Signal recording-reproducing apparatus

Abstract: There is provided a digital signal recording and/or reproducing apparatus which converts an input analog signal into a digital signal and records the digital signal on a recording medium, or which reproduces a digital signal recorded on a recording medium and converts the digital signal into an analog signal. Since the apparatus has an analog signal level control means and a digital signal level control means disposed in front and at the rear of an A/D converter or a D/A converter and controlled in correlation with each other, the S/N ratio of a recording signal or a reproducing signal can be improved.

Description: A well-known type of signal recording and/or reproducing apparatus, for example, a digital audio tape recorder (DAT) using a rotary head, converts an analog signal into a digital signal and records the digital signal on a recording medium, such as a magnetic tape, and reproduces the digital signal recorded on the recording medium, and converts it into an output analog signal. In such an apparatus the analog signal, e.g. the audio signal, is adjusted to a certain signal level by an analog volume control circuit, before digital conversion and recording. On reproduction, the digital signal reproduced from the magnetic tape is converted into an analog signal, adjusted to a certain signal level with the analog volume control circuit, and output. Fig. 1 of the accompanying drawings shows the construction of a signal recording system 1 of a conventional digital signal recording and/or reproducing apparatus. An analog audio signal S1 supplied from a microphone or an external device is input to a line input terminal 3 and is supplied to an analog volume control circuit to be adjusted to a volume value set by the user and is then supplied to an analog-digital conversion circuit 6 via an amplifying circuit 5, for conversion into a digital audio signal S3. The signal S3 is output to a recording circuit 7, modulated to a certain recording signal S4, and recorded on a magnetic tape 9 by a magnetic head 8. Fig. 2 shows the construction of a signal reproducing system 2 of the conventional digital signal recording and/or reproducing apparatus. A digital signal recorded on a magnetic tape 10 is read out by a magnetic head 11, and supplied as a reproduction signal S5 to a reproducing circuit 12 which modulates the reproduction signal S5 to a digital audio signal S6. The digital signal S6 is converted into an analogue audio signal S7 by a digital-analog conversion circuit 13, and supplied via an amplifying circuit 14 to an analog volume control circuit 15, which has, for example, a variable resistor and controls the volume by change of resistance. The analog volume control circuit 15 adjusts the signal S7 to a volume value desired by the user. A signal S8 output from the analog volume control circuit 15 is amplified by an amplifying circuit 16, and output as an audio signal S9. However, with the apparatus shown in Figure 1, noise components which signal processing circuits of the amplifying circuit 5, the analog-digital conversion circuit 6 and so on naturally add are superimposed on the input signal S1, and recorded as the recording signal S4. Thereby, the S/N ratio is lowered when the volume value is made small. Also, when an audio signal output from the analog volume control circuit 4 is monitored in recording, if a recording volume value is gradually decreased in, for example, a fade-out operation, since a monitor level decreases with the decrease in the volume, the input signal cannot be easily monitored. Furthermore, with the reproducing circuit of Fig. 2, noise components arising from signal processing circuits of the digital-analog conversion circuit 13, the amplifying circuit 14 and so on are superimposed on the output audio signal S8. Therefore, when an audio signal recorded on a particularly low volume level is reproduced while the set volume value of the analog volume control circuit 15 is increased, since noise components arising from signal processing circuits of the digital-analog conversion circuit 13, the amplifying circuit 14 and so on are amplified together with the audio signal, the S/N ratio of the output audio signal S8 is lowered. US-A-4,745,298 discloses a digital signal reproducing apparatus which has a D/A converter and digital and analog signal level control means which are controlled by volume control means so that the digital signal level control means is active during high volume settings and the analog signal level control means is active during low volume settings. EP-A-0 322 777 discloses apparatus for recording signals with a large dynamic range by switching of the analog gain depending on the analog signal level and recording the signal A/D converted together with a gain switching control signal. In view of the foregoing, an object of the present invention is to provide a digital signal recording and/or reproducing apparatus in which the S/N ratio of a recording and/or reproducing signal is not lowered. It is also an object to allow an input signal to be monitored at a certain volume even if a signal recording level is lowered by a recording signal level control means in signal recording. According to the present invention, there is provided a digital signal recording apparatus comprising analog-digital conversion means for converting an input analog signal into a digital signal and digital signal recording means, for recording said digital signal obtained by said analog-digital conversion means on a recording medium and analog signal level control means for controlling the signal level of said analog signal supplied to said analog-digital conversion means; characterised by: digital signal level control means for digitally controlling the signal level expressed in a digital signal output from said analog-digital conversion means; and volume control means for controlling the signal levels of said analog signal level control means and said digital signal level control means such that the signal level of the input signal is mainly controlled by said digital signal level control means when the volume level set by said volume control means is less than a certain level, and the signal level of the input signal is mainly controlled by said analog signal level control means when the volume level set by said volume control means is more than a preset level. For example, if a signal recording level set by the user is within a range lower than a certain level, a control level of an analog signal level control means is held on a certain level in order to adjust the signal recording level. Then, the signal level is adjusted by a digital signal level control means. In other words, the digital signal level control means mainly operates in accordance with the signal recording level within such a range set by the user. It is possible to prevent the S/N ratio from being lowered due to noises and minute signal outputs arising from the analog signal level control means itself by outputting an analog signal on a certain signal level. The noises from the analog signal level control means can be turned down together with the input signal in the digital signal level control means, and thus the S/N ratio of the recording signal can be further increased. In this case, a preferable gain of the digital signal level control means is less than 1. Furthermore, the invention may be applied in signal recording and reproducing apparatus which includes recording apparatus as above together with reproducing apparatus for reproducing a digital signal recorded on a recording medium in which a digital signal reproduced from a recording medium by a reproducing means is converted into an analog signal by a digital-analog conversion means and then adjusts and outputs the analog signal to a certain signal level by an analog signal level control means, is provided with a digital signal level control means disposed between the reproducing means and the digital-analog conversion means for adjusting the signal level of the digital signal to a certain signal level. If the signal reproducing level set by the user is more than a certain level, the signal level is adjusted by the digital signal level control means while holding the control level of the analog signal level control means at a certain level. Therefore, a reproduction signal can be obtained without increasing noises arising from analog signal processing circuits of the analog signal level control means and so on. The invention will be further described in the following detailed description in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters. Fig. 1 is a block diagram of a digital signal recording system of a conventional digital signal recording and/or reproducing apparatus; Fig. 2 is a block diagram of a digital signal reproducing system of the conventional digital signal recording and/or reproducing apparatus; Fig. 3 is a block diagram of a digital signal recording system of a digital signal recording and/or reproducing apparatus according to the present invention; Figs. 4(A) and 4(B) are graphs showing the signal recording level control characteristics of an analog volume control circuit and a digital volume control circuit in the digital signal recording system shown in Fig. 3; Fig. 5 is a block diagram of a digital signal reproducing system of the digital signal recording and/or reproducing apparatus according to the present invention; and Figs. 6 (A) and 6(B) are graphs showing the signal reproducing level control characteristics of an analog volume control circuit and a digital volume control circuit in the digital signal reproducing system shown in Fig. 5. Preferred embodiments of this invention will be described with reference to the accompanying drawings: Referring to Fig. 3 in which components corresponding to those shown in Fig. 1 are denoted by the same numerals, a signal recording system 20 of a DAT adjusts an audio signal S1 input from an audio signal input terminal 3 through a microphone or a line to a certain signal level based on volume data, described below, set by the user in an analog volume control circuit 4A. An output signal S21 from the analog volume control circuit 4A is transmitted to a monitor amplifying circuit 21 and an analog-digital conversion circuit 6. The monitor amplifying circuit 21 amplifies the analog signal S21 output from the analog volume control circuit 4A to a certain signal level, and outputs the analog signal S21 to a monitor terminal (not shown), such as a head phone, thereby making it possible for the user to monitor the audio signal S21 output from the analog volume control circuit 4A. The analog-digital conversion circuit 6 converts the analog signal S21 output from the analog volume control circuit 4A into a digital signal, and then transmits the digital signal as a digital audio signal S22 to a digital volume control circuit 22. The digital volume control circuit 22 is composed of a digital multiplier having a gain less than, for example, 1, and adjusts a signal level of the digital audio signal S22 to a desired signal level by changing a signal level coefficient based on recording volume data set by the user. An output signal S23 output from the digital volume control circuit 22 is transmitted to a recording circuit 7, modulated to a recording signal S24, and recorded on a magnetic tape 9 through a recording head 8. The analog volume control circuit 4A and the digital volume control circuit 22 control the analog audio signal S21 and the digital audio signal S22 to certain volume values, respectively, based on volume data set by the user's manipulation of a recording volume controller 24. In other words, when a plus volume (increase) key 24A or a minus volume (decrease) key 24B of the recording volume controller 24 is operated, an input control circuit 25 receives a signal from the key 24A or 24B, and transmits a volume control signal S26 for increasing or decreasing the volume value to a volume control circuit 26. The volume control circuit 26 displays a volume value set based on the volume control signal S26 in a volume display portion 28, transmits an analog volume control signal S27 to the analog volume control circuit 4A based on the volume control signal S26, and transmits a digital volume control signal S28 to the digital volume control circuit 22. In other words, the amplitude level of the digital audio signal S23 obtained through the analog volume control circuit 4A and the digital volume control circuit 22 is controlled in accordance with the volume value set by the user's manipulation of the recording volume controller 24. Figs. 4(A) and 4(B) show the signal level control characteristics of the analog volume control circuit 4A and the digital volume control circuit 22, respectively. In the analog volume control circuit 4A shown in Fig. 4(A), if the volume data set by the user is within a range of level from 0 to 10 (in this embodiment, the minimum volume data corresponding to the minimum volume value is at the level 0 and the maximum volume data corresponding to the maximum volume value is at the level 30), control is made to maintain a certain volume value. In short, even if the volume data changes, the output signal level from the analog volume control circuit 4A does not change. When the volume data is within a range of level from 10 to 30, the signal level is controlled so as to change proportional to the volume data set by the user. On the other hand, in the case of the digital volume control circuit 22 shown in Fig. 4(B), if the volume data set by the user is between the level 0 and the level 10, control is performed so that the output signal level of the digital volume control circuit 22 changes in accordance with the volume data. If the volume data is between the level 10 and the level 30,control is made so that a certain, for example, the maximum volume value is maintained, that is, the output signal level is constant. Therefore, for example, if the user continuously increases a set volume value by operating the plus volume (increase) key 24A of the recording volume controller 24, when the volume data is between the level 0 and the level 10, the signal level of the output signal S21 from the analog volume control circuit 4A does not change since a set volume value VOL1 of the analog volume control circuit 4A is kept a certain value. On the other hand, a set volume value VOL2 of the digital volume control circuit 22 gradually increases from the minimum volume value. If the volume data is set within a range from the level 10 to the level 30, the set volume value VOL1 of the analog volume control circuit 4A sequentially increases in accordance with the volume data set by the user. The set volume value VOL2 of the digital volume control circuit 22 is kept a substantially constant value even if the volume data changes. According to the above construction, when the recording volume value is within a relatively low range (from 0 to 10), the signal level of the digital volume control circuit 22 is controlled while the signal level of the analog volume control circuit 4A is kept constant. It is thereby possible to decrease the signal level value on the side of the digital volume control circuit 22 while keeping the value of the signal output from the analog volume control circuit 4A at a certain level other than 0 so as not to lower the S/N ratio. Therefore, the S/N ratio of the output signal S21 from the analog volume control circuit 4A can be prevented from being lowered, the noise components of analog circuit systems of the analog-digital conversion circuit 6 and so on disposed between the analog volume control circuit 4A and the digital volume control circuit 22 can be decreased together with the audio signal in the digital volume control circuit 22, and the S/N ratio can be further increased. Since the level of the signal output from the analog volume control circuit 4A does not become less than a certain level, even if the level of the recording volume data is lowered to 0, the input signal can be monitored based on the signal S21 output from the analog volume control circuit 4A. Therefore, the analog volume control circuit 4A in the former stage and the digital volume control circuit 22 in the latter stage are mounted, and the recording volume values of the analog volume control circuit 4A and the digital volume control circuit 22 are controlled in correlation with each other in accordance with the recording volume level set in a signal level control operation. If the recording signal level set by the user is below a certain signal level, the volume level can be controlled by the digital volume control circuit 22 while keeping the volume signal level of the analog volume control circuit 4A a certain level, a recording signal S24 in accordance with the volume value set by the user can be obtained, and the S/N ratio can be further increased. Furthermore, since the level of the signal output from the analog volume control circuit 4A is controlled not to be less than a certain level and the input signal can be monitored when the level of the recording volume data falls to the level 0, the state of the input voice can be monitored to the end by headphones or the like even if the recording volume is gradually decreased to the level 0 in, for example, a fade-out operation. As a result, convenience of use of the apparatus can be further improved. Although the set volume values of the analog volume control circuit 4A and the digital volume control circuit 22 are controlled as shown in Fig. 4 in accordance with the volume data set by the user in the above embodiment, the present invention is not limited to the embodiment. The main point is to substantially control the volume value on the side of the digital volume control circuit 22 when the signal level is controlled within a range lower than a certain signal level. Therefore, if the recording signal levels of both the volume control circuits are within a low range, it is only necessary to mainly operate the digital volume control circuit. The signal level control characteristics of the volume control circuits are not limited to ones shown in Fig. 4. The analog volume control circuit 4A in the above embodiment may switch the gain of the amplifying circuit in response to the operation of the recording volume controller 24 rather than adjust volume using a variable resistance. The construction of a signal reproducing system of the embodiment will be described. Referring to Fig. 5 in which components corresponding to those shown in Fig. 2 are denoted by the same numerals, a reproduction signal S5 output from a magnetic tape 10 through a magnetic head 11 is sent to a reproducing circuit 12 by a signal reproducing system 30 of a DAT, and a digital audio signal S6 is modulated and sent to a digital volume control circuit 31. The digital volume control circuit 31 is composed of a multiplier having a gain more than 1 whose coefficient is changed based on reproduction volume data set by the user. Therefore, it adjusts the level of the digital signal S6, and then sends a signal S31 output from the digital volume control circuit 31 to a digital-analog conversion circuit 13. The digital-analog conversion circuit 13 obtains an analog signal S32 by converting the digital signal S31 output from the digital volume control circuit 31 into an analog signal, and transmits the analog signal S32 to an analog volume control circuit 15A, for example, which can adjust the signal level by controlling a variable resistor. A volume value (that is, a signal level) of the analog audio signal S32 is adjusted by the analog volume control circuit 15A (e.g. by using a variable resistance) based on the reproduction volume data set by the user, described blow, an output signal S33 from the analog volume control circuit 15A is amplified to a certain level through an amplifying circuit 16, and the amplified signal is output as an output signal S34. The digital volume control circuit 31 and the analog volume control circuit 15A adjust the digital signal S6 and the analog signal S32 to respective certain levels based on the volume data set by the user's manipulation of a reproduction volume controller 34. In other words, when a plus volume (increase) key 34A or a minus volume (decrease) key 34B of the reproduction volume controller 34 is operated, an input control circuit 32 transmits a volume control signal S36 for increasing or decreasing a signal level value to a volume control circuit 33. The volume control circuit 33 transmits a digital volume control signal S37 to the digital volume control circuit 31 based on the volume control signal S36 and transmits an analog volume control signal S38 to the analog volume control circuit 15A, thereby adjusting the signal level of the analog signal S33 obtained through the digital volume control circuit 31 and the analog volume control circuit 15A to a signal level set by the user's manipulation of the reproduction volume controller 34. As shown in Fig. 6(B), in the case of the digital volume control circuit 31, when volume data set by the user (in this embodiment, the minimum volume data corresponding to the minimum volume value is on the level 0 and the maximum volume data corresponding to the maximum volume value is on the level 30) is within a range from the level 0 to the level 24, a certain volume value is maintained. When the volume data is within a range from the level 24 to the level 30, the volume value is changed in accordance with the volume data. On the other hand, in the analog volume control circuit 15A as shown in Fig. 6(A), when the volume data set by the user is within a range from the level 0 to the level 24, the volume value is controlled so as to change in accordance with the volume data. When the volume data is within a range from the level 24 to the level 30, the maximum volume value is maintained. Therefore, for example, if the user continuously changes a set volume value by operating the plus volume (increase) key 34A of the recording volume controller 34, when the volume data is within a range from the level 0 to the level 24, a set volume value VOL2 of the digital volume control circuit 31 is kept a certain value and a set volume value VOL1 of the analog volume control circuit 15A is increased from the minimum volume value to a certain volume value. At this time, if the volume data is within a range from the level 24 to the level 30, the set volume value VOL2 of the digital volume control circuit 31 is increased to a certain volume value in accordance with the volume data, and the set volume value VOL1 of the analog volume control circuit 15A is kept a certain volume value. When the set volume value in the analog volume control circuit 15A is within a range where it continuously changes from the minimum value to a certain value (that is, the reproduction volume data set by the user is within a range from the level 0 to the level 24), the set volume value in the digital volume control circuit 31 is kept a certain value except for 0. Furthermore, if data of a level more than that of the reproduction volume data (level 24), where the set volume value in the analog volume control circuit 15A is constant, is set by the user, the set digital volume value of the digital volume control circuit 31 is increased while keeping the set volume of the analog volume control circuit 15A a certain value. Thereby, the analog volume control circuit 15A mainly controls the level of the audio signal S34 in accordance with the volume value set by the user in a range between the level 0 and a certain volume data (the level 24). In a volume data range higher than the certain volume data (the level 24), the digital volume control circuit 31 mainly controls the level of the signal S34 in accordance with the volume data set by the user. In the above construction, if an audio signal is output at a volume value higher than a certain volume value capable of being set by the analog volume control circuit 15A, the signal reproducing system 30 of the DAT can widen a variable volume range of the output signal S34 by increasing the set volume value of the digital volume control circuit 31 while keeping the set volume value of the analog volume control circuit 15A a certain value, and can obtain the output signal S34 in accordance with the volume value set by the user without increasing the noise components in signal processing circuits of the digital-analog conversion circuit 13, the analog volume control circuit 15A and so on. Therefore, for example, the voice recorded at a low level can be reproduced at a higher level by increasing the set volume value on the side of the digital volume control circuit 31 while preventing the S/N ratio from being lowered. The digital volume control circuit 31 has an overflow limiter, and can clip a suddenly input signal of a high level at a certain value. According to the above construction, the digital volume control circuit 31 in the former stage and the analog volume control circuit 15A in the latter stage are mounted, and the reproduction volume values of the digital volume control circuit 31 and the analog volume control circuit 15A are controlled in correlation with each other in accordance with the reproduction volume level set by operating a volume. If the reproduction volume level set by the user is higher than a certain value, the S/N ratio of the audio signal S34 can be prevented from being lowered by adjusting the volume level by the digital volume control circuit 31 while keeping the volume level of the analog volume control circuit 15A a certain level. Although the set volume values of the digital volume control circuit 31 and the analog volume control circuit 15A are controlled as shown in Fig. 6 in accordance with the volume data set by the user in the above embodiment,the present invention is not limited to the embodiment. It is necessary to control the signal level on the side of the digital volume control circuit 31 when the level of the set volume data is more than a certain set value. Furthermore, although the analog volume control circuit 15A in the above embodiment changes the signal level by using a variable resistor, it may switch the gain of the amplifying circuit. Although the present invention is applied to a digital audio tape recorder in the above embodiment, it is applicable to various kinds of digital audio recorders. Still furthermore, although the present invention is applied to a recording and/or reproducing apparatus which records and/or reproduces audio signals in the above embodiment, the present invention is not limited to such an apparatus.

Claims: A digital signal recording apparatus comprising analog-digital conversion means (6) for converting an input analog signal into a digital signal and digital signal recording means (8), for recording said digital signal obtained by said analog-digital conversion means (6) on a recording medium (9) and analog signal level control means (4A) for controlling the signal level of said analog signal supplied to said analog-digital conversion means (6) ; characterised by: digital signal level control means (22) for digitally controlling the signal level expressed in a digital signal output from said analog-digital conversion means (6); and volume control means (24, 25, 26) for controlling the signal levels of said analog signal level control means (4A) and said digital signal level control means (22) such that the signal level of the input signal is mainly controlled by said digital signal level control means (22) when the volume level set by said volume control means (24, 25, 26) is less than a certain level, and the signal level of the input signal is mainly controlled by said analog signal level control means (4A) when the volume level set by said volume control means (24, 25, 26) is more than a preset level. The digital signal recording apparatus acocrding to claim 1, wherein said analog signal level control means (4A) holds a certain signal level with respect to the volume control by said volume control means (24, 25, 26) where said volume level is less than said certain level. The digital signal recording apparatus according to claim 1 or 2, wherein said digital signal level control means (22) holds a certain signal level with respect to the volume control by said volume control means when said volume level is more than the preset level. The digital signal recording apparatus according to claim 1, 2 or 3, wherein, when the volume level set by said volume control means (24, 25, 26) is less than a certain volume level, a control signal for keeping an analog signal level at a certain value is supplied to said analog signal level control means (4A) and a control signal in accordance with the set volume level is supplied to said digital signal level control means (22). The digital signal recording apparatus according to claim 1, 2, 3 or 4, wherein, when the volume level set by said volume control means (24, 25, 26) is more than a certain level a control signal for keeping a digital signal level at a present value is supplied to said digital signal level control means (22) and a control signal in accordance with the set volume level is supplied to said analog signal level control means (4A). A digital signal recording and reproducing apparatus comprising recording apparatus according to any one of claims 1 to 5 and signal reproducing means (12) for reproducing a digital signal recorded on a recording medium (10) and digital-analog conversion means (130) for converting a digital signal output from said signal reproducing means (12) into an analog signal, and analog signal level control means (15A) for controlling the signal level of said analog signal output from said digital-analog conversion means (13); characterised by digital signal level control means (31) for digitally controlling the signal level of said digital signal supplied to said digital-analog conversion means (13); volume control means (32, 33, 34) for controlling the signal levels of said digital signal level control means (31) and said analog signal level control means (15A) in a predetermined relation with each other. The digital signal recording and reproducing apparatus according to claim 6, wherein the said level of the signal output from said digital signal reproducing means (12) is mainly controlled by said analog level control means (15A) when the volume level set by said volume control means (32, 33, 34) is less than a certain level. The digital signal recording and reproducing apparatus according to claim 7, wherein said digital signal level control means (31) holds a certain signal level with respect to the volume control by said volume control means (32, 33, 34). The digital signal recording and reproducing apparatus according to claim 6, 7 or 8, wherein the signal level of the signal output from said digital signal reproducing means (12) is mainly controlled by said digital signal level control means (31) when the volume level set by said volume control means (32, 33, 34) is more than a preset level. The digital signal recording and reproducing apparatus according to claim 9, wherein said analog signal level control means holds a certain signal level with respect to the volume control by said volume control means when the volume level is more than the preset level. The digital signal recording and reproducing apparatus according to claim 6, 7, 8, 9 or 10, wherein, when the volume level set by said volume control means (32, 33, 34) is less than a certain volume level, a control signal for keeping a digital signal level at a certain value is supplied to said digital signal level control means (31) and a control signal in accordance with the set volume level is supplied to said analog signal level control means (15A). The digital signal recording and reproducing apparatus according to claim 6, 7, 8, 9, 10 or 11, wherein, when the volume level set by said volume control means (32, 33, 34) is more than a preset level, a control signal for keeping an analog signal level a certain value is supplied to said analog signal level control means (15A) and a control signal in accordance with the set volume level is supplied to said digital signal level control means (31).

Inventors: HIMENO TAKUJI, MATSUMOTO KISSEI, HIMENO, TAKUJI, MATSUMOTO, KISSEI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G11B20
IPC codes: G11B20, G11B27