squarewave

¿FM without RF?
« on: February 27, 2020, 05:44:28 PM »
Consider using high frequency modulation to transmit multiple channels of audio over a single 50 ohm coax cable.

How would distortion / noise compare to a conventional line driver / receiver?

No, I haven't been drinking!


ruffrecords

Re: ¿FM without RF?
« Reply #1 on: February 27, 2020, 06:10:39 PM »
Also known as frequency division multiplex (FDM). Used in the not so distant past for transatlantic phone cables I believe.

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Ian
www.customtubeconsoles.com
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'The only people not making mistakes are the people doing nothing'

JohnRoberts

Re: ¿FM without RF?
« Reply #2 on: February 29, 2020, 09:05:08 AM »
Back several decades ago I investigated piggybacking an RF modulated audio signal on mains wiring, for a fixed install application. If it worked it would have saved a lot of labor and wiring to distribute sound in existing offices/factories. I eventually abandoned the project because the sound was too easily corrupted by machinery sharing the same power circuits.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

squarewave

Re: ¿FM without RF?
« Reply #3 on: February 29, 2020, 11:02:40 AM »
The first question would have to be if the noise / distortion is low enough. With enough bandwidth could it rival a regular line driver? If yes, then ...

Imagine a floor box that sends / receives a few channels over one relatively cheap quad shield coax cable. You could deliver pretty decent power through a simple RCL filter.

There is the obvious benefit of increasing distances without adding delays. Imagine an aux send to outboard gear in another studio across the street or maybe even down the road.

If analog gates are used for switching, would they not provide some immunity to the usual problems of noise / distortion / frequency loss?

abbey road d enfer

Re: ¿FM without RF?
« Reply #4 on: March 02, 2020, 10:47:38 AM »
The first question would have to be if the noise / distortion is low enough. With enough bandwidth could it rival a regular line driver? If yes, then ...
A good FM link is capable of 80dB S/N and 0.1% THD, so it's not as good as copper but good enough in most cases.

Quote
If analog gates are used for switching,
For switching what? Do you mean the RF signal? Beware that impedance has to be preserved. You don't want standing waves in your cable.

Quote
  would they not provide some immunity to the usual problems of noise / distortion / frequency loss?
I don't follow; why would including a lossy element improve quality? Signal degradation in an RF link comes mainly from the mod/demod process and heterodyning when signal level is correct.
RF relays are commonly used, cost about $5.

Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

gyraf

Re: ¿FM without RF?
« Reply #5 on: March 02, 2020, 10:49:24 AM »
..MADI will run many audio channels over a 50 Ohm coax..

/Jakob E.
..note to self: don't let Harman run your company..

abbey road d enfer

Re: ¿FM without RF?
« Reply #6 on: March 02, 2020, 11:07:06 AM »
..MADI will run many audio channels over a 50 Ohm coax..

/Jakob E.
Indeed, as Dante, AVB or UMAN, but squarewave seems to be concerned with latency.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

Re: ¿FM without RF?
« Reply #7 on: March 02, 2020, 11:39:10 AM »
Consider using high frequency modulation to transmit multiple channels of audio over a single 50 ohm coax cable.

How would distortion / noise compare to a conventional line driver / receiver?

No, I haven't been drinking!
So you would use direct baseband FM. That is quite difficult because in order to have the large excursion that is required for good S/N, you have two choices:
Either you use a carrier at low frequency, typically about 1-10MHz, but the process of direct frequency modulation over an excursion that makes about 1-10% of the carrier is not very linear
Or you use a carier at about 100MHz, but then direct demodulation is extremely difficult.
Typical RF links use multipliers (transmitter) and heterodyning (receiver).
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

squarewave

Re: ¿FM without RF?
« Reply #8 on: March 02, 2020, 12:25:26 PM »
If it were possible to use an IC for the important bits, could it be reduced to a bunch of tiny chip inductors and ceramic caps, an op amp or two, a specialized IC and a driver amp? That would be a lot easier and cheaper than MADI. Although if it's only 80dB S/N that's a drag.

For a concrete example, how do you think SA602A could perform as a receiver?

  https://www.nxp.com/docs/en/data-sheet/SA602A.pdf

If you can't tell, I know nothing of HF signal processing. I'm just spit-balling.

abbey road d enfer

Re: ¿FM without RF?
« Reply #9 on: March 02, 2020, 12:56:48 PM »
If it were possible to use an IC for the important bits, could it be reduced to a bunch of tiny chip inductors and ceramic caps, an op amp or two, a specialized IC and a driver amp? That would be a lot easier and cheaper than MADI. Although if it's only 80dB S/N that's a drag.
That would be for a super-duper implementation; typical chip solutions quote 65dB S/N and 1% THD.

Quote
For a concrete example, how do you think SA602A could perform as a receiver?
This is just the RF end, you need an IF strip and demod. The app shows a 455kHz output so it looks more like an AM system or low BW FM.

Ther used to be many chips doing the whole shebang from antenna to audio output that had pretty good performance. I don't think they exist anymore.
In order to get an idea of possible directions, check the documentation for TDA7021, the new generation of integrated receivers that don't use heterodyning, and a very modern implementation of a classical solution, the Si4836-A10.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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