Circuit for PCM conversion of an analog signal, with improvement in gain-tracking

The circuit includes a filter to which an analog signal is applied, a quantizer driven by the filter, a sampler at a desired frequency driven by the quantizer and a PCM encoder driven by the sampler. The quantizer generates a quantize signal according to the received analog signal and further generates a difference signal according to the difference between a quantized signal and the analog signal. A feedback circuit feeds back the difference signal from the quantizer to a stage of the filter so that the overall transfer function from the input of the feedback circuit to the output of the filter is equivalent to a low pass filtering.

BACKGROUND OF THE INVENTION 
1. FieId of the Invention 
The present invention relates to a circuit for PCM conversion of an analog 
signal, such as a voice signal, with improvement of gain-tracking. 
2. Prior Art 
In recent years PCM (Pulse Code Modulation) coding has become very 
widespread, until it has become the modulation method most used in 
telephone systems. Among the various international recommendations (such 
as CCITT in Europe) which set rules on every possible telephone 
connection, those related to the PCM converter (normally constituted by an 
integrated circuit known as "Combochip") prescribe, among other things, a 
mask (described hereinafter) for limiting the gain error (i.e. 
gain-tracking) as the level of the input signal, assumed sinusoidal, 
varies. 
Analog-digital conversion is performed with a quantizer which forces the 
analog signal to assume only preset discrete values, and with a sampler 
which samples the quantized signal and feeds the successive samples to an 
encoder. Since analog-digital conversion introduces, as is known, a 
quantization error in the signal to be encoded, this reflects in a 
corresponding error in the encoded signal, equivalent to a gain error, 
even for an ideal "combochip". 
SUMMARY OF THE INVENTION 
The aim of the invention is now to provide a PCM conversion circuit 
("combochip") having a gain-tracking better than that of known circuits, 
and actually better than the one considered ideal. 
The invention achieves the above described aim, as well as other objects 
and advantages which will become apparent hereinafter, with a circuit for 
PCM conversion of an analog circuit, comprising a filter to the input 
whereof the analog signal is applied, a quantizer driven by the filter, a 
sampler at a desired frequency driven by the quantizer and a PCM encoder 
driven by the sampler, characterized in that the circuit furthermore 
comprises circuit means for detecting the difference between the analog 
signal and the quantized one, and for injecting in feedback the difference 
in a stage of the filter in such a manner that the transfer function from 
the input of the circuit means to detect the difference at the output of 
the filter corresponds to a low-pass filtering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, which is a diagram of the variation in gain in dB as a function 
of the input signal level, also in dB, the two broken lines define a mask 
within which, according to CCITT recommendations, the gain variation curve 
must be comprised. The mask is referred to the gain at -10 dBmO, it 
assumes that the gain is measured when the sinusoidal input signal is 
equal to -10 dBmO as absolute reference. 
FIG. 2 illustrates a PCM conversion circuit according to the prior art. It 
comprises a high-pass filter 10 which receives an analog input signal 
V.sub.in, typically a voice signal. It is assumed that the signal V.sub.in 
has been previously filtered in a low-pass filter, not illustrated, with 
cutoff frequency lower than half the sampling frequency f.sub.s, according 
to criteria known to the experts in the field. The output signal V.sub.out 
of the high-pass filter 10 enters a PCM converter 11 comprising a 
quantizer 12, known to the expert in the field, which produces an output 
signal V.sub.Q in steps, forced to assume values in a discrete series, a 
sampler 14, which receives the output signal V.sub.Q of the quantizer and 
samples it at a desired frequency f.sub.s, typically 8 kHz, to generate a 
quantized sampled signal V.sub.QS, and an encoder 16, which encodes the 
signal V.sub.QS in PCM pulses. 
PCM encoders typically operate by successive comparisons of the sample of 
signal with gradually more proximate references, and in general at the end 
of the conversion there is available, on an adder node of the circuit, the 
residual difference, or remainder, between the sample and the reference. 
An example of PCM encoder of this type is described for example in the 
article "A Segmented .mu.-255 Law PCM Voice Encoder Utilizing NMOS 
Technology", by Yannis P. Tsividis et al., in IEEE J. Solid-State 
Circuits, vol. SC-11, pp. 740-747, Dec. 1976. 
According to the invention, as illustrated in FIG. 3, the PCM conversion 
circuit comprises, besides the circuits 10, 12, 14 and 16 of FIG. 2, a 
remainder feedback circuit 18, connected to receive said residual 
difference from the adder node 20 of the PCM converter and inject it, with 
or without further processing such as appropriate filtering, in a stage of 
the high-pass filter 10 such that the overall transfer function from the 
input of the remainder feedback circuit 18 to the output of the high-pass 
filter 10 is equivalent to a low-pass filtering. 
With the circuit arrangement described above with reference to FIG. 3, a 
gain-tracking as illustrated in FIG. 4 is obtained, having the same 
meaning of FIG. 1. As is apparent, the gain-tracking is improved with 
respect to the one considered ideal illustrated in FIG. 1. 
Though the inventors do not have a full understanding of the theoretical 
reasons for this result, the feedback of the remainder seems to have the 
effect of distributing more uniformly the quantization error, eliminating 
the error peaks which occur at the transitions of the signal from a 
discrete quantization level to another. 
FIG. 5 illustrates a more specific example of the invention. The high-pass 
filter 10 of FIG. 3 has an input terminal IN and an output terminal OUT, 
and is provided with three operational amplifiers stage 30, 32 and 34, 
interconnected in a known manner with coupling and integration capacitors 
36, and with switched capacitors 38. The remainder feedback circuit 18 is 
provided as a simple switched capacitor 40, which alternately charges from 
the adder node 20 of the PCM converter 11 and discharges onto the input of 
the third operational amplifier 34 of the filter 10. It can be seen that 
the transfer function from the adder node 20 to the OUT terminal of the 
filter corresponds to a low-pass filtering. 
The remainder feedback circuit 18, instead of being provided with a simple 
switched capacitor, may also be a complex network, such as to impart to 
the signal a different filtering, and the feedback of the remainder may be 
executed in another stage of the filter, for example at the input. 
Preferred embodiments of the invention have been described, but it is 
understood that they are susceptible to modifications and variations on 
the part of the expert in the field, according to the given teachings, 
without thereby abandoning the scope of the invention.