Video reproduction apparatus having automatic black and white calibration

• US 4,802,024 A
• Filed: 11/12/1986
• Issued: 01/31/1989
• Est. Priority Date: 11/12/1986
• Status: Expired due to Fees

• Current Assignee: Eastman Kodak Company
• Original Assignee: Eastman Kodak Company
• Inventors: Knowlton, James R.
• Primary Examiner(s): PSITOS, ARISTOTELIS
• Application Number:
  US06/929,574
• Time in Patent Office: 811 Days
• Field of Search: 360/9.1, 360/33.1, 360/36.1, 360/27, 358/171
• US Class Current: 386/204
• CPC Class Codes: H03D 3/04 by counting or integrating cycles of oscillations arrangements for measuring frequencies G01R23/10
• CPC Class Codes: H04N 5/919 by dividing samples or signal segments, e.g. television lines, among a plurality of recording channels
• CPC Class Codes: H04N 5/931 for restoring the level of the reproduced signal
• CPC Class Codes: H04N 5/95 Time-base error compensation H04N5/932 takes precedence

Abstract

Video reproduction apparatus in which a plurality of parallel channels of frequency modulated video information signals reproduced from magnetic tape are automatically calibrated during demodulation to constrain each video information signal between the same peak to peak black and white analog signal range. The demodulator for each video channel includes a circuit for comparing demodulated white and black reference level signals at the beginning of the reproduced video signal with fixed white and black reference levels. White and black correction signals are produced which are representative of any differences between the reproduced and fixed reference level signals. The white and black correction signals are applied to the demodulator during demodulation of the video information signal to constrain the analog video signal between the fixed white and black reference levels. This results in a high degree of precision of peak-to-peak video information from channel to channel within a block and from block to block in a block sequential multichannel video signal. It also eliminates the need for manual calibration of each of a plurality of video demodulators, eliminates block to block intensity shifts of a displayed frame and permits interchangeability of tapes between different multitrack video reproduction apparatus.

Claims

1. A demodulator for a frequency modulated (FM) video signal comprising;

means for reproducing an FM video signal from recording media;

integrator means for producing a ramp signal which varies substantially linearly over a predetermined integration period between first and second signal levels representing intensity limits of a scene;

sample and hold means for sampling and holding the signal level produced by said integrator means;

detector means for detecting the zero crossings of said reproduced FM video signal and for producing a pulse of fixed sampling period which terminates production of said ramp signal by said integrator means and which causes said sample and hold means to sample said integrator means signal level and to hold said sampled signal level after termination of said sampling pulse;

switch means for resetting the signal level of said integrator means to a reset level less than said first signal level and for maintaining said reset signal level for a predetermined offset period;

means for comparing first and second reference signal levels of said demodulated reproduced video signal with respective fixed first and second reference signal levels and for producing a first signal level correction signal for changing the slope of said ramp signal and a second signal level correction signal for changing the duration of said offset period as a function of differences in the compared reproduced reference video signal levels and said fixed reference levels so as to constrain said demodulated video signal between said fixed first and second signal levels.

2. The demodulator of claim 1 wherein said integrator means includes an integrating capacitor and an integrating current source means for supplying an integrating current to said capacitor and wherein said first signal level correction signal changes said integrating current to said integrating capacitor to constrain said demodulated video signal to said first signal level.

3. The demodulator of claim 1 including means for producing a reset signal, which actuates said switch means for the offset period of said reset signal, said reset signal producing means including an offset period timing capacitor and an offset current source means for supplying an offset current to said capacitor and wherein said second signal level correction signal changes said offset current to said offset period timing capacitor to change said offset period so as to constrain said demodulated video signal to said second signal level.

4. The demodulator of claim 1 wherein said reproducing means simultaneously reproduces from recording media both an FM video signal and a timing signal which are subject to corresponding flutter error components, and includes means for producing a flutter correction signal from said timing signal, and further includes means for applying said flutter correction signal (1) to said integrator means to change the slope of said ramp signal, and (2) to said switch means to change the duration of said offset period to cancel said flutter error in said demodulated video signal.

5. A multitrack magnetic tape video reproduction apparatus comprising:

means for reading out an area imager in sequential blocks of a plurality of parallel channels of analog video information signals wherein each channel of video signal includes a reference period with white-and-black reference level signals followed by a video period with video information signals;

a plurality of frequency modulator means for converting said plurality of analog video information signals into a plurality of frequency modulated video information signals;

means for recording blocks of said plurality of frequency modulated video information signals on a plurality of parallel longitudinal tracks on magnetic tape so that a frame of video information is recorded in sequential blocks on said tape;

means for simultaneously reproducing a plurality of parallel channels of FM video information signals from said magnetic tape;

a plurality of FM demodulator means for demodulating said plurality of reproduced FM video information signals into a plurality of analog video information signals;

wherein each of said plurality of FM demodulator means includes

(a) integrator means for producing a ramp signal which varies substantially linearly over a predetermined integration period between first and second signal levels representing intensity limits of a scene;

(b) sample and hold means for sampling and holding the signal level produced by said integrator means;

(c) detector means for detecting the zero crossings of said reproduced FM video signal and for producing a pulse of fixed sampling period which terminates production of said ramp signal by said integrator means and which causes said sample and hold means to sample said integrator means signal level and to hold said sampled signal level after termination of said sampling pulse;

(d) switch means for resetting the signal level of said integrator means for a reset level less than said first signal level and for maintaining said reset signal level for a predetermined offset period;

(e) means for comparing first and second reference signal levels of said demodulated reproduced video signal with respective fixed first and second reference signal levels and for producing a first signal level correction signal for changing the slope of said ramp signal and a second signal level correction signal for changing the duration of said offset period as a function of differences in the compared reproduced reference video signal levels and said fixed reference levels so as to constrain said demodulated video signal between said fixed first and second signal levels; and

wherein said reproduction apparatus further includes means for displaying said plurality of demodulated analog video information signals such that the white-to-black range of the displayed image is uniform.

6. The apparatus of claim 5 wherein, in each of said demodulator means, said integrator means includes an integrating capacitor and an integrating current source means for supplying an integrating current to said capacitor and wherein said first signal level correction signal changes said integrating current to said integrating capacitor to constrain said demodulated video signal to said first signal level.

7. The apparatus of claim 5 wherein, each of said demodulator means includes means for producing a reset signal, which actuates said switch means for the offset period of said reset signal, said reset signal producing means including an offset period timing capacitor and an offset current source means for supplying an offset current to said capacitor;

and

wherein said second signal level correction signal changes said offset current to said offset period timing capacitor to change said offset period so as to constrain said demodulated video signal to said second signal level.

8. The apparatus of claim 5 wherein said reproducing means simultaneously reproduces from said magnetic tape both said plurality of channels of FM video information signals and a timing signal, said video information signals and said timing signal being subject to corresponding flutter error components;

wherein said apparatus includes means for producing a flutter correction signal from said timing signal and means for applying said flutter correction signal to each of said demodulator means to change the slope of said ramp signal produced by said integrator means and to change the duration of said offset period of said switch means so as to cancel any flutter error in said demodulated video signal.