Television – Synchronization – Locking of video or audio to reference timebase
Reexamination Certificate
1999-12-07
2002-08-06
Miller, John (Department: 2714)
Television
Synchronization
Locking of video or audio to reference timebase
C348S512000, C348S464000, C348S515000, C348S824000, C375S355000, C375S362000, C375S363000, C375S364000, C375S365000
Reexamination Certificate
active
06429902
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to end-to-end signal synchronization between an encoder and a decoder.
BACKGROUND OF THE INVENTION
Moving images and associated sound may be transmitted as audio/visual (A/V) signals and represented to viewers by receivers having video displays and audio speakers. A Set-Top Box (STB) represents an example device commonly used to receive such A/V signals. The STB, so called because this standalone signal converter is typically placed above the video display of a television receiver, may receive transmitted encoded A/V signals (sent by cable, broadcast or via satellite) and decode these signals for displaying on the television. For A/V signals to be enjoyed by a consumer without degradation in image frames or sound fidelity, the audio and video bitstreams must be synchronized based on standards used in the industry.
In order for two waveforms to be synchronized, their wavelengths and their phase must be matched. The time difference between corresponding events may be described as “skew”, and
FIG. 1
illustrates an example. A first square-wave
10
has a first step rise event
12
corresponding to a particular time
14
. The wavelength of the first square-wave provides a measure of its period
16
. A second square-wave
18
has a second step rise event
20
a finite time interval later than the first step rise event
12
, and this difference may be described as the skew
22
.
The transmission of A/V data into a single bitstream as shown in
FIG. 2
requires several processes. Video data
24
are input to a video encoder
26
yielding a video elementary stream
28
that is input to a video packetizer
30
, producing a video packetized elementary stream (PES)
32
. The corresponding audio data
34
are input to an audio encoder
36
to produce an audio elementary stream
38
that is input to an audio packetizer
40
producing an audio PES
42
. The video PES
32
and audio PES
42
are input to a stream multiplexer (MUX)
44
from which a resulting bitstream
46
is transmitted. Upon reception, the bitstream must be separated into audio and video streams and synchronized for decoding.
Synchronization of an encoder and a decoder involves sending program clock reference (PCR) time stamps (or counts) embedded by the encoder in the A/V transport bitstream and received by the decoder on the STB. The PCR time stamps provide a sample of the encoder clock count sent in the transport stream packet. The encoder clocks drive a constantly running binary counter. The value of these counters is sampled periodically and placed in the header adaptation fields as the PCR. The decoder compares the received PCR time-stamps from the packet header to its corresponding time stamps from a local time counter (LTC), in order to synchronize the A/V presentation for decoding. The short-term history of the PCR increments relative to their LTC counterparts provides the relation between the local decoder's clock and the encoder's clock. The difference may represent skew or phase error. Typically, the decoder's system clock is adjusted to match that of the encoder to avoid loss of A/V bitstream data.
The synchronization of system coding is defined by the International Standards Organization (ISO) in ISO-13818-1 titled
Information Technology—Generic Coding of Moving Pictures and Associated Audio Information, Part
1
: Systems
(November 1994). ISO-13818-1 specifies a multiplexed structure for combining audio and video data along with the representation of the timing information needed to replay synchronized sequences in real-time. Compression of the A/V bitstream is standardized by the Moving Photographers Expert Group (MPEG), e.g., standards such as MPEG-1, MPEG-2, MPEG-4 and MPEG-7. The target timebase frequency for the ISO-13181-1 system clock is 27 megahertz (MHz) with a variation of ±30 parts per million (ppm).
The PCR time stamp, used for the MPEG-2 transport standard, represents a small portion of a 188-byte packet as illustrated in
FIG. 3
showing the packet
48
divided into a header
50
and a payload
52
. The header is subdivided into several fields, in the first expanded row
54
, including an adaptation field
56
. Expansion of the adaptation field
56
yields a second expanded row
58
, which includes an entry for optional fields
60
. Expansion of the optional fields
60
yields a third expanded row
62
, within which is contained the PCR
64
.
The common synchronization method for coder/decoder (codec) end-to-end communication uses an external VCxO component to control the STB master clock frequency that establishes the encoder operating frequency, and to synchronize the decoder operating frequency so as to match the encoder operating frequency. The VCxO is an oscillator with a dynamic range connected to a voltage control input pin. The decoder can regulate its clock frequency by altering the VCxO control input voltage in response to the encoder clock PCR time stamps received by the decoder.
The codec clock comparison may be illustrated in
FIG. 4
in which a remotely located video encoder
66
receives a input video signal
68
and outputs an elementary stream
70
that is combined with the frequency of the encoder clock
72
for the transport stream formation device
74
. The bitstream
76
includes a first packet
78
and a second packet
80
separated by n bits of data
82
transmitted over the time between packets t
n
. PCR time stamps labeled “X”
84
and “X+t
n
”
86
are embedded within the first and second packets
78
and
80
respectively. At the local STB, the bitstream
76
is received by a transport stream decoder
88
, which forwards the PCR
90
and the LTC
92
from the local clock
94
to the time-difference comparison device
96
. After sending the time-difference to a low pass filter
98
, the time-difference is sent to a variable oscillator
100
that uses the filtered time-difference to adjust the local clock
94
so as to be synchronized with the encoder clock
72
.
The arrangement of synchronizing a local clock to a remote clock requires an external component with multiple-pin connections to be installed on the STB. The incorporation of a variable voltage oscillator at current prices on a STB having only modest sophistication represents an additional expense not commensurate to the value of the synchronization function and its application for STB signal decoding.
SUMMARY OF THE INVENTION
A method and apparatus for synchronization of an audio/visual bitstream is transmitted by an encoder and received by a decoder by employing duplication or elimination of audio samples and video pixels. The invention enables clock synchronization between the encoder and a decoder with an unregulated clock oscillator so as to control the data reader by skipping ahead (eliminating a data element) or to pause (duplicating a data element) depending on whether the encoder clock is faster or slower than the decoder clock.
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Cohen Ariel
Har-Chen Dror
LSI Logic Corporation
Miller John
Natnael Paulos
Thelen Reid & Priest LLP
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