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Improving Wireless Voice Quality with ADM Part I

Thu, 06/30/2005 - 8:29am

While ADM is a mature modulation technique and has been around for a while, pressure on cost and times to market breathes new life into this modulation scheme.

By Ron Hunter
Glossary of acronyms

3B4B— an encoding scheme that translates 3-bit bytes into 4-bit coded words using running disparity.

ADM— Adaptive Delta Modulation

ADPCM— Adaptive Delta Pulse Code Modulation

CBUS— Control Bus (Nortel)

CVSD— Continuously Variable Slope Delta

DCO— Digitally Controlled Oscillator

FLASH— a type of electronically erasable programmable read-only memory.

IF— Intermediate Frequency

LDO— Low Dropout

MOS— Mean Opinion Score

PCB— Printed Circuit Board

PCM— Pulse Coded Modulation

PLL— Phase-Locked Loop

RAM— Random Access Memory

SNR— Signal-to-Noise Ratio

SPI— Serial Peripheral Interface

TDD— Time-Division Duplexing

USART— Universal Synchronous/Asynchronous Receiver/Transmitter (also called SPI port)

Short-range wireless digital voice transmission is used extensively in contemporary consumer electronics. Products such as cordless telephones, wireless headsets (for mobile and landline telephones) and baby monitors are a few items that use digital techniques to wirelessly communicate voice information. Wireless environments are inherently noisy, so the voice coding scheme chosen must be robust in the presence of bit errors. PCM and its derivatives are commonly used in wireless consumer products for their compromise between voice quality and implementation cost, but these schemes are not particularly robust in the presence of bit errors. ADM is a mature technique that should be considered for these applications because of its bit error robustness and its low implementation cost.



click the image to enlarge

Figure 1. MOS comparison for various voice coding methods.3

ADM 101

ADM quantizes the difference between the current sample and the predicted value of the next sample. ADM uses a variable "step height" to adjust the predicted value of the next sample so that both slowly and rapidly changing input signals can be faithfully reproduced. One bit (i.e., "1" or "0") represents each sample in ADM.1 The one-bit-per-sample ADM data stream requires no data framing, thereby minimizing the workload on the host microcontroller.




Wireless environments are inherently noisy, and bit errors are present in any wireless application. Most voice coding techniques provide good audio quality in an ideal operating environment, but the challenge is to provide good audio quality in the presence of bit errors.



click the image to enlarge

Figure 2. ADM reference design block diagram.

Traditional performance metrics (e.g., SNR) do not accurately measure perceived audio quality for various voice coding methods and input signals.2 MOS testing overcomes the limitations of other metrics by successfully quantifying perceived audio quality. MOS testing uses a scale of 1 to 5 to represent audio quality, with 1 representing very bad speech quality and 5 representing excellent speech quality. A MOS score of 4 or higher represents "toll quality" speech, which is equivalent to audio quality obtained during a traditional telephone call.3


Figure 1 shows the relationship between MOS scores and bit errors for three voice coding schemes, CVSD, µ-law PCM and ADPCM (note CVSD coding is a ADM voice coding schemes). As the perceived audio quality (i.e., MOS score) of all three schemes degrades as the number of bit errors increases, the graph indicates that ADM sounds better than the other schemes as bit errors increase.


Because ADM provides robust performance in the presence of bit errors, error detection and correction typically are not used in an ADM design, and this contributes further to a reduction in host processor workload. The superior noise immunity, coupled with a significantly reduced workload for the host processor, strongly supports consideration of ADM as a voice coding method for wireless applications.



click the image to enlarge

Figure 3. Data processing flow path for the ADM reference design.

In Part II, coming next month, the advantages of ADM for wireless applications will be demonstrated in an example reference design. This small form-factor, low-power design includes all of the building blocks necessary for a complete wireless voice product, including:

• ADM voice codec

• microcontroller

• RF transceiver

• power supply including rechargeable battery

• microphone, speaker and amplifiers

• schematics, board layout files and microcontroller code written in C.


Part II delves into the technical depths of the design and discusses the voice coder, microcontroller and transceiver. It ties together the components and presents a reference design that can act as a "seed" for wireless voice projects. It contains the critical data and details the pertinent numbers so the reader has the basic concept and values to understand the complete design.


About the Author

Ron Hunter is an applications engineer for CML Microcircuits. He received a B.S.E.E. from the University of North Carolina-Charlotte and has completed multiple graduate engineering courses from North Carolina State University.

Footnotes

1. Steele, R., Delta Modulation Systems, Pentech Press, London, 1975. 2. CML Microcircuits Application Note, "Continuously Variable Slope Delta Modulation (CVSD): A Tutorial," www.cmlmicro.com.

3. Jayant, N.S., and P. Noll, Digital Coding of Waveforms; Principles and Applications to Speech and Video, Prentice-Hall, Englewood Cliffs, NJ, 1984.

4. CML Microcircuits Application Note, "Using Two-Point Modulation To Reduce Synthesizer Problems When Designing DC-Coupled GMSK Modulators," www.cmlmicro.com.

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