Post by dbsteff on Oct 13, 2008 9:10:49 GMT -5
Virtually all FM stations today transmit in “Stereo”… such that they don’t even make mention of it. But in 1968 Stereo was a very big deal. There were many FM stations on the air, at that time, but they were transmitting in Mono. Some were quick to get on board wit the Stereo craze, others lagged behind the herd. So, it was a Huge marketing point of distinction to proclaim that one was “Broadcasting in STEREO”
If you listen to some of my audio snippets, as posted by John Quincy on the WLRS page, you’ll hear stingers proclaiming that WLRS was indeed “BROADCASTING IN STEREO”.
The technical significance of Stereo Multiplexing was lost on the listener. Only those of us in the technical community really had an appreciation for it. In part, the Stereo signal was quite an engineering coup.
When the notion of stereo transmission first came into the Radio community, several competing techniques were put forth for consideration and ultimate adoption by the FCC.
The FCC proclaimed that any such technique must be compatible with the Mono signal already in place. There were, after thousands of existing receivers out there, and the FCC wanted to ensure that they were not rendered obsolete by the Stereo concept. Color TV faced a similar challenge when it was introduced into a well established Monochrome market. There was to be no clean break, which would have been the outcome of many of these other techniques……much as we are about to see in the Digital TV changeover coming in Feb, 2009.
Anyway, the challenge was to come up with a technique that would be useable by existing Mono FM receivers, but could then be decoded to produce Stereo in the new Stereo-equipped receivers.
I may be wrong, it’s been 40 years…but I believe it was RCA that solved the problem.
It was pretty darned clever, as to how they did it.
In order to keep the Mono receivers happy, one was constrained to transmit in Mono.
So how did these technical magicians provide Stereo over the same transmission?
Well, by means of transmitting a difference signal on a sub carrier, riding along side of the main signal.
It’s really pretty darned smart. I’ll offer a greatly simplified explanation, and hope it makes sense…..here goes:
If one considers the two signal involved in Stereo…..a Left Channel (L) and a Right Channel (R) The equivalent Mono signal is a simple combination of the two: L+R. This is the signal that was transmitted on the main frequency of the station. Thus, existing receivers could decode and present this mono signal as they always had. First problem solved.
Now came the clever piece of engineering.
I won’t dwell at length on how the subcarrier was generated or the 19 KHz pilot tone upon which it was referenced, as we’d be out into the weeds quickly. I will mention, however that the presence of the 19 KHz pilot tone was what the Stereo-Equipped receivers detected to illuminate the “STEREO” lamp on the receiver as well as to provide the reference for the demodulation of the difference signal that allowed Stereo to be produced.
I’ll try to explain this in the easiest math model possible:
The L+R signal was the Mono equivalent as described earlier. The R signal was run through an inverter amplifier to produce –R. The –R was then combined with the L signal to produce what is called the difference signal, or L-R. This signal was transmitted on the subcarrier.
In the receiver the difference signal, L-R was combined with the L+R in the following fashion:
L+R
L-R
= 2L
Now one has L (the left channel) decoded at twice the volume level. (Easily attenuated back to L)
Now the receiver would run the difference signal through an inverter amp to produce signal: –L+R. This signal was then combined, in similar fashion, with the Mono signal L+R, as before. This produces the result:
L+R
-L+R
= 2R
Now one has R (the right channel) decoded at twice the volume level.
The receiver now has decoded both L and R and can present them as Stereo to the speakers of the radio. In the absence of a 19 KHz pilot tone, the receiver would simply process the L+R (Mono) and present it to both speakers. Also the “STEREO” lamp on the receiver would be extinguished.
PRETTY CLEVER, HUH?
Anyway, at a minimum, a station transmitting in Mono would require a new exciter to convert to Stereo….not to mention that all the studio equipment would have to be converted to stereo, as would any STL (Studio to Transmitter Link) equipment. So it was a major undertaking for an existing station.
So it was a very BIG DEAL, in those days, for a station to proclaim itself to be “BROADCASTING IN STEREO”
Most stations, including WLRS much ballyhooed the fact, as evidenced by our Station IDs and other stingers.
FYI: The Color TV challenge was solved in the same fashion. The main signal transmitted the image in Black & White, while a difference signal (much more complicated than the one used in FM Stereo) was used to decode the R-G-B color signal and paint color onto the Black & White picture.
Regards
Dan
If you listen to some of my audio snippets, as posted by John Quincy on the WLRS page, you’ll hear stingers proclaiming that WLRS was indeed “BROADCASTING IN STEREO”.
The technical significance of Stereo Multiplexing was lost on the listener. Only those of us in the technical community really had an appreciation for it. In part, the Stereo signal was quite an engineering coup.
When the notion of stereo transmission first came into the Radio community, several competing techniques were put forth for consideration and ultimate adoption by the FCC.
The FCC proclaimed that any such technique must be compatible with the Mono signal already in place. There were, after thousands of existing receivers out there, and the FCC wanted to ensure that they were not rendered obsolete by the Stereo concept. Color TV faced a similar challenge when it was introduced into a well established Monochrome market. There was to be no clean break, which would have been the outcome of many of these other techniques……much as we are about to see in the Digital TV changeover coming in Feb, 2009.
Anyway, the challenge was to come up with a technique that would be useable by existing Mono FM receivers, but could then be decoded to produce Stereo in the new Stereo-equipped receivers.
I may be wrong, it’s been 40 years…but I believe it was RCA that solved the problem.
It was pretty darned clever, as to how they did it.
In order to keep the Mono receivers happy, one was constrained to transmit in Mono.
So how did these technical magicians provide Stereo over the same transmission?
Well, by means of transmitting a difference signal on a sub carrier, riding along side of the main signal.
It’s really pretty darned smart. I’ll offer a greatly simplified explanation, and hope it makes sense…..here goes:
If one considers the two signal involved in Stereo…..a Left Channel (L) and a Right Channel (R) The equivalent Mono signal is a simple combination of the two: L+R. This is the signal that was transmitted on the main frequency of the station. Thus, existing receivers could decode and present this mono signal as they always had. First problem solved.
Now came the clever piece of engineering.
I won’t dwell at length on how the subcarrier was generated or the 19 KHz pilot tone upon which it was referenced, as we’d be out into the weeds quickly. I will mention, however that the presence of the 19 KHz pilot tone was what the Stereo-Equipped receivers detected to illuminate the “STEREO” lamp on the receiver as well as to provide the reference for the demodulation of the difference signal that allowed Stereo to be produced.
I’ll try to explain this in the easiest math model possible:
The L+R signal was the Mono equivalent as described earlier. The R signal was run through an inverter amplifier to produce –R. The –R was then combined with the L signal to produce what is called the difference signal, or L-R. This signal was transmitted on the subcarrier.
In the receiver the difference signal, L-R was combined with the L+R in the following fashion:
L+R
L-R
= 2L
Now one has L (the left channel) decoded at twice the volume level. (Easily attenuated back to L)
Now the receiver would run the difference signal through an inverter amp to produce signal: –L+R. This signal was then combined, in similar fashion, with the Mono signal L+R, as before. This produces the result:
L+R
-L+R
= 2R
Now one has R (the right channel) decoded at twice the volume level.
The receiver now has decoded both L and R and can present them as Stereo to the speakers of the radio. In the absence of a 19 KHz pilot tone, the receiver would simply process the L+R (Mono) and present it to both speakers. Also the “STEREO” lamp on the receiver would be extinguished.
PRETTY CLEVER, HUH?
Anyway, at a minimum, a station transmitting in Mono would require a new exciter to convert to Stereo….not to mention that all the studio equipment would have to be converted to stereo, as would any STL (Studio to Transmitter Link) equipment. So it was a major undertaking for an existing station.
So it was a very BIG DEAL, in those days, for a station to proclaim itself to be “BROADCASTING IN STEREO”
Most stations, including WLRS much ballyhooed the fact, as evidenced by our Station IDs and other stingers.
FYI: The Color TV challenge was solved in the same fashion. The main signal transmitted the image in Black & White, while a difference signal (much more complicated than the one used in FM Stereo) was used to decode the R-G-B color signal and paint color onto the Black & White picture.
Regards
Dan