gary wrote:Well that's not true - as far as I am aware SSTV does not normally exceed 3kHz and could feasibly be much lower, whereas 32 line 12.5 f/s NBTV requires 10kHz.
I stand corrected there must be a little bit of difference between the many different modes well thats the case i will look more closely at the transducers but i think i should not blame them just yet as theres a bit of Harry electronics between so i know who i'd blame first ....
!
Well yes you will hear the characteristic 400Hz NBTV sound as that is within the 1kHz bandwidth. Although how it is being demodulated is beyond me - I read somewhere that the nonlinearity of the US passing through air can actually demodulate it - weird.
I don't see the sync that well on your software scopes but yes hearing it means little and really does so far trying to view it i see no hint of shapes out lines ,i was just thinking the sync was not making it .
I assume you say that because your approach is empirical rather than analytical - let me explain how I came up with those parameters:
OK my little 1980s xt processor in my head will try
The bandwidth required to transmit an nbtv signal of:
lines = n
aspect ratio = a
frame rate = fr
is given by:
bw = n*n*a*fr/2
so for NBTVA video bw = 32 * 32 * 1.5 * 12.5 / 2 = 9,600 Hz or 9.6 kHz (we normally call that 10kHz)
so for the parameters I gave you the bandwidth is:
16 * 16 * 1.5 * 4 / 2 = 768 Hz
Given that a narrowband FM signal bw is 2fm then 2 * 768Hz fits into the available bw of the transducer.
Oh i am glad you did that i would have no hope ...but i understand now the sstv fm is narrower than the NBTV.
The same formula above applies to sstv - now you know that a frame of sstv may take many seconds - lets say 10 seconds - so the frame rate is 1/10 = 0.1 frames per second.
Plug that into the formula above and you can see that the BW is very small indeed even for much higher resolution pictures.
Does that explain things?
Yes sir i can see they are not the same so its not so much a mystery due to them not using the same bandwidth .
But i will try a bit more as i said need to make sure its not the harry factor more than the transducers .
I note that you don't define NBTV - if you don't define it I will always assume that it is NBTVA 32 line NBTV.
Well yes 32 line standard .
Well I can certainly assure you that NBTV CAN be transmitted ultrasonically providing the carrier wave can be modulated/demodulated appropriately and that the bandwidth of the transducer is sufficient.
Yes my reasoning for trying Just using what i had handy ,i want to try to do it as it has my interest at the moment .
So you have to make a decision to either reduce the resolution and frame rate of the NBTV signal, or find a transducer with a wider bandwidth - they must exist surely!??? I can't imagine they would be cheap though.
I will look into that i have read of tweaters being used they are at about 2khz to 20khz perhaps that might be a better idea for this ..some thing to look into .
There is another, very complex alternative whereby you split the signal bandwidth over a number of transducers with different carrier frequencies - that would be fun! not!
Not fun ... but speakers and tweeters are used together for the same sort of idea one device has a frequency bandwidth they can put out and the other does what the other can't ...
Also you have to decide on the modulation scheme AM or FM - I would suggest AM as an FM demodulator is fairly complicated. Also you have the circuits available to you as you indicated.
Heres the thing i don't have a Fm demodulator for the sstv i would of thought it was a no goer as is but it works for Fm...but it must be doing it .
the receiver should be a Am receiver ... the transmitters are FM...i did try a little am modulation of the carrier on my first transmitter and i did get a result i was thinking it might have been stray RF the receiver was picking up but i need to redo the experiment .
As for the scanning... well you are pretty much on your own there - I see no end of physical problems no less than, as you mentioned, the focussing of the pressure wave without significant loss - my brain hurts thinking about it.
Oh yes...i think they use banks of transducers for sonar and ultrasound imaging i would not expect great results perhaps seeing shapes perhaps...... and you would be doing it blind really ...focusing and all.
I enjoy trying stuff even if i fail.
In addition, I should say there is no real similarity between transmitting an NBTV signal ultrasonically and scanning with ultrasound (other than the use of ultrasound). There is not much that you learn in the former that can be applied to the latter.
Its just an idea to try if things fall into place turn sound into light more than likely a poor version of a ultra sound in air but just an idea so far but who knows .
Hmmm I am not sure I am following you there - there has to be signal loss or there would be no image - when scanning an object ultrasonically you are more or less measuring the density of the object under each "sound spot" - that is a hard non absorbing material will reflect the sound with far less attenuation than a soft absorbing material - you won't be able to use the piece of paper with NBTV written in large letters trick.
Sorry gary when i had the idea it was point a transducer at a nipkow and do a sort of sound flying spot camera but i can see a lot of signal loss more than likely it would not work at all...... rather nothing in its way i was thinking perhaps a pointing the transducer at a polygon shape rotating sort of like my laser nbtv octagon project a while back and do a sort of sound version instead of mirrors just a flat reflecting surface for the sound to bounce off.
Have the receiver and transducer near the object being scanned just a sound fly spot scanner idea really...works in my head but thats my head OMG !
Finally, given the average age of the NBTVA membership I am not entirely sure that your "ultrasonic" transducer couldn't just be a reasonably good quality speaker. For instance here is a full NBTVA BW sweep modulated up to 13kHz - I can just barely hear the low frequencies.
I can hear one fourth of it rest is beyond my big ears !
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.