Old shortwave radios: coherent detection vs non coherent

[mr ed]

Been looking at some overpriced vintage sw receivers. Some boast coherent detectors. I know what coherent detection is ( phase and amplitude like  I Q , QUAM etc. )  So, how does this make mono audio better? Something to do with removing carrier noise and getting better signal to noise? 

[radiolistener]

I don't think that it can improve sound quality, it allows to receive DSB modulated signal with no carrier, which require much less power for transmission, but at the same time it requires very stable LO and very stable carrier of transmitter. As a result, this also requires the transmitter and receiver to remain stationary relative to each other to avoid the Doppler effect.

[antenna]

I don't think that it can improve sound quality, it allows to receive DSB modulated signal with no carrier, which require much less power for transmission, but at the same time it requires very stable LO and very stable carrier of transmitter. As a result, this also requires the transmitter and receiver to remain stationary relative to each other to avoid the Doppler effect.

You'd have to be going much faster than any automobile can go for doppler shift to make a difference at shortwave listening frequencies.  At 60mph (96.6kph), you will have a 1.16Hz shift at 13MHz.

[radiolistener]

at shortwave listening frequencies.  At 60mph (96.6kph), you will have a 1.16Hz shift at 13MHz.

this is enough to get 1 Hz fading for DSB. Here is example of 1 Hz offset with DSB demodulator.

[wofritz]

It can help receiving AM with carrier in situations with heavy selective fading by providing a stable carrier when the received carrier drops out.

[radiogeek381]

Was this feature used to recover the carrier for a DSBSC (suppressed carrier) signal. (This became a thing in SW broadcasting.)

Though one can tune an LO carefully to the nominal carrier frequency, unless the LO is phase locked to the carrier, you'll here some odd audio effects that a friend once referred to as "gas music from Jupiter."

To some it may even sound like selective fading, but it isn't.  The fading is caused by phase shift between the two sidebands causing cancellation and reinforcement alternately.

[Dave Wise]

It can help receiving AM with carrier in situations with heavy selective fading by providing a stable carrier when the received carrier drops out.

That's why I used a synchronous detector when I was listening to AM shortwave broadcasts.  Without it, every fade lost audio; with it, I barely noticed.  I was using a Hammarlund SP600 (with a long AGC time constant) with IF output routed to a Sherwood SE-3.

[tggzzz]

Been looking at some overpriced vintage sw receivers. Some boast coherent detectors. I know what coherent detection is ( phase and amplitude like  I Q , QUAM etc. )  So, how does this make mono audio better? Something to do with removing carrier noise and getting better signal to noise?

Are you sure they aren't coherer detectors? Very different

https://en.wikipedia.org/wiki/Coherer
https://www.geojohn.org/Radios/MyRadios/Coherer/CohererMobile.html

[mr ed]

Dsb Suppressed carrier is new to me, I have not been active for decades. I can see the need for a precise carrier injection else one sideband side and goes up in frequency and the other down without it. Donald duck speaks!

For regular am, I guess it helps fooling the agc?  Why not just turn off the agc then?

As a side note, it seems there is a price premium for many 30 to 40 year old sw receivers. No bargains on ebay for sure. I'm used to getting top quality test gear, way better built or much higher spec as better deals.