The present invention generally relates to vehicular audio systems. In particular, the present invention relates to vehicular audio systems equipped to receive both AM/FM audio and audio from a digital broadcast in the AM/FM bands.
It is known in the art that AM and FM radio stations are allowed to simulcast their AM/FM audio content using an accompanying digital broadcast. One form of digital broadcast in the AM/FM bands is commercially available under the tradename HD RADIO® from iBiquity Digital Corporation of Columbia, Md. Unlike the gradual changes in reception quality common with AM/FM audio, reception quality of digital broadcast audio is nearly perfect until the signal quality falls below a certain threshold, and then audio is lost entirely.
At the receiver 100, an AM/FM signal 110 and a digital broadcast signal 108 share the same antenna 102, RF front end tuner 104, and A/D converter 106 before the digital broadcast signal 108 splits from the AM/FM signal 110. At this stage, the AM/FM signal 110 enters an AM/FM detector and decoder 112 as the digital broadcast signal 108 enters a digital broadcast decoder 114. After exiting the AM/FM detector and decoder 112, AM/FM audio 116 undergoes processing at block 118 to reduce noise under impaired signal conditions. Then, processed AM/FM audio 120 exits the processing block 118 to a blend function 125 that implements gradual transitions between digital broadcast audio 122 and the processed AM/FM audio 120.
The digital broadcast audio 122, on the other hand, travels from the digital broadcast decoder 114 directly to the blend function 125. If the digital broadcast decoder 114 detects an imminent reception loss, the digital broadcast decoder 114 provides a digital broadcast audio dropout indicator 124 to the blend function 125. The blend function 125 then responds by initiating a gradual transition from the digital broadcast audio 122 to the processed AM/FM audio 120. When digital broadcast audio 122 is reacquired, the blend function 125 initiates a gradual transition to return to the digital broadcast audio 122 from the processed AM/FM audio 120.
The conventional receiver 100 attempts to disguise the transitions between AM/FM audio 120 and digital broadcast audio 122 through static time alignment and volume equalisation of the two audio sources. When the transition occurs, the receiver 100 linearly fades from one audio source to the other audio source in the blend function 125. Despite these attempts to disguise the transition between the AM/FM and digital broadcast audio, the transitions are still rather noticeable, largely due to differences between the two audio sources in the areas of frequency content, stereo separation, and volume.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring to
According to one aspect of the invention, the receiver 10 includes improved digital broadcast audio processing, which is referenced generally at reference numeral 50. The processing 50 includes a controller block 11a and a multi-stage processing block 11b, which includes first and second processing stages 12, 14. According to an embodiment, the processing 50 represents a digital signal processing (DSP) core including software that resides on a baseband processor integrated circuit (IC). As illustrated, the processing 50 outputs processed digital broadcast audio, which is generally referenced at 120b, to the blending blend function 125. The processing 50 is designed such that, at the moment of transition between the digital broadcast audio 120b and AM/FM audio 120a, the processed digital broadcast audio 120b has audio characteristics substantially similar to those of the processed AM/FM audio 120a. Therefore, the audio transitions between the digital broadcast audio 120b and the AM/FM audio 120a are more effectively disguised.
As discussed above, the processing block 11b has two stages. The first stage accounts for inherent differences between AM/FM audio 116 and digital broadcast audio 122 at block 12, which compensates for frequency content and stereo separation differences between the AM/FM audio and digital broadcast audio streams. For example, the digital broadcast audio 122 has slightly more extensive frequency content than FM audio 116 (e.g., FM audio 116 contains no frequencies above 19 kHz and is usually limited to approximately 15 kHz, whereas audio from digital broadcasts 122 in the FM band contains frequencies up to 20 kHz) and significantly more extensive frequency content than AM audio 116 (e.g., AM audio 116 is typically limited to about 8 kHz, whereas audio from digital broadcasts in the AM band contains frequencies up to 15 kHz). Furthermore, digital broadcast audio 122 is typically in full stereo whereas most AM audio is mono and FM audio stereo separation is slightly more limited than that of digital broadcast audio.
The second stage, which is referenced generally at block 14, accounts for the changes made to the AM/FM audio 116 in the AM/FM audio processing block 118. These changes are communicated to the processing block 50 as a number of audio processing parameters 18. These parameters include coefficients for the blend to mono function, the frequency reduction function, and volume attenuation function, which are commonly used in AM/FM audio processing to reduce noise during weak RF signal conditions. Using these parameters, block 14 applies the same levels of blend to mono, frequency reduction, and volume attenuation to the digital broadcast audio 122 when a digital broadcast audio dropout is imminent. It should also be noted that AM/FM audio is conveyed to block 14 for dynamic volume equalization.
The degree of processing in block 50 is controlled by a digital broadcast signal quality estimate 20 and the digital audio dropout indicator 124, both of which originate from the digital broadcast decoder 114. When digital broadcast audio 122 is available and the digital broadcast signal quality estimate 20 corresponds to a low probability of a digital broadcast audio dropout, the controller block 11a disables the processing in blocks 12, 14. Accordingly, when blocks 12, 14 are disabled, the processing block 50 seems transparent to the digital broadcast audio 122, and essentially disappears. However, as the digital broadcast signal quality worsens and the signal quality estimate 20 corresponds to a high probability of a digital broadcast audio dropout, the controller block 11 a permits a gradual increase in the amount of processing at processing blocks 12, 14 in an attempt to match the processed digital broadcast audio 120b to the processed AM/FM audio 120a. When the signal quality estimate 20 corresponds to the highest probability of a digital broadcast audio dropout, processing blocks 12, 14 cause the processed digital broadcast audio 120b to sound nearly identical to the processed AM/FM audio 120a in anticipation of the impending blend from processed digital broadcast audio 120b to processed AM/FM audio 120a at block 125.
According to an embodiment, the digital broadcast signal quality estimate 20 may be a carrier to noise ratio (CNR). For example, with regard to a digital broadcast in the FM band, a CNR of fifty-four or lower would correspond to the highest probability of a digital broadcast audio dropout over an ensuing brief time period, such as, for example, approximately ten seconds. A CNR of fifty-nine or higher would correspond to a very low probability of a digital broadcast audio dropout over the ten-second period. For CNRs of fifty-five, fifty-six, fifty-seven, and fifty-eight, the probability of a digital broadcast audio dropout over a ten-second period may be approximately 70%, 50%, 30%, and 10%, respectively, over the ten-second time period. Thus, there is a low probability of a digital broadcast audio dropout when the CNR falls within an approximate range of fifty-seven to fifty-nine, and there is a high probability of a digital broadcast audio dropout when the CNR falls within an approximate range of fifty-four to fifty-six. It will be appreciated that time periods other than the ten second time period discussed above may be applied when approximating the dropout probability; however, relatively short time periods, such as one ms, may consistently return a low dropout probability, whereas relatively long time periods, such as one hour, may consistently return a high dropout probability. It will also be appreciated that the CNR values listed above have meaning for one particular embodiment and are provided for explanatory purposes only.
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As illustrated, the digital broadcast signal quality estimate 20 and the digital broadcast audio dropout indicator 124 are input to the averager 28. The averager 28 uses the digital broadcast signal quality estimate 20 to calculate the audio dropout probability estimate 30. The audio dropout probability estimate 30, which is a scaled and averaged version of the digital broadcast signal quality estimate 20, controls the degree to which the digital broadcast audio processing is active at processing blocks 12, 14. When the audio dropout probability estimate 30 indicates that the digital broadcast audio 122 is acceptable (i.e., a low dropout probability), no effective processing is applied to the digital broadcast audio 122 at processing block 12. Consequently, the digital broadcast audio 122 is passed unchanged to block 125. However, as signal conditions of the digital broadcast audio 122 worsen (i.e., a high dropout probability), the audio dropout probability estimate 30 gradually and increasingly activates processing of the digital broadcast audio 122 at processing blocks 12, 14 to more closely match the processed digital broadcast audio 120b with the processed AM/FM audio 120a prior to an audio transition at block 125. Accordingly, the slow attack/fast decay averager 28 prevents rapid changes in audio quality, even when repeated digital broadcast audio dropouts occur under what would otherwise appear to be strong signal conditions.
When digital broadcast audio dropouts occur unexpectedly, the digital broadcast audio dropout indicator 124 acts as an override function so as to quickly set the output of the averager 28 to its minimum value, thereby fully engaging the controller block 11a. Accordingly, the digital broadcast audio dropout indicator 124 acts as a safety indicator when the digital broadcast signal quality estimate 20 fails to accurately predict digital broadcast audio dropouts (e.g., when a vehicle travels under an overpass and the signal quality changes rather abruptly). As such, when the digital broadcast audio dropout indicator 124 activates, the controller block 11a fully engages and remains fully engaged until digital broadcast audio 122 is reacquired. After reacquisition, the controller block 11a slowly disengages, even if signal conditions rapidly improve, to prevent a noticeable and abrupt transition from the AM/FM audio 116 to the digital broadcast audio 122.
FM receivers employ techniques to reduce noise during weak RF signal conditions. One such technique commonly used in automotive FM receivers is audio processing called “weak-signal handling” that gradually reduces stereo separation, frequency content, and volume as RF signal conditions worsen. Shown generally at 200 in
Shown generally at 300 in
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The present invention has been described with reference to certain exemplary embodiments thereof. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the exemplary embodiments described above. This may be done without departing from the spirit of the invention. The exemplary embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is defined by the appended claims and their equivalents, rather than by the preceding description.