Microphone with associated amplifier

The problem addressed by the invention, to develop a microphone with assoted amplifier which has a better dynamic response than analog microphones, requires less space, can replace analog microphones by using cables already in place, and has a low-cost construction, is solved by the invention in that the microphone is made up of an electroacoustic transducer, two analog-digital converters, two preamplifiers connected to the output of the electroacoustic transducers on the one hand and on the other hand each with preamplifiers connected each to an analog-digital converter, and with a driver stage connected to the analog-digital converters for a two-channel digital audio format and in that the amplifier is made up of a receiver for a two-channel digital audio format and a signal processor which generates a one-channel signal from the two-channel audio format as an image of the signal of the electroacoustic transducer. The field of application is in musical production. The invention is illustrated by FIGS. 1 and 2.

The invention relates to a microphone with associated amplifier, these 
being of digital construction. The field of application of the invention 
lies in studio technology, and radio and television technology, as well as 
theatrical and musical production. 
It is known that sound signals are increasingly being stored and processed 
digitally. This entails advantages in the quality and price of the 
apparatus used, in comparison to analog apparatus. However, it has not 
been possible heretofore to digitalize the signals of microphones in 
sufficient quality, since in contrast to telecommunication, for example, 
the available dynamics of the analog-digital converters are in some cases 
considerably poorer than the useful dynamics of analog microphones. It is 
therefore common practice to connect microphones with their often weak 
analog signals through long cables to controllable microphone amplifiers 
which are set by the sound engineer so that analog-digital converters then 
connected will be optimally modulated. The result is two large problems. 
The first problem is that interference can easily be picked up by the 
cables between the microphone and the microphone amplifier, so that 
special routing rules have to be observed, especially when power lines or 
lighting control lines run parallel. 
Second a great deal of experience is needed for the adjustment of the 
microphone amplifier in order on the one hand to leave sufficient reserve 
for the clipping limit of the analog-digital converter, and on the other 
hand to keep the quantization noise of the analog-digital converter 
sufficiently low with respect to the signal. An incorrect estimate can 
make an entire recording unusable. 
An attempt to digitalize the signal of a microphone has already been made 
by a microphone manufacturer. In spite of using the best available 
analog-digital converter circuit the only dynamic response that was 
obtained was about 10 dB lower than that of the corresponding analog 
microphone. In microphones with a very great dynamic range an impairment 
of about 25 dB must be expected. An analog-digital converter has been 
proposed, and one has been disclosed in DE-OS 4420713 A1, that permits a 
substantially greater dynamic response, but is has not built so far. This 
analog-digital converter is based on a plurality of lower dynamic 
analog-digital converters fed by preamplifiers with different values of 
gain. Subsequently, one analog-digital converter is working with high 
resolution at low level input signals and another analog-digital converter 
is working with high resolution at high level input signals. For this 
purpose a signal processor is needed to eliminate all errors based on 
different signal paths. 
The cables necessary for the operation of a digital microphone constitute 
an additional problem. In the experiment referred to above, three cables 
were used: one cable to carry the digital audio values, a second cable to 
carry a sampling cycle to the microphone, and a third cable for power 
supply. The sampling cycle is necessary for the synchronous sampling of 
the audio levels by the other connected digital processing apparatus. 
Operating the microphone as a sampling source with a fixed crystal 
oscillator is not possible, since the connection of several microphones 
must be possible and therefore the connected apparatus cannot be 
synchronized with the microphone. The separate power supply cable is 
necessary due to the required power of about 0.5 to 1 watt. For the 
microphone user, a plurality of microphone cables represents a big 
problem, since in changing over from analog to digital technology, not 
only must the microphone and the corresponding amplifiers be replaced, but 
also the installations in the different housings. In addition, another 
kind of spare cables is necessary, and compatibility or quick changeover 
is impossible. Modulating the necessary signals onto the digital audio 
cable, which may be plug-compatible with the analog audio cable, founders 
on the high cost and the amount of space required for the purpose in the 
microphone. Just the expense of obtaining the low-jitter sampling cycle, 
which with a PLL is additionally increased to a multiple of the sampling 
cycle in order to drive the analog-digital converter, represents a great 
technical problem in view of the great number of standardized sampling 
frequencies. 
A theoretical possibility for eliminating the sampling cable is the use of 
so-called sampling rate converters which can be connected between the 
microphone and the associated amplifier. At the present time such sampling 
rate converters have less usable dynamic range than available 
analog-digital converter circuits, so that this would lead to a further 
degradation of the qualities of a digital microphone. 
The problem to which the invention is addressed is the development of a 
microphone with associated amplifier in digital technology, in which the 
dynamic range of the microphone is not limited by the analog-digital 
converter, in which operation with the microphone cables of an analog 
microphone will be possible, and in which the cost involved and space 
requirements are low. 
This problem of developing a microphone with corresponding amplifier 
offering a better dynamic range than analog microphones, occupying less 
space, interchangeable with analog microphones using available cables, and 
involving less costly construction, is solved by the invention in that the 
microphone and the associated amplifier have digital circuits which are 
interconnected by a shielded symmetrical cable, while the circuit for the 
microphone contains a sound converter, two preamplifiers, two 
analog-digital converters and a driver stage for a two-of channel digital 
audio format, the preamplifiers being connected to the output of the sound 
converter and connected each with an analog-digital converter, the 
analog-digital converters being connected to the digital driver stage, and 
the associated amplifier containing a receiver for a two-channel digital 
audio format and a signal processor which generates from the two-channel 
digital format a single-channel signal which represents an image of the 
signal of the sound converter, and which is used for computing the 
amplification and filtering commonly used in microphones. 
An advantageous embodiment of the microphone additionally has a switch and 
a calibration oscillator which are wired such that the sound converter is 
alternately connected with the two preamplifiers or the calibration 
oscillator is connected to the two preamplifiers. The switch independently 
cuts off the calibration oscillator by remote control a given time after 
the microphone is turned on and connects the sound converter to the 
preamplifiers, the given time being made such that the signal processor 
can determine the coefficients which effect on differences of different 
signals and are are necessary for generating the single-channel signal 
from the two-channel digital audio format. 
In an additional embodiment of the invention, an identification code can be 
entered via the microphone into the two-channel digital audio format to 
indicate the position of the switch, causing a muting in the associated 
digital amplifier of the signal of the balancing oscillator by means of 
the signal processor. 
To avoid synchronization problems or problems with the external sampling 
connection, the microphone can have its own audio sampling oscillator. The 
necessary sampling synchronization by means of a two-channel sampling rate 
converter is performed in the associated digital amplifier which is 
arranged between the receiver of the digital audio format and the signal 
processor. 
The two-channel digital audio format advantageously complies with the 
AES/EBU standard which permits the transmission of digital audio 
information via cable and plug connector which are used for the 
transmission of analog microphone signals. 
The average differential AES/EBU signal can have a voltage with respect to 
the shielding, which will serve for supplying power to the digital 
microphone without additional cables. 
In the associated amplifier, pulses can be modulated onto the voltage of 
the power supply of the microphone which serve for the remote control of 
microphone settings.

DETAILED DESCRIPTION 
The microphone is connected to an associated amplifier through a shielded 
symmetrical cable. In the microphone there is contained the 
electroacoustic transducer 1, a condenser microphone cartridge and its 
impedance converter which is connected to the switch 3. After the 
microphone is turned on the switch 3 can present, instead of the 
transducer signal, the signal from an equalization oscillator 2 through 
the transmission chain to the signal processor 14 to obtain starting 
values of an iterative computation. The signal from switch 3 is fed to the 
two different preamplifiers 4 and 5 which in turn control each one channel 
of the two-channel ADC 6. The ADC 6 transfers the two digitalized audio 
values to the transmitter component 8 which gives a two-channel digital 
sound signal in coded form to a symmetrical two-wire conductor, in accord 
with the published AES/EBU standard. Both the ADC 6 and the AES/EBU 
transmitter are supplied with the sampling cycle f.sub.SM of a quartz 
oscillator, the sampling cycle oscillator 7. The AES/EBU standard provides 
for the use of pulse transmitters both on the transmission and on the 
receiving end in professional applications. These pulse transformers 9 are 
here additionally provided with a center tap for the power supply. In the 
microphone the center tap of the pulse transformer 9 is connected to the 
input of the voltage regulator 10 which in turn supplies the entire 
microphone with the necessary voltage VCC. 
The shielded symmetrical cable 11 can be connected to the pulse transformer 
9 by the XLR plug connections common in studio technology. 
The shielded symmetrical cable 11, which comes from the microphone, is 
connected in the microphone amplifier to the pulse transformer 9. The 
center tap of the pulse transmitter 9 is connected through a decoupling 
diode to the power supply VDC. The decoupling diode permits the use of a 
plurality of microphone amplifiers on one microphone. The side of the 
pulse transformer 9 remote from the cable 11 is connected to the inputs of 
an AES/EBU receiver component 12 which decodes the two digital audio 
signals and recovers the sampling cycle f.sub.SM used in the microphone. 
By means of the recovered sampling cycle f.sub.SM and the microphone 
amplifier sampling f.sub.S, which is identical with the sampling of all 
other digital processing apparatus, the sampling rate converter 13 can 
convert the two digital audio signals of the AES/EBU receiver 12 into 
digital audio signals which are synchronous with the sampling rate 
f.sub.S. The converted digital audio signals are applied to the input of 
the signal processor 14 which computes the single-channel digital audio 
signal that corresponds to that of the electroacoustic transducer, sampled 
with the sampling frequency f.sub.S. The signal processor 14 furthermore 
continues to perform microphone signal amplification and filtration the 
same as it does in analog microphone amplifiers. 
The use of the invention provides not only an improvement of the quality of 
transmission but also the advantage that a digital microphone can be 
operated with the microphone cables of an analog microphone. Also, the 
cost of the construction of the digital microphone configured according to 
the invention, as well as the space it requires, are low, so that analog 
microphones can be replaced by digital microphones in a simple manner. 
In a further development of the invention, the 2-channel audio format is 
converted still in the microphone into a 1-channel audio signal with the 
help of an additional signal processor. As a result, when standardized, 
2-channel, digital audio driver and receiver circuits are used, on the one 
hand, a transmission, conforming to standards, becomes possible and, on 
the other, an otherwise usable second transmission channel arises. The 
1-channel, digital audio format must, however be split once again by a 
digital amplifier in the amplifier allocated to the microphone, in order 
to solve problems of the scanning rate conversion. 
The advantage of this variation of the solution lies therein that a 
transmission, conforming to standards, takes place with the 1-channel 
technique.