Robust continuous variable slope delta demodulation

Details Robust continuous variable slope delta demodulation Robust continuous variable slope delta demodulation

2000-11-21

2002-06-04

Wamsley, Patrick (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C375S247000

Reexamination Certificate

active

06400296

ABSTRACT:

BACKGROUND
The present invention relates to communication systems supporting synchronous voice services, more particularly to the use of continuous variable slope delta modulation/demodulation in such systems, and even more particularly to techniques for making continuous variable slope delta modulation-based systems more robust in the presence of interference.
In the last decades, progress in radio and Very Large Scale Integrated Circuit (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radios, can now be produced having acceptable cost, size and power consumption.
Although wireless technology is today focused mainly on cellular communications, in which a user is connected to a fixed infrastructure via radio base stations and portable handsets, a new area of radio communications is emerging that provides short-range connectivity between nomadic devices like laptop computers, mobile phones, Personal Digital Assistants (PDAs) and digital notebooks. Further advances in technology will provide very inexpensive radio equipment, which can be easily integrated into many devices. This will reduce the number of cables currently use to interconnect devices. For instance, radio communication can eliminate or reduce the number of cables used to connect master devices with their respective peripherals. The aforementioned radio communications will require an unlicenced band with sufficient capacity to allow for high data rate transmissions. A suitable band is the Industrial, Scientific and Medical (ISM) band at 2.45 GHz, which is globally available. The ISM band provides 83.5 MHz of radio spectrum.
By definition, unlicensed bands allow all kinds of radio systems to operate in the same medium. This gives rise to mutual interference. To reduce interference and allow a fair access for every user, signal spreading is usually applied. Spreading provides immunity to other systems to other systems and jammers sharing the band. In fact, the Federal Communications Commission (FCC) in the United States currently requires radio equipment operating in the 2.45 GHz band to apply some form of signal spreading when the transmit power exceeds about 0 dBm. Spreading can either be at the symbol level by applying a direct-sequence (DS) spread spectrum technique, or at the channel level by applying a frequency hopping (FH) spread spectrum technique. The latter is attractive for the radio applications mentioned above because it more readily allows the use of cost-effective radios. A radio interface, called BLUETOOTH™ wireless technology, has recently been introduced to provide pervasive connectivity, in particular, between portable units like mobile phones, laptop computers, PDAs, and other nomadic devices. The BLUETOOTH™ system applies frequency hopping to enable the implementation of lowpower, low-cost radios having small physical dimensions (“a small footprint”). The system supports both data and voice services. The latter is optimized by applying fast frequency hopping with a nominal rate of 800 hops per second (hops/s) through the entire 2.45 GHz ISM band in combination with a robust voice coding. An introduction to the BLUETOOTH™ system can be found in “BLUETOOTH—The universal radio interface for as hoc, wireless connectivity,” by J. C. Haartsen, Ericsson Review No. 3, 1998.
The default voice coding scheme in the BLUETOOTH™ system is based on Continuous Variable Slope Delta (CVSD) modulation. CVSD modulation is a type of delta modulation. More generally, delta modulation is a waveform coding technique in which an analog signal is sampled and the difference between successive samples (delta step) is represented by a binary word (bit sequence) that is subsequently transferred to the recipient. The coded bits in the delta modulation do not represent an absolute signal level but rather a derivative. In fact, the bits typically only tell the recipient to go one step up (+d) or one step down (−d) with respect to the previous value.
FIG. 1
shows a simple example of a conventional delta modulator/demodulator configuration
100
. An analog signal is supplied to an input, sampled by a sample and hold circuit
110
, and digitized by a digitizer
120
. The digital samples are then supplied to a non-negating input of a subtractor
130
. A negating input of the subtractor
130
receives a reconstructed signal (image). The subtractor
130
then determines the difference between the supplied digital sample and a sample from the reconstructed signal (image). This difference is supplied to a limiter
140
. Only the sign of the difference is of interest. Consequently, the output of the limiter
140
supplies bits
0
/
1
where, for example, a 0 means that a negative difference was detected and 1 means that a positive difference was detected. The output of the limiter
140
is the output of the delta modulator
180
, and is therefore transferred to a delta demodulator
190
. Within the delta modulator
180
, the output of the limiter
140
is also used to control a feedback circuit that includes an accumulator
150
. The output of the limiter
140
indicates whether a step &dgr; is to be added (bit=1) or subtracted (bit=0) from the value stored in the accumulator
150
. The output of the accumulator
150
is the reconstructed signal (image) that is supplied to the negating input of the subtractor
130
.
The delta demodulator
190
simply contains an accumulator
160
that stores a value to which either a step &dgr; is added (if the received input bit=1) or subtracted (if the received input bit=0). Finally, the digitized output of the accumulator
160
is filtered in a low-pass filter
170
. The output of the low-pass filter
170
is an analog signal that should substantially resemble the one initially supplied to the sample and hold circuit
110
. The modulation and demodulation processes may further contain upsampling and downsampling, but these functions are not shown in order to simplify the explanation of the pertinent details.
The simple delta modulation technique described above does not give acceptable performance when a voice signal is considered. The dynamics in the voice signal are large and it is difficult to find a suitable step size in the delta modulator that is both small enough to hide quantization noise, and large enough to be able to follow large escapes in the voice signal. If the step size is chosen small enough to reduce noise, a phenomenon called “slope overload” typically results, in which the signal reconstructed from the delta values is unable to accurately reproduce the original signal. An example of slope overload is illustrated in the graph of FIG.
2
. An original analog signal
201
is shown. The step-wise signal
203
is generated at the outputs of the accumulators
150
and
160
. The derivative of the original signal
201
is much larger than the step size in the delta modulator. Consequently, a long sequence of positive or negative delta values is generated, such as the sequence of negative values
205
. However, the relatively small step size in the modulator prevents the reconstructed step-wise signal
203
from following the original signal
201
.
To avoid slope overload problems, a modification in the delta modulator has been developed in which the step size dynamically changes: If the signal involves large variations, the step size is increased; and if the signal is rather stable, the step size is decreased. In this way, the instantaneous signal-to-quantization noise ratio is kept fairly constant whereas large changes in the signal can be followed. Such a system is referred to as a Continuous Variable Slope Delta (CVSD) modulator, since the step size varies depending on the slopes in the input signal.
A CVSD modulator/demodulator configuration is shown in FIG.
3
. An analog signal is supplied to an input, sampled by a sample and hold circuit
110
, and digitized by a digitizer
120
. The digital samples are then supplied to a CVSD encoder
301
. Within the CVSD enco

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