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Backward Compatibility
13 years ago
General
Alright, so lets say you've got an information transmission system and decide you want to expand what information is transmitted. What can you do to make sure the receivers you have in the field can handle the new, expanded format? I suppose in this day and age, you'd issue a patch, or a firmware update and life would be good.
But let's make it interesting: your receivers are simple solid-state devices with no firmware. It's all hard-wired or etched on a circuit board. Maybe you decide to offer a patch in the earlier use of the word, involving wire wraps and changed traces. Good solution, except you've got millions of these devices out in the field.
I imagine this is what the FCC faced when deciding they could upgrade FM radio from a mono signal to stereo. How do you alter the existing mono broadcast so that all your mono receivers already out there can still work, while allowing new sets to decode stereo? Well, they started by adding a 38kHz sub-carrier to the existing signal so they could send another channel of audio. Now what?
You can't send left channel on the carrier and right on the sub-carrier: existing mono sets would then only get the left channel. And if you've ever listened to some of the early Beatles recordings in stereo, you know that's a good way to end up without lead vocals, or only half the instruments, or other weirdness. (Trust me, the music PA in my gym only does one channel from a stereo XM Radio receiver... I've heard weird things first hand)
The answer... is really quite brilliant. First, you keep the mono signal (which is the sum of left and right, or L+R) on the main carrier for all the mono radios out there. They've been made to expect audio on the carrier, and they keep on chugging right along like normal. And on this 38kHz sub-carrier you put the difference between the two channels (L-R).
How do you get separate left and right from all this? By adding and subtracting the sum and difference signals. Strap on your algebra goggles, we're goin' in!
(L+R) + (L-R) = L+R+L-R = L+R+L-R = L + L = 2L = LEFT
(L+R) - (L-R) = L+R-L+R =L+R-L+R = R + R = 2R = RIGHT
Behold! Simple addition and subtraction of the two signals yields pure separated audio channels, while still keeping the summed channel available for the original mono radios to use.
Adding signals is easy to do in electronics, and subtracting signals is simply adding the opposite of the second signal. And an opposite signal could be as easy as reversing the connections from the decoder to the summing circuit. This is all stuff that can be done with simple, non-programmable electronics. (Oversimplified, I'm sure, but bear with me)
They faced a similar task when TV went from black and white to color... Plus, transmitting a linear signal is easy. How do you transmit a vector? Stay tuned...
But let's make it interesting: your receivers are simple solid-state devices with no firmware. It's all hard-wired or etched on a circuit board. Maybe you decide to offer a patch in the earlier use of the word, involving wire wraps and changed traces. Good solution, except you've got millions of these devices out in the field.
I imagine this is what the FCC faced when deciding they could upgrade FM radio from a mono signal to stereo. How do you alter the existing mono broadcast so that all your mono receivers already out there can still work, while allowing new sets to decode stereo? Well, they started by adding a 38kHz sub-carrier to the existing signal so they could send another channel of audio. Now what?
You can't send left channel on the carrier and right on the sub-carrier: existing mono sets would then only get the left channel. And if you've ever listened to some of the early Beatles recordings in stereo, you know that's a good way to end up without lead vocals, or only half the instruments, or other weirdness. (Trust me, the music PA in my gym only does one channel from a stereo XM Radio receiver... I've heard weird things first hand)
The answer... is really quite brilliant. First, you keep the mono signal (which is the sum of left and right, or L+R) on the main carrier for all the mono radios out there. They've been made to expect audio on the carrier, and they keep on chugging right along like normal. And on this 38kHz sub-carrier you put the difference between the two channels (L-R).
How do you get separate left and right from all this? By adding and subtracting the sum and difference signals. Strap on your algebra goggles, we're goin' in!
(L+R) + (L-R) = L+R+L-R = L
(L+R) - (L-R) = L+R-L+R =
Behold! Simple addition and subtraction of the two signals yields pure separated audio channels, while still keeping the summed channel available for the original mono radios to use.
Adding signals is easy to do in electronics, and subtracting signals is simply adding the opposite of the second signal. And an opposite signal could be as easy as reversing the connections from the decoder to the summing circuit. This is all stuff that can be done with simple, non-programmable electronics. (Oversimplified, I'm sure, but bear with me)
They faced a similar task when TV went from black and white to color... Plus, transmitting a linear signal is easy. How do you transmit a vector? Stay tuned...
I love digging into these things, thanks!
That is a very interesting work solution. Not something that would have occurred to me initially.
Speaking of signals and sub-channels reminds me of all the stuff I have to read about with wireless tech. Of course, now days all that stuff is running on digital, so it is usually talking about how much data can be encoded per signal (phase shift keying and all that stuff). But talk of using existing equipment brings up another interesting topic. Like how do companies or entities like the FCC deal with the rapid pace of technology? Not everything that gets into public or commercial use is going to be "backward compatible." At some point there is going to be the decision that "old" equipment is going to have to be replaced. This threshold is what we are seeing with the 4G networks that are still in the process of being built/expanded/brought into operation. 4G is not compatible with 3G; it is a whole different way of sending and receiving data. Sure 4G is evolved from 3G tech, but the two do not speak the same language. Although, most phones today have all the internal bits to make sure they can work with 2G/3G/4G networks. So in the case of wireless (mainly cellular) it is the infrastructure that is going to need the "can we make this work with new handsets?" scrutiny.
Excellent topic! I always like seeing how you approach these fascinating discussions. And good to see you posting again.
I'd be interested to hear about what you're learning about wireless (cellular) networking. About all I know is that 4G LTE is not really 4G as it was originally spec'd, it's 4G Long Term Evolution... it's still working toward the goal. Heck, I my phone is just 3G (and I barely ever wish it was faster). But the whole deal about making phones that speak 2G/3G/4G all at once sounds like the solution of making DVD writers that all spoke +R, -R, +RW and -RW... it works, but seems clumsy somehow.
Of course, going back to the FCC forcing TV stations to digital (it's about money... but not for the TV stations), I wonder if cellular carriers would ever do the same thing, like tell all their 3G users to suck it up and get a 4G phone. I figure you gotta weigh how bad you wanna bone your established user base versus getting more modern. Remember the Sprint "hybrid" phones that came out after they bought Nextel? They used the Sprint PCS network for calls and data, but also had an IDEN radio in them for Direct Connect on Nextel's network? I figure 'cause so many businesses came to depend on DC, they couldn't just close up shop and say, "Sorry, you lose!" to all those customers...
Maybe it's already happening... I'm sure the rise in data prices have to do with 4G's cost and popularity. Like if I'm gonna pay $30 for 2GB of data a month, I might as well go up to 4G, right? Hey Verizon, how about I stay on 3G and you keep giving me unlimited data for $30 a month? Heh, they'll probably make me pay $50 a month for 1GB of 3G because it's a burden for them to keep the old system alive.
And as far as the iDEN network goes, it *is* closing down. Sprint has already decided to phase out the Nextel network (more commonly referred to as shutting it down). So guess what? All those people who like and use DC? Too bad, it is going bye bye. Sprint does have a similar service being implemented, but it is nowhere near as good as the iDEN system was. But that really is not the point, hm? Sprint says if you want to have DC, then you must switch services (which OF COURSE requires that users get a new handset, money money money MONEY..money!)
And you are on the right track with LTE. Technically, it IS 4G...but that is the problem. "Technically" and "finished" are not part of the same sentence when talking about 4G. As I'm sure most of us know, the 4G networks are far from complete, and are not quite ready to take over the same coverage areas that 3G has. Which is why for the most part, 3G still makes up most of how we access the cellular networks (both voice and data).
I've actually used this link in my class discussions, and this article gives a quick and dirty breakdown of the evolution of cellular. http://www.androidauthority.com/hspa-vs-lte-which-one-is-better-78120/
It seems like the looming trend is that carriers will be shifting over to data-centric plans, instead of voice-centric plan. Instead of how many minutes you get a month, it will be how many GBs you get per month. This is because even though we make "voice" calls, it is essentially taking place over data packets. I suppose it all comes down to how you track or classify these packets, since the network cannot tell the difference between a packet sent during a voice call and a packet sent when you run that google search.