Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1999-12-29
2003-07-08
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C386S349000, C386S349000
Reexamination Certificate
active
06590733
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to servo positioning the read heads of a video cassette recorder (VCR), and particularly to detecting pilot-tones from adjacent tracks or stripes and digitally processing pilot-tone information for centering the head on the track being followed.
Video cassette recorders record audio and video signals in stripes across the width of the tape, the stripes being arranged in parallel at an incline to the direction of the tape travel. As illustrated in
FIG. 1
, the stripes are recorded such that every other stripe along the length of the tape contains information modulated to produce a tone at either of two pilot-tone frequencies, f
1
and f
2
. The f
0
stripes are interleaved between the f
1
and f
2
stripes such that each f
0
stripe is bounded by adjacent f
1
and f
2
stripes. Each f
0
stripe contains specific notches at the f
1
and f
2
frequencies. Track following occurs only when the head is attempting to track the f
0
stripes. As shown in
FIG. 1
, the head senses the pilot-tones of the adjacent f
1
and f
2
stripes. The amplitude of each pilot-tone f
1
or f
2
sensed is largely affected by the position of the head in relation to the f
1
and f
2
stripes. If the head detects an f
1
pilot-tone amplitude greater than an f
2
pilot-tone amplitude, the head is off the center of the f
0
stripe toward the f
1
stripe. Differential averaging techniques are employed to quantify the off-center condition of the head and to operate a servo mechanism based on the f
1
and f
2
pilot-tone amplitudes to move the head relative to the f
0
stripe until the head is centered on the f
0
stripe and the f
1
and f
2
pilot-tone amplitudes are equal.
Prior pilot-tone detection and processing employed analog technology using bandpass filters that pass signal amplitudes centered on the pilot-tone frequencies. The amplitude of each filtered signal was detected and the difference between them was a measure of servo mis-position. The difference signal was ultimately converted to a digital form to operate the servo control microprocessor. However, pilot-tone frequency can vary by as much as ±10% due to variations in tape speed, such as in a “trick” play mode. Therefore, bandpass filters associated with pilot-tone detection necessarily had to be wide enough to accommodate expected frequency variations of the pilot-tones. Wider bandpass filters resulted in a greater likelihood of detection of noise with the pilot tone, resulting in inaccurate amplitude detection of the pilot-tone. Inaccuracies in pilot tone amplitude detection resulted in errors in operating the servo to center the head on the f
0
stripe. Moreover, certain servo algorithms cannot process wide bandwidths, making the wide bandpass filters infeasible in some systems. Narrow bandpass filters could be employed but required switching to define the bandwidths of the actual tones, thereby adding to the complexity of the detection and processing of pilot-tones. The present invention is directed to a digital technique for digitally processing the pilot tone frequencies to inaccuracy of less than about 0.5%, over a range of ±10% of the nominal pilot-tone frequency. As a result, pilot-tone amplitude is more accurately detected, resulting in greater accuracy of the head positioning over the f
0
stripes, and the quantity of expensive analog circuitry can be reduced.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, the amplitudes of at least two signals having mutually exclusive frequencies are digitally determined from an input digital representation of an input signal set containing the two signals. Digital representations are locally derived representing the nominal frequencies of the two signals, from which digital representations of the sine and cosine functions of each locally derived frequency are derived. The derived digital representations of the sine and cosine functions of each frequency are mixed with the input digital representation to determine a product of the input signal with the sine and cosine functions of each of the locally derived frequencies. The resulting four digital representations are processed to derive digital representations of the amplitudes of each of the two signals.
In one form of the invention, the digital representations of the frequencies are derived by establishing a number representative of the respective frequency and repeatedly advancing a count in an accumulator by the number to establish a digital ramp having a slope representative of the frequency. One accumulator is provided for each frequency f
1
and f
2
. The digital representations of the sine and cosine functions of each of the two locally derived frequencies are stored in a look-up table or an array of look-up tables, and the sine or cosine value is based on an instantaneous value of the count in the accumulator. The sine and cosine functions of each frequency are multiplied with the input signal, and the output of each multiplier is processed in a separate digital low-pass filter to extract the baseband component thereof. The outputs of the low-pass filters processing the sine and cosine functions of frequency f
1
are combined in each channel in root-sum-of-squares fashion to yield a signal representative of the amplitude of the f
1
component of the input signal. In similar fashion, the outputs of the low-pass filters processing the sine and cosine functions of frequency f
2
are combined in root-sum-of-squares to yield a signal representative of the amplitude of the f
2
component of the input signal. The signals representative of the amplitudes of f
1
and f
2
are subtracted and the result is provided in digital form to the servo control microprocessor of the VCR that regulates head position.
Another form of the invention employs time-sharing techniques to derive the digital representations of the sine and cosine functions and to perform the mixing, low-pass filtering and root-sum-of-squares combining operations, thereby achieving significant savings in the hardware volume utilized to implement the invention. The amplitudes of at least two signals having mutually exclusive frequencies are digitally determined from an input digital representation of an input signal set containing the two signals. Digital representations are derived representing the frequencies of the two signals at frequencies f
1
and f
2
. As in the first form of the invention, these representations are obtained by incrementing an accumulator at an appropriate rate for each frequency, and two accumulators are therefore provided. A sine/cosine look-up table, mixer, low-pass filter and root-sum-of-squares combinor are employed sequentially and in pipeline fashion to process the four channels of information representing sine and cosine functions of signals for both frequencies f
1
and f
2
. State memory is employed to store the states associated with each channel.
REFERENCES:
patent: 5060092 (1991-10-01), van der Kop
patent: 5875065 (1999-02-01), Shimura
“An Experimental Digital Consumer Recorder For MPEG-Coded Video Signals” by Saeijs, et al,Transactions On Consumer Electronics, vol. 41, No. 3, pp 651-660, Aug. 1995.
“DVS Full-Digital VTR Servo Interface” by Motorola Semiconductor.
“Channel Code With Embedded Pilot Tracking Tones for DVCR” by Kahlman et al.,Transactions on Consumer Electronics, vol. 41, No. 1, pp 180-185, Feb. 1995.
“Computer Arithmetic Principles, Architecture and Design” by Hwang,School of Electrical Engineering, Purdue University, pp 359-365.
Spaur Michael R.
Wilson Rosser S.
Agere Systems Inc.
Hudspeth David
Kinney & Lange , P.A.
Slaviti Mitchell
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