Television – Image signal processing circuitry specific to television – Dc insertion
Patent
1994-12-05
1996-08-20
Kostak, Victor R.
Television
Image signal processing circuitry specific to television
Dc insertion
348696, H04N 518
Patent
active
055483437
DESCRIPTION:
BRIEF SUMMARY
The invention relates to transmission of an analogue or digital television signal by radio channel, by satellite or by cabled network, and its subject more particularly is a method of alignment in television with reduced noise and a device for implementing it.
It is known that an alignment device in television, also called clamp, allows the DC component of a television signal to be restored before modulation or after demodulation. This device is used in all the presently known television standards: PAL, SECAM, NTSC, D2 MAC, HD MAC, etc. A television signal undergoes various processings before transmission by a television transmitter and on reception by a television demodulator receiver. During these processings, the DC component of the television signal is spoiled. This is represented, for example on reception, by a slow fluctuation of the level of a composite video signal on which transmission noise is superimposed. In order to get around this fluctuation, it is necessary to stabilize the composite video signal in order to make use of it before a modulation function or after a demodulation function.
A conventional alignment device uses a feedback device, also called feedback loop, the mathematical model and a theoretical diagram of which, taken as example, are described later and correspond respectively to FIGS. 1 and 2.
The summarized theory of an alignment device in television is based on sampling, at the start of each line, a part of the television signal taken over a plateau of constant amplitude containing no video information, chosen as an alignment reference level, and on holding of the average of the level of the signal sampled over the duration of one line, i.e. 64 .mu.s. This signal is next subtracted from the television signal coming into the alignment device so as to restore its DC component. This average is in fact an estimate of the average level of the plateau of the television signal of limited duration on which an error signal is superimposed. This error signal is related to the noise arising from various origins including: carrier-to-noise ratio, and on the filters associated with the demodulator, from energy dispersion, from a dislocation in frame on the signal, from a low-frequency noise from the transposition oscillators, from the residual 50 Hz, etc.
The alignment device with feedback is generally well suited to the requirements of rejection, of energy dispersion and of very-low-frequency noise in the case of modulation and demodulation. However, during the sampling of a part of the television signal, the device generates noise in the range of frequencies lower than 15 kHz. This frequency range corresponds to the region of the spectrum of very high sensitivity of the eye. The noise generated by the alignment device is manifested visually by the appearance of horizontal streaks on an image when the television signal is very noisy before modulation or demodulation.
A known device developed in the European Patent Application No. 0 281 175 includes, at the front end of the feedback loop, a low-pass filter which filters the high frequencies and the noise which is associated with these frequencies. The filter is activated at start-of-line, after the line synchronization pulse.
Although the device reduces the transmission noise superimposed on the television signal, it does not, however, reduce the noise generated by the loop itself during estimation of the plateau.
By way of example, the 25 Hz and 50 Hz rejection characteristics, as well as the effective signal-to-noise ratio in a band of frequencies from 0 to 10 kHz of a conventional alignment device with feedback are:
The specific 50 Hz and 25 Hz frequencies correspond respectively to the frame synchronization frequency and to the frequency of the triangular energy-dispersion signal in satellite transmission. The effective signal-to-noise ratio of an alignment device is given in the frequency range (0 to 10 kHz) of high sensitivity of the eye.
After experimentation, the threshold of visual perception of the noise of the alignment device
REFERENCES:
patent: 2936335 (1960-05-01), Urtel
Guillevic Dominique
Paquier Jean-Luc
Kostak Victor R.
Thomcast
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