Abstract:
An analog-to-digital conversion system for converting an analog signal to a digital signal for recording is configured to detect a low level period of the analog signal, amplify a low noise in the analog signal in the detected period by a predetermined multiplying ratio or by an amount corresponding to the signal amplitude, and subtract from the converted digital signal a number of bits corresponding to the amplification degree in the same period after analog-to-digital conversion, so as to reduce the influence of a noise entering in the analog signal before analog-to-digital conversion.

Description:
FIELD OF THE INVENTION 
     This invention relates to an analog-to-digital conversion system, and more particularly to an improvement thereof capable of reducing &#34;influence of noises&#34; which enter in an analog signal before its analog-to-digital conversion. 
     BACKGROUND OF THE INVENTION 
     In a case where a signal converted into a digital code is returned to an original analog signal (audio signal, for example), and the restored analog signal is further changed to a digital code and recorded on a recording medium (for example, a digital audio tape - hereinafter called &#34;DAT&#34;), the analog signal is often masked by &#34;noises&#34; of an amplifier, etc. when its signal level is low. Due to this, an analog-to-digital converter cannot maintain its original level resolution (16 bits in a DAT), and invites deterioration in the digitalized recording signal. 
     Generally speaking, if any noise enters in an analog signal, expected level resolution is not sufficiently effective in analog-to-digital conversion of the analog signal. In most cases, if a digitalized signal is converted to an analog signal, the converted analog signal itself includes less noise to establish high signal-to-noise ratio. However, noises often enter in the signal before subsequent digital recording. 
     The solid line curve in FIG. 5(a) shows an analog signal S. When it is converted to a digital signal, its bit levels represent the distribution shown by the solid line in FIG. 5(c). If a noise N shown in in FIG. 5(b) enters in the signal before analog-to-digital conversion, the analog signal takes a waveform S 2  shown by the dotted line in FIG. 5(a) which has a considerable difference from the solid line curve. The difference does not matter to human ears if the signal level is high. However, human ears hear it if the signal level is low. 
     The analog signal may be amplified by a low noise amplifier before analog-to-digital conversion so as to subsequently convert the amplified analog signal to a digital signal, eliminating low level bits of the digital signal. For example, when an input analog signal shown by the solid line of FIG. 4(a) is applied to an analog-to-digital converter, the signal level in a period T is low, and a difference in period T is sensed by human ears. Therefore, the signal is amplified as shown by the dotted line in FIG. 4(a) (doubled in the illustrated example). This increases the amplitude in period T, and hence reduces the influence of the noise. However, since this amplification extends to the entire length of the signal, the hatched region, for example, overlaps the non-linear region of the amplifier and invites clipping or distortion caused by saturation. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the invention to provide an analog-to-digital conversion system capable of reducing the influence of noises at low levels (small amplitudes) of an analog signal to totally establish digital recording and reproduction of a high signal-to-noise ratio. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided an analog-to-digital conversion device comprising: 
     a detection means for detecting a period of an input analog signal in which the amplitude of the analog signal does not exceed a predetermined value; 
     a gain control means responsive to a detection signal from said detection means to increase the gain of the input analog signal in said period; 
     an analog-to-digital conversion means responsive to an output from said gain control means in the form of analog signal to convert it to a digital signal; and 
     a subtraction means for subtracting from said digital signal a number of bits corresponding to the gain in said period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an analog-to-digital conversion system which is a first embodiment of the invention; 
     FIGS. 2a to 2e are diagrams showing waveforms of signals at various points of the system of FIG. 1; 
     FIG. 3 is a block diagram of an analog-to-digital conversion system which is a second embodiment of the invention; 
     FIGS. 4a to 4c are diagrams showing waveforms of signals at various points of the system of FIG. 3; and 
     FIGS. 5a to 5c show waveforms for explanation of digitalization and entrance of noises. 
    
    
     DETAILED DESCRIPTION 
     The invention is hereinbelow described in detail, referring to preferred embodiments illustrated in the drawings. 
     FIG. 1 is a block diagram of an analog-to-digital conversion system according to the invention configured to amplify an audio signal in a low level period by a given ratio. The conversion system includes rectifying circuits 1 and 6, comparators 2 and 7, reference voltage applying terminals 3 and 8, amplifying and amplitude-limiting circuits 4 and 9, differentiating circuit 5, AND circuit 10, gain control circuit 11, analog-to-digital (A/D) converter 12, bit subtracting circuit 13, bit gate 14, N bit generator 15, output terminal 16 and input terminal 16&#39;. Assume here that an audio signal entering in the input terminal 16&#39; has a waveform shown by the solid line in FIG. 2(a), and that the level in a period T 0  is lower than a given value Δ 1 . The amplitude in period T 0  is multiplied by n (corresponding to N bits). In this connection, the audio signal is rectified by the rectifying circuit 1 into uniform-polarity, i.e. forward d.c. voltage, and subsequently compared by the comparator 2 with the reference voltage Δ 1  applied to the reference voltage input terminal 3, so that an output having a level lower than the reference voltage Δ 1  is extracted to establish the waveform of FIG. 2(b). The output is amplified within a limited amplitude by the amplifying and amplitude-limiting circuit 4. On the other hand, the audio signal is differentiated by the differentiating circuit 5 to obtain a signal having the waveform of FIG. 2(c). The signal of FIG. 2(c) is rectified by the rectifying circuit 6. The resulting output d.c. voltage is compared to a reference voltage Δ 2  applied to the reference voltage input terminal 8, and an output having a level lower than the reference voltage Δ 2  is extracted to establish the waveform shown in FIG. 2(d). The signal of FIG. 2(d) is shaped by the amplifying and amplitude-limiting circuits 9. Outputs from the amplifying and amplitude-limiting circuits 4 and 9 are applied to the AND circuit 10 which in turn produces an output having the waveform shown in FIG. 2(e) which has a level lower than the reference voltage Δ 1  and does not include large-sloping part of the input analog signal. The output of the AND circuit 10 is applied to the gain control circuit 11, which amplifies the input analog signal in period T 0  into the waveform shown by the dotted line in FIG. 2(a). A noise 20 enters in the output of the gain control circuit 11 before analog-to-digital conversion, and the signal including the noise 20 is converted to a digital signal by the analog-to-digital converter 12. In this configuration, however, the amplitude in period T 0  is n times the original amplitude as shown in FIG. 2(a), and the analog signal having the n-times amplitude is converted to a digital signal. Therefore, the N bit generator 15 generators N bits corresponding to the n-times amplitude component and applies them to the bit subtracting circuit 13 via the bit gate 14, so that the bit subtracting circuit 13 subtracts bits of period T 0  from the output of the analog-to-digital converter 12 (this corresponds to 1/n division of an analog signal). 
     Assuming that n=2 N , the subtraction corresponds to removal of the final (least significant N bits of an output of the analog-to-digital converter 12 and rightward displacement of the remainder bits to the end of the output. In this fashion, a digital signal corresponding to the solid line of FIG. 2(a) is obtained at the output terminal 16, and the signal-to-noise ratio (with respect to the noise 20) in the low level period T 0  is sufficiently increased. 
     Assuming that n=4 and N=2 in a six-bit analog-to-digital converter, the subtraction may be the following two-bit displacement. ##STR1## 
     Therefore, what is requested to the N bit generator 15, bit subtraction circuit 13 and bit gate 14 is to merely shift the bits in period T 0 . In this fashion, the influence of the noise in period T 0  is reduced to 1/n. 
     In order to advantageously use dynamic ranges of the transmission, it is preferable to effect a gain control shown in FIG. 4(c) against the input analog signal shown by the solid line in FIG. 4(b) and amplify the amplitude in the low level period shown by the solid line in figure 4(b) to establish a substantially uniform amplitude. More specifically, while the gain in the aforegoing embodiment is uniform in period T 0 , the embodiment of FIG. 4 is configured to change the gain from gain A 1  in period T 1  to gain A 2  in period T 2 . This gain control established less influence of the noise to input analog signal of FIG. 4(b) in period T 1  than that in period T 2 . This method is called AGC system. FIG. 3 is a block diagram of a further embodiment of the invention employing the AGC system. 
     The device of FIG. 3 is substantially equal to that of FIG. 1 except that the gain control circuit 11 of FIG. 1 is replaced by an AGC (automatic gain control) circuit 17, so that the AGC circuit 17 is activated in periods T 1  and T 2  of the output of the AND circuit 10 shown in FIG. 4(c). An output 19 of the AGC circuit 17 includes gain control outputs A 1  and A 2  which are converted to bit outputs by an analog-digital converter 18 (for gain bit conversion). The bit subtraction circuit 13 subtracts the bits from the bit output of the converter 12 to reduce the number of bits of the digital signal by an amount corresponding to the amplitude increase in periods T 1  and T 2  of FIG. 4(b) to establish the solid line waveform of FIG. 4(b). 
     As described above, since the invention effects analog-to-digital conversion after amplifying a low noise in an analog signal in a low level period thereof, the influence of the noise entering in the signal before analog-to-digital conversion is significantly reduced. Beside this, since the analog signal is amplified only in a limited period and not along its entire length, the invention device never invites distortion caused by a non-linear characteristic in the amplification step upon signal amplification. Further, since the invention device amplifies a limited signal component in a low signal level period, an analog-to-digital converter of a predetermined number of bits suffices, and it does not require an analog-to-digital converter having a great level resolution which is indispensable when the analog signal is amplified throughout its entire length before analog-to-digital conversion.