# Patent

## Title
System and method for estimating channel condition based on filtered pilot energy signal

## Abstract

A receiver may estimate a channel condition based on a difference between a pilot energy signal and a filtered pilot energy signal. The receiver may further switch to a stereophonic mode when both the difference and an energy level of the pilot energy signal satisfy a predetermined requirement of the receiver, and switch to a monophonic mode when either the difference or the energy level of the pilot energy signal fail to satisfy the predetermined requirement of the receiver.

## Background
INCORPORATION BY REFERENCE
This application is a continuation of U.S. patent application Ser. No. 11/871,777, filed on Oct. 12, 2007, which claims the benefit of U.S. Provisional Patent Application No. 60/829,202, “Method and Algorithm to Estimate the Channel Condition Based on Filtered Pilot Energy Signal,” filed on Oct. 12, 2006, incorporated herein by reference in their entireties.


BACKGROUND
This present disclosure relates to a system and method for estimating a channel condition based on a filtered pilot energy signal, and more particularly, a system and method capable of improving the stereophonic/monophonic detection of broadcast audio signals.
Receivers that detect stereophonic/monophonic signals are incorporated into a vast number of devices used in everyday life. For example, such receivers are used in automobile radios, a variety of communication systems such as cellular telephones, and even children's toys. Unfortunately, many modern receiver systems suffer from such performance shortfalls as frequent switchover between monophonic and stereophonic modes.
In order to receive FM audio signals, be they music or any other type of information, a receiver must be robust enough to handle changes in the channel wherein the transmission could become very noisy and/or must overcome interference. A pilot tone can be transmitted as part of the baseband signal that is used to modulate an FM carrier signal in order to indicate the nature of the transmission to be stereophonic. The energy of the pilot tone may fluctuate significantly in a harsh channel scenario. Simply comparing the pilot tone energy, estimated at the receiver, against a predetermined threshold may cause the receiver to switch between monophonic and stereophonic mode too frequently and degrade the entertainment quality of the audio program delivered to the consumer.
SUMMARY
An audio receiver is disclosed that includes a mono/stereo detector that causes the audio receiver to output either a monophonic or a stereophonic signal based on an energy level and a noise level associated with a pilot energy signal. The noise level can be based on a residual signal indicative of a difference between a pilot energy signal and a filtered pilot energy signal.
The audio receiver may also include a subtractor that calculates the difference between the pilot energy signal and the filtered pilot energy signal to generate a residual signal. The residual signal is then processed by a signal adder that sums up the residual signal in a time interval to generate a total residual energy signal.
A noise energy comparator, which may also be included in the audio receiver, compares the total residual energy signal with a noise energy threshold to generate a channel condition signal.
Furthermore, a pilot energy comparator, which may also be included in the audio receiver, compares the pilot energy signal with a pilot energy threshold to generate an energy level signal.
Based on the channel condition signal and the energy level signal of the pilot energy signal, a decoder of the audio receiver outputs either the monophonic signal or the stereophonic signal. In particular, if either the channel condition or the energy level of the pilot energy signal do not meet the threshold requirements of the audio receiver, the decoder outputs the monophonic signal. Otherwise, the decoder outputs the stereophonic signal.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the receiver incorporating the mono/stereo detector (MSD) will be described with reference to the following drawings, wherein like numerals designate like elements, and wherein:
FIG. 1 shows a block diagram of an exemplary receiver incorporating MSD;
FIG. 2 shows a block diagram of exemplary MSD shown in FIG. 1;
FIG. 3 shows a block diagram of an exemplary pilot tone energy extractor of FIG. 2;
FIG. 4 shows a block diagram of an exemplary threshold comparator of FIG. 2;
FIG. 5 shows a block diagram of an exemplary decoder of FIG. 2; and
FIG. 6 shows a flow-chart of an exemplary MSD process for detecting a stereophonic signal.



DETAILED DESCRIPTION OF EMBODIMENTS
FIG. 1 shows a receiver 100 having a mono/stereo detector (MSD) 120 that is capable of robust and flexible detection of stereophonic signals. As shown in FIG. 1, mono/stereo detector 120 is coupled to a demodulator 110, which receives a base line signal BS from a transmitter (not shown). Demodulator 110 demodulates base line signal BS into a multiplex signal MPX and then transmits multiplex signal MPX to mono/stereo detector 120. Mono/stereo detector 120 receives and processes multiplex signal MPX and outputs the processed signal in the form of a left signal L and a right signal R, which may then be amplified by an amplifier (not shown) and used to drive a left and a right speaker (not shown), respectively. Depending on the presence and the state of a pilot tone component in the multiplex signal MPX, the mono/stereo detector 120 may output either left and right signals L and R in a stereophonic form or a monophonic form.
FIG. 2 shows a block diagram of mono/stereo detector 120. As can be seen, mono/stereo detector 120 can include a pilot tone energy extractor 220, a threshold comparator 230, and a decoder 240, which are described in greater detail with reference to FIGS. 3, 4, and 5, respectively.
During operation, when multiplex signal MPX is transmitted to mono/stereo detector 120, it is received by pilot tone energy extractor 220 and decoder 240. Pilot tone energy extractor 220 extracts and processes a pilot tone signal and converts it into a sample pilot energy signal SPE, which is a measure of pilot energy per audio sample. Sample pilot energy signal SPE is then transmitted to threshold comparator 230, which compares signal SPE to various pre-set parameters (e.g., energy and noise thresholds) and outputs a mono/stereo indicator signal MSI that controls the output of decoder 240.
As mentioned above, decoder 240 also receives and processes multiplex signal MPX. Decoder 240 utilizes various filters and algorithms to extract left and right signals L and R from multiplex signal MPX for output by receiver 100. However, whether left and right signals L and R are stereophonic or monophonic will depend on what MSI signal decoder 240 receives from threshold comparator 230. For example, if the MSI signal indicates that the energy of the detected pilot tone signal is sufficient and meets a predetermined noise requirement, decoder 240 will output left and right signals L and R in stereophonic form. Whereas, if the MSI signal indicates that the energy of the detected pilot tone signal is insufficient or fails to meet the predetermined noise requirement, decoder 240 will output left and right signals L and R in monophonic form. As such, a robust and flexible detection of stereophonic signals can be achieved.
FIG. 3 shows a block diagram of an exemplary pilot tone energy extractor 220 of FIG. 2. In particular, pilot tone energy extractor 220 can include a band pass filter 310, a squarer 320, a low pass filter 330, a convolution filter 340, and a decimation filter 350.
The band pass filter 310 defines a predetermined shape of the frequency response over a band of frequency, such that its output is the appropriately shaped version of the MPX input. For example, in one embodiment, band pass filter 310 may be tuned to the 19 kHz pilot tone signal. The signal extracted from the squarer 320 mainly includes a DC component representing the pilot signal power and a second harmonic tone of 38 kHz along with residue of the channel noise and interference. Next, the low pass filter 330 outputs the DC component along with the low pass filter shaped noise component. The convolution filter 340 accumulates or averages the signal over a predetermined and programmable time interval governed by No samples, where No is an integer >0.
The decimation filter 350 outputs every Noth sample of its input. As a combined processing entity, the convolution filter 340 along with the decimation filter 350, make up a window accumulator wherein the window defines a time duration such that the successive windows can be contiguous and at non-overlapping time intervals. The output of the decimation filter 350 are the pilot energy samples averaged over the window time duration of No samples.
The signal samples output by decimation filter 350 are the sample pilot energy signal SPE which can be subsequently fed to threshold comparator 230. The SPE samples are further processed in the threshold comparator 230 and converted into mono/stereo indicator signal MSI for transmission to decoder 240.
FIG. 4 shows a block diagram of threshold comparator 230 that processes and converts sample pilot energy signal SPE into mono/stereo indicator signal MSI. As shown, threshold comparator 230 can include pilot energy comparator (PEC) 410 for comparing sample pilot energy signal SPE with a predetermined pilot energy threshold EPT, a low pass filter 420 for smoothing the sample pilot energy signal, a signal subtractor 430 for obtaining a difference signal, an absolute value filter 435 for taking the absolute value of the difference signal and producing a residual signal, a noise energy comparator (NEC) 440 for comparing the noise energy of the residual signal with a predetermined noise energy threshold ENT, and an MSI generator 450 for performing a logic function on the outputs of comparators PEC 410 and NEC 440.
During operation, sample pilot energy signal SPE is received by threshold comparator 230, and then transmitted to pilot energy comparator 410 and low pass filter 420. Pilot energy comparator 410 compares sample pilot energy signal SPE with predetermined pilot energy threshold EPT, and outputs a signal to MSI generator 450 indicating whether the sample pilot energy signal SPE has sufficient energy to be indicative of a reliable stereophonic signal. For example, if sample pilot energy signal SPE is below predetermined pilot energy threshold EPT, then pilot energy comparator 410 can transmit a “mono-on” signal, a signal indicating that receiver 100 should remain in or switch to monophonic mode, to MSI generator 450. On the other hand, if sample pilot energy signal SPE is at or above predetermined pilot energy threshold EPT, then pilot energy comparator 410 can transmit a “stereo-on” signal, a signal indicating that receiver 100 should remain in or switch to stereophonic mode, to MSI generator 450.
Low pass filter 420, signal subtractor 430, absolute value filter 435, and noise energy comparator 440 collectively constitute a channel condition measurement stage, where the noise energy level of the sample pilot energy signal SPE is measured, and a signal is generated that indicates whether the noise energy is acceptable for stereophonic output by receiver 100.
Sample pilot energy signal SPE is received by low pass filter 420 where it is “smoothed” and transmitted to signal subtractor 430 as a smooth pilot energy signal. It should be appreciated that low pass filter 420 can be a 1-tap IIR filter, or any other kind of low pass filter suitable for “smoothing” or reducing the noise level of sample pilot energy signal SPE. Signal subtractor 430 receives smooth pilot energy signal and the original sample pilot energy signal SPE, and calculates the difference between these two energy signals, which is then processed by the absolute value filter 435 and transmitted as a residual signal to noise energy comparator 440.
Residual signal can be used to indicate the relative condition of the channel. For example, when the channel is clean, residual signal tends to be weak. Otherwise, when the channel is noisy, residual signal tends to be strong and thus, has high energy. Noise energy comparator 440 computes a total energy of the residual signal in a given time block or interval (e.g.,  ̃0.0232 seconds) of the base line signal with, for example, an L1-noun filter (not shown) that sums up absolute values of the residual signal, and compares this energy with predetermined noise energy threshold ENT. For example, if the total energy of the residual signal in the block is equal to or greater than that of noise energy threshold ENT, then noise energy comparator 440 can transmit a “mono-on” signal to MST generator 450. On the other hand, if the total energy of the residual signal of the block is less than that of noise energy threshold ENT, then noise energy comparator 440 can transmit a “stereo-on” signal to MSI generator 450.
MST generator 450 receives “mono-on” or “stereo-on” indicator signals from pilot energy comparator 410 and noise energy comparator 440 and performs a logic operation on the signals. MSI generator 450 can, for example, be a logic gate that performs a logic AND operation, or the like. If either of the signals from PEC and NEC 410 and 440 is a “mono-on” signal, then MSI generator 450 will output MSI signal instructing decoder 240 to remain in or switch to monophonic mode. MSI generator 450 will output MSI signal instructing decoder 240 to remain in or switch to stereophonic mode only when both output signals from PEC and NEC 410 and 440 are “stereo-on” signals.
FIG. 5 shows decoder 240 that decodes multiplex signal MPX into left and right signals L and R. Decoder 240 can include two low pass filters 510 and 530, a signal multiplier 520, a signal adder 540, a signal subtractor 550, and a multiplexer (MUX) 560.
In operation, multiplex signal MPX is processed in two signal paths. In a first signal path, low pass filter 510 isolates a sum signal M, which contains half of the sum of left and right signals L and R, from multiplex signal MPX. In a second signal path, signal multiplier 520 multiplies multiplex signal MPX by a 38 kHz auxiliary carrier (obtained, for example, from 19 kHz pilot tone), and low pass filter 530 isolates a difference signal S, which contains half of the difference between the left and right signals L and R. Then, M and S signals are summed and subtracted by signal adder 540 and signal subtractor 550, respectively, and transmitted to a stereo input section (Stereo) of multiplexer 560. Furthermore, signal M is transmitted to a mono input section (Mono) of multiplexer 560. Multiplexer 560 then receives mono/stereo indicator signal MSI, and, based on signal MSI, selects either the stereo signals, or the mono signals for output on the L and R channels. As such, a robust and flexible detection of stereophonic signals can be achieved.
FIG. 6 shows a flow-chart 1000 of an exemplary mono/stereo detection process. The exemplary MSD process can be implemented in a receiver where frequent switching between monophonic and stereophonic modes needs to be reduced, such as the receiver shown in FIG. 1.
The process begins and proceeds to step S1002 where a determination is made as to whether a signal is received. If a signal is not received, then the process returns to step S1002. Otherwise, the process proceeds to step S1004.
In step S1004, the process demodulates the received signal and proceeds to step S1006. For example, in receiver 100, demodulator 110 demodulates base line signal BS into a multiplex signal MPX.
In step S1006, a determination is made as to whether a pilot energy of a detected pilot signal is greater than or equal to pilot energy threshold EPT. If pilot energy of detected pilot signal is less than pilot energy threshold EPT, then the process proceeds to step S1010. Otherwise, the process proceeds to step S1008.
In step S1008, a determination is made as to whether a noise energy of detected pilot signal is less than noise energy threshold ENT. If noise energy of detected pilot signal is greater than or equal to noise energy threshold ENT, then the process proceeds to step S1010. Otherwise, the process proceeds to step S1014.
In step S1010, a determination is made as to whether the receiver is in mono mode. If the receiver is not in mono mode, the process proceeds to step S1012. Otherwise, the process proceeds to step S1018.
In step S1012, the process switches the receiver to mono mode and proceeds to step S1018. For example, in threshold comparator 230 of receiver 100, if sample pilot energy signal SPE has an energy level less than that of pilot energy threshold EPT, which means that the pilot signal is absent or very weak, MSI generator 450 will generate a mono/stereo indicator signal MSI instructing the receiver to switch to mono mode if it is not already in mono mode. Also, even if sample pilot energy signal SPE is equal to or greater than pilot energy threshold EPT, but has a residual signal having noise energy greater than or equal to that of noise energy threshold ENT, MSI generator 450 will similarly generate a mono/stereo indicator signal MSI instructing the receiver to switch to mono mode if it is not already in mono mode.
In step S1014, a determination is made as to whether the receiver is in stereo mode. If the receiver is not in stereo mode, the process proceeds to step S1016. Otherwise, the process proceeds to step S1018.
In step S1016, the process switches the receiver to stereo mode and proceeds to step S1018. For example, if sample pilot energy signal SPE satisfies both energy and noise threshold requirements of comparators PEC 410 and NEC 440, MSI generator 450 will generate a mono/stereo indicator signal MSI instructing the receiver to switch to stereo mode if it is not already in stereo mode.
In step S1018, the process determines whether the receiver is powered off. If the receiver is not powered off; then the process returns to step S1002. Otherwise, the process ends.
Pilot energy thresholds EPT and noise energy threshold ENT may be either fixed or variable. Depending on design parameters, operating conditions, or the mere preference of a user, EPT and ENT may be either manually or automatically adjusted to maximize the performance of the receiver system incorporating the mono/stereo detector. For example, if need be, the user can raise the noise energy threshold ENT to enjoy a stereophonic sound at the expense of an increase in noise.
Furthermore, it should be appreciated that mono/stereo detector MSD can detect pilot energy signals of stereophonic broadcasts at both 50 kHz and 75 kHz rated maximum system deviation RMSD.
For purposes of explanation, in the above description, numerous specific details are set forth in order to provide a thorough understanding of the mono/stereo detector MSD. It will be apparent, however, to one skilled in the art that MSD can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the MSD.
While the MSD has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, embodiments of the MSD as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the spirit and scope of the invention.

## Claims

1. A receiver comprising:
an extractor configured to extract a pilot energy signal from an input signal; and
a detector configured to cause the receiver to output a monophonic signal based on a residual signal indicative of a difference between the pilot energy signal and a filtered pilot energy signal.

2. The receiver of claim 1, further comprising:
a subtractor configured to calculate the difference between the pilot energy signal and the filtered pilot energy signal to generate the residual signal.

3. The receiver of claim 2, further comprising:
a first comparator configured to compare the residual signal with a noise energy threshold to generate a first signal, and cause the receiver to output the monophonic signal based on the signal.

4. The receiver of claim 3, further comprising:
a second comparator configured to compare the pilot energy signal with a pilot energy threshold to generate a second signal, and causes the receiver to output the monophonic signal further based on the second signal.

5. The receiver of claim 4, further comprising:
a indicator configured to generate a third signal based on the first signal and the second signal, and cause the receiver to output the monophonic signal based on the third signal.

6. A method for switching a mode of a receiver, the method comprising:
extracting a pilot energy signal from an input signal; and
outputting a monophonic signal based on a residual signal that is indicative of a difference between the pilot energy signal and a filtered pilot energy signal.

7. The method of claim 6, further comprising:
calculating the difference between the pilot energy signal and the filtered pilot energy signal to generate the residual signal.

8. The method of claim 7, further comprising:
comparing the residual signal with a noise energy threshold to generate a signal; and
outputting the monophonic signal based on the signal.

9. The method of claim 8, wherein the signal is a first signal, further comprising:
comparing the pilot energy signal with a pilot energy threshold to generate a second signal; and
outputting the monophonic signal based on the second signal.

10. The method of claim 7, further comprising:
summing the residual signal in a time interval to generate a total residual energy signal;
comparing the total residual energy signal with a noise energy threshold to generate a signal; and
outputting the monophonic signal based on the signal.

11. A receiver comprising:
an extractor configured to extract a pilot energy signal from an input signal; and
a detector configured to cause the receiver to output a stereophonic signal based on a residual signal indicative of a difference between the pilot energy signal and a filtered pilot energy signal.

12. The receiver of claim 11, further comprising:
a subtractor configured to calculate the difference between the pilot energy signal and the filtered pilot energy signal to generate the residual signal.

13. The receiver of claim 12, further comprising:
a comparator configured to compare the residual signal with a noise energy threshold to generate a signal, and cause the receiver to output the stereophonic signal based on the signal.

14. The receiver of claim 13, wherein the comparator is a first comparator and the signal is a first signal, further comprising:
a second comparator configured to compare the pilot energy signal with a pilot energy threshold to generate a second signal, and causes the receiver to output the stereophonic signal further based on the second signal.

15. The receiver of claim 14, further comprising:
a indicator configured to generate a third signal based on the first signal and the second signal, and cause the receiver to output the stereophonic signal based on the third signal.

16. A method for switching a mode of a receiver, the method comprising:
extracting a pilot energy signal from an input signal; and
outputting a stereophonic signal based on a residual signal that is indicative of a difference between the pilot energy signal and a filtered pilot energy signal.

17. The method of claim 16, further comprising:
calculating the difference between the pilot energy signal and the filtered pilot energy signal to generate the residual signal.

18. The method of claim 17, further comprising:
comparing the residual signal with a noise energy threshold to generate a signal; and
outputting the stereophonic signal based on the signal.

19. The method of claim 18, wherein the signal is a first signal, further comprising:
comparing the pilot energy signal with a pilot energy threshold to generate a second signal; and
outputting the stereophonic signal based on the second signal.

20. The method of claim 17, further comprising:
summing the residual signal in a time interval to generate a total residual energy signal;
comparing the total residual energy signal with a noise energy threshold to generate a signal; and
outputting the stereophonic signal based on the signal.