Both AM and FM In-Band On-Channel (IBOC) broadcasting systems utilize a composite signal including an analog modulated carrier and a plurality of digitally modulated subcarriers. The audio signal can be redundantly transmitted on the analog modulate carrier and the digitally modulated subcarriers. The analog audio is delayed at the transmitter by the diversity delay.
In the absence of the digital audio signal (for example, when the channel is initially tuned) the analog AM or FM backup audio signal is fed to the audio output. When the digital audio signal becomes available, a blend function smoothly attenuates and eventually replaces the analog backup signal with the digital audio signal while blending in the digital audio signal such that the transition preserves some continuity of the audio program. Similar blending occurs during channel outages which corrupt the digital signal. In this case the analog signal is gradually blended into the output audio signal by attenuating the digital signal such that the audio is fully blended to analog when the digital corruption appears at the audio output. Corruption of the digital audio signal can be detected during the diversity delay time through cyclic redundancy check (CRC) error detection means, or other digital detection means in the audio decoder or receiver.
The digital signal has characteristics such that the digital audio is either virtually perfect or not received at all, whereas the analog signal generally experiences a degraded quality as the signal quality (e.g. signal to noise ratio (SNR)) degrades. Therefore the analog signal is a good backup when the digital signal is lost. Furthermore it is required that the receiver output the analog audio signal whenever the digital signal is not present.
The concept of blending between the digital audio signal of an IBOC system and the analog audio signal has been previously described in U.S. Pat. Nos. 6,178,317; 6,590,944; and 6,735,257, the disclosures of which are hereby incorporated by reference. The diversity delay and blend allow the receiver to fill in the digital audio gaps with analog audio when digital outages occur. The diversity delay ensures that the audio output has a reasonable quality when brief outages occur in a mobile environment (for example, when a mobile receiver passes under a bridge). This is because the time diversity causes the outages to affect different segments of the audio program for the digital and analog signals.
Both FM and AM Hybrid In-Band On-Channel (IBOC) HD Radio™ receivers require an audio blend function for the purposes of blending to the FM or AM analog backup signal when the digital signal is unavailable. The maximum blend transition time is limited by the diversity delay and receiver decoding times, and is typically less than one second. Frequent blends can sometimes degrade the listening experience when the audio differences between the digital and analog are significant.
This invention provides a method and apparatus for processing the digital audio during these frequent blend occurrences to make the blending less annoying to the listener.