity mode, switching circuit 25 selects the combined 
output from summer 21 (i.e., the FM MPX MIX signal) for input into RDS 
modulator 26 to assure the best quality signal for recovering RDS data for 
the user selected program. 
When microcontroller 16 determines that the receiver should be in 
non-diversity mode, the elements of FIG. 1 are reconfigured so that one 
selected tuner is used to search for predetermined alternate frequency 
information in the RDS signal (e.g., determining the quality of each 
available AF or detecting RDS data at each AF) and the other tuner is used 
to reproduce the user selected program. In non-diversity mode, 
microcontroller 16 configures forcing circuit 23 in a way which forces one 
of amplifiers 20 or 22 to have a maximum gain and the other amplifier to 
have a minimum gain. Thus, the tuner which is selected as the RDS tuner 
can then be retuned to any other frequency in order to gather RDS data or 
signal strength or other information at alternate frequencies without this 
action affecting the audio quality of the other tuner providing the audio 
output of the receiver. Also while in non-diversity mode, microcontroller 
16 configures switching circuit 25 to couple the selected tuner output to 
RDS demodulator 26. The selection of which tuner to use as the RDS 
alternate frequency tuner can be arbitrary or may be done according to an 
optimized method such as the one discussed below in connection with FIG. 
6. 
Switching and conditioning circuit 18 operates in a conventional manner to 
introduce high-cut and stereo blend in stereo decoder 24 in response to 
signal strength SS and noise detection ND in tuners 10 and 12. This helps 
eliminate any residual multipath distortion during diversity mode and 
improves signal reproduction during any multipath events that occur during 
non-diversity mode. 
A preferred method for coordinating receiver operation between diversity 
and non-diversity modes is shown in FIG. 2. The method begins at block 30 
when the radio receiver is either turned on or when it is manually retuned 
to a new frequency by the user. In step 31, a check is made to determine 
whether RDS data is present within the currently tuned-in broadcast. If no 
RDS data is present, then the tuner enters diversity mode in step 40. If 
the station being listened is not an RDS broadcast, then the receiver can 
spend full time in diversity mode. 
If RDS data is present in step 31, then the receiver initially enters 
non-diversity mode in step 32. While in non-diversity mode, 
microcontroller 16 gathers RDS data at the selected broadcast frequency 
including alternate frequencies carrying the same broadcast audio program. 
Signal quality of the AFs is collected and stored in memory based on 
signal strength and noise detection at each AF in step 33. After signal 
quality is gathered for all the AFs, a check is made in step 34 to 
determine whether the manually selected or current frequency is the best 
(i.e., strongest) one to receive. If not, then a switch is made to that 
best AF in step 35. 
In step 36 (while still in non-diversity mode), a check is made to 
determine whether a multipath event is present by examining signal 
strength and noise detection signals. If a multipath event is detected in 
step 36, then the receiver enters diversity mode in step 40. If no 
multipath event is detected, then additional lower priority RDS data may 
be collected and/or monitored in step 37. In step 38, a check is made to 
determine whether all possible RDS data has been gathered. If not, then a 
return is made to step 36. Otherwise, the receiver enters diversity mode 
in step 40. Thus, under good signal conditions (indicated by the tuner SS 
and ND signals), the receiver can spend as much time as necessary 
gathering RDS data. 
After entering diversity mode in step 40, the receiver checks for worsening 
reception quality in step 41. At the point where reception quality worsens 
to an unacceptable level, a return is made to step 31 which allows the 
receiver to reenter non-diversity mode if RDS data is present, thereby 
allowing a search to be conducted for a better signal at an alternate 
frequency. If reception quality has not worsened in step 41, then a check 
is made in step 42 to determine whether a predetermined period of time has 
passed (e.g., several minutes) in which reception quality of alternate 
frequencies may have changed and the memory of the receiver should be 
updated. If the predetermined delay has not expired, then a return is made 
to step 41 and the receiver remains in diversity mode. If the 
predetermined delay has passed, then a return is made to step 31. 
FIG. 3 shows forcing circuit 23 in greater detail. A pair of multiplexers 
45 and 46 receive gain control signals G.sub.1 and G.sub.2, respectively, 
from mix control circuit 17. A maximum gain signal G.sub.max and a minimum 
gain control signal G.sub.min are provided to additional multiplexer 
inputs as shown. The outputs of multiplexers 45 and 46 provide modified 
gain control signals G'.sub.1 and G'.sub.2 as determined by control 
signals from microcontroller 16. Thus, either gain control signals G.sub.1 
and G.sub.2 are passed unchanged, or G.sub.1 is forced to maximum gain 
while G.sub.2 is forced to minimum or no gain, or G.sub.1 is forced to 
minimum gain while G.sub.2 is forced to maximum gain depending upon the 
control signals. 
FIG. 4 shows one embodiment for detecting multipath in step 36 of FIG. 2. 
In step 50, signal strength SS (from the tuner being used to provide the 
audio reproduction) is compared with a predetermined signal strength 
threshold. If instantaneous signal strength falls below this threshold, 
then a multipath event is detected. Otherwise, a check is made in step 51 
to determine whether the noise detection signal indicates an amount of 
noise present greater than a predetermined noise threshold. If the 
threshold is exceeded then multipath is present, otherwise there is no 
multipath event taking place. Many other ways are known in the art for 
detecting a multipath event and any may be acceptable for purposes of this 
invention. 
FIG. 5 shows one embodiment of a method for detecting worsening reception 
quality as used in step 41 of FIG. 2. Likewise, many other acceptable 
methods could be used for determining when reception quality has worsened. 
In this preferred embodiment, average signal quality (SQ) for a current 
time period is compared with average SQ for a previous time period. Signal 
quality as used herein means a determination based upon signal strength 
SS, noise detection ND, or both. Noise added to a signal may consist of 
multipath distortion or other noise such as intermodulation or adjacent 
channel noise. In the preferred embodiment, signal quality is measured in 
direct proportion to signal strength ss and in inverse proportion to noise 
detection ND. Signal quality can be determined by counting how many times 
signal strength SS and noise detection signal ND cross their respective 
thresholds during a specified time period. 
In step 55, a number of times, x, that SS and/or ND are worse than their 
respective thresholds during a time block #1 is recorded. In step 56, a 
number of times, y, that SS and/or ND are worse than their respective 
thresholds during a time block #2 is recorded. Time blocks #1 and #2 are 
of equal time (each on the order of at least several seconds) and are 
consecutive. The values of x and y are indicators of signal quality SQ 
wherein the lower the value of x or y, the higher the signal quality. A 
check is made in step 57 to determine whether the count of threshold 
crossings has increased by 10 percent (i.e., whether x&gt;110% of y). If so, 
then the signal quality has unacceptably worsened and a search for a new 
broadcast frequency can be initiated. Otherwise, a determination is made 
that reception quality has not worsened and is still acceptable. It may 
also be desirable to compare running averages of SS and ND with their 
respective thresholds to also detect unacceptable or worsened conditions. 
In a typical vehicle installation, although tuners 10 and 12 have 
substantially identical electrical characteristics, antennas 14 and 15 
will not have identical electrical properties for a variety of reasons. 
Aesthetic and styling requirements dictate the kind of antennas used and 
where they are placed on the vehicle. Commonly used types of antennas 
include a whip antenna mounted on a vehicle panel or roof, and an on-glass 
antenna. Since the antennas will most often differ in gain and radiation 
patterns, the signal strength received by the tuners will vary from each 
other even when multipath or other noise is not present. Although it may 
be possible to designate one tuner as a main tuner and the other as a 
subsidiary tuner, it may not be possible to know in advance which tuner 
input is likely to be connected to the better antenna. Thus, a further 
method is provided for selecting which tuner to use as the RDS tuner and 
which to use as the audio program reception tuner when entering 
non-diversity mode as shown in FIG. 6. In general, under strong signal 
conditions, it may be desirable to use the strongest signal to perform RDS 
data gathering since the audio signal from the other tuner is then of good 
quality. However, whenever medium or weak signal conditions are present, 
it is generally desirable to use the stronger signal for audio 
reproduction and use the weaker signal for RDS data gathering. Thus, a 
check is made in step 60 to determine whether signal strength signals SS1 
and SS1 are each greater than a threshold T.sub.1. If both are greater 
than the threshold then a strong signal is detected in step 61 and the RDS 
tuner is selected as the one with the bigger signal strength signal. 
Otherwise, a weak signal is detected in step 62 and the RDS tuner is 
selected as the one with the smaller signal strength signal. Non-diversity 
mode is entered in step 63 wherein the selected RDS tuner is scanned tuned 
while the other tuner is tuned to the desired frequency for the selected 
program. 
Yet another advanced feature of the present invention relates to making a 
completely smooth, inaudible transition when switching between alternate 
frequencies. In prior art receivers, an audible click or transient can 
often be heard during retuning to the new frequency. In the present 
invention, the forcing circuit can be modified to provide ramping signals 
for controlling the gain control signals to smoothly transition 
reproduction from one tuner to the other. Before performing such a 
transition, however, the receiver needs to verify that the content at each 
frequency is the same. Thus, even though the program information (PI) RDS 
code may indicate that the two frequencies have the same program audio, 
there are instances where the program audio may in fact be different. 
Furthermore, in some instances under weak signal conditions, the RDS 
decoder may have difficulty obtaining the PI code or may take an 
unacceptably long period of time to obtain the PI code. Thus, audio 
correlator 27 in FIG. 1 is used to verify identical program audio. Audio 
correlator 27 is a known circuit for indicating whether or not the two 
multiplex signals have their inputs correlated. 
As shown in FIG. 7, a check is made in step 65 to determine whether the 
audio signals are correlated. If the signals are not correlated, then the 
transitioning between alternate frequencies is skipped. Otherwise, the 
receiver enters non-diversity mode in step 66. In step 67, the selected 
RDS tuner is retuned to the new frequency. Then, the amplifier gains are 
ramped in step 68 such that audio reproduction transitions from the other 
tuner to the selected RDS tuner. In step 69, the other tuner is retuned to 
the new frequency and the receiver enters diversity mode in step 70.