EP0067585A2
Expired / Lapsed
EP
⏱️ 0.0 years remaining
Noise sensitivity reduction for a TV receiver AGC system
Assignee: RCA Corp
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Filing: 1982-05-27
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Granted: —
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Expires: 2002-05-27
Abstract
Abstract Video is applied via a gain controlled IF amplifier (11) to a video detector (13) and amplifier (15). Sync and equalising pulses are fed from the amplifier to an active filter (21-26) and then to a comparator (30), which produces an output variation for changing an AGC voltage produced by generator (40) and AGC capacitor (41) if the pulse peaks exceed a threshold. The filter (21-26) delays and attenuated noise impulses, but can also cause noise impulses to remain above the threshold for a period of time defined by pulse stretching in the filter. A transistor (32) is responsive to a potential difference of predetermined sense and magnitude between the input and output of the filter to curtail the output variation of the comparator (30). Such a difference exists when a delayed leading edge of a noise pulse is present at the output of the filter and the trailing edge is at the input.
Inventors
- 👤 Jack Rudoplh Harford
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-11-09 20:38
CPC Classifications
H; H04; H04N; H04N5/00; H04N5/14; H04N5/21; H04N5/44; H04N5/52
Claims Analysis
Total Claims: 10
Claim 1
Independent Claim
247 words
In a television receiver including an IF amplifier having a gain subject to control in accordance with a gain control potential supplied thereto, and a video detector responsive to the output of said gain controlled IF amplifier; an automatic gain control system being characterized by a low pass filter having an input and an output, and exhibiting a given cutoff frequency; means (15) responsive to an output of said video detector for supplying a composite video signal to said input of said low pass filter; a source (+2.4 V) of reference potential; voltage comparison means having a first input coupled to said source of reference potential and a second input coupled to said output of said low pass filter for developing an output variation when the potential at said second comparator input reaches a threshold potential related to said reference potential; means (32) having an input responsive to said output of said video detector, and having an output (emitter 32) coupled to said second comparator input, for bypassing said low pass filter; said bypassing means being enabled only in the presence of a difference between the potentials at the input and output of said bypassing means of a given sense and of a magnitude exceeding a given magnitude; said bypassing means have a passband encompassing the passband of said low pass filter and extending beyond said cutoff frequency; and means (40) responsive to output variations of said voltage comparison means for developing said gain ' control potential.
Claim 2
Dependent Claim
247 words
In a television receiver including an IF amplifier having a gain subject to control in accordance with a gain control potential supplied thereto, and a video detector responsive to the output of said gain controlled IF amplifier; an automatic gain control system being characterized by a low pass filter having an input and an output, and exhibiting a given cutoff frequency; means (15) responsive to an output of said video detector for supplying a composite video signal to said input of said low pass filter; a source (+2.4 V) of reference potential; voltage comparison means having a first input coupled to said source of reference potential and a second input coupled to said output of said low pass filter for developing an output variation when the potential at said second comparator input reaches a threshold potential related to said reference potential; means (32) having an input responsive to said output of said video detector, and having an output (emitter 32) coupled to said second comparator input, for bypassing said low pass filter; said bypassing means being enabled only in the presence of a difference between the potentials at the input and output of said bypassing means of a given sense and of a magnitude exceeding a given magnitude; said bypassing means have a passband encompassing the passband of said low pass filter and extending beyond said cutoff frequency; and means (40) responsive to output variations of said voltage comparison means for developing said gain ' control potential.
Claim 3
Dependent Claim
44 words
Apparatus in accordance with claim 1 ' , characterized in that said bypassing means include a first transistor (32) disposed as an emitter-follower and having a base electrode coupled to receive said composite video signals and an emitter electrode coupled to said filter output.
Claim 4
Dependent Claim
110 words
Apparatus in accordance with claim 2, characterized in that said filter is active and comprises a second transistor (26) having base and emitter electrodes, a first resistance (21,22) and a second resistance (24) connected in series between the output of said video signal supplying means and the base electrode of said second transistor, a first capacitance (23) coupled between the junction of said first and second resistances and the emitter electrode of said second transistor, and a second capacitance (25) coupled between the base electrode of said second transistor and a point of fixed potential; the output of said active filter appearing at said emitter electrode of said second transistor.
Claim 5
Dependent Claim
28 words
Apparatus in accordance with claim 3, characterized in that the base electrode of said first transistor (32) is connected to an intermediate point of said first resistance (21,22).
Claim 6
Dependent Claim
87 words
Apparatus in accordance with claim 4, characterized in that said voltage comparison means includes a third transistor (30) having a base electrode connected to said reference potential source, an emitter electrode connected to the emitter electrode of said second transistor (26), and a collector electrode connected to a load impedance (31) across which said output variations of said voltage comparison means appear; said apparatus also including a fourth transistor (28) disposed as a current source coupled to the interconnected emitter electrodes of said second and third transistors.
Claim 7
Dependent Claim
105 words
Apparatus in accordance with claim 1, characterized in that said composite video signals supplied to said low pass filter input include periodic synchronizing pulses extending in a given direction to a peak desirably maintained at a given level during normal operation of said gain control system; said signals being undesirably subject to occasional accompaniment by noise impulses extending in said given direction beyond said given level; said given direction and given level being so related to said given sense and given magnitude that said bypassing means is subject to enablement during the appearance of the trailing edges of said noise impulses at said filter input.
Claim 8
Dependent Claim
78 words
Apparatus in accordance with claim 6, characterized in that said composite video signals supplied to said filter input include equalizing pulses of a width narrower than said periodic synchronizing pulses and of a peak level equal to the peak level of said synchronizing pulses, and wherein said given cutoff frequency is sufficiently high so that said equalizing pulses in the output of said filter attain substantially the same peak level as is attained by said wider synchronizing pulses.
Claim 9
Dependent Claim
192 words
A television receiver wherein a video detection means (13,15) is coupled to receive a composite video signal from an IF amplifying means (11) which is subject to gain control by a gain control means including comparison means (30,31 + 2.4 V) arranged to produce an output variation when the peaks of at least sync pulses provided to the comparison means exceed a threshold value, means (40,41) responsive to the output variation to produce a gain control signal for the amplifying means (11) and low pass filter means (21,26) having a predetermined cut-off frequency and arranged to couple the pulses to the comparison means and to delay and attenuate high frequency noise, and characterised by means (32,33) responsive with a delay less than the delay of the filter means to a difference of a predetermined sense and of a magnitude exceeding a predetermined magnitude between the leading portion of a noise impulse as supplied by the filter means to the comparison means and the trailing portion of the noise impulse as supplied to the filter means to curtail the output variation due to the delayed and attenuated noise impulse, of the comparison means.
Claim 10
Dependent Claim
192 words
A television receiver wherein a video detection means (13,15) is coupled to receive a composite video signal from an IF amplifying means (11) which is subject to gain control by a gain control means including comparison means (30,31 + 2.4 V) arranged to produce an output variation when the peaks of at least sync pulses provided to the comparison means exceed a threshold value, means (40,41) responsive to the output variation to produce a gain control signal for the amplifying means (11) and low pass filter means (21,26) having a predetermined cut-off frequency and arranged to couple the pulses to the comparison means and to delay and attenuate high frequency noise, and characterised by means (32,33) responsive with a delay less than the delay of the filter means to a difference of a predetermined sense and of a magnitude exceeding a predetermined magnitude between the leading portion of a noise impulse as supplied by the filter means to the comparison means and the trailing portion of the noise impulse as supplied to the filter means to curtail the output variation due to the delayed and attenuated noise impulse, of the comparison means.
References Cited (Backward Citations)
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