Digital signal conversion method and apparatus for converting photoelectrically converted video signals

A digital signal conversion apparatus converts an input video signal having a first dynamic range into a converted video signal having a dynamic range which is smaller than the first dynamic range. In particular, the conversion apparatus includes a digital signal processor (DSP) and a knee point determiner. The DSP inputs the input video signal and converts such signal into the converted video signal based on a selected one of a plurality of input/output (I/O) characteristic functions stored in the DSP. Furthermore, the DSP generates brightness information based on the magnitude of the first dynamic range of input video signal. The knee point determiner inputs the brightness information and generates knee point information based on the brightness information. As a result, the DSP selects one of the I/O characteristic functions in accordance with the knee point information such that the DSP can appropriately convert the input video signal into the converted video signal.

FIELD OF THE INVENTION 
The present invention relates to a digital signal conversion method and 
apparatus for converting photoelectrically converted video signals. More 
particularly the invention relates to a digital signal conversion method 
and apparatus for converting video signals such that the signals have 
magnitudes which fall within a dynamic range of a signal processing 
system. 
BACKGROUND OF THE INVENTION 
In order to process data corresponding to a video image, a video camera 
inputs optical signals corresponding to the image, and the optical signals 
are photoelectrically converted into electrical signals via charge coupled 
devices. Subsequently, the electrical signals are input to a signal 
processing system and processed in accordance with a particular 
application. However, since the dynamic range of the charge coupled 
devices is relatively large, the converted electrical signals typically 
have dynamic ranges which are broader than a dynamic range of the signal 
processing system. Accordingly, processing all of the electrical signals 
in such processing system is difficult. 
In order to overcome the problem above, an apparatus has been developed 
which converts electrical signals having values which fall outside of the 
dynamic range of the signal processing system into electrical signals 
having all values within such range. Such a signal conversion apparatus is 
called an "automatic knee" apparatus and has a dynamic range of 
approximately 50-70 dB so that it is not significantly affected by noise. 
An example of an analog signal conversion apparatus which is used in an 
analog video camera will be described with reference to FIG. 1. As shown 
in the figure, the conversion apparatus comprises a signal converter 11 
and an excessive light quantity detector 15. The signal converter 11 
inputs red, green, and blue analog signals R, G, and B which correspond to 
optical signals that have been photoelectrically converted by charge 
coupled devices (not shown). Then, the converter 11 determines a maximum 
value from among the signals R, G, and B and generates a "NAM" signal 
corresponding to the maximum value. 
The excessive light quantity detector 15 inputs the NAM signal and detects 
an amount by which the maximum value of the signals R, G, and B exceeds 
the dynamic range of the converter 11. In other words, the detector 15 
determines an "excessive light quantity range" relating to the maximum 
value. In the present example, the dynamic range of the device 11 is equal 
to a range of output values between 0 and Q' (FIG. 2). 
After the excessive light quantity range is determined, the detector 15 
outputs a corresponding signal to the signal converter 11, and the 
converter 11 determines the value of a knee point KP and an input/output 
(I/O) characteristic based on the excessive light quantity range. For 
instance, the converter 11 may calculate the value of the knee point KP 
such that it is inversely proportional to the size of the excessive light 
quantity range. Specifically, if the size of the excessive light quantity 
range is relatively large, then the value of the knee point KP may 
correspond to a relatively small value within the dynamic range. 
Conversely, if the size of the excessive light quantity range is 
relatively small, then the value of the knee point may correspond to a 
relatively large value within the dynamic range. 
FIG. 2 illustrates an example of a knee point KP and an I/O characteristic 
determined by the converter 11. As shown in the figure, the I/O 
characteristic has a portion A and a portion B, and the portions A and B 
are connected at the knee point KP. Accordingly, if the converter 11 
inputs a signal having a value between O and P, the signal is converted to 
an output signal having a value between O and P' in accordance with 
portion A of the I/O characteristic. On the other hand, if a signal having 
a value between P and Q is input, the signal is transformed into an output 
signal having a value between P' and Q' based on portion B of the I/O 
characteristic. Thus, even if, an input signal has a value larger than the 
dynamic range of the signal processing system, the converter 11 
manipulates the signal such that its value falls within such dynamic 
range. As a result, all of the input electrical signals R, G, and B having 
values beyond the dynamic range are converted into signals R', G', and B' 
having values within the dynamic range. 
Even though the analog signal conversion apparatus is capable of 
effectively converting various input signals, such apparatus has several 
disadvantages. For instance, the analog apparatus comprises both passive 
elements (such as resistors and capacitors) and active elements (such as 
transistors). Therefore, the quality of the signals output from the 
apparatus is degraded due to discrepancies among the temperature 
characteristics of the passive and active elements. In addition, since the 
signal conversion apparatus processes the red, green, and blue signals R, 
G, and B via particular devices, uniformly processing the signals and 
limiting the excessive quantity range is difficult. 
In order to solve the problems inherent in an analog signal conversion 
apparatus, a digital signal conversion apparatus has been developed for a 
digital video camera by the Matsushita company in Japan. The apparatus 
uses a plurality of look-up tables stored in a memory to convert an input 
signal into a signal within a dynamic range. However, since the device 
requires a plurality of different types of look-up tables to adequately 
convert signals based on variable knee points KP and I/O characteristics, 
a relatively large memory capacity is needed. As a result, such digital 
conversion device is relatively expensive. 
SUMMARY OF THE INVENTION 
In order to solve the problems above, one of the objects of the present 
invention is to provide a digital signal conversion method for converting 
an input video signal having a first dynamic range into a converted video 
signal within a predetermined dynamic range. Furthermore, such conversion 
is preferably based on one of a plurality of input/output characteristics 
which are programmed, such that an excessive light quantity range varies 
in accordance with each of the input/output characteristics. 
Another object of the present invention is to provide a digital signal 
conversion apparatus for performing a method similar to the method above. 
In order to accomplish one of the above objects of the present invention, a 
digital signal conversion method for converting an input video signal 
having a magnitude within a first dynamic range into a converted video 
signal having a magnitude within a second dynamic range is provided. 
Specifically, the digital signal conversion method comprises the steps of: 
(a) storing input/output characteristics which respectively correspond to 
knee point information values; 
(b) generating a particular knee point information value according to 
brightness information of said input video signal, wherein said brightness 
information corresponds to said first dynamic range of said input video 
signal; and 
(c) converting said input video signal into said converted video signal 
having said magnitude within said second dynamic range based on a 
particular input/output characteristic which corresponds to said 
particular knee point information value. 
In order to accomplish another object of the present invention, a digital 
signal conversion apparatus is provided and comprises: knee point 
determination means for determining knee point information based on 
brightness information; and signal processing means for inputting said 
input video signal, outputting said brightness information according to 
said input video signal, and converting said input video signal into said 
converted video signal, wherein said input video signal has a first 
dynamic range which corresponds to said brightness information, wherein 
one of said signal processing means and a separate signal processing 
system has a second dynamic range, wherein an amount by which said first 
dynamic range exceeds said second dynamic range corresponds to an 
excessive light quantity range, wherein said signal processing means 
converts said input video signal into said converted video signal by using 
an input/output characteristic which corresponds to said excessive light 
quantity range, and wherein a size of said excessive light quantity range 
varies according to said knee point information.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present invention will be described below in 
more detail with reference to FIGS. 3, 4, and 5. 
FIG. 3 shows a digital signal conversion apparatus according to a preferred 
embodiment of the present invention. In particular, the apparatus 
comprises analog-to-digital (A/D) converters 31, 32, and 33, a digital 
signal processor (DSP) 34, digital-to-analog (D/A) converters 35, 36, and 
37, and a knee point determiner 40. 
Optical signals corresponding to an image are photoelectrically converted 
into analog red, green, and blue signals R, G, and B by charge coupled 
devices (not shown). Afterwards, the analog signals R, G, and B are 
respectively converted into digital component data R, G, and B by the A/D 
converters 31, 32, and 33, and the digital component data R, G, and B are 
output to the DSP 34. 
Subsequently, the DSP 34 generates brightness information BRT based on a 
video data block of the component data and outputs the brightness 
information BRT to the knee point determiner 40. The determiner 40 
comprises a maximum value detector 41 and a knee point information 
generator 45. The detector 41 inputs the brightness information BRT for 
the video data block, determines the maximum value of the brightness 
information BRT, and outputs maximum brightness information MBRT based on 
the maximum value. The knee point information generator 45 inputs the 
maximum brightness information MBRT and outputs knee point information N 
based on such information MBRT. 
Subsequently, the DSP 34 inputs the knee point information N and selects 
one of a plurality of prestored I/O characteristic functions based on the 
information N. Then, when the DSP 34 inputs another group of digital 
component data R, G, and B, the DSP 34 converts the data R, G, and B into 
converted digital component data Rc, Gc, and Bc based on the selected I/O 
characteristic function. The D/A converters 35, 36, and 37 input the 
digital data Rc, Gc, and Bc and transform the data Rc, Gc, and Bc into 
analog red, green, and blue signals Rc', Gc', and Bc'. 
An example of the pre-stored I/O characteristic functions f.sub.o through 
f.sub.n are illustrated in FIG. 5. As shown in the figure, the functions 
f.sub.1 to f.sub.n respectively correspond to knee points T.sub.1 to 
T.sub.n, and the knee points T.sub.1 to T.sub.n correspond to the possible 
values of the knee point information N (N=1 to n). 
TABLE 1 
______________________________________ 
Knee point 
Condition information 
______________________________________ 
MBRT .ltoreq. D N = 0 
D &lt; MBRT .ltoreq. D + k 
N = 1 
D + k &lt; MBRT .ltoreq. D + 2k 
N = 2 
D + 2k &lt; MBRT .ltoreq. D + 3k 
N = 3 
. . 
. . 
. . 
C - k &lt; MBRT .ltoreq. C 
N = n 
______________________________________ 
For example, as illustrated in Table 1, if the value of the maximum 
brightness information MBRT output from the detector 41 indicates that the 
maximum value of the digital component data R, G, and B is less than the 
value "D" (i.e. within the dynamic range of the signal processing system), 
the knee point information generator 45 outputs knee point information N 
which equals "0". As a result, the DSP 34 converts the component data R, 
G, and B into the digital data Rc, Gc, and Bc based on the I/O 
characteristic function Y=f.sub.0 (X). 
On the other hand, if the information MBRT indicates that the maximum value 
of the digital component data is between the values "D" and "D+k" (i.e. 
slightly beyond the dynamic range of the processing system), the generator 
45 outputs information N which equals "1". Accordingly, the DSP 34 
converts the component data R, G, and B into the digital data Rc, Gc, and 
Bc based on the function Y=f.sub.1 (X) and the knee point T.sub.1. In 
other words, the DSP 34 converts all of the component data having values 
less than a critical value "D-1" corresponding to the knee point T.sub.1 
based on the function Y=f.sub.0 (X). Furthermore, all of the component 
data having values greater than the critical value "D-1" are converted 
according to the function Y=f.sub.1 (X). 
In addition, if the information MBRT indicates that the maximum value of 
the digital component data is between the values "C-K" and "C" (i.e. 
substantially beyond the dynamic range of the signal processing system), 
the generator 45 outputs knee point information N which equals "n". 
Accordingly, the DSP 34 converts the component data R, G, and B into the 
digital data Rc, Gc, and Bc based on the function Y=f.sub.n (X) and the 
knee point T.sub.n. Specifically, the DSP 34 converts each of the 
component data R, G, and B having a value less than a critical value A 
corresponding to the knee point T.sub.n based on the function Y=f.sub.0 
(X). Moreover, all of the component data having values greater than the 
critical value A are converted according to the function Y=f.sub.n (X). 
In FIG. 5, the range of input values from "0" to "D" (or the range of 
output values "0" to "D") represents the dynamic range of the signal 
processing system. Furthermore, the range of input values "D" to "C" 
represents the excessive light quantity range which exceeds the dynamic 
range. In addition, as shown in Table 1 and FIG. 5, the number n 
corresponds to the number of predetermined intervals k into which the 
excessive light quantity range is divided. 
Also, as shown by the I/O characteristic functions Y=f.sub.o (X) to 
Y=f.sub.n (X), the value of the maximum bright information MBRT is 
directly proportional to the value of the knee point information N. In 
particular, as the value of the information MBRT increases (or decreases), 
the value of the knee point information N also increases (or decreases). 
Furthermore, the value of the knee point information N and the maximum 
brightness information MBRT are each directly proportional to the size of 
the excessive light quantity range. Specifically, as the information N and 
MBRT become larger (or smaller), the size of the excessive light quantity 
range likewise becomes larger (or smaller). For example, if the knee point 
information N equals "2", the excessive light quantity range spans from 
"D" to "D+2k", but if the information N equals "n" (n&gt;2), the range spans 
from "D" to "C". 
An operation of the digital signal conversion apparatus will be further 
described with reference to FIG. 4 and 5. As previously mentioned, a video 
camera inputs optical signals corresponding to an image, and the optical 
signals are photoelectrically converted into analog red, green, and blue 
signals R, G, and B via a photoelectric converter (not shown). 
Furthermore, the photoelectric converter (not shown) has a broader dynamic 
range than the dynamic range of the DSP 34 and/or various signal 
processing systems located downstream from the DSP 34. 
The analog signals R, G, and B are converted into digital component data R, 
G, and B by the A/D converters 31, 32, and 33, and the digital data R, G, 
and B are input to the DSP 34. Then, the DSP 34 generates a NAM signal 
indicating the maximum value among the digital data R, G, and B. In the 
present embodiment, the NAM signal represents the maximum value among the 
digital data R, G, and B corresponding to respective pixels. Subsequently, 
the DSP 34 generates brightness information BRT which corresponds is to 
the average value of the NAM signals relating to respective video data 
blocks, each of which has a predetermined magnitude. In the present 
embodiment, each of the video data blocks is obtained by dividing one 
field or frame of a video image signal such that the field or frame has 
the above-mentioned predetermined magnitude. 
The brightness information BRT represents the brightness of the respective 
video data blocks and is input to the maximum value detector 41. The 
detector 41 detects the maximum brightness information MBRT from among the 
brightness information BRT corresponding to each video block, and the 
information MBRT is output to the knee point information generator 45. 
Subsequently, the generator 45 determines the knee point information N 
corresponding to the maximum brightness information MBRT according to 
Table 1. 
FIG. 4 illustrates a flow chart of the conversion operation performed by 
the DSP 34. In particular, the DSP 34 inputs the current knee point 
information N output from the generator 45. Furthermore, the DSP 34 inputs 
the digital component data R, G, and B corresponding to the field or frame 
of a video signal which follows the field or frame from which the current 
knee point information N was determined (step 101). Since the DSP 34 
processes the data R, G, and B relating to frame or field which is after 
the frame or field used to calculate the knee point information N. the DSP 
34 can process the data R, G, and B without experiencing any delay. In 
other words the digital signal conversion apparatus can process signals in 
real time. 
Subsequently, the DSP 34 determines if the knee point information N equals 
"0" (step 102). If the information N does equal "0", the DSP 34 converts 
the component data R, G, and B into converted digital component data Rc, 
Gc, and Bc based solely on the I/O characteristic function Y=f.sub.o (X) 
shown in FIG. 5 (step 103). In a preferred embodiment of the present 
invention, the function Y=f.sub.o (X) is equivalent to the function Y=X. 
As a result, the data Rc, Gc, and Bc generated in step 103 are equal to 
the red, green, and blue component data R, G, and B input to DSP 34 in 
step 101. 
On the other hand, if the knee point information N does not equal "0", the 
DSP 34 compares the red component data R with the critical value "D-N" 
corresponding to the knee point T.sub.N (step 104). Similarly, the DSP 34 
compares the green and blue component data G and B with the critical value 
"D-N" (steps 107 and 120). 
Since the operation performed in steps 104-106 is the same as the 
operations performed in steps 107-109 and 120-121, only the operation 
performed in steps 104-106 will be described. In particular, if the value 
of the red component data R is smaller than the critical value "D-N" (step 
104), the DSP 34 converts the component data R into the component data Rc 
in accordance with the function Rc=f.sub.o (R) (step 105). On the other 
hand, if the value of the red component data R is greater than or equal to 
the critical value "D-N" (step 104), the DSP 34 converts the component 
data R into the component data Rc based on the function Rc=f.sub.N (R) 
(step 105). 
For example, if the value of the knee point information N applied from the 
knee point determiner 40 equals "2", the DSP 34 compares the value of the 
red component data R with the critical value "D-2". If the value of the 
data R is less than the critical value "D-2", the DSP 34 converts the data 
R into the data Rc based on the function Rc=f.sub.o (R). However, if the 
value of the data R is greater than or equal to the critical value "D-2", 
the DSP 34 converts the data R into the data Rc in accordance with the 
function Rc=f.sub.2 (R). 
In step 123, the converted digital component data Rc, Gc, and Bc generated 
by the DSP 34 are respectively output to the D/A converters 35, 36, and 
37. Then, the converters 35, 36, and 37 convert the digital data Rc, Gc, 
and Bc into analog signals Rc', Gc', and Bc'. 
As illustrated above, the red, green, and blue component data R, G, and B 
processed by the DSP 34 correspond to the frame or field which follows the 
frame or field used for determining the current knee point information N. 
Furthermore, when new knee point information N is output from the 
determiner 40, the DSP 34 processes the red, green,. and blue component 
data R, G and B relating to field or frame that follows the field or frame 
used to calculate the new knee point information N. 
On the other hand, the above embodiment may be modified such that the field 
or frame which corresponds to the currently processed component data R, G 
and B is the same field or frame used to determine the current knee point 
information N. In this case, since the current data R, G, and B and the 
current knee point information N used during the processing of the current 
data R, G, and B correspond to the same field or frame, the input data R, 
G, and B can be converted more accurately. 
As described above, the digital signal conversion apparatus compresses the 
values of the data which extend into the excessive light quantity range by 
using an internal program. Accordingly, the present invention can be 
implemented with a relatively small amount of hardware and does not 
require a vast amount of memory. Furthermore, the invention provides 
flexibility with respect to the development of a signal conversion 
apparatus which is specifically adapted to a particular system. 
Furthermore, as the brightness information with respect to one field or 
frame of a video signal becomes larger, the data R, G, and B having values 
which extend further beyond the dynamic range of the system can be 
compressed such that the values fall within the dynamic range. As a 
result, the signal conversion apparatus can be easily modified based on 
the degree by which the value of the data R, G, and B extend beyond the 
dynamic range (i.e. based on the size of the excessive light quantity 
range). 
In addition, the digital signal conversion apparatus of the present 
invention does not contain as many passive and active elements as a 
conventional analog apparatus. Accordingly, the reliability of the digital 
apparatus is relatively high. 
Furthermore, the present invention is not limited any specific 
applications. For example, the signal conversion apparatus can be 
incorporated into a broadcasting camera, a commercial video camera, and a 
digital camcorder. 
Also, it is to be understood that the above described embodiments of the 
invention are only illustrative and that modifications thereof may occur 
to those skilled in the art. Accordingly, the present invention is not to 
be regarded as limited to the embodiments disclosed herein, but is to be 
limited only as defined by the appended claims.