Can two DSB transceivers communicate?
What if I scramble (frequencies inversion) the transmitted audio in one of the two transceivers, and tune the other a bit out of frequency, so that the upper sideband of one is at the same side as the lower sideband of the other?
Will the other DSB transceiver be able to recover the audio ok then?
What do you mean frequency inversion ?
What is the inverse of 3.25 Khz ?
Surely DSB is AM with the carrier filtered out. Reinserting the carrier at the receiver, would fool the envelope detector in to thinking it got ordinary AM?.
Frank
It certainly would.
But the problem is that the reinserted carrier has to not only be at the right frequency but the correct phase. otherwise the two sidebands do not add to reproduce the original modulation.
With SSB, only the frequency need be correct, or nearly correct.
DSB is far less forgiving.
The usual technique is to filter out only one sideband in the receiver, and then treat it as SSB.
If you try to do it with both sidebands, all you get is an unintelligible distorted mess.
What you describe is the usual problem of why two DSB transceivers cannot communicate directly, but only to other SSB transceivers (that are tuned either on USB or LSB).
The problem is that the USB and the LSB interfere with each other at the receiver so that the detected signal cannot be recognized.
What I propose, is to modulate transmitter A, with a scrampled audio (the lower frequencies become higher and the higher lower, based on an audio carrier) eg. this ../imgqa/eboard/Antenna/rf-adkzciiqez3.png
In the RF domain, the USB of A will still be moved away from the carrier as the audio frequencies get higher in frequency, but now these audio frequencies carry an inverted audio signal.
Then receiver B will receive the A signal and in the RF domain, it will be tuned so that the B carrier is below the supposed original A carrier. This will make the USB of receiver B to lie on the frequencies of the LSB of the transmitted signal of the A.
After detection, the recovered audio will be scrambled and it needs to be discrambled (re-inverted) so that the audio is recognized.
All that will do is invert the sideband so that anyone with an SSB receiver just switches to the opposite sideband and offsets the carrier frequency by the amount of your "audio inversion gizmo".
O/k so you have an original two tone test signal 300 Hz and 1 Khz.
You feed that through your magic 5Khz audio inversion gizmo box so that the frequencies that come out are (say) 4.7 Khz and 4.0 Khz
If you modulate a 10 Mhz carrier you get
USB 10.0047 and 10.004 frequencies
LSB 9.9953 and 9.996 frequencies.
If you transmit just the USB I can receive that perfectly clearly if I use a receiver tuned the LSB but tuned 5khz higher
10.005 - 10.0047 = 300Hz
10.005 - 10.004 = 1Khz
The same trick works if you transmit on LSB, I just switch to USB and tune 5 Khz lower.
9.995 + 9953 = 300Hz
9.995 + 996 = 1Khz
You have scrambled nothing.
Either signal would sound perfectly normal, and anyone would never even realize it was supposed to be scrambled.
Thanks Tony, but my goal is NOT to scramble the conversation so that it cannot be heard.
My point is to be able to make two homemade DSB transceivers to communicate together, whereas they normally cannot, because of the opposite sideband problem at the reception.
One of them has to be tuned to an offset so thet the TX USB and the RX LSB lie together. But in that case you cannot recover the audio because you try to recover a USB signal on the LSB.
By the audio inversion, I try to propose a system (if it can be done) to do so.
For example:
DSB transmitter A, transmits a DSB signal, but the content of the sidebands (audio) will be scrambled.
Explaining only what happens for the USB sideband for the shake of the example, a 1KHz audio tone will appear 1KHz away from the suppressed carrier, whereas a 2KHz audio tone, will appear 2KHz away from the suppressed carrier.
If the audio tones are previously scrambled (inverted), prior to modulation, the 2KHz audio tone will appear closer to the suppressed carrier than the 1KHz, is that right?
If the above line is true, then the DSB receiver will receive the DSB signal and in the RF domain, it must be tuned so that the suppressed carrier frequency is above the suppressed transmitted carrier frequency. This will make the LSB of the receiver to lie on the frequencies of the USB of the transmitted signal.
The transmitter LSB and the receiver USB will never interfere, as they are apart, due to the offset of the receiver carrier (tuned frequency).
The transmitter USB will lie to the receiver LSB frequencies.
In a normal audio modulation this would be heard as trying to receive an USB signal on the LSB, but in our case, because the audio is inverted, a descrambler on the receiver can recover the original audio tones.
That is my idea, how does it sound?
After detection, the recovered audio will be scrambled and it needs to be discrambled (re-inverted) so that the audio is recognized.
I can see what you are trying to do, use one 'frequency' but the two sidebands independently. The first problem is the idea that inverting 2KHz might produce 1KHz displacement. You can't invert a frequency unless you analysed the original, did some math on it and recreated a spectrally inverted version of it. What you could do though is use two different carriers, spaced say 5KHz apart to generate two SSB signals, taking the LSB of the higher carrier and USB of the lower one. With sufficient filtering you could in theory restrain them into the bandwidth a single DSB signal mid way between them. I'm not sure it has any benefits though, to a receiver it would be like listening to 20m during an SSB contest but with statons using a mix of sideband modes, in other words, even more chaos than normal .
Brian.
Why Is that Brian?
and why is this a problem?
The only two ways to mirror a spectrum are:
1. Mix the base signal with a carrier and use their subtractive result (spectral inversion)
2. analyze the base signal (maybe using FFT) and synthesize a new signal based on ('maximum possible' - actual frequency).
Method 1 is similar to what I suggested, using a second carrier a few KHz higher in frequency than you want then modulating to SSB and filtering to accept only the lower sideband. The LSB will 'grow downwards' giving the same effect as reversing the audio spectrum when seen from the original carrier frequency.
Method 2 requires conversion of the audio into separate frequency and amplitude domains, then subtracting the frequency from a top limit and re-combining it with the original amplitude. Its a complex mathematical process but can be done if you have enough processing power.
For both methods you need to know the upper and lower ends of the audio bandwidth window and ensure no significant signals fall outside it. Its like saying you want reverse the order of only part of the frequency domain, you know you can't reverse ALL frequencies so you have to define cut-off points to work between. For example in method 2, you would have to specifiy say 4KHZ as the highest frequency so that 4KHz in becomes 0KHz out, 1KHZ in becomes 3KHz out, 2KHz stays the same, 3KHz becomes 1KHz and so on.
Brian.
The whole concept relies on filtering out one of the sidebands in the receiver so the other unwanted sideband does not interfere and mutilate our demodulated signal.
If you are going to do that (by whatever method) you might as well do it on the transmit side, generate SSB, and be done with it.
That has two advantages, transmission bandwidth will be halved, effectively halving the required transmitted power level for an equivalent receiver signal to noise ratio.
And the same oscillators and filters can be used for both transmit and receive, a very big advantage in a transceiver.
Transmitting and receiving on split frequencies makes it rather difficult to hear both sides of a conversation where there are multiple users needing to all be able to communicate with each other.
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