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Audio levels for digital modes on FM
Some
knowledgeable ATV Speak
SSTV Repeater
Australian Analogue Television Broadcast Standards
Australian Digital Television Broadcast standards
Audio levels for digital modes on FM (PDF)
Audio levels for digital modes on FM
Analogue SSTV modes are generally tolerant of incorrect audio levels. The reason this is so is that only one audio frequency is generated at any one time. The optimum audio level into the Transmitter is just a little below what would give maximum deviation (about 3 kHz is good). This is the balance between there being insufficient audio recovered by the receiver and added noise due to the receiver IF filter removing some of the significant RF sidebands. Inputting excessive audio into the transmitter in most cases does not significantly degrade the result. Digital modes using AFSK (Audio Frequency Shift Keying) are similarly tolerant. The reason for this is that the only distortion products are audio harmonics which are outside the receiver band-pass.
The newer digital modes are not nearly as tolerant. The popular DIGTRX program uses 8 simultaneous audio carriers and each of them uses Phase Shift Keying. To say the least the resulting waveform is quite complex. When there is distortion on such a signal there are not only harmonics but also intermodulation products. These IMD (Intermodulation Distortion) products fall inside the audio band being decoded. IMD increases rapidly as transmitter audio input is increased beyond a threshold. The result is the loss of data as the decoder at the receiver struggles to recover the original data that was sent. A degree of error detection and correction is available as an inbuilt part of the program (this is because the program can be also used on HF). However it is best to minimise any errors in the first place; this also reduces decoding times.
Having access to calibrated test equipment is ideal for setting up the best levels but not many of us have this to hand. all testing is based on comparisons with a reference. The reference we all have available is the un-muted (un-squelched) noise from an FM receiver. Proceed as follows:
Set the volume of your receiver to a comfortable level so that the audio is not distorted on the loudest audio you receive (ie from the guy with the loudest audio). Then with no RF received signal. listen to the un-muted noise. Check that this is also not too loud. Now feed this signal into your sound card (line input). Run DIGTRX and note the receive level indicator in the program. Go to "setup", "Sound card", "Sound Card input control RX" and set the slider for the selected input so that the RX level indicator is as close as possible to 0 dB. You have now set up your receiver and should not change these settings.
Get another station to send to you. Initially the 2 tone test. If the level received is greater than -10 dB (i.e. -7 or such) then the other station is sending too high a level. Have a look at the left hand side of the Scope screen and check the IMD figure. To have any chance of decoding easily this needs to be lower than about 10 dB. A small reduction in transmit audio input from this point will result in a rapid improvement of the IMD to -20 dB or better. This will ensure very accurate decoding. Note that the received level should not disappear entirely (somewhere between -10 ans -20 dB ia a fair level although even down to -30 dB seems to work well). Remember at this stage do NOT change any settings at the receiver end. Only adjust the transmitter end. The transmitter settings are accessed in DIGTRX from "setup", "Sound card", "Sound card output control TX". Adjust the sliders for "Wave" or "Volume control".
Now get the station to send a file or picture. Watch the waterfall display for the 8 PSK carriers. When the carriers are seen check the receive level. It may be slightly higher than the previous 2 tone test but this is OK. If it peaks above about -10 dB you will probably be getting some IMD though the figure in the IMD box is not a reading of this signal.
The procedure above is just one way to set up levels for DIGTRX transmission, getting the IMD good on the 2 tone test is really what matters. Good luck!
Bruce VK4EHT
Some knowledgeable ATV Speak
And in just case you haven't had enough of ATV, here is some more, but this time
some technical stuff to think about.
We keep referring to 444.25 MHz ATV living on channel 16 which isn't really
correct,
because you see there really isn't a channel 16.
So how come you can see pictures there ? well if you can you are watching the LF
end of the UHF band on your tuner, told you it was going to technical.
However, Gary VK3KHB has kindly offered his advice to put us straight on a few
misconceptions, and also add to our ATV knowledge bank.
Channel 16 is not the official ATV channel number, it doesn't really have one. It's just that people got so used to a knob with Channel numbers, that for some
odd reason they still need one, even though modern TV sets no longer have knobs
or preset TV channels.
You now "scan" for TV signals, so when you get your new TV set, you
ask Mr Antenna or your six year old Poke'mon expert to set the Channels
all up for you.
ATV became identified as Channel 16 if you count backwards in 7Mhz steps from Channel 28. Or to put it another way, from the VHF end you
started at Channel 12 VHF and count upwards in 7Mhz to Channel 43/44.
Confused ?
well who isn't ?
And so, to help reduce confusion (as if we can) we adopt Channel 16 as it
is not clearly defined on any TV sets, and also counting backwards seem to
make better sense for a UHF TV signal.
And now for Bandwidth of Antennas; the common commercial TV antenna is very
broad, around 100Mhz bandwidth per TV band.
VHF Lo, VHF Hi and UHF band 4 all span 100Mhz, so your common TV antenna,
if it can receive all bands has 3 groups of 100Mhz bandwidth.
Now while ATV is outside that designed TV bandwidth, some designs do enhance the
LF end of the UHF part of the antenna, hence why some people can receive ATV
with a
"Cable ready TV set"
So if your new TV handbook states that it is "Cable ready" then it has
a very good chance of being able to pick up ATV.
"Cable Ready" usually means continuous coverage from 45Mhz through to
800Mhz. and it's continuous so that you can receive the cable TV signals.
As "Cable TV" is a captured spectrum, inside a coaxial cable, it can
use all of the frequencies for TV, but because the cable TV companies in OZ want
you to pay per view, you still need that "set top box" to convert the
modified TV signal to a watchable format.
Even the audio carriers are moved, however, their technicians do have a few un encoded Channels not receivable with the set top box, but Receivable by your
cable ready TV set... Something for cable TV subscribers to hunt for.
And so, if you happen to know how, it does enable you to become a pirate cable
TV station within your own street or local cable branch.
And if some of that was not too clear to you, we'll try to explain it again next
time when we meet on Channel 16.
(Gary VK3KHB/Keith VK3JNB)
John VK4ET is operating an
experimental SSTV analogue repeater on
7.055 MHz.
His location is at Brackenridge. The power output is
100 watts and his antenna is
single a 1/2 wave sloper at about 13 meters in height. Operation times are 9AM
to 9PM daily. His beacon is transmitting at 30 minute intervals.
A 1750 Hz tone is required to access his repeater. Pictures are to be sent
within 10 seconds from his repeaters reply.
Current indications are that good signals are being received within the Brisbane
area to within a 30 km radius on ground
wave from his location and encouraging signals by sky wave from further beyond.
Operating times are 9AM to 7PM daily (storms permitting).
Information regarding this repeater is subject to change. Times and other data may change without notice.
Trevor VK4ZU's website
VK4ZU December 2005
Australian Analogue Television Broadcast Standards
VHF PAL B
UHF PAL G
7 MHz wide channels, 4:3 aspect
ratio.
625 lines and a field frequency of 50 Hz, colour subcarrier is 4.43361875 MHz,
phase alteration line (PAL).
Australian Digital Television Broadcast Standards
DVB-T transmission standard
Modulation 64-QAM
Code rate 2/3
C/N 20db
Guard Interval 1/8
Carrier Mode 8k
Australian receivers are capable of 1705 carriers (2k mode), and 6817 carriers
(8k mode).
Standard Definition Digital TV
576 lines x 720 active pixels @ 50Hz interlaced (576i).
MPEG-2 digital stereo sound
Widescreen 16:9 aspect ratio
High Definition Digital TV
Australian broadcasters are currently using three different levels:-
1440 active pixels x 1080 lines @ 50Hz interlaced
1280 active pixels x 720 lines @ 50Hz progressive
720 active pixels x 576 lines @ 50Hz progressive
Widescreen 16:9 aspect ratio
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