Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
2000-06-01
2003-12-09
Chin, Stephen (Department: 2734)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
Reexamination Certificate
active
06661861
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and a system for synchronizing related functions in a communication system apparatus and to a method and a system for synchronizing between a transceiver and a voice encoder-decoder unit in frame based communication in particular.
BACKGROUND OF THE INVENTION
In conventional communication methods, such as TDMA and CDMA, the link between a mobile unit and a base unit, is frame based. The base conventionally includes a very precise and relatively expensive crystal clock which is used to time its operations and synchronize every unit therein. A mobile unit which communicates with this base often includes a low quality crystal clock which is of lower precision than the crystal clock of the base unit.
Therefore, the mobile unit includes correction mechanisms which are used to synchronize the frequency produced by the mobile unit crystal to the frequency produced by the base unit crystal.
Reference is made now to
FIG. 1
which is a schematic illustration of a mobile unit and a base unit known in the art. Base unit
2
includes a data interface
14
, an FEC unit
16
connected to the data interface
14
, a transmitter
18
connected to the FEC unit
16
and an antenna
20
connected to the transmitter
18
.
The base unit
2
also includes clock mechanism which includes a crystal
13
for generating a basic high frequency and a PLL unit
15
connected thereto. PLL unit
15
is further connected to the transmitter
18
and provides it with a frequency which is
m
n
of the basic frequency. It will be appreciated by those skilled in the art, that the values set for m and n can be any natural number (including the number 1).
Mobile unit
4
includes an antenna
30
, a channel sampler
26
connected to antenna
30
, a demodulator
27
connected to the channel sampler
26
, a time tracking unit
28
connected to demodulator
27
, a voice decoder
24
connected to the demodulator
27
, a digital to analog converter (DAC)
22
connected to voice decoder
24
and a speaker
32
connected to DAC
22
.
Mobile unit
4
also includes a timing mechanism which includes a crystal oscillator
23
and
2
PLL units
21
and
25
. PLL unit
25
is further connected to channel sampler
26
and provides it with a frequency which is
m
n
of the basic frequency provided by crystal oscillator
23
. PLL unit
21
is further connected to the DAC
22
and provides it with a frequency which is
l
k
of the basic frequency provided by crystal oscillator
23
.
The mobile unit
4
also includes a sampling and transmitting section, including a microphone
42
, a voice sampler
34
connected to the microphone
42
, a voice encoder
36
connected to the voice sampler
34
, a modulator
38
connected to the voice encoder
36
and transmitter DAC
40
connected to the modulator
38
.
The timing mechanism of the mobile unit is also connected to the transmitting section so that PLL
21
also times and controls voice sampler
34
and PLL
25
also times and controls the transmitter DAC
40
.
Accordingly, transmitter DAC
40
is affected by PLL unit
25
and sampler
34
has to adjust accordingly. The sampler
34
has to provide a block of a predetermined number of samples N
BLOCK
, for each frame produced by modulator
38
. Since the timing of transmitter DAC
40
is dynamic, wherein its frequency may increase or decrease, the voice sampler
34
may provide more or less than N
BLOCK
samples in a block, for each frame.
According to frame based communication standards such as TDMA and CDMA, each frame includes a predetermined number of information bits which may be the information data, voice data and the like. According to TDMA standard, each frame lasts 20 ms. A voice information frame includes 160 voice samples.
Accordingly, the data interface
14
provides blocks to the FEC unit, which include N
BLOCK
samples, in each block, every 20 ms. The FEC unit
16
produces a frame which is then provided to transmitter
18
. The transmitter
18
transmits this frame via antenna
20
.
At the mobile unit
4
, the channel sampler
26
detects the transmitted frame via antenna
30
and provides it to demodulator
27
. The demodulator
27
analyzes the frame, extracts voice coded data and provides it to voice decoder
24
. The voice decoder
24
decodes this data and provides the decoded signal to DAC unit
22
. The DAC unit
22
converts the decoded signal into analog signal and provides it to speaker
32
which in turn produces sounds.
In the mobile unit
4
, the channel sampler
26
and the DAC
22
have to be synchronized too. Thus, for every frame detected by channel sampler
26
, the DAC
22
has to convert N
BLOCK
samples. It will be appreciated that the synchronization mechanisms, of both base unit
2
and mobile unit
4
, have to be completely synchronized. Accordingly, the channel sampler
26
has to be synchronized with FEC
16
so that each frame produced by FEC
16
will be received as such in channel sampler
26
.
Conventional communication systems include a calibration mechanism which constantly calibrates PLL
25
so as to synchronize it according to the transmitting rate of the base unit
2
. It will be appreciated that since DAC
22
is dependent on channel sampler
26
, changing the frequency on which channel sampler
26
operates will affect DAC
22
. For example, if the frequency of channel sampler
26
increases, then the DAC
22
may be provided with more than N
BLOCK
samples in 20 ms.
On the other hand, if the frequency of channel sampler
26
is decreased then the DAC
22
might be provided with less than N
BLOCK
samples per frame. Both of these situations are not allowed in TDMA and for that matter in most frame base communication standards. Another method known in the art deletes or estimates the last samples.
If the current number of samples which are to be provided for a block exceeds N
BLOCK
samples than N
BLOCK
samples are selected and provided as a block and the rest of the samples are deleted. If, on the other hand, there are less than N
BLOCK
samples and time comes to provide them to the voice encoder then samples are estimated according to the existing samples so as to provide complementary samples which will complete the block to N
BLOCK
samples. It will be appreciated that according to this method, the quality of speech is reduced since an additional element of distortion or noise is introduced, which did not exist in the original sampled or the decoded signal.
SUMMARY OF THE PRESENT INVENTION
It is an object of the present invention to provide a novel method and a novel system for providing synchronized sampling frequencies, which overcomes the disadvantages of the prior art.
In accordance with the present invention there is thus provided a synchronized frequency generating system which includes a main crystal clock, for producing a basic frequency F
B
, a channel sampling phase locked loop (PLL) unit, connected to the main crystal clock, for converting the basic frequency F
B
into a channel sampling frequency F
CS
, a voice sampling PLL unit, connected to the main crystal clock, for converting the basic frequency F
B
into a voice sampling frequency F
VS
, a time tracking unit, connected to the channel sampling PLL unit, for detecting signal characteristics so as to determine a channel sampling frequency phase change value &Dgr;&phgr;
CS
and a frame timing phase change value &Dgr;&phgr;
FRAME
and a frequency controller, connected to the voice sampling PLL.
The frequency controller receives channel sampling frequency phase adjustment data and determines a voice sampling frequency phase change value &Dgr;&phgr;
VS
. The frequency controller provides the voice sampling frequency phase change value &Dgr;&phgr;
VS
to the voice sampling PLL.
Then, the frequency controller receives channel sampling frequency phase adjustment data from the time tracking unit and the frequency controller receives channel sampling frequency phase adjustment data from the channel sampling PLL.
The system according to the invention may further inclu
Pianka Boaz
Shatit Biniamin
Chin Stephen
Eitan, Pearl, Latzer & Cohen Zedek LLP
Kim Kevin
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