Time-alignment measurement for hybrid HD radio™ technology

A method for processing audio signals in a radio transmitter, the method comprising:

• receiving an analog audio sample stream and a digital audio sample stream obtained from an analog audio stream and a digital audio stream;
• determining offsets in time between the analog audio sample stream and the digital audio sample stream using a normalized cross-correlation of audio envelopes of the analog audio sample stream and the digital audio sample stream prior to transmitting the analog audio stream and digital audio stream using a transmitting amplifier of the radio transmitter, wherein determining offsets in time further comprises:
• using a coarse envelope cross-correlation computed over a first range of lag values to locate a vicinity of the time offset;
• and
• subsequently using a fine envelope cross-correlation computed over a second range of lag values, where in the second range of lag values is narrower than the first range of lag values;
• filtering the determined offsets in time to produce filtered offset values;
• determining an alignment slip adjustment value as a function of the filtered offset values;
• aligning the analog audio sample stream and the digital audio sample stream using the determined alignment slip adjustment;
• and
• generating a hybrid radio signal for broadcast that includes time-aligned analog audio and digital audio.

An apparatus for generating a hybrid radio signal for broadcast, the apparatus comprising:

• a digital input port configured to receive a digital audio sample stream;
• an analog input port configured to receive an analog audio sample stream;
• processing circuitry configured to:
• compute a normalized cross-correlation of audio envelopes using a vector of bandpass samples of the analog audio sample stream and a vector of bandpass samples of the digital audio sample stream;
• determine offsets in time between the analog audio sample stream and the digital audio sample stream using the normalized cross-correlation of audio envelopes;
• filtering the determined offsets in time to produce filtered offset values;
• determine an alignment slip adjustment value as a function of the filtered offset values;
• align the analog audio sample stream and the digital audio sample stream using the determined alignment slip adjustment value;
• adjust a relative offset between the analog audio sample stream and the digital audio sample stream using limiting and hysteresis;
• and
• generate a hybrid radio signal for broadcast that includes time-aligned analog audio and digital audio.