sceiv

eoonstruel those sa~ into vottages,
you will see tner the resu lt Is JUSI a
slraight IWle! Thai dOesn'l eescee the
orignal signal at ai , sees II? If you sample
at say, 1.51mes the signallrequen­
C'f, iI gets even weirder. Try iI. Draw a
bunch 01 Sine waves and !hen, using a
ruler, piCk points on it 9Y13fY 1.5 cycles.
Now, connect the dots. Whal have you
got1 Garbage! This Is called ·aliasing ;
and 1\ sounds terrible. ThaI'S why CD
players sample a lillie bll laster than
twice the deSired 20 kHz Inpul bandwidth;
the Input and outpul Iilters don'
have peflectfy steep slopes, and keeping
Ihe music signals aWly from the
NyqlJst Lmt~ eimI'late alasing..
ThaI's Deep
So now you know whaf s involved In
the "tIorlzonlar aspect of sampling . In
other words, how often you have to do if.
BUI, there's a 'verncar aspect as well.
The more bits (digital "ons" and ·offs")
you use In each sample, lhe more peecisely
you can describe each voltage
rreasuremenl. II you only have two bits,
you can only describe lour possible vo.­
ages, because there are onfy lour p0ssible
COITOinations of two bits. They are:
00, 0 1, 10 and 11 . Thaf s not rruch res0­
lutiOn! If you have 16 bits. though. you
can break the measlRm6nI inlo 65,536
parts. (No, I'm 001 gonna list them all
hefe!!) With that kind of precision, the inherent
diStortion is redliCed to a very tiny
fraction 01 a percent. Of course, that
means you have to send lots more bits,
so you need lots more bandwidth. A CD
player reads about 2 megabits per second
olf the esc Not all of that is music,
Itlough; some is lor the time counter and
some is lor error correclion, But « .1
kHz limes 16 bils Umes two channels
equals ovtll'" 1.4 megabits per second, so
lorgeI set ldillQ it over 20 meters, al leaSl
in teal tme!
What'. the Point?
Of course, ham communiCations don'
require CD quality. In order to digitize an
HFlIrade voce signal, we need to take
about 6,000 samples per second. Four
bits 01 sample "depth" giV1;l us 16 levels
of vollage resolution, which will proooce
lislenable speech. SiK or eig hl bits are
much better. So, we're sliDtalking aboUt
24,000 to 48,000 bits per secced, which
Is an awful lot Remembel, though, that
a bit is not the same It1l1'1g as a Hz. n is
possible 10 send many bits per second
over a limited bandWidth. atlhough it
gets bicky if you push it too lar. 9600­
bits-per-second modems are increasingly
common over the telephone anel
VHF/UHF cecset links, but even that Is
too slow for reet-ttrre speech 01 decent
grade, Is there anolher way out?
very workable. and there are far more
sophisticated systems which can reocce
anyfiJe by an average of 50 percent!
There are other methods of data rel1ICIion
which are particularly applicable
10 VOiCe and vDeo data. A geal one is
"delta modulaliorl.- In thiS scheme, only
the changes In the incoming analog signal
are coded. This works especia1ty
well for video because pktures usually
contain lots of Identical or similar areas.
Instead 01 sending the same byte over
and over, me delta modulator simply
sends it once: analog wilt1 a code teling
the demodulator how many limes it is 10
receet
A ~w~,~..~· 18tion of della modulatiOn and
data ~ession can tremendously rewee
the amounl of data required 10 reproduce
adequate sound. In fact. Sony's
new Minidisc pocket eligital recorder reduces
me data by 80 percent belore
recording II! And that's for hi-fi sound .
Imagine what we could do with voicegrade
signals.
Oops, Missed
One problem with digital eocoding is
Ihal missing data causes far worse
gti1ches in the recoostruetect aucr.o than
a Simlar amount 01 missing analog In\ormation.
I know that seems oootraetidory
to the performance of CDs versus l Ps
(remember those?). bl.It n's true. The
problem was recognized early in the development
of the disc and solved with a
technique we may be able 10 use. It's
called "interleaving·
Spread It Mound
On a CD. uere's lots of rewndanl in­
Iorrnati:ln, along with checksums simiar
10 those used In packet ratio. A d1edl:-
sum is sinllfY a number which tels the
decoder how many bits !here should be
l they all are COfteclly read . Thus, the
system knows when something's missing.
but not what if is. More sophiSticaled
techniques actually lei the decoder fil
in and correct some missing Inlormation,
But st ill. If a decent Chunk gets lost,
there's gonna be a nasty noise in the resulting
audio. Inlerleavlng is simpler
Ihan it sounds . It just means that the data
is not recorded sequentially. For example.
the first bit of a byte may be f0llowed
with lt1e fll"Sl: tHI: 01 the ren byte.
After, say, eighl of them. the second bit
of the lirst byte is stored, followed by the
second bit of the second byte, and so
on. Why 00 it? Because, by spreading
each byte over a larger area of the esc,
Ot a longer span of lime over the air, the
chances ollosiog a significant JXll1ion 01
any one byte are tremendously redlJOBd.
That's why scratches in a CD don't mess
up the soun d at all unless they're bad
enough to cause the laser to skp an enlire
track. Of course, if takes longer to
retrieve an interleaved byte, so !here's a
delay between the time you slart reading
it and the time you can turn if back inlo
analog. On a CD. it ccese't maner, because
you don' know and don't care
when iI was read off the esc.
If we apply the same joea 10 radio
transmission. though, il does maner. A
delay 01 o nly a quarter 01 a second
should be acceptable, but a one-second
delay would make conversations very
awkward. On the omer hand, the longer
tile Interleave period, the better if works
at keeping noise bursts and a RM from
destroying the data .
Well. there's more 10 discuss. but I'm
out 01 room.. See you ne_1 tme. III
TTENTION:
EA-FAX USERS
From~~: ....,_.....
OverView Sy.stems
AMFAX-lO is a hardware AM to FM
FAX converter designed espec ial ly
for the "'AEA-FAX package,
---- Features: ...::._--
• Copy vivid polar orbiting, Geostationary
, and GOES-TAP images.
• Easy to connect and use!
• Operates from 12 -14 VDC.
• Select original FM FAX or new AM
FAX modes by the flip of a sw itch.
• Fully assembled and tested.just
plug in...and go!
• Works ID1h existing "'AEA-FAX
adapter and software.
• Cost: AMFAX-IO only $99,95 plus
$6,00 shipping, optional 12 VDC
~rter $8.95.
To order send check or money order to:
Fe.- IIl