Hardware Autopsy
specifi cation, however, enable a consumer to
build a 200Mb/s network using the existing
power lines in their home.
The MAC layer uses Time Division Multiple
Access (TDMA) and Carrier Sense Multiple
Access (CSMA) with AC line-cycle synchro-
nization. TDMA enables several devices con-
nected to the same network to transmit over
the network and share its capacity; specifi cally,
it allows these devices to share the same fre-
quency channel by dividing the signal into dif-
ferent time slots. Each device then takes turns
transmitting in rapid succession. The use of
TDMA enables HomePlug AV to deliver Quality
of Service (QoS) guarantees so that each
network application gets the bandwidth and
transmit/receive time that it needs.
CSMA is a protocol under which a device
about to transmit fi rst listens to the channel
it’s about to use to determine if it is in use. If
the device senses that the channel is busy,
it will defer its transmission; if the channel is
idle, the device sends a message to all the
other devices on the network not to use that
channel. It then sends its data packet, waits
for an acknowledgement that the packet was
received, and releases the channel.
AC line-cycle synchronization enables a
HomePlug AV network to identify and work
around noise in power lines. General noise
tends to fl uctuate, but the impulse noise
injected into the power lines by appliances
such as refrigerators and air conditioners tends
to be synchronous and of limited duration.
HomePlug AV adapters use multiple time slots
that are synchronized with the AC cycle, and
they analyze line noise before loading bits into
the carrier waves. This allows a HomePlug AV
network to minimize the impact of power-line
noise: If line conditions are optimal, each car-
rier wave can be loaded with different data to
yield the highest possible bit rate; if line condi-
tions are at their worst, every carrier wave can
be loaded with the same data to ensure that
the data arrives at its destination.
Where the original HomePlug spec used
only QAM (which has eight unique analog
phase/amplitude symbols), HomePlug AV uti-
lizes a variety of modulation techniques based
on line conditions: Binary Phase Shift Keying,
Quadrature Phase Shift Keying, and 16-, 64-,
256-, or 1024-QAM. As its name implies, 1024-
QAM has 1,024 unique analog phase/ampli-
tude symbols and can represent 10 digital bits.
HomePlug AV equipment is compatible
with HomePlug 1.0 equipment, but neither
of these standards is interoperable with gear
based on DS2’s technology (or the relatively
ancient Passport hardware, for that matter).
You can, on the other hand, mix power-line
networking technology with wired and wireless
Ethernet products.r & d BREAKING DOWN TECH —PRESENT AND FUTURE
White Paper: Power-Line Networking
Soundcard
The soundcard still rules the audio fidelity and performance roost. Here’s a look at what
makes the popular and powerful X-Fi series tick.Any requests? What hardware—new or old—would you like to see go under
Maximum PC’s autopsy knife? Email your suggestions to [email protected].http://www.maximumpc.com | FEB 08 | MAXIMUMPC 59
ADC The analog-to-digi-
tal converter receives an
analog input and turns it
into a digital signal. The
ADC determines how
accurately the soundcard
will record audio and at
what levels. The Wolfson
ADC here will record with
24-bit resolution and a
96kHz sample rate.DAC The opposite of the ADC, the digital-to-analog
converter turns a digital value into an analog waveform.
The DAC is largely responsible for giving the soundcard
its audio character and dictating how many channels the
card supports. The Cirrus Logic CS4382 lets you output
up to eight channels with 24-bit resolution and a sample
rate of 96kHz, or with 24-bit resolution and 192 kHz
sample rate in stereo mode.DSP A digital signal processor,
the meat of any true hardware
soundcard, lies under this heat-
sink. The X-Fi’s DSP packs 51
million transistors and 10,000
MIPS of power. This lets the
soundcard, instead of the CPU,
perform audio calculations.RAM Creative’s X-Fi
series includes a tiny
amount of RAM (64MB
in this case) that devel-
opers can use to store
audio samples.PORTS These ports connect to the
standard 1/8-inch speaker jacks com-
mon in multimedia designs. One port can
also be used for digital I/O.AD LINK The AD Link port con-
nects expansion-bay items such
as Creative’s X-Fi I/O Drive.