MULTIMEDIA

The wireless Web is an exciting new development, but it is not the only one.
For many people, multimedia is the holy grail of networking. When the word is
mentioned, both the propeller heads and the suits begin salivating as if on cue.
The former see immense technical challenges in providing (interactive) video on
demand to every home. The latter see equally immense profits in it. Since multimedia
requires high bandwidth, getting it to work over fixed connections is hard
enough. Even VHS-quality video over wireless is a few years away, so our treatment
will focus on wired systems.
Literally, multimedia is just two or more media. If the publisher of this book
wanted to join the current hype about multimedia, it could advertise the book as
using multimedia technology. After all, it contains two media: text and graphics
(the figures). Nevertheless, when most people refer to multimedia, they generally
mean the combination of two or more continuous media, that is, media that have
to be played during some well-defined time interval, usually with some user interaction.
In practice, the two media are normally audio and video, that is, sound
plus moving pictures.
However, many people often refer to pure audio, such as Internet telephony or
Internet radio as multimedia as well, which it is clearly not. Actually, a better
term is streaming media, but we will follow the herd and consider real-time
audio to be multimedia as well. In the following sections we will examine how
computers process audio and video, how they are compressed, and some network
applications of these technologies. For a comprehensive (three volume) treatment
on networked multimedia, see (Steinmetz and Nahrstedt, 2002; Steinmetz and
Nahrstedt, 2003a; and Steinmetz and Nahrstedt, 2003b).

OSCILLATORS

THIS OSCILLATORS TUTORIALS SITE
This section devoted to oscillators is one of the most interesting for newcomers. You need a thorough grasp of oscillators to fully understand the later and much more complicated electronics tutorials on radio receivers and transmitters.
OSCILLATOR BASICS
Here we explain a lot of the basics involved. We discuss the principles of oscillator operation, briefly look at two popular types, Hartley Oscillators and Colpitts Oscillators as well as Frequency or Phase Stability of an oscillator. Then we go on and consider reducing Phase Noise in oscillators, discuss the effects of ambient changes on stability in oscillators and finally minimizing frequency drift in oscillators.
CLAPP OSCILLATORS
A clapp oscillator is in effect a series tuned version of the colpitts oscillator. Perhaps the simplest Colpitts oscillator to construct and get running is the "series tuned" version, more often referred to as the "Clapp Oscillator". Because there is no load on the inductor a high "Q" circuit results with a high L/C ratio and of course much less circulating current. This aids drift reduction. Because larger inductances are required, stray inductances do not have as much impact as perhaps in other circuits.
COLPITTS OSCILLATORS
Colpitts oscillators are somewhat similar to the shunt fed Hartley circuit except the Colpitts oscillator, instead of having a tapped inductor, utilises two series capacitors in its LC circuit. With the Colpitts oscillator the connection between these two capacitors is used as the centre tap for the circuit.
CRYSTAL OSCILLATORS AND CRYSTAL GRINDING
Crystal oscillators are oscillators where the primary frequency determining element is a quartz crystal. Because of the inherent characteristics of the quartz crystal the crystal oscillator may be held to extreme accuracy of frequency stability. Temperature compensation may be applied to crystal oscillators to improve thermal stability of the crystal oscillator.
What is crystal grinding? There was an exchange of emails asking about crystal grinding on the 'Flying Pigs' list. Karl Kanalz, W8TIF gave his insight and experience on the topic of crystal grinding FT-243 crystal blanks and has graciously consented to it being reprinted here.
HARTLEY OSCILLATORS
Hartley oscillator are inductively coupled, variable frequency oscillators where the oscillator may be series or shunt fed. Hartley oscillators have the advantage of having one centre tapped inductor and one tuning capacitor. This arrangement simplifies the construction of a Hartley oscillator circuit.
VOLTAGE CONTROLLED OSCILLATORS
A voltage controlled oscillator or as more commonly known, a vco, is an oscillator where the principal variable or tuning element is a varactor diode. The voltage controlled oscillator is tuned across its band by a "clean" dc voltage applied to the varactor diode to vary the net capacitance applied to the tuned circuit.
OSCILLATOR DRIFT AND DRIFT CORRECTION CIRCUITS
Oscillator drift can be related directly to frequency stability. Drift is the unwanted and unwarrented change in frequency measured over seconds, minutes or hours. Just how stable should an oscillator be? Your oscillator or any electronic project should be as state-of-the-art as is possible, consistent with your design goals.
An oscillator drift correction circuit is any circuit which automatically brings an oscillator back onto it's assigned or tuned frequency. Such circuits might be frequency synthesisers with phase locked loops, automatic fine tuning circuitry AFT or, AFC in AM Receivers and automatic temperature compensation built into an oscillator circuit.