SSB Carrier Supression

[Mikebits]

I see what your saying and I am feeling your vibe, but it is not so straight forward as you imply.

Delays, delays and phase shifts. These all come into play. Far end sends. x msec delay to hit rx, Near end rcvs some x msec later. Rx signal propagates through RX unit, lets say 2 milli sec. Near end speaks 1 msec after far end sent. We have a 1msec error. Now how is near end to tell vox circuit to not send when far end sent 1 msec earlier. What you end up with is one sender is cutoff.


SenderA ----><-----SenderB ;collision, VOX is confused so clips sender or receiver.

It is hard to explain. I guess I am not doing good job.

 

[Mikebits]

I think I give up, and return this thread to the Varmint.

 

[xanadunow]

Sure Mike

I do feel that way too.. Just to answer your question - VOX is for the "lazy" people, I do rarely use it. You can "tune" the VOX/ANTIVOX to your content. You are looking at the "envelope" of two audio signals (mic and speaker) and making the decision to cut-out or not your vox (mic), and - the TX or RX action of your rig info, is also provided.

And PTT has this "retro' feeling I do like

xanadunow

 

[Nigel Goodwin]

Consider this. RX gets a signal in. ~~~ TX at the same time sends ~-~-

Now how is this VOX circuit going to differentiate between the two? Only with complex DSP is this going to happen.

It doesn't distinguish between them, and has no need to.

A VOX circuit transmits when you talk, it's up to YOU not to talk when someone else is - it's no different to not pushing the PTT when someone else is talking.

 

[xanadunow]

And the Antivox will give you the insurance that you would not "cut-in", unless ofcourse - you press the PTT.

 

[Nigel Goodwin]

And the Antivox will give you the insurance that you would not "cut-in", unless ofcourse - you press the PTT.

No, the anti-VOX is just to stop the speaker triggering the microphone.

 

[flat5]

I don't think milliseconds are part of the issue.
There are time constants to these circuits.

**broken link removed**

 

[Nigel Goodwin]

I don't think milliseconds are part of the issue.
There are time constants to these circuits.

As is usual with audio circuits, fast attack, slow decay - just like audio AGC.

 

[audioguru]

VOX?
The Motorola speakerphone ICs "sit in the middle of the road" so they receive a little and transmit a little at idle with the transmit and receive audio paths attenuated a little. When a strong fluctuating mic signal occurs then it applies full gain to the Tx and reduces the gain of the Rx. If the speech stops then it sits in the middle of the road again. When a strong Rx occurs then it switches the Rx to full gain and reduces the gain of the the Tx. Since it is sensitive only to a fluctuating mic input then it is almost immune to being switched by continuous noise.

The Rx and Tx acoustical paths must have attenuation so that a speaker output does not activate the Tx.
Speakerphones have an additional problem of the 4-wires to 2-wires hybrid cancellation which is not perfect since each telephone line has different impedances at different frequencies which causes some Tx signal to be received.

Full duplex over a long distance causes echoes if both ends have an open mic. Good quality speakerphones and video conferencing units use a sophistocated DSP echo-canceller circuit that makes a model of the entire long distance frequency response, delays and phases including a model of the acoustical conditions of the room it is in.
Then received sound from a speaker is cancelled from being re-transmitted back to where the speech came from.

 

[xanadunow]

No, the anti-VOX is just to stop the speaker triggering the microphone.

I can not see any contradiction here. The anti-vox will "cut-in" to cut out the mic as you said (and I).

 

[flat5]

Here is how Collins Radio described their circuit for the 32S-3/3A Transmitter (drool)

VOX and Anti-VOX Circuits

Output from second audio amplifier V1B is fed to the grid of VOX amplifier V14A through VOX GAIN control R74. This audio input is amplified by V14A and rectified by VOX rectifier V10B. When the positive output of V10B is high enough to overcome the negative bias on V11A grid, the VOX relay is actuated to turn the transmitter on. Receiver output is fed from J13 through ANTI VOX GAIN control R85 to the grid of anti-VOX amplifier V14V. Output from V14B is rectified by anti-VOX rectifier V10A to provide the negative bias necessary to keep the transmitter disabled during receive periods. The anti-VOX circuit provides a threshold voltage to prevent loudspeaker output (picked up by the microphone) from tripping the VOX circuit into transmit. ANTI VOX GAIN control R85 adjusts the value of the anti-VOX threshold so that loudspeaker output will not produce enough positive dc output from the VoX rectifier to exceed the negative dc output from the anti-VOX rectifier and cause V11A to actuate VOX relay K1. Speech energy into the microphone will cause the positive VOX voltage to overcome the negative anti-VOX voltage and produce the desired action of K1. Contacts of relay K1 control relay K2, key line, PA and driver screens, receiver muting circuits, and oscillator plate voltages.

 

[xanadunow]

Here is how Collins Radio described their circuit for the 32S-3/3A Transmitter (drool)

..provides a threshold voltage to prevent loudspeaker output (picked up by the microphone) from tripping the VOX circuit into transmit

Sorry, it is not the microphone "pickup" that decides on prevention of the VOX triggering, it must be some poor japanese translation to english. it is the treshold from the receiver's audio (speaker).

xanadunow

 

[xanadunow]

microphone just does not know the speaker (hardware) from the speaker (men)

 

[xanadunow]

I believe they did mean it right (at Collins) - just did not say it straight. I am Sorry for the "Japaneese" interlude (I have just seen to many of those)

 

[Nigel Goodwin]

I can not see any contradiction here. The anti-vox will "cut-in" to cut out the mic as you said (and I).

No, it doesn't 'cut out' the mike, it just prevents the mike picking up the speaker and switching to transmit.

Even if the speaker is quite happily playing away to itself, you speaking in the microphone will set it to transmit.

 

[flat5]

"The anti-VOX circuit provides a threshold voltage to prevent loudspeaker output (picked up by the microphone) from tripping the VOX circuit into transmit."

Is exactly what anti-vox is for
How it does this is not completely explained.
I believe attenuating the mic signal is the usual way.
To force (receive to) transmit you have to speak very loudly into the mic.

 

[xanadunow]

Would it not defeat the purpose of the ANTI-VOX?

There would be a number of solutions and as flat5 said. If you control the gain of the mic - then you take the chance that you will trigger the VOX but if you "cut it out" (the mic), it doesnt matter how loud would you want to speak to the mic.

 

[xanadunow]

the ANTIVOX is the "governer" of the VOX and the vox will only do what the antivox will let it to do..

 

[xanadunow]

Would it not defeat the purpose of the ANTI-VOX?

.. it doesnt matter how loud would you want to speak to the mic..

because you are receiving; the ANTIVOX - knows it.

 

[Nigel Goodwin]

Would it not defeat the purpose of the ANTI-VOX?

There would be a number of solutions and as flat5 said. If you control the gain of the mic - then you take the chance that you will trigger the VOX but if you "cut it out" (the mic), it doesnt matter how loud would you want to speak to the mic.

Way back near the beginning of this discussion I explained it, the speaker output is fed back in anti-phase to cancel the same signal from the microphone. This doesn't affect the normal microphone working in any way, and will switch to transmit when you talk, even if you are currently receiving audio from the speaker.