Methods and systems for blending between analog and digital broadcast signals

#This invention relates to digital radio receivers and, more particularly, to blending between analog and digital broadcast signals.

#Analog frequency modulation (FM) broadcast band receivers can be impaired by noise, multipath and interference from blocker signals. These impairments will often show up as static in the stereo audio output for the tuned analog FM channel. Analog frequency demodulated audio signals degrade gradually with noise and channel impairments. Therefore analog FM receivers apply gradual mitigation techniques such as stereo blend (removing stereo content) and hi-cut (attenuating high frequency audio components) to the demodulated audio signal. The amount of mitigation is gradual and based on received signal quality metrics such as signal-to-noise ratio (SNR), received signal strength indicator (RSSI) and multipath indicator. SNR and multipath metrics can be computed, for example, by analyzing the amplitude modulation in the received FM signal.

#FIG. 1 illustrates an example of a conventional stereo blending relationship for the demodulated audio signal of an analog FM receiver, showing percentage stereo as a function of radio frequency (RF) SNR. In FIG. 1, full stereo (100% stereo) is output at the RF SNR value corresponding to point b, and full mono (or 0% stereo) is output at the RF SNR value corresponding to point a, with percentage stereo output varying from 0% to 100% at SNR values between point a and b according to the relationship shown. FIG. 2 illustrates an example of conventional hi-cut filter attenuation (demodulated audio corner frequency) as a function of RF SNR for a conventional FM receiver that results in a gradual drop in high frequency audio signal components with decreasing SNR. In FIG. 2, full audio signal frequency bandwidth is produced at the SNR value corresponding to point d (corner frequency maximum), while a reduced minimum audio signal frequency bandwidth is produced at the SNR value corresponding to point c (corner frequency minimum). The amount of high frequency components decreases between the SNR values of point d and c according to the relationship shown. For the illustrated case of FIGS. 1 and 2, the SNR values corresponding to points a, b, c and d that define the region over which mitigation occurs are programmable at circuit design time. Information on receiver signal processing and signal mitigation techniques may be found in U.S. Pat. No. 7,272,375; U.S. Pat. No. 8,023,918; U.S. Pat. No. 8,358,994; and U.S. Pat. No. 8,417,206, each of which is incorporated herein by reference in its entirety.

#It is noted that the appended drawings illustrate only example embodiments of the invention and are, therefore, not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

#FIG. 1 illustrates an example of a conventional stereo blending relationship for FM audio signals.

#FIG. 2 illustrates an example of conventional hi-cut filter attenuation for FM audio signals.

#FIG. 5 illustrates a block diagram of one exemplary embodiment of a digital radio receiver system 500 that includes analog receiver circuitry. As shown, system 500 includes an antenna 502 that is coupled to provide a received RF spectrum to RF or analog front end circuitry 504, which may include a mixer to downconvert incoming RF signals to a lower frequency (e.g., intermediate frequency (IF) signal, low-IF signal, baseband signals, etc.) as well as any other processing tasks suitable for the particular given system embodiment. In this embodiment, received RF signals may include digital modulated broadcast signals (e.g., such as HD™ digital broadcast signals) that are simulcast with analog modulated broadcast signals (e.g., FM broadcast signals) that include the same audio program content as the digital modulated broadcast signals. Although this exemplary embodiment is described in relation to an RF spectrum including a combination of analog FM broadcast signals and simulcast digital broadcast signals (e.g., such as HD™ digital broadcast signals) present as one or more sidebands to the analog FM carrier signal, it will be understood that the disclosed systems and methods may be implemented to similarly receive and process a RF spectrum including any combination of simulcast digital modulated and analog modulated signals that share at least some of the same information such as audio and/or other program content. Examples of suitable RF spectrum communications include those made in accordance with various standards such as Digital Audio Broadcasting, Digital Radio Mondiale or other standard. Other examples may include digital broadcast signals that are present in one or more sidebands to an AM carrier signal, or any other combination of digital modulated and analog modulated broadcast signals that are received in the same RF spectrum.

#Still referring to the illustrated embodiment of FIG. 5, the output of RF front end circuitry 504 may be provided to analog-to-digital conversion (ADC) circuitry 506, which provides a digitized signal output to digital front end circuitry 508 that may perform tasks such as channelization and filtering. As shown, digital front end circuitry may in turn provide radio channel information as processed output signals both to an analog demodulation path that includes analog demodulation circuitry in the form of FM discriminator (demodulation) circuitry 510 and to a digital demodulation path that includes digital demodulation circuitry in the form of digital signal (e.g., HD™ signal) demodulation circuitry 530 as shown. In this regard, DFE circuitry 508 may in one exemplary embodiment provide two channelized outputs, a first output 509 for analog FM in which the HD sidebands are filtered out, and a second output 511 for HD in which the analog FM carrier is filtered out.

#FM discriminator circuitry 510 in turn provides analog demodulated (multiplex) signals to FM multiplex (MPX) decoder circuitry 512 that may in turn produce separate demodulated L+R and L−R signals therefrom as shown. Digital demodulation circuitry 530 may demodulate the processed digital information from digital front end circuitry 508 and provide digital demodulated left and right signal pair (e.g., as an I2S serial bus output or other suitable form of signal pair) that may be received and further processed in adder and subtraction circuitry 532 to produce separate demodulated L+R and L−R signals therefrom as shown. Although FIG. 5 illustrates particular examples of analog and digital demodulation circuitries, it will be understood that analog demodulation circuitry and digital demodulation circuitry may be implemented with any configuration of demodulation circuitry suitable for demodulating the given types of analog and digital modulated signals processed in a given embodiment.