Pulse or digital communications – Bandwidth reduction or expansion
Reexamination Certificate
1997-05-14
2001-02-20
Kelley, Christopher S (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
C380S044000
Reexamination Certificate
active
06192074
ABSTRACT:
The invention relates to communication systems in general, and more particularly, the invention relates to a method and apparatus for utilizing a substantially fixed frequency oscillator within a timing processor.
BACKGROUND OF THE DISCLOSURE
Synchronization of a decoding and presentation process for received bitstreams is a particularly important aspect of real-time digital data delivery systems. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172-1 (Systems), 11172-2 (Video), 11172-3 (Audio), 11172-4 (Compliance Testing), and 11172-6 (Technical Report). The second, known as MPEG-2, refers to ISO/IEC standards 13818-1 (Systems), 13818-2 (Video), 13818-3 (Audio), 13818-4 (Compliance).
The MPEG standards address the timing and synchronization issues for decoders of MPEG data streams (e.g., video, audio, data, and the like) as follows. A sample of a 27 MHz clock is transmitted in a program clock reference (PCR) field of a transport stream packet. The PCR indicates a time when the transport decoder is expected to have completed reading the PCR field. The phase of the local clock running at the decoder is compared to the PCR value in the bit stream at the instant at which it is obtained to determine whether the decoding process is synchronized.
In general, the PCR from the transport bit stream does not directly change the phase of the system clock of the decoder, but only serves as an input to adjust the clock rate of a Voltage Controlled Crystal Oscillator (VCXO). This adjustment is accomplished by comparing the PCR sample to the output frequency of the VCXO and responsively adjusting the VCXO until the two frequencies match. The system clock of the decoder is then derived from the VCXO. Thus, the system clock of the decoder is adjusted and locked to the 27 MHz clock of the encoder or transmitter via the transmitter clock sample included in the PCR.
A disadvantage of the prior art decoder arrangement is the relatively high cost of the VCXO. Therefore, a need exists in the art for a method and apparatus which is able to perform timing functions in a data delivery system using an inexpensive oscillator, e.g., a substantially fixed frequency oscillator.
SUMMARY OF THE INVENTION
Part of the invention resides in the recognition by the inventor that, in some real time digital data delivery systems, there are portions of the data which may be modified without reducing the efficacy of the system.
The timing method and apparatus receives an information stream comprising timing information and respective payload information. A substantially fixed clock source is used to generate local timing information, which is used to decode the payload information. A difference signal representing the timing difference between the local timing reference and the received timing information is calculated and used to adjust the decoded payload information signal.
In the case of a decoded payload information signal having critical and non-critical portions, the adjustment may be made during non-critical portions of the signal such that the critical portions are suitably time aligned.
In the case of an MPEG encoded video information signal, a decoder may use the fixed frequency oscillator to decode an MPEG-compliant bitstream. The presentation timing of the video may be adjusted to compensate for differences between the local clock and an encoder clock. The adjustment may be made by selectively increasing or decreasing the length of one or more horizontal display lines during a vertical blanking interval.
REFERENCES:
patent: 5113440 (1992-05-01), Harney et al.
patent: 5473385 (1995-12-01), Leske
patent: 5771075 (1998-06-01), Rim et al.
An Shawn S.
Burke William J.
Kelley Christopher S
Sarnoff Corporation
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