Set-Top Box Audio Decoder Using FIFO Buffer for Clock Synchronization
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Solution Overview
Problem
Existing digital TV broadcast decoding methods, such as the PCR and VBV methods, are prone to errors and unreliability due to incorrect or missing system clock references, leading to issues like video freeze and audio cuts, especially when broadcast conditions do not comply with standards.
Innovation Solution
A set-top box uses a FIFO buffer to extract and process the digital TV audio stream, comparing FIFO fullness and bit rate information to adjust the local clock, allowing for reliable decoding without relying on encoded clock or video buffer information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the PCR method is used to synchronize the STB local clock to the system clock, then clock synchronization is achieved, but video freeze, audio cuts, and other artifacts occur when the system clock reference is incorrect or missing
Solution Approach 1:
The patent introduces a FIFO buffer as an intermediary component between the audio stream input and the audio decoder. This buffer absorbs timing variations and clock synchronization errors, preventing them from directly causing video freeze and audio cuts. The buffer acts as a mediator that decouples the timing requirements of different system components.
Solution Approach 2:
The patent implements a feedback mechanism that continuously monitors the FIFO buffer fullness and adjusts the audio decoder's processing rate accordingly. When the buffer is nearly empty, the decoder slows down to prevent underflow; when the buffer is nearly full, the decoder speeds up to prevent overflow. This closed-loop feedback system maintains stable operation despite clock synchronization issues.
2Speed
If the STB decodes the DTV bit stream at a different speed than broadcasted due to clock desynchronization, then the STB local clock runs faster or slower than system time, but the internal buffer eventually runs out of data or overflows
Solution Approach 1:
The patent makes the audio decoder's processing speed dynamic rather than fixed. The decoder continuously adjusts its processing rate based on real-time feedback from the FIFO buffer fullness status. This dynamic speed adjustment allows the system to adapt to clock desynchronization and prevent buffer overflow or underflow conditions.
Solution Approach 2:
The system employs feedback control where the FIFO buffer fullness level is continuously monitored and used to regulate the audio decoder's processing rate. This feedback mechanism ensures that the decoding speed automatically adapts to maintain optimal buffer levels, preventing both overflow and underflow scenarios.
3Reliability
If the VBV method is used as an alternative to PCR for clock synchronization, then clock synchronization may be achieved, but the method is unreliable because VBV data is often not present in the DTV stream
Solution Approach 1:
The patent makes the system self-sufficient by eliminating its dependence on external clock synchronization references (both PCR and VBV methods). Instead, the system uses its own internal FIFO buffer status and feedback control mechanism to maintain stable operation. This self-service approach allows the system to operate reliably regardless of whether VBV data is present or PCR references are correct.
Data Source
AI summary
For use as or in connection with a set-top box, a digital TV broadcast signal is decoded by processing its audio stream into and read from a FIFO buffer. According to certain embodiments, fullness of the FIFO buffer is compared to a reference fullness, which can be updated based on changes to the bit rate of the audio stream. In response to the FIFO buffer fullness, the set top box regulates the rate at which the audio stream is decoded. The disclosed decoding methods do not need to rely on the accuracy or presence of a system time encoded into the broadcast signal.