Reference generator for demodulator calibration

Details Reference generator for demodulator calibration Reference generator for demodulator calibration

1999-09-01

2001-09-25

Tokar, Michael (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C341S152000, C341S131000, C341S142000, C341S147000, C341S166000, C341S150000, C341S155000, C341S126000, C341S118000, C329S319000, C329S345000, C329S318000, C329S351000, C375S230000, C375S213000, C375S224000, C375S316000

Reexamination Certificate

active

06295015

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a reference generator for generating a substantially distortion free ATSC (Advanced Television Systems Committee) compliant VSB (vestigial sideband) reference signal which may be used, for example, to calibrate a digital demodulator.
BACKGROUND OF THE INVENTION
Demodulators are used in television applications in order to demodulate the intermediate frequency (IF) output of a tuner down to baseband for further processing by a receiver. Professional demodulators are special examples of demodulators that are typically used in a laboratory or on a production line in order to test modulators, transmitters, and translators. However, professional demodulators have other uses such as monitoring broadcasts or cable transmissions, testing receivers, field testing television transmissions, and as exhibits used in demonstrations and seminars. In such cases, the professional demodulator receives an input signal to be monitored and provides various outputs representing the input signal which is received over a channel.
When testing transmission equipment, for example, there are generally two sources of distortion in the output of a professional demodulator. One source of distortion is the transmission equipment itself. The professional demodulator is typically provided with distortion correcting components, such as equalizers, in order to reduce distortion caused by the transmission equipment being tested.
The other source of distortion is the professional demodulator itself. This type of distortion is generally caused by variations in component performance characteristics and by component performance characteristics which change over time. For example, the tuner of a professional demodulator contributes to such demodulator caused distortion. In the case of tuner caused distortion, not only does the performance characteristics of the tuner change over time, but the performance characteristics of the tuner also change from channel to channel.
It is standard practice to calibrate a professional demodulator in the factory in order to reduce demodulator caused distortion. However, such calibration, although satisfactory at the time the professional demodulator leaves the factory, can soon become unsatisfactory because of the time related changes of the demodulator's performance characteristics.
Moreover, calibrating the professional demodulator in the factory poses the additional problem of determining the channel to which the professional demodulator should be calibrated. That is, a professional demodulator that is optimally calibrated in the factory at one channel is not optimally calibrated at another channel because, as discussed above, the performance characteristics of the demodulator's tuner change from channel to channel. Thus, a professional demodulator that is optimally calibrated in the factory at one channel may not be optimally calibrated when being used to test equipment operating at a different channel.
The present invention is directed to a reference generator which may be used to supply a substantially distortion free ATSC compliant VSB reference signal to a digital demodulator so that the digital demodulator can be calibrated in the field. However, it should be understood that the reference generator of the present invention may be used for purposes other than calibrating a digital demodulator.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a reference generator for generating an ATSC compliant VSB reference signal comprises a memory, a digital-to-analog converter, a clock, and a low-pass reconstruction filter. The memory stores reference data representing a modulated ATSC compliant VSB signal having substantially no distortion. The clock emits a clock signal that clocks the stored reference data out of the memory to the digital-to-analog converter to produce a continuous converter output. The low-pass reconstruction filter filters the converter output.
In accordance with another aspect of the present invention, a computer readable storage medium has a block of reference data stored thereon. The block of reference data, when repeatedly clocked out of the computer readable storage medium by a clock signal, functions as a continuous ATSC compliant VSB reference signal producing substantially no sub-harmonics of the clock signal.
In accordance with a further aspect of the present invention, a method of preparing ATSC compliant VSB reference data for storage in a memory comprises the following steps: a) modulating data with a carrier so that the data is modulated using an integral number of carrier cycles, wherein the modulated data has ATSC synchronization and DC bias; and, b) pre-distorting the modulated data in order to compensate for distortion imposed on the modulated data during a digital-to-analog conversion.


REFERENCES:
patent: 5481564 (1996-01-01), Kakuiishi et al.
patent: 5495504 (1996-02-01), Yasuda
patent: 5614903 (1997-03-01), Oyama
patent: 5859605 (1999-01-01), Raghavan et al.
patent: 6166594 (2000-12-01), Nielsen et al.

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