Method and apparatus for generating an auxiliary timing signal in the horizontal blanking interval of a video signal

An auxiliary signal including a burst of oscillations and a blacker-than-black horizontal synchronizing signal is inserted in the horizontal blanking intervals of an HDTV video signal by first generating a digital auxiliary signal in a PROM operating at a clock frequency of 27 MHz. If the video signal from which the clock frequency and other control signals are derived is an analog video signal, the digital auxiliary signal is converted to an analog signal, filtered and clamped before it is additively combined with the analog video signal which has been similarly clamped. In that case the digital auxiliary signal stored in the PROM is predistorted so as to compensate for the damping effects of digital-to-analog conversion and low-pass filtering. When the video signal is available as a digital signal, the PROM outputs are clocked through a register enabled only in the horizontal blanking intervals and the digital video signal is clocked through a register disabled in the horizontal blanking intervals, so that the outputs of the registers can then go to a common digital-to-analog converter. In this case the PROM handles 9 bits, the most significant one of which goes to a MSB input of the digital-to-analog converter, to which the video signal has no connection, in order to produce the blacker-than-black synchronizing signal.

This invention concerns the generation of an auxiliary signal in the 
horizontal blanking interval of a video signal for facilitating the 
elimination of time-base errors that may arise, for example, in magnetic 
tape recording, especially in the case of HDTV video signals. 
In the published German Patent Publication DE OS 37 36 741 of the assignee 
of this application, a system was disclosed for measuring time-base errors 
in a HDTV video signal reproduced from a magnetic tape, in which the HDTV 
video signal was provided with a reference signal in the region of the 
horizontal blanking interval which, like the color carrier synchronizing 
signal of a color television signal, consists of burst of a number of 
sinusoidal oscillations. In contrast to the color synchronizing signal, 
which is transmitted on a rear black shoulder of the horizontal 
synchronizing pulse, the reference signal is superimposed on a DC voltage 
of a medium gray value within the horizontal blanking interval. The 
frequency of the reference signal is locked to the frequency of a clock 
signal in accordance with the following relation 
EQU f.sub.CLK =f.sub.AUX (2n+4) where n =0,1,2,3. . . 
so that at a clock frequency of f.sub.CLK =27MHz and where n=2, the 
frequency of the reference signal f.sub.AUX =3.375MHz. 
The clock signal of the frequency fCLK is, furthermore, frequency-locked 
with 864 times the horizontal scan frequency of a HDTV 1250 line video 
signal. A reference frequency oscillation which is added in the horizontal 
frequency blanking interval, which may be done in the recording portion of 
a magnetic tape recording and playback equipment for video signals, serves 
for determining time errors found in the reproduced HDTV video signal. The 
timing error is determined by reference to the phase position of the 
reproduced reference signal in a special phase detector. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide in a method and system 
of the above-described kind to produce reference-oscillations of 
sinusoidal curve shape with great phase and amplitude accuracy and to add 
the reference oscillations together with a horizontal synchronizing signal 
as an auxiliary signal inserted into a video signal. 
Briefly, during every horizontal frequency blanking interval an address 
signal is generated for a programmable read only memory (PROM) under 
control of a clock signal which is an integral multiple of the reference 
oscillation frequency and which is locked to the horizontal synchronizing 
signal of the video signal. The PROM stores the shape of the auxiliary 
signal and preferably also data for generating a synchronizing signal 
preceding the auxiliary signal. The PROM then contains a first memory 
portion storing the reference oscillations and a pedestal therefor, as 
less significant bits, and a second portion storing data for the synch 
pulse including a most significant bit designating a blacker than black 
level. The output signal of the PROM is converted from digital to analog 
form either before or after insertion into the video signal, according to 
whether the video signal is originally digital or analog and is separately 
low-pass filtered before insertion into the video signal in the horizontal 
frequency blanking interval when the video signal is an analog signal at 
the time of insertion. 
In a one embodiment it is desirable for the low-pass filtering to be 
performed with a Bessel function low-pass filter having a cut-off 
frequency of about twice the frequency of the reference signal portion of 
the auxiliary signal. 
It is advantageous for the contents of the PROM to provide pre-correction 
for the damping of the auxiliary signal in conversion from digital to 
analog form and in low-pass filtering when the conversion or filtering is 
performed prior to insertion of the auxiliary signal into the video 
signal. It is also desirable for the frequency of the clock signal to be 
locked to an even multiple of the reference frequency, preferably the 
8-fold multiple. 
In a preferred embodiment a first PROM is used for storing the reference 
oscillations for the auxiliary signal and a second PROM for a 
synchronizing pulse in the auxiliary signal, in which case the first PROM 
serves for the shaping of the reference oscillations and its output is put 
into a data stream of a digital video signal as the 8-bit data values of 
less significance. The data signal obtained from the second PROM to 
provide the synchronizing pulses is added in as the most significant data 
value when the data stream with its 8-bit insertions is converted from 
digital to analog form. 
The corresponding apparatus of the present invention will be better 
understood in connection with an illustrated description that follows 
further below. 
The method and system of the invention have the advantage that the digital 
and therefore highly precise generation of the auxiliary signal prior to 
recording reduces to a few nanoseconds the residual time errors that 
remain after correction of time errors during playback. The second 
embodiment has the advantage that the dynamic range of a digitalized video 
signal, established by 8-bit wide data words, does not need to be limited 
to an analog picture range from 0 volt for the black value to 0.7 volt for 
the white value, if supplementary synchronizing pulses having a negative 
amplitude of -0.3 volt are found necessary which lie outside the dynamic 
range of the 8-bit system.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS: 
At the terminal 1 shown in FIG. 1 the chrominance component of an analog 
video signal is supplied into which an auxiliary signal in the form of a 
negative synchronizing pulse S of a horizontal frequency synchronizing 
signal and a reference signal R provided on a gray level step are to be 
inserted in the horizontal blanking interval A. The reference signal R 
comprises 10 sinusoidal oscillations of a frequency of 3.375MHz. The video 
signal in the illustrated case is a HDTV video signal. 
FIG. 2a shows one of the horizontal frequency blanking intervals A 
containing a forward achromatic region P.sub.v, a synchronizing pulse S, a 
reference signal R and a rear achromatic region P.sub.h. The two 
achromatic regions P.sub.v and P.sub.h are at 50% of the picture amplitude 
range, which corresponds to 0.35 volt, since the picture voltage range 
from 0% to 100% extends from the 0 volt level to the 0.7 volt level, shown 
in the scale at the right of FIG. 2a. The synchronizing pulses S of the 
synchronizing signal occupy an amplitude region from 0 volt to -0.3 volt 
of the aggregate amplitude range of one volt peak-to-peak. 
As already mentioned the reference signal is to be locked in frequency and 
phase with the horizontal synchronizing signal of a HDTV video signal. For 
this reason a horizontal synchronizing signal H.sub.D (32.mu.s per line 
period) transmitted in parallel with the analog video signal to a terminal 
2 is supplied to a phase-locked loop 3, at the output of which a clock 
signal CLK having a clock frequency of 27MHz is made available which is 
locked in phase to the 864-multiple of the horizontal frequency of a HDTV 
video signal. 
The clock signal CLK, as well as a horizontal blanking signal A.sub.H 
present at a terminal 4 and a vertical synchronizing signal 2V supplied at 
a terminal 5, are all combined into a control signal in a logic stage 6. 
The control signal is supplied through a reset stage 7 to the control 
inputs of an address counter 8. The outputs of the counter 8 are connected 
over a 9-bit wide address bus, on one hand, to the address inputs of a 
PROM 9 and, on the other hand, to a control stage 10. The control stage 10 
detects an address value that is present at the end of the blanking 
interval, in order to reset, through the reset stage 7, the address 
counter 8 to a definite initial address value. 
In the PROM 9 data words with the amplitude values of the curve shape of 
the auxiliary signal are stored at the appropriate addresses. In the 
illustrated case 8 amplitude values are stored in the PROM 9 for each 
oscillation period of the reference signal R. Furthermore data are stored 
in the PROM 9 for the curve shape of the horizontal frequency 
synchronizing pulse S and the two achromatic regions P.sub.v and P.sub.h. 
In reading out the PROM 9 in response to a sequence of address signals the 
auxiliary signal is made available in the form of data words for further 
signal processing. These data words are then supplied over an 8-bit wide 
data bus to a digital to analog converter 11 where they are converted into 
corresponding analog values. The output of the digital to analog converter 
11 is connected through a low-pass filter 12 with a clamping stage 13, 
which clamps the now analog auxiliary signal to a definite DC voltage 
potential in the rear achromatic region P.sub.h by means of a horizontal 
frequency clamping pulse signal H.sub.C. Another clamping stage 14 is 
inserted in the transmission channel provided for transmitting the HDTV 
video signal. 
The signals made ready at the outputs of the clamping stages 13 and 14 are 
added together in an addition stage 15, so that at an output terminal 16 
an HDTV video signal is made available into the horizontal blanking 
intervals of which an auxiliary signal is inserted which is highly precise 
with reference to its phase and amplitude values. 
A clock signal CLK prepared in the logic stage 6 is supplied to the reset 
stage 7, the address counter 8, the PROM 9, the control stage 10 and the 
digital to analog converter 11. 
The digital to analog converter 11 has an amplitude damping characteristic 
that conforms to 
##EQU1## 
function 
The low-pass filter 12 which follows the converter 11 likewise has a 
certain amplitude-damping characteristic which in the illustrated example 
falls off towards higher frequencies in accordance with a Bessel function. 
The cut-off frequency of the low-pass filter 12 constituted as a Bessel 
low-pass filter lies at about 6MHz, thus about twice as high as the 
frequency of 3.375MHz of the oscillatory portion of the auxiliary signal, 
which may be referred to for short as the frequency of the auxiliary 
signal. By the choice of such a high cut-off frequency compared to the 
frequency of the auxiliary signal, the group propagation time errors of 
the Bessel low-pass filter can be treated as negligible. Means for 
correction of group velocity errors can therefore be left out, particular 
since practically only a single frequency is to be transmitted. 
The amplitude-damping losses produced by the digital-to-analog converter 11 
and the low-pass filter 12 can be compensated by providing correspondingly 
"predistorted" amplitude values in the data words stored in the PROM 9. 
The broken line signal curve of the reference signal R in FIG. 2a is 
intended to show this compensation procedure. The voltage/time diagram of 
FIG. 2b, on a different time scale, shows two complete line periods H of 
the analog video signal provided at the terminal 1. The timing diagrams 
shown in FIGS. 2c and 2d respectively show voltage/time diagrams of the 
horizontal synchronizing signal H.sub.T and of the horizontal blanking 
signal A.sub.H. The address signal, as shown in FIG. 2e, appears only 
during horizontal frequency blanking intervals A, during which intervals 
it is applied to the address inputs of the PROM 9. 
By virtue of the digital derivation of the auxiliary signal which includes 
the reference oscillation signal and the horizontal synchronizing signal 
and the resulting fixed time relation between defined sampling instants in 
the auxiliary signal that is generated a signal shape is provided that is 
essentially independent of the manufacturing tolerances of analog circuit 
components. In consequence there is no effect on the phase position and 
the amplitude course of the generated auxiliary signal resulting from the 
unavoidable variations between mass produced components of the same type. 
In the block circuit diagram of FIG. 3, relating to a second embodiment of 
the invention, the video signal at the input terminal 1 is a digital video 
signal with a word width of 8 bits. Again, in the horizontal frequency 
blanking interval A of this digital video signal an auxiliary signal, 
comprising negative synchronization pulses S of a horizontal frequency 
synchronizing signal and reference oscillations R on a gray pedestal G, is 
to be inserted. The reference oscillations R are sinsusoidal oscillations 
of a frequency of 3.375MHz. 
FIG. 4a shows one of the horizontal blanking intervals A with a region for 
a forward black shoulder P.sub.v, a synchronizing pulse S, a reference 
signal R, a gray pedestal G and a rear black shoulder P.sub.n. The gray 
pedestal G is again at 50% of the picture amplitude, which is a level of 
0.35, volt and the picture range corresponding to the 0 volt to 0.7 volt 
levels is shown on a percentage scale at the right. The sync pulses S of 
the synchronizing signal occupy an amplitude range, again, from 0 volt to 
-0.3 volt. 
The reference oscillations and their frequency are, again, locked in 
frequency and phase to the synchronizing signal of a HDTV video signal. 
In a manner similar to FIG. 1 a horizontal synchronizing signal H.sub.D is 
supplied to a terminal 102 from which it proceeds to a phase locked loop 
103 and to a logic circuit 106. A horizontal blanking signal A.sub.H and a 
vertical blanking signal 2V are respectively supplied through terminals 
104 and 105 to the logic circuit 106 where they are combined to provide a 
control signal which is supplied to a reset circuit 107. Again the phase 
locked loop 103 produces a clock signal CLK of a clock frequency of 27MHz 
which is locked in phase to an 864-multiple of the horizontal frequency of 
a HDTV video signal. The address counter 108 and the control circuit 111, 
as well as the reset circuit 107 operate in the same manner as the 
corresponding components of FIG. 1. 
In this case a 9-bit wide output of the address counter 108 addresses two 
PROMs 109 and 110. The PROM 109 provides an 8-bit wide output defining the 
wave-shape of the 10-cycle burst of reference frequency sinusoidal 
oscillations. The second PROM 110 provides a one-bit sync signal output 
which is the most significant bit of the combination of the outputs of the 
PROMs 9 and 10, both of which outputs go to a register 12 which is enabled 
by the inverse A.sub.H of the horizontal blanking signal and is clocked by 
the clock signal CLK. The digital video signal is also clocked through a 
register, in this case the register 113, which is enabled by the 
horizontal blanking signal A.sub.H. The 8 less significant bits clocked 
out of the register 112 and the 8-bits clocked out of the register 113 
(which do not overlap in time and can form a single stream) are combined 
to provide the 8 least significant bits for the digital to analog 
converter 114, to which the most significant bit is supplied from the 
register 112 to define the negative-going horizontal synchronizing pulse. 
The output of the digital to analog converter 114 is passed through the 
low-pass filter 115, a distortion compensation stage 116 and a clamping 
stage 117 which clamps the now analog video signal with a horizontal 
frequency clamping pulse H.sub.C in the region of the forward black 
shoulder P.sub.v to a definite the d.c. signal base level. 
In this case the low-pass filter 115 is a 7-pole Tschebyscheff filter 
having a linear frequency characteristic up to about 10.5MHz. The 
distortion compensation stage 116, by means of a passive frequency 
characteristic that rises towards the cut-off frequency, compensates for 
the amplitude damping losses produced in the analog to digital converter 
114 and the low-pass filter 115. At the output of the clamping stage 117 
an analog video signal is made available which contains, in the region of 
the horizontal blanking interval, as shown in FIG. 4a, a synchronizing 
pulse S, the reference signal R and the gray pedestal G. 
FIG. 4b shows, on a different time scale, two complete horizontal periods 
of the video signal output of the circuit of FIG. 3 at the terminal 118. 
On the same time scale FIG. 4c shows a horizontal synchronizing signal H 
and FIG. 4d shows a horizontal blanking signal A.sub.H defining a blanking 
interval A and a line period H. 
With either the embodiment of FIG. 1 or the embodiment of FIG. 3, the 
residual errors remaining after a timing error correction on the playback 
side of a recording and playback magnetic tape equipment for HDTV 
recording and playback can be limited to about 3 nanoseconds. This 
corresponds to about one-sixth of a pixel in a HDTV video signal. 
Although the invention has been described with reference to a particular 
illustrative examples, it will be understood that variations and 
modifications are possible within the inventive concept.