can you send video over hf?
Hi,
Good idea. Call it "TV". Analog TV.
Klaus*
i mean does high frequency radio waves have enough bandwidth to send a live video, not a recording, without satellites or relay stations, over a distance of say 600 miles?
Hi,
TV is video.
It does not matter wheter live or recorded. Via sattelite, via relay stations or direct.
Klaus
so this means, there is no reason high frequency radio does not have enough bandwidth to send live video?
Hi,
Again a repeated question.
Counterquestion: Does analog TV exist?
Simply accept it.
Klaus
There is slow-scan tv. Frames are smaller, and take a few seconds to transmit each frame.
To go 600 miles distance, consider using a shortwave frequency at night because that is when those frequencies bounce off the ionosphere.
so you are saying that if analog television exists that means high frequency radio does have enough bandwidth to send live video? and the only reason you cannot send live video over high frequency is because the fcc does not authorize an emission mode for high frequency radio? when i say high frequency i mean 30 megahertz to 3 megahertz.
Not that I'm certain but...
A circuit transmitting at a very high frequency only needs to modulate a tiny percentage when you incorporate complex information (such as a tv image).
However say you transmit at a low frequency. Then the circuit is required to modulate several percent. It's difficult to design such circuitry so it is sufficiently versatile, and to adjust it so it runs in a stable manner.
Likewise for the receiving unit. Its circuitry must not be taxed by wide modulations in the broadcast. Therefore its job is easier if the frequency modulates by only a small percent.
Shortwave radio starts just above commercial AM radio at 1.7 MHz. Once commercial AM stations were established (and regulated), the shortwave spectrum became the international place for individuals (amateur and expert). The FCC made a few rules and guidelines particularly for US transmission. However seeing that shortwave is worldwide the FCC doesn't necessarily govern everything that goes on.
to 30MHz spectrum .
Unlikely, please review your specification. You mean 3 GHz?
There's sufficient bandwidth for analog video in the 2.4 GHz band, but as you have to share the band with different services, it's unlikely to achieve good quality.
Please take also a look at this recent thread about RF video transmission https://www.edaboard.com/showthread....smitter-for-TV
No, he is talking about short wave radio in all his threads.
Even if we ignore regulations, and even if we have a theoretical transmitter and antenna that could theoretically transmit analog TV on short wave, that covers a lot of bandwidth in the short wave spectrum, several MHz. And the signal path attentuation over that bandwidth might be very different.
If you want to make it realiable over 600 miles, without having large antenna towers on the receiver side, that is difficult even for normal audio signals. Look at the large transmit sites for RF radio. And it gets much more difficult for TV. I don't think that will ever work, even if you invest millions of dollars into transmitter equipment
I recommend reading a book on short wave communication first, that will answer many questions.
Thanks, somehow I still expect technically plausible descriptions like 30 megahertz down to 3 megahertz, or 10 m to 100 m wavelength.
The answer for HF band is simple, you can't send analog video with its several MHz bandwidth.
As an RF Engineer, I can absolutely back up what you say FvM ... aside from the issue of hogging bandwidth used by everyone else within that 600 miles radius (and probably beyond), it simply isn't feasible to transmit a normal TV signal on the HF band using conventional techniques on that band, which is why old-fashioned Analogue TV signals were sent at UHF, where the bandwidth they occupied is a relatively small fraction of the carrier frequency. That makes modulation, filtering, spectral efficiency and transmitter antenna design a lot easier, as well as less antisocial!
There are some techniques to compress digital video to smaller bandwidths that can allow transmissions of digital video at VHF and UHF that are used by amateur TV hobbyists - but not as a 24/7 spectrum hogging broadcast, only for limited experiments.
The HF band (3-30MHz) is great for narrowband long distance communications using the ionosphere to bounce around the world, but can be lousy for reliable, repeatable point to point propagation, in particular above say about 10MHz (the ionosphere varies a lot with time of day, etc) - and fading would also be an issue for signals with any significant bandwidth.
If you want to communicate occasional Video signals over that sort of distance, you'd be better off using satellite communications such as Oscar 10, if you are in its footprint (that one doesn't cover USA, but covers a huge area over the surface of the Earth including Europe and most of Asia - with none of the skip zones you'd get with HF communications). It can also be used for narrowband comms.
I think dl09 is not understanding the difference between frequency and bandwidth.
Frequency is a single point in the radio spectrum, it could be 1.2345KHz, 1.2345MHZ, 123.4MHz or 123.4GHz or any other fixed number you can think of.
Bandwidth is at least two frequencies and all the frequencies between them, a section of the radio spectrum rather than a single point.
The only method of using a single frequency is to turn a signal at that frequency off or on, similar to Morse code so it is very slow to send any information.
All data, voice or vision signals require a wider bandwidth, they still have a center frequency but they occupy other frequencies around it as well. The distance from the lower edge to the higher edge is what we call the transmission bandwidth and it varies from a few KHz for voice up to many MHz for high speed data. Video lies somewhere in the middle, around 4MHz for 'VHS' quality up to around 21MHz for high definition pictures.
It follows that you can't use a wide bandwidth at a low frequency, there isn't enough space for it. This is why domestic broadcasts use the lowest practical frequency that is usable. AM radio has the poorest quality and has a bandwidth of about 9KHz so it is used on low frequencies, typically from around 450KHz up to 30MHz, FM domestic broadcasts are higher quality and have two channels (stereo) and are used on VHF, typically 85MHz - 110MHz and televison, which needs much wider bandwidth from about 450MHz and up.
Brian.
As Brad mentions, there is slow scan TV. I once witnessed SST with an acquaintance who was a radio amateur.
Slow scan meaning something like 0.3 FPS.
Low definition TV also. Perhaps QVGA quality at best, 320X240 pixels.
Ah! and monochrome only.
Early broadcast television done by RCA in 1936 used 343 lines/30 frames, with 1.5MHz video bandwidth.
The frequency bands for the public broadcast were between 42 MHz and 80 MHz, but the very first experiments (done before 1936) were made in 20 MHz band.
https://www.radiomuseum.org/r/rca_rr_359rr35.html
So when you start transmitting narrow video bandwidth TV signals in HF frequencies, let me know to see if I can demodulate your signal
Current color TV with low bit-rate H264 encoded video is being sent regularly in a 1.5MHz bandwidth on VHF. Stereo hi-quality audio is sent commercially using DRM as low as 603KHz and various short wave frequencies (see https://drm.org). Even encoded voice signals (DV mode) can now occupy under 1KHz.
SSTV typically requires about 2.5KHz but needs around 15 seconds to scan a single frame at low resolution.
All require special digital compression and encoding though, I don't think they qualify for defining normal bandwidth requirements.
I've got all the modes here!
Brian.
did the very first experiments in 20 megahertz send live video? could they have sent live video?
Of course, was live transmission.
All the TV transmissions until early 50ties were live transmissions, because before that time were no video recorders..