Soft Modems:
Structured Design and Implementation

Provisional and Incomplete!

Progress

Chaps.	2, 3, 4, 5, 6	- finished
Chap.	1	- partly written
Chaps.	7 .. 11	- not started yet

Table of Contents

Preface

1.   What Is a Modem?

Why modulate?

The ITU Recommendations

Hard modems and soft modems

The modem, fmodem and zmodem programs

Coding conventions

2.   Basic Concepts

The telephone network
Introduction
Voltages and currents
Plugs and sockets
Signalling
Transmission
The line termination unit


Time-domain and frequency-domain signals

Sampling and reconstruction
Choice of sample rate
Complex-valued signals
The Fourier transform


Bits, symbols and constellations

Nyquist's limit on symbol rate
Shannon's limit on bit rate
Quadrature amplitude modulation
Encoding bits into symbols
Decoding symbols into bits


3.   Digital Filters

How they work
The frequency response function
Filtering real sequences
The impulse response and the z notation Filtering complex sequences


Filtering and convolution

A taxonomy of digital filters

Finite and infinite impulse response
Lowpass, highpass, bandpass and bandstop


Infinite impulse response filters

Order
Brick-wall, Butterworth, Bessel and Chebyshev
Resonators


Finite impulse response filters

Moving-average filters
Raised-cosine pulse-shaping filters
x / sin x compensation


Designing digital filters

Commercial software
mkfilter and fifi


4.   Basic Software Components

Debugging a modem
The debug class
The co_debug class


Coroutines

The need for multi-tasking
The coroutine package


Audio input and output

Input, output and buffering
The ``ping-pong'' functions


Terminal input and output

The filter classes

Tone generators

Direct digital synthesis
Digital sine oscillator
Infinite-Q resonator


Tone detectors

Tone detection using the fast Fourier transform
Tone detection using digital filters
Hard limiting
Handling phase reversals
Implementation
Call progress monitoring
DTMF recognition


5.   The ``Dinosaur'' Modes: Frequency-Shift Keying (V.21, V.23)

Frequency-shift keying
Introduction
Transmitting FSK
Receiving FSK


Asynchronous communication

Definition
Asynchronous transmission: bytes into bits
Asynchronous reception: bits into bytes


Synchronous communication

Definition
Synchronous transmission: bytes into bits
Synchronous reception: bits into bytes


6.   Adaptive Algorithms

Equalization
Introduction
Compensating for channel distortion
Adjustment of coefficients
Choice of /D and N
Implementation
Training and scrambling
Example: the V.32 bis training sequence
Symbol sample timing
Carrier phase synchronization and automatic gain control
Short updates
Summary


Echo cancellation

Introduction
Adjustment of coefficients
Choice of /D and N
Interaction with audio sample buffering
Implementation


7.   Putting It All Together

V.29

V.22, V.22 bis

V.32

Round-trip delay estimation


8.   Trellis Coding and the Viterbi Algorithm

Convolutional encoding

The trellis

The Viterbi algorithm

Coding gain

V.32 bis and V.33

9.   The V.34 Standard

Line probing and pre-emphasis

Shell mapping

10.   Facsimile (T.4, T.30)

11.   Other Topics

Compression and error correction (V.42, V.42 bis)

Start-up and mode selection (V.8, V.25)

Auto-moding

Text telephony for the deaf (V.18)

Simultaneous voice and data (V.61)

Voicemail

The ITU ``56k'' standard (V.90)

Appendix:   List of ITU recommendations referred to