Method and apparatus for reducing quantization noise

In a quantization noise reduction circuit (200), a feedback signal (W)is added to an input signal (X) to the quantization circuit to reduce quantization noise. The feedback signal is generated as a filtered difference between a sample of a N bit signal (X') and a time coincident sample of a M bit quantized signal, where M<N. The feedback signal is subtracted from the input signal (X) prior to quantization thereby introducing out of band noise into the input signal for reducing in band noise in the quantized signal (Y).

FIELD OF THE INVENTION 
The present invention relates generally to digital signal processing, and 
more particularly, to a method and apparatus for reducing quantization 
noise in digital signal processing applications. 
BACKGROUND OF THE INVENTION 
Digital signal processing is evolving as the preferred implementation in 
many signal processing applications. The advent of improved, higher speed 
and lower cost digital signal processors (DSPs) and other digital circuit 
elements coupled with increased flexibility and accuracy of digital 
circuits is driving a move to converting a number of signal processing 
applications from the analog forum to the digital forum. Digital signal 
processing, while offering the above mentioned advantages and other 
advantages, does not come without some drawbacks. For example, some 
applications, particularly in the field of radio frequency (RF) 
communications, are inherently analog. Signal processing for RF 
applications often require converting an analog signal, for example an RF 
or intermediate frequency (IF) signal, to a digital signal and likewise 
converting digital signals to analog signals. An example of such an 
application is in wideband digital transceivers such as shown and 
described in commonly assigned U.S. patent application Ser. No. 
08/366,283, the disclosure of which is hereby expressly incorporated 
herein by reference. 
In many digital processing applications, including those accomplished in a 
wideband digital transceiver, the precision of a signal must be converted 
from a high level of precision to a lower level of precision. For example, 
a signal represented as 32 bits of information may have to be reduced to a 
signal represented as 16 bits of information. This is due to the limited 
capabilities of certain digital processing elements such as, for example, 
digital-to-analog converters (DACs). In making such a conversion, however, 
there is a loss of information. One will appreciate in the above example 
that 32 bits can represent more information than 16 bits at a given data 
rate. The result of this loss of information is quantization noise. 
Referring to FIG. 1, a typical example is shown to illustrate the effects 
of quantization noise. In the application illustrated, a 16 bit digital 
signal X of given frequency is to be converted to an analog signal by DAC 
10. However, DAC 10 is only a 12 bit device. Therefore, the signal X must 
be first converted to a 12 bit signal. A typical approach is to use a hard 
quantizer 12 which truncates the least significant bits (LSBs), in this 
case the 4 LSBs, of signal X to create a 12 bit signal Y. The noise 
relative to the carrier signal in decibels (dBc) of this application is 
given as: 
EQU noise (dBc)=20 log 2.sup.-n 
where n is the number of bits of the DAC. Thus, the noise level is (-72) 
dBc for the 12 bit DAC and would be, for example, (-78) dBc for a 13 bit 
DAC, etc. Often the noise is distributed over the entire Nyquist bandwidth 
and the noise power per Hertz is negligible. However, frequently the noise 
appears at discreet frequencies, like second and third harmonics of the 
signal, which pose significant problems. 
To overcome the problem of noise dwelling at particular frequencies, it has 
been proposed to introduce psuedorandom noise to the signal, often 
referred to as dithering. A number of dithering techniques are taught in 
U.S. Pat. Nos. 4,901,265, 4,951,237, 5,073,869, 5,228,054 and 5,291,428. A 
major disadvantage of dithering is the requirement of having to provide 
pseudorandom noise generator circuitry which is often complex making the 
application implementation intensive and costly. 
Therefore, a need exists for a method and apparatus for reducing 
quantization noise without significantly increasing the cost and 
complexity of the digital signal processing circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to the present invention, a feedback signal is provided to the 
input of a quantization circuit to reduce quantization noise. The feedback 
signal is generated as a filtered difference between a sample of the N bit 
signal and a time coincident sample of a M bit quantized signal, where 
M&lt;N. The feedback signal is subtracted from the input signal prior to 
quantization thereby introducing out of band noise into the input signal 
for reducing in band noise in the quantized signal. 
With reference to FIG. 2, a N bit to M bit, where M&lt;N, quantization circuit 
200 in accordance with the present invention is shown. A N bit signal X, 
is coupled to a summer 202 where a N bit feedback signal W is subtracted. 
The resulting signal X' is then sampled in a N bit latch 204 and 
concomitantly quantized in a M bit hard quantizer 206. Hard quantizer 
truncates the N-M LSBs of signal X', effectively setting the N-M LSBs to a 
value of zero. A N bit error signal E, is generated in summer 208 as the 
difference between the M most signification bits (MSBs) of the N bit 
sample of X' contained in latch 204 and the M bit quantized sample 
contained in hard quantizer 206. The LSBs of the N bit sample of X' pass 
unchanged. Error signal E is filtered through filter 210 creating N bit 
feedback signal W. It should be appreciated, however, that any M bits of 
signal X' may retained in hard quantizer 206 depending on the particular 
application. 
Further shown in FIG. 2 is a 12 bit DAC 212 for converting hard quantizer 
output signal Y to an analog signal. It should be understood, however, 
that quantization circuit 200 of the present invention is useful in any 
digital signal processing application requiring a conversion from a high 
precision information signal to a lower precision information signal where 
it is critical to avoid introduction of quantization noise. 
Filter 210 is chosen to pass only components of error signal E which are 
out of band with respect to input signal X. In the preferred embodiment, 
filter 210 is a low pass filter which substantially maintains the noise 
components introduced into signal X' by feedback signal W at low 
frequencies and away from the band of interest. This is illustrated in 
FIGS. 4 and 5. As can seen in the FIG. 4, without the present invention, 
spurious noise components, illustrated at .function..sub.s, having 
signicant energy are present around the signal of interest illustrate at 
.function..sub.X. As can be seen in FIG. 5, while there is a substantial 
amount of energy below a frequency .function..sub.fco, the cut off 
frequency of filter 210, there is only a low level of noise which is 
substantially evenly distributed about the signal of interest at frequency 
.function..sub.X. In testing the present invention, a noise floor of (-93) 
dBc was observed about .function..sub.X as compare to (-72) dBc as may be 
typically expected from a 12 bit quantizer without the present invention. 
These data were generated referencing the analog signal output of DAC 212. 
Another feature of the quantization circuit 200 is that when signal X is 
not present, or is substantially zero, there is no noise output. With 
prior art dithering techniques, psuedorandom noise is continuously input 
to the quantization circuit. When no input signal is present, the output 
signal of the quantization circuit is the pseudorandum noise. In the 
present invention, when input signal X is absent or substantially zero, 
the difference taken between the N bit sample of X' and the M bit 
quantized sample is substantially zero. Hence, the output of quantization 
circuit 200 is zero when no input signal is present. 
As described with respect to a preferred implementation of quantization 
circuit 200, error signal E is a 16 bit signal. However, since it is the 
N-M LSBs which primarily contribute to error signal E, a N-M bit signal 
could be substituted. In such an implementation, the sign information of 
error signal E will be lost. Hence, it may be more desirable to implement 
a (N-M)+1 bit error signal which retains the sign bit from signal X'. Such 
an implementation simplifies the data path for error signal E as well as 
reduces the size of filter 210. 
With reference to FIG. 3, a transfer function for a preferred 
implementation of filter 210 is shown. As can be seen in FIG. 3, filter 
210 is a 3 real pole filter which can be implemented using three full 
adders 302, 304 and 306 and one delay element 308. In the preferred 
embodiment of the present invention, the poles of filter 210 are selected 
to be at 15/16 which allows for the simplified implementation shown in 
FIG. 3. As can be seen, this implementation advantageously eliminates the 
need for multipliers which allows a simplified implementation of filter 
210 in an application specific integrated circuit (ASIC). Filter 210 
further includes an overall gain factor, in the preferred embodiment 
approximately 100 dB. Gain is provided at each stage of filter 210 which 
enhances the level of feedback signal W with respect to input signal X and 
hence the noise generating effect of feedback signal W on input signal X. 
As will be appreciated from the foregoing, the quantization circuit 200 of 
the present invention provides for a greatly simplified implementation 
particularly with respect to ASIC implementation. The elimination of the 
pseudorandom noise generator previously required for dithering techniques 
and advantageous selection of filter design minimize required gates in the 
ASIC. These and many other advantages and uses of the present invention 
will be appreciated by those of ordinary skill in the art from the 
foregoing description and the following claims.