Dolby FM Broadcasting?

[kve777]

I know the format died in infancy, but is there any way to take a modern Low Power FM transmitter and add a Dolby circuit to it, just like in the '70's? There are still some high end Sansui and Marantz receivers with the circuitry to receive the broadcasts, it would be fun to build and try it out. Most of us never heard a decoded Dolby signal, I'd venture to guess. Who knows anything about this? A transmitter circuit schematic would be golden!

 

[KentTeffeteller]

You can't easily. And you'd need the Dolby FM encoder equipment, which is not easily found.

 

[Lesterbest]

Dolby FM died an early death. Late 70 Yammy Recievers had the capability to access it. They were feature laden, to say the least.

 

[I LIKE MUSIC]

A while back I decided to built a few FM stereo transmitters and as a source to test the transmitters sections, I generated the FM multiplex audio signal in software just for something to do. I looked at Dolby processing in software and tried a few thing and it worked okay, but this was just something to be doing and has been since put on the back shelf. The receiver that I used was a 4 channel Marantz receiver with FM Dolby built in and metering and controls for calibration to the signal being broadcast.

Not to go too technical on this but it was not all that difficult. To start I made sure that I had a good match for the pre-emphis and de-emphis of the signal. Then I took a look at what happened in the receiver in the Dolby mode and set up the software to do the inverse transfer function. This way I had a good real world match to the hardware decoding built into the receiver.

Did it work? Its measured performance was what you would expect and it sounded okay. Friends that listened to it thought it sounded very good for FM stereo. As already mentioned, not much in the way of application, except for old guys like me that just like to tinker with this stuff.

 

[steerpike2]

You can't easily. And you'd need the Dolby FM encoder equipment, which is not easily found.

You can chop the Dolby B encoder out of any cassette deck.
Or just buy one of the ICs from a vintage parts supplier.

FM transmitter circuits are all over the www. You need a fairly good, linear one with a wide bandwidth, or the decode Dolby will not track the encode Dolby well, and then the frequency response will be odd.

 

[evsentry3]

Dolby used in other areas used 2:1 or 3:1 compression and then complimentary expansion. It also used pre and de-emphasis at a few different curves. In Dolby FM it was a 25uS. The typical chip I was exposed to had either 2 or 3 pots that were adjusted for minimum distortion as a system. Sorry, I can't remember more specifically, it's been about 25 years since I worked with it.

So the two things make up Dolby FM. Rather than the typical American standard of 75uS pre-emphasis, the curve used was less, the more gentle 25uS was used. So the fixed companding is the only other thing at play. Since all stations use some and usually many forms of level controls in the form of slow rate gain control, medium rate limiting and a variety of fast rate of clipping on audio including peak control, the question is how do those play with the fixed rate of the Dolby compression that then followed that? Did stations using Dolby adapt anything to do with other processing otherwise to accommodate? Just how clean was the chip design used for the fixed rate compression and expansion cycle.

So was there any advantage sonically to the system? Did it fail because it didn't show a sonic advantage that simply using good (and light) processing would have matched? I don't know as I never heard it as it predated me.

EV3

 

[kve777]

I guess I should just forget it, if it's just a Dolby B equivalent. Using a Bob Carver TX1-11 is probably better.

 

[KentTeffeteller]

Try an outboard Dolby B NR box for an encoder, and see if your small transmitter can have the pre-emphasis set to the European standard and you should do just fine. Rethought it out.

 

[steerpike2]

A good FM tuner can get 80dB signal to noise ratio. The extra 10dB Dolby B gets - over a portion of the spectrum - is probably not worth the effort. When Dolby FM was proposed, tuners may have been a little noiser, but the source material at the station would not have had a s/n ratio of 80dB anyway. The FM transmission was not the noisiest part of the signal chain.

 

[KentTeffeteller]

Dolby FM was a great idea, and in the day should have caught on. Regrettably it didn't.

 

[rcs16]

All i can say is that modern day DSP tuners like the Si4735 eliminates much of the noise that a traditional tuner can not ever achieve.
i keep trying to find an old tuner that can compare. I just bought a Crown FM-2, it does not compare against my Si4735 design.
Next on the test block, is the kenwood KT-3800 which I plan to use the FM-2 RF FE and the KT-8300 IF and decoder sections as part of the comparison.
I look at all the circuitry, space in the FM-2/KT-8300 and compare it against a small 20 pin SSOP. mW of power vs watts, it really is quiet amazing.
Try a Si4735 design out and let me know what you think.
Dolby FM is a waste of time, they figured that out years ago. I hope IBOC has the same fate.

 

[Ricktptman]

I second the Carver TX-1-11 idea. Forget the Dolby encoding angle. By all accounts it was a mercy killing anyway. I shallow out the Dolby B on all four of my Pioneer CT-F1250's. I really like Dolby S (A) and I'm okay with C as well, but B's well.....just not ideal...at least to my ears....but then I'm no fan of DBX either. The only versions of it I heard (the ubiquitous DAK NX-40 or on Technics decks I sold) breathed and wheezed like it was on its honeymoon. YMMV.....

 

[Lesterbest]

FM Dolby died in the womb.

 

[sKiZo]

Rumor has it quite a few stations still broadcast dolby, but that's more to get the extra PUNCH the FCC allows in the rules for that. One way to tell is if you have more splatter to either side of center channel on an older analog receiver.

A dolby decoder equipped receiver should still be able to get the benefits originally intended, even if the encoding isn't happening for the intended reason. I suppose eventually, the FCC will catch up and stop that sort of thing, but in the meantime, enjoy!

PS ... one way to know for certain a station is NOT broadcasting in dolby is if they have HD sub carriers. The main carrier ends up to wide to allow that to happen.

....but then I'm no fan of DBX either. The only versions of it I heard (the ubiquitous DAK NX-40 or on Technics decks I sold) breathed and wheezed like it was on its honeymoon.

dbx done right is a joy to behold. I recently dove into digitizing the vinyls and saved the dbx encloded disks for last, as my 224 encoder/decoder had developed "issues" over time. Picked up on a standalone decoder (dbx 10) and ... wOw ... just wOw. I'd forgotten just how good those sounded. Easily rival (and sometimes even surpass) the hires digital stuff in the collection.

 

[steerpike2]

Rumor has it quite a few stations still broadcast dolby, but that's more to get the extra PUNCH the FCC allows in the rules for that. One way to tell is if you have more splatter to either side of center channel on an older analog receiver.

A dolby decoder equipped receiver should still be able to get the benefits originally intended, even if the encoding isn't happening for the intended reason. I suppose eventually, the FCC will catch up and stop that sort of thing, but in the meantime, enjoy!

PS ... one way to know for certain a station is NOT broadcasting in dolby is if they have HD sub carriers. The main carrier ends up to wide to allow that to happen.

Where does this "information" come from?
A Dolby encoded broadcast will sound unnaturally bright - too much treble, as do Dolby cassettes played without a Dolby decoder. The effect is not subtle on a high fidelity receiver. (it may go unnoticed on a battery portable radio).
The stereo image will not be affected.

And why keep something so inappropriate as a Dolby encoder in the transmitter when there are things like Optimod and Stereomaxx that really gave a broadcast an "in your face" sound.

Also, how can a Dolby encoded audio feed modulate the carrier to any greater deviation than any other source? The peak level is limited to keep a strict control on the carrier deviation. All Dolby does is boost the treble WHEN the overall signal level is low.

 

[amptramp]

We already have pre-emphasis and de-emphasis in the normal FM audio signal. Was Dolby enough of an improvement that people want to revive it?

 

[hphase]

Even with pre-emphasis, there is still some noise on weak signals with quiet audio, such as on classical music stations. Most of the stations that went with Dolby FM were classical stations. Dolby FM used the consumer B-type noise reduction system (originally developed for consumer audio tape) and appllied it to FM broadcasting.

Because un-decoded Dolby B audio sounds bright, the standard FM 75 microsecond pre-emphasis curve was changed to 25 microseconds when broadcasting with the Dolby system. Reducing the pre-emphasis had an additional benefit of preventing overmodulation of the FM carrier by high frequency signals. This allowed a more dense signal that didn't sound dull when received on non-Dolby receivers. This ends up being similar to how Dolby-encoded tapes sound brighter when left un-decoded. To some (most?) this was a pleasing result, especially on cassette tapes.

Dolby FM was an improvement f FM broadcasting. It just didn't catch on in the marketplace. "Regular" FM was good enough, at least after using the Optimod (and other) afterburners to 'improve' the sound.

 

[vwestlife]

The Optimod 8100 made Dolby FM obsolete. Instead of improving the S/N ratio by transmitting a Dolby-encoded signal and decoding it at the receiver, which required the listener to have special equipment and calibrate it, the Optimod achieved the same benefit of improved S/N ratio by allowing FM stations to increase the average modulation level (and thus loudness) of their transmitted audio without the audible drawbacks of previous FM audio processor designs, and without requiring the listener to have any special equipment.

This interview with Bob Orban is highly recommended viewing. He discusses the history of FM radio, FM stereo, AM stereo, HD Radio, and the Optimod:

 

[N8Nagel]

Rumor has it quite a few stations still broadcast dolby, but that's more to get the extra PUNCH the FCC allows in the rules for that. One way to tell is if you have more splatter to either side of center channel on an older analog receiver.

A dolby decoder equipped receiver should still be able to get the benefits originally intended, even if the encoding isn't happening for the intended reason. I suppose eventually, the FCC will catch up and stop that sort of thing, but in the meantime, enjoy!

PS ... one way to know for certain a station is NOT broadcasting in dolby is if they have HD sub carriers. The main carrier ends up to wide to allow that to happen.



dbx done right is a joy to behold. I recently dove into digitizing the vinyls and saved the dbx encloded disks for last, as my 224 encoder/decoder had developed "issues" over time. Picked up on a standalone decoder (dbx 10) and ... wOw ... just wOw. I'd forgotten just how good those sounded. Easily rival (and sometimes even surpass) the hires digital stuff in the collection.

Interesting... Well most of my local stations are HD, and I've probably got more HD tuners than the vast majority of the population! (one in the AVR, one in the spare bedroom, one in the basement, and one in each of two vehicles)

Another format that seems to be on life support... Everyone's broadcasting in it but I bet most people don't even know it exists and all the tuners still for sale are either eye wateringly expensive or in little table radio things. Go figure.

 

[for_p1]

Interesting... Well most of my local stations are HD, and I've probably got more HD tuners than the vast majority of the population! (one in the AVR, one in the spare bedroom, one in the basement, and one in each of two vehicles)

Another format that seems to be on life support... Everyone's broadcasting in it but I bet most people don't even know it exists and all the tuners still for sale are either eye wateringly expensive or in little table radio things. Go figure.

Almost all car radios support HD transmission. Many AV receivers do too. Today there is no need in standalone tuner - no one will buy it. But table radio do have their niche, though still diminishing with proliferation of network media players.