Digital microphonic device

Abstract: The invention concerns a microphone device of the digital type, which comprises at least one analog input interface (IN) having input terminals arranged to receive an analog voltage signal. This microphone device further comprises at least one digital output interface (OUT) having at least one input terminal arranged to receive a digital voltage signal, and at least first and second output terminals for transmitting said digital voltage signal in a serial format. Lastly, the microphone device includes at least one converter block (C) connected between the input interface (IN) and the output interface (OUT).

Description: This invention relates to devices for processing an audio signal, in particular to a microphone device of the digital type. As is known, the processing of audio signals has undergone considerable improvement in recent years. More particularly, the ability to integrate digital functions by technologies of the VLSI type has made it possible to translate many functions which are typical of the analog domain into a digital format. It is currently possible to reproduce in the digital format effects that, in the past, could only be achieved in the analog form. This is the case, for example, with the so-called equalizations, the term being understood here to encompass general filtering, surround effects, reverberations, and echoes. These effects can be of far better quality than in the past, when obtained with digital methods. Also, new and more advanced sub-micrometric manufacturing processes have resulted in smaller and more powerful digital integrated circuits being produced at ever lower cost. Thus, great importance is attached, in this field, to the conversion devices whereby signals of the analog type can be converted to a digital format, and vice versa. The trend toward a digital domain in the processing of audio signals is so marked that even signal sources, understood as reproduction devices, are now implemented in the digital format. In the light of the foregoing, it is to be expected that microphone devices will soon be the sole items to survive in the analog form. Currently available microphone devices comprise a telephone transmitter operative to produce a voltage analog signal which is proportional to an audio signal generated by a sound source. In certain apparatus, this voltage analog signal is amplified before being transmitted over a cable or broadcast. In case of the signal being broadcast, e.g. in cordless applications, it would have to be frequency-modulated before its transmission, using carrier frequencies in the 170 MHz range. Other applications of microphone devices are disclosed in WO-A-95/19085, in US-A-4,548,082 and in DE-A-3935308. More particularly, WO-A-95/19085 discloses an echo-cancelling system 700 used in a cellular radiotelephone 600. The echo-cancelling system 700 comprises an analog to digital converter 780 for converting a first electrical signal generated by a microphone 605, in a first digital electrical signal 706. The first digital electrical signal 706 includes two components: a first component is the voice signal which is produced by the user into microphone 605 and a second component is the echo of the voice signal which is picked up by microphone 605. The echo-cancelling system further comprises a digital-to-analog convert 790 for producing a second electrical signal 708, from a second digital electrical signal 712 which is produced by a receive digital signal processor 625. The second electrical signal 708 is provided to a loudspeaker 610 and includes two components: a first component is the received signal from a remote caller and the second component is an echo of the originating signal which was received by microphone 605. The echo-cancelling system 700 also comprises a transmit DSP 620 to remove the residual echo signal. US-A-4,548,082 discloses a hearing aid including a microphone 75 for generating on a line 101 connected to a signal conditioning circuit 103 an electrical output from sounds external to a user of the hearing aid. Signal conditioning circuit 103 produces a preemphasized band limited (anti-alising) and compressed output which is converted into discrete digital samples by combined actions of a multiplexer 105, a sample-and-hold circuit 109 and an analog-to-digital converter 111. The analog-to-digital converter 111 is connected to a digital signal processor 113. DE-A-3935308 discloses a speech recognition method in which individual spoken words, generated by a user into a microphone, are converted into analogue electrical signals and these, after amplification and normalisation are digitalised by an analog-to-digital Delta modulator. These prior art microphone devices, while being advantageous in many ways, still have some drawbacks, however, as are typical of analog apparatus, among which coupling noise from the electromagnetic waves that surround the devices, attenuations, and filtering due to the transmitting means, for example. All this generally detracts from the quality of the transmitted signal. The underlying technical problem of this invention is to provide a microphone device of the digital type which features high quality of the transmitted signal and low manufacturing cost, while overcoming the limitations and/or drawbacks mentioned above. This technical problem is solved by a device as indicated in the preamble and defined in the characterizing parts of the appended claims. The features and advantages of a device according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. In the drawings: Figure 1 shows diagramatically the device of this invention; Figure 2 is a block diagram of a detail of the device in Figure 1; Figure 3 is a breakdown diagram of a portion of the block diagram shown in Figure 2; and Figure 4 is a breakdown diagram of another portion of the block diagram shown in Figure 2. With reference to the drawing views, and in particular to Figure 1, generally and schematically shown at 1 is a microphone device of the digital type which embodies this invention. The device 1 comprises an analog input interface IN having a plurality of input terminals for receiving an analog voltage signal from a transducer TR. The transducer may, for example, be a telephone transmitter coupled to a signal source S. This input interface IN comprises at least a first OA' and a second OA" amplifier circuit, each provided with at least first and second input terminals and at least one output terminal, and each comprising a circuit portion devoted to automatic gain control (AGC). The device 1 also includes a timer block T coupled to a quartz element Xtal, and a converter block C of the analog-to-digital type. Specifically, the timer block T comprises at least first and second output terminals, and the converter block C comprises at least first, second and third input terminals and at least one output terminal. The first and second input terminals of the converter block C are connected to the output terminals of the first OA' and the second OA" amplifier circuit, while the third input terminal of this block is connected to the first output terminal of the timer block T. The device 1 further comprises a digital output interface OUT of the parallel-serial type which has first and second input terminals, respectively connected to the output terminal of the converter block C and to the second output terminal of the timer block T, and at least first and second output terminals for transmitting a digital voltage signal to a user apparatus, designated USER. More particularly, and as shown in Figure 2, the converter block C comprises a first or signal modulator sub-block MOD of the Sigma-Delta type which is cascade connected to a second or signal sampler block CAM. Referring now to Figure 3, the first signal modulator sub-block MOD has first 2 and second 3 circuit portions. More particularly, the first circuit portion 2 of the first signal modulator block MOD comprises a first integrator block I1, a first amplifier block A1, a first summing node S1, a second integrator block I2, a second amplifier block A2, and a first quantizer block Q1, all connected in cascade with one another. The first quantizer block Q1 is feedback coupled to the first integrator block through a second summing node S2. This quantizer block Q1 is also coupled to an output terminal of the signal modulator sub-block MOD, through a shunter block D1 and a third summing node S3 in cascade. The second circuit portion 3 of the first signal modulator sub-block MOD comprises a third integrator block I3, a third amplifier block A3, and a second quantizer block Q2, all connected together in cascade. The second quantizer block Q2 is feedback coupled to the third integrator block I3 by a fourth summing node S4. This second quantizer block Q2 is further coupled to the output terminal of the signal modulator sub-block MOD, through a fourth amplifier block A4, a second shunter block D2 and the third summing node S3, also connected in cascade with one another. Lastly, the second amplifier block A2 is coupled to the fourth summing node S4 through a cascade of a fifth A5 and a sixth A6 amplifier block interconnected by a fifth summing node S5. With reference now to the example of Figure 4, the second signal sampler sub-block CAM comprises a clipping circuit DEC which is cascade connected to a filter F of the FIR type. It is worth mentioning that today's technologies allow the converter block C to be fabricated to high quality standards, e.g. with a signal to noise ratio of better than 90 dB, and at low cost e.g. using Sigma-Delta techniques for low power consumption, such as for operation on a supply voltage of 3.3 volts. The operation of the device 1 will now be described with particular reference to an initial state in which the transducer TR is outputting an analog voltage signal which is proportional to the audio signal generated by the signal source S. This analog voltage signal is amplified through the first OA' and the second OA" amplifier circuit, each having high input impedance. Further, the circuit portion devoted to automatic gain control, is effective to match the amplification to the current sound level. The analog output signal is then converted, by the first modulator sub-block MOD, to a digital voltage signal, namely into a string of bits. This digital signal contains, at a low frequency, the information contained in the analog signal, and at a high frequency, the quantization noise which has been generated during the talk carried out at a high speed by the first modulator sub-block MOD. Specifically, the analog signal being input to the first modulator sub-block MOD is sampled at the rate of 128fs, where fs is the speed of the digital signal being output from the device 1. The first circuit portion 2 of the first modulator sub-block MOD carries out, on the input signal, a second order integration followed by a quantization at two levels only. The effect of the feedback between the direct path and the signal difference before and after the first quantizer block Q1, results in the output signal from this first quantizer block Q1 containing the input signal unchanged, but with the quantization noise introduced by the first block Q1 added to it, and undergoing filtration by a differentiation of the second order. On the other hand, the second circuit portion 3 of the first modulator sub-block MOD will process the signal difference before and after the first quantizer block Q1. The integration with feedback, applied by this second circuit portion 3, plus the provision of the second quantizer block Q2, allows an output signal to be obtained which is the sum of the quantization noise introduced by the first block Q1 and a second quantization noise introduced by the second block Q2 and filtered by a first order differentiation. The outgoing logic functions from the two circuit portions 2 and 3, as suitably summed, allow an output signal from the first modulator sub-block MOD to be obtained in which the quantization noise introduced by the first block Q1 has been fully suppressed, the quantization noise introduced by the second block Q2 undergoing filtration by a differentiation of the third order. The last-mentioned differentiation provides a clean baseband output signal, and concentration of the quantization noise at a high frequency. The second sampler sub-block CAM is operative to sub-sample, through the clipping circuit DEC, the digital voltage signal to reduce it to correct resolution (e.g., 16 bits for a signal of the audio type). Furthermore, this sampler sub-block CAM suppresses, by means of the filter F, the quantization noise present in the digital signal, thereby providing an output signal of a high quality which can be measured by the signal-to-noise ratio. Lastly, the output interface OUT is operative to turn into a serial format the digital voltage signal from the converter block C, thereby speeding up the communication with the user apparatus USER. Thus, in order to transmit the digital signal without deteriorating its quality, a simple single-pole cable and a ground return lead will be required. Transmissione standards have been established for the purpose, such as the AES-EBU standard, which allow the digital signal to be transmitted over a single electric or optical lead. Were the microphone device to be used without cable connections, a digital channel modulation system could be utilized to broadcast the signal. The signal transmission could also be effected in the infrared range using LEDs. To summarize, the device of this invention allows the transmission from the microphone to the reproduction apparatus to be digital, with the advantage that all the problems which typically associate with the analog mode, such as disturbation of the transmissive medium (screen-offs, cable attenuations, RF noise) can now be obviated. Also, the signal transmission format (AES-EBU in this case) enables errors to be corrected, conferring superior quality features on the microphone device for the same cost. Not least in importance is the fact that, by having all the signal conversion and transmission apparatus integrated to the same element, the manufacturing costs of the microphone device can be lowered.

Claims: A microphone device of the digital type, characterized by : at least one analog input interface (IN) having input terminals arranged to receive an analog voltage signal; at least one digital output interface (OUT) having at least one input terminal arranged to receive a digital voltage signal, and at least first and second output terminals for transmitting said digital voltage signal in a serial format; at least one converter block (C) connected between the input interface (IN) and the output interface (OUT), wherein said converter block (C) is of the analog-to-digital type and comprises at least a signal modulator sub-block (MOD) connected in cascade with at least a signal sampler sub-block (CAM). A device acccording to Claim 1, characterized in that the analog input interface (IN) comprises at least a first (OA') and a second (OA") amplifier circuit having output terminals connected to input terminals of the first signal modulator sub-block (MOD). A device according to Claim 1, characterized in that the digital output interface (OUT) is of the parallel-serial type. A device according to Claim 1, characterized in that the signal modulator sub-block (MOD) is of the Sigma-Delta type. A device according to Claim 1, characterized in that the signal sampler sub-block (CAM) comprises at least one clipping circuit (DEC) connected in cascade with at least one filter (F). A device according to Claim 5, characterized in that said filter (F) is of the FIR type. A device according to Claim 1, characterized in that it comprises at least one timer block (T) having at least first and second output terminals respectively connected to at least one input terminal of the converter block (C) and to at least one input terminal of the digital output interface (OUT).

Inventors: BIANCHESSI MARCO, DALLE FESTE SANDRO, SERINA NADIA, BIANCHESSI, MARCO, DALLE FESTE, SANDRO, SERINA, NADIA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H04R1
IPC codes: H04R1, H04R3