Image Communications.pdf

(5760 KB) Pobierz

Contents

1 Fast-Scan Amateur Television Overview

1.1 ATV Activities

1.2 Comparing Analog AM, FM and Digital ATV

1.3 How Far Does ATV Go?

2 Amateur TV Systems

2.1 Analog AM ATV

2.2 Frequency Modulated ATV

2.3 Digital ATV

2.4 ATV Antennas

2.5 ATV Identification

2.6 ATV Repeaters

3 ATV Applications

3.1 Public Service

3.2 Radio Control Vehicles

3.3 ATV from Near Space

4 Video Sources

5 Glossary of ATV Terms

6 ATV Bibliography and References

7 Slow-Scan Television (SSTV) Overview

7.1 SSTV History

8 SSTV Basics

8.1 Computers and Sound Cards

8.2 Transceiver Interface

8.3 Transceiver Requirements

8.4 SSTV Operating Practices

9 Analog SSTV

9.1 Color SSTV Modes

9.2 Analog SSTV Software

9.3 Resources

10 Digital SSTV

10.1 Digital SSTV Setup

10.2 Operating Digital SSTV

10.3 Future of SSTV

11 Glossary of SSTV Terms

12 SSTV Bibliography and References

Image Communications

1 Fast-Scan Amateur Television Overview

Fast-scan amateur television (FSTV or just ATV) is a wideband mode that is based on the

analog

NTSC

(National Television System Committee) standards used for broadcast televi-

sion in the US for many years, before most broadcasters switched to digital transmission in

2009. Analog AM and FM ATV use standard NTSC television scan rates. It is called “fast

scan” only to differentiate it from slow-scan TV (SSTV) or digital TV (DTV). In fact, no

scan conversions or encoders/decoders are necessary with analog ATV.

Any standard TV set capable of displaying analog NTSC broadcast or cable TV signals

can display the AM Amateur Radio video and audio signals. New consumer digital television

(DTV) sets sold in the US are designed to receive high definition television (HDTV) broadcasts

since the changeover in 2009 using the 8-VSB (8-level Vestigial Side Band) standard from

the Advanced Television Systems Committee (ATSC). DTV televisions will also include

analog AM and Digital Cable (D-CATV) TV channel tuners per FCC Rule 15.118(b). It is

a good idea however, to not dispose of your old analog-only televisions, but keep them for

ATV. Analog AM ATV on the 70 cm band will be the primary ATV system for some time

given its low cost, size, and ease to get on.

To transmit ATV signals, standard RS-170 composite video (1-V peak-to-peak into 75

and line audio from home camcorders, cameras, DVD/VCRs or computers is fed directly into

a transmitter designed for the ATV mode. The audio goes through a 4.5 MHz FM subcarrier

generator in the AM ATV transmitter that is mixed with the video. It is the same for FM ATV,

but the sound subcarrier can be anything from 4.5 MHz to 6.8 MHz, with 5.5 MHz typical.

Picture quality is about equivalent to that of a VCR, depending on ATV RF signal level

and any interfering carriers. All of the sync and signal-composition information is present

in the composite-video output of modern cameras and camcorders. Most camcorders have

an accessory cable or jacks that provide separate audio/video (A/V) outputs. Audio output

may vary from one camera to the next, but usually it has been amplified from the built-in

microphone to between 0.1 and 1 V P-P (into a 10-kW load).

This supplement covers two

popular communication modes

that allow amateurs to exchange

still or moving images over the

air. Advances in technology have

made image communications

easier and more affordable,

resulting in a surge of interest.

The first part of this chapter, by

Tom O’Hara, W6ORG, describes

fast-scan television (FSTV), also

called simply amateur television

(ATV). ATV is full-motion video

over the air, similar to what

you see on your broadcast TV.

Because of the wide bandwidth

required for ATV signals,

operation takes place on the UHF

and microwave bands.

The second part of this chapter,

prepared by Dave Jones, KB4YZ,

describes slow-scan television

(SSTV). Instead of full motion

video, SSTV requires a few

seconds per picture. SSTV is a

narrow bandwidth image mode

that is popular on the HF bands

using SSB voice transceivers.

SSTV operation can take place

on FM, repeaters and satellites

too. Unless otherwise noted,

references to other chapters refer

to chapters in the print version of

the

ARRL Handbook.

1.1 ATV Activities

Amateurs regularly show themselves in the shack, zoom in on projects, show home video

recordings, televise ham club meetings and share just about anything that can be shown live

or by tape (see

Figs 1

and

and application notes at

www.hamtv.com/info.html).

Whatever

the camera “sees” and “hears” is faithfully transmitted, including full motion color and sound

information. Computer graphics and video special effects are often transmitted to dazzle

viewers. Several popular ATV applications are described in detail later in this chapter.

1.2 Comparing Analog AM, FM and Digital ATV

Receiving ATV using any of the three ATV modes is relatively easy using consumer

televisions and receivers directly, or with the addition of an amateur band receive converter

(downconverter). Much of the activity in an area depends on the first few hams who experiment

with ATV, and on the cost and availability of equipment for others to see their first picture.

Most older analog-only TVs that do not have active channel memory scan setup and

Image Communications

Fig 3 — When the info or display button is pushed on newer digi-

tal TV’s remote control, the D-CATV channel number may be a

Virtual Channel designation from 1 to 999 (and not the actual

transmitted channel programmed in the DTV modulator), along

with the station call and other information. Channel 59.1 is

shown, but the actual transmitter channel was 58 and does not

change if the frequency is changed. Analog channels are shown

without the decimal point or with a decimal point or dash and a

zero. Digital channels are shown with a major number, decimal

point or a dash and a minor number. The minor numbers indicate

that this channel may have more than one video or program.

Fig 1 — Students enjoy using ATV to com-

municate school to school or between

classrooms (top). The ATV view shows the

aft end of the Space Shuttle cargo bay

during a mission (bottom).

Fig 2 — The one-way ATV DX record is

held by KC6CCC in San Clemente,

California, for reception of 434-MHz video

from KH6HME in Hawaii during a tropo

opening in 1994. The distance is 2518

miles. See www.hamtv.com/atvdxrecord.

html.

don’t have video squelch that goes to blank

or blue screen when no signal is present are

preferred by ATV DXers as they can continu-

ously search or monitor for ATV signals in

the snow easily. Newer TV’s have an active

channel scan setup that searches for analog

and digital channels and skips those that are

found not to be active. If you have cable, most

likely it will have active cable channels 57-60

Image Communications

(direct 70 cm ATV), 3 and 8 (downconverter

output) and be available for use on ATV if

you have done the setup scan while connected

to your cable service. If you do not have a

cable service, your TV may or may not let

you manually add a channel if no signal is

present. You may have to get a local ATVer

to send you a signal that is strong enough as

you do the channel scan. This will also be

the case with over the air digital 8-VSB and

cable QAM transmissions.

The frequency reference for an analog AM

transmission is the video carrier which is

1.25 MHz up from the lower channel

edge. A digital transmission is referenced

as the center frequency — for instance,

cable channel 57 AM is 421.25 MHz, digi-

tal is 423.0 MHz in the same 420-426 MHz

range. Note that D-CATV stations can pres-

ent as Virtual Channels or Logical Channel

Numbers which may not be the actual trans-

mitted channel when detected by the TV

setup channel scan. If your TV info overlay

says you have a valid digital channel 59.1, it

could actually be transmitted on a different

channel. The station ID, channel and info are

transmitted with the video and any text can

be programmed into the modulator as seen in

Fig 3

for whatever purpose the cable opera-

tor has.

Analog AM ATV on the 70 cm band is the

easiest mode. The 70 cm ham band contains

the same frequencies as cable channels 57

through 61, so a TV that has an analog cable

tuner can be used for receiving ATV signals

as well. Simple 70 cm ATV transmitters are

available that are low cost, small, and draw

less current than the other ATV modes. This is

desirable for R/C (radio control), high altitude

balloon, rocket and portable emergency com-

munications or public service events. Call

letter IDs with 70 cm AM ATV can be seen

at much greater range, although with a lot

of snow in the picture, and is the favorite of

DXers. Adding a linear amplifier requires

an initial drive setup so as to keep the video

modulation within the linear range.

FM ATV also uses simple transmitters, but

most activity is on the 13 cm, 23 cm and 33

cm amateur bands. This is in part because

of the wide occupied bandwidth needed for

FM ATV signals, and in part because of the

availability of receivers originally designed

for commercial C-band satellite or unlicensed

Part 15 use. The FM receivers have analog

A/V outputs that must be connected to a video

monitor and not a channel 3 analog RF modu-

lator to a TV in order to see the best picture

resolution possible. Less care need be given

to driving power amplifiers than when using

AM or digital modulation.

Digital ATV (D-ATV) signals using 8-VSB

or QAM, can also be received simply by using

new televisions in the same manner as ana-

log cable televisions, but the transmitters are

complex and currently expensive. Quadrature

Phase Shift Keying (QPSK) may also be used

but it requires a receiver box that is more

common in other parts of the world to receive

Digital Video Broadcast — Satellite (DVB-

S). D-ATV is still an emerging technology

among hams who like to experiment. The

Fig 4 — The Drake DSE24 D-CATV 64-QAM

modulator used for cable TV can be adap-

ted for ATV with the addition of linear am-

plifiers. Maximum output is about –8 dBm

which can drive high linearity MMIC,

MOSFET and LDMOS amplifiers to usable

power levels for ATV on the 70 cm and

33 cm bands. Transverters can be used for

operation on higher frequency bands.

cost of board level components and software

is coming down and there are cable head-

end D-CATV modulators using the North

American ITU-T J.83B protocol version of

Quadrature Amplitude Modulation (QAM),

such as the Drake DSE24, shown in

Fig 4,

that are available for about $1000 and can be

used directly on the 70 cm or 33 cm ham band

with the addition of very linear amplifiers.

Digital transmission requires an MPEG-2

compression and digital transport encoder

and a digital RF modulator/exciter. 8-VSB

is an amplitude-modulated, 8-level baseband

signal that is processed and filtered to occupy

5.38 MHz bandwidth. This signal fits in a

standard 6-MHz channel with guard bands.

D-CATV QAM also fits in a standard 6-MHz

channel. RF amplifiers for DTV are more

critical as to drive level, linearity and low

intermodulation distortion than with analog

AM or FM ATV modes.

A good reference on the technical char-

acteristics of an ATSC 8-VSB transmission

can be found in an article by David Sparano

entitled “What Exactly Is 8-VSB Anyway?”

and available online from

www.broadcast.

net/~sbe1/8vsb/8vsb.htm.

8-VSB and QAM

can have a little higher resolution (Fig

than AM because they do not have a sound

subcarrier that limits the video bandwidth.

Information on QAM can be found at

www.

ni.com/white-paper/3896/en.

D-ATV using

QPSK resolution varies trading off with band-

width, data rate, bit error correction and other

factors.

PICTURE QUALITY

Experimentally, using the US standard,

FM ATV gives increasingly better picture-

to-noise (snow) ratios than AM analog ATV at

receiver input signals greater than 5 µV. That’s

also about the signal level for the 8-VSB DTV

“cliff effect” where the signal disappears. The

DTV all-or-nothing cliff effect occurs because

the digital signal detector and processing in

the receiver require a signal-to-noise ratio

(SNR) of at least 15 dB for 8-VSB. Above

15 dB, you get an excellent picture, and at

14 dB SNR — nothing. Other DTV types —

D-CATV QAM or Digital Video Broadcast

— Satellite (DVB-S) QPSK — are a few dB

better in the presence of noise.

Because of the wider noise bandwidth and

FM threshold effect, AM analog video can be

seen in the noise well before FM and DTV.

For DX operation, it has been shown that AM

signals are recognizable in the snow (noise)

at up to four times (12 dB) greater distance

than FM or 8-VSB DTV signals, with all

other factors equal. Above the FM thresh-

old, however, FM rapidly overtakes AM. FM

snow-free pictures occur above 50 µV, or four

times farther away than with AM signals. The

crossover point is near the signal level where

sound and color begin to appear for AM, FM

and 8-VSB systems. 64-QAM pops up snow-

free at 20 µV.

Fig 6

compares analog AM, FM

and digital ATV picture quality levels across

a wide range of received signal strengths.

Fig 5 — These two images compare P5 test patterns from an AM analog transmitter and

a D-CATV 64-QAM transmitter. The 4.8 MHz multiburst vertical lines can be seen in the

64-QAM 480i picture ,but the AM is rolled off to grey and is limited to resolving the 3.8

MHz burst lines

1.3 How Far Does ATV Go?

Fig 6 — Four approaches to ATV receiving. This chart compares AM, FM and two digital

ATV modes as seen on a TV receiver and monitor. Signal levels are into the same

downconverter with sufficient gain to be at the noise floor. The FM receiver bandwidth

is 17 MHz, using the US standard. The straight vertical line for DTV around –93 dBm for

8-VSB and –80 dBm for D-CATV 64-QAM illustrates the cliff effect described in the text.

QPSK DTV will be somewhere between –80 and –93 dBm depending on the selected

bandwidth and other modulation factors.

The theoretical snow-free line-of-sight dis-

tance for 20 W PEP 70 cm analog AM ATV,

given 15.8 dBd gain antennas and 2 dB of

feed line loss at both ends, is 150 miles. (See

Fig 7)

In practice, direct line-of-sight ATV

contacts seldom exceed 25 miles due to the

curvature of the Earth with hills and buildings

Image Communications

P5 — Excellent

Fig 7 — This graph shows the possible line-of-sight distances for P5 (snow free) video

reception for various analog AM ATV transmitter levels in the 70 cm band. Power levels

shown are in PEP. The Total Gain is calculated by adding the antenna gain (dBd) for

both the receive and transmit antennas and then subtracting the feed line loss (in dB)

at both ends. For other bands: 33 cm, subtract 6 dB; 23 cm, subtract 9 dB; and 13 cm,

subtract 15 dB. For FM ATV (4 MHz deviation, 5.5 MHz sound), add 12 dB. For ATSC

8-VSB digital TV, the sudden loss of picture “cliff effect” distance is found by adding 26

dB. If the noise figure of the first stage in the downconverter is greater than 2 dB, sub-

tract for each dB over 2. See the example in Table 1.

P4 — Good

Table 1

ATV DX Graph Example

Transmit antenna

Receive antenna

Transmit feed line

Receive feed line

6 dB noise figure

+10 dBd

+12 dBd

–1 dB

–2 dB

–4 dB

Total Gain for 70 cm is 15 dB

With 20 W PEP, range is 35 miles (Fig 7)

For 23 cm (–9 dB) gain is 6 dB

With 1 W PEP, range is 3 miles (Fig 7)

blocking the direct path. Longer distances

are possible with over-the-RF-horizon tropo

openings, reflections, or through high hilltop

repeaters. A 2518-mile reception record is

shown in Fig 2. (See the

Propagation of

Radio Signals

chapter.)

The antenna is the most important part of

an ATV system because it affects both receive

and transmit signal strength. For best DX, use

low-loss feed line and a broadband high-gain

antenna, up as high as possible.

A snow-free, or “P5,” picture rating (see

Fig 8)

requires at least 200 µV (–61 dBm)

of signal at the input of the analog AM ATV

receiver, depending on the system noise figure

and bandwidth. The noise floor increases with

bandwidth. Once the receiver system gain and

noise figure reaches this floor, no additional

gain will increase sensitivity. At 3-MHz band-

width the noise floor is 0.8 µV (–109 dBm)

at standard temperature in a perfect receiver.

Image Communications

Analog TV luminance bandwidth starts roll-

ing off around 3 MHz to separate it from the

color subcarrier at 3.58 MHz. If you compare

this 3 MHz bandwidth to an FM voice receiver

with 15 kHz bandwidth, there is a 23 dB dif-

ference in the noise floor.

Much like the ear of an experienced SSB

or CW operator, however, the eye can pick

out sync bars in the noise below the noise

floor. Sync lock and large, well contrasted

objects or lettering can be seen between 1

and 2 µV with AM ATV. Color and subcar-

rier sound come out of the noise between 2

and 8 µV depending on their injection level

at the transmitter and characteristics of your

TV set. For the ATV DXer, using an older

analog TV that does not go to blue screen or

go blank (like a video squelch) with weak

signals is a must, especially when rotating

the antenna for best signal.

Operators must take turns transmitting on

the few available channels. Two meter FM

is used to coordinate ATV contacts, and the

2 meter link allows full-duplex audio commu-

nication between many receiving stations and

the ATV transmitting station speaking on the

sound subcarrier. This is great for interactive

show and tell. It is also much easier to monitor

a squelched 2 meter channel using an omni-

directional antenna rather than searching out

each station by rotating a beam. Depending

on the third-harmonic relationship to the

video on 70 cm, 144.34 MHz and 146.43

MHz (simplex) are the most popular frequen-

cies. The 2 meter audio is often mixed with

the subcarrier sound on ATV repeater outputs

so all can hear the talkback.

P3 — Fair

P2 — Poor

P1 — Barely perceptible

Fig 8 — An ATV quality reporting system.

Image Communications.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: