Even though it has been quite some time since the FCC's Narrowband mandate has occurred, I still see some confusion on exactly what happened when this occurred.
Let's start out with a brief explanation of how a normal analog FM voice signal "looks" like. Draw a horizontal line, then draw a wavy line up and down across that line that extends equidistant above and below the line. You've just drawn the space that a voice signal takes up. Let's say the center of that line depicts your Fire dispatch frequency, or 155.000 Megahertz (MHz).
Prior to narrowbanding, basically, each radio transmission was allowed a 000.025 MHz (25 Kilohertz/KHz) total space around it's frequency that it could operate in. Without going into specifics, that means that your frequency of 155.000 was "allowed" from 154.9875 MHz to 155.0125 Mhz to operate. That is a 25 Khz space. We call this 25 KHz channel spacing.
After narrowbanding, all VHF (for the purposes of public safety, 150-174MHz), and UHF (421-512MHz), transmissions were only allowed to take up half that space, or 12.5 KHz (000.0125 MHz) instead of 25 KHz. Your fire dispatch frequency of 155.000 now could only take up from 154.99375-155.00625 (compared to 154.9875-155.0125). When you consider the frequency range of 150-174 MHz as a "spectrum", your transmissions after narrowbanding were only allowed to take up half the space in the spectrum as they did before. The wavy line you drew earlier is now half as wide.
Alright, cool, but why? To make room for more available licensed frequencies. That's it. When all the existing frequencies now take up half as much space in the spectrum as they did before, we've now got room in that same spectrum for TWICE as many licensed frequencies to give out to the end user like PD's, FD's, EMS, etc...
"So why did we HAVE to go digital?" You didn't. But, Many agencies took advantage of the narrowband requirement to purchase new and update radios and radio systems. Many OLD radios didn't even have the capability to DO narrowband, so they were not compliant with the narrowband mandate and could not be used, forcing agencies to purchase new radios. Some agencies with equipment like this said "OK, we have to buy new radios and possibly new equipment for our radio system to narrowband. Do we want to spend a bunch of money only to keep the junky same old type of system that we had, or do we want to spend money and update our radio system to something more effecient? We have to spend the money either way, we either stay with what we have, or upgrade and prepare for the future".
In the case of local agencies where I live, that's exactly what happened. They had some narrowbandable equipment, and some that wasn't narrowband compliant. They also were having trouble communicating between agencies on different types of radio systems. So all the agencies said "Well, we HAVE to spend the money to narrowband, lets use this opportunity to upgrade to P25 trunking and get everybody in the county on the same system at the same time". Still, many agencies across the country chose only to narrowband their current system and be done with it. In many cases, all that took (with modern enough equipment) was a contractor or city/county radio tech to come in, change all the current frequencies from 25khz spacing to 12.5khz spacing in your portables and mobiles, push the "program" button on the programming software, and be done with it. (There was also work at tower sites to narrowband the tower site equipment as well). Some agencies also took this time to license new frequencies for additional use, or update their current analog systems, or kept their existing conventional (non trunking) radio systems from analog voice to digital voice. The ONLY requirement was narrowbanding. Any upgrades past that were left entirely up to agencies.
I see a lot of people saying "Yeah we did the digital requirement". Not true. there was no requirement to go digital. Some agencies just knew they were going to have to spend money anyway, and took the opportunity to do massive upgrades instead of merely complying with the narrow band requirement.
Narrowband does not mean digital.
Let's start out with a brief explanation of how a normal analog FM voice signal "looks" like. Draw a horizontal line, then draw a wavy line up and down across that line that extends equidistant above and below the line. You've just drawn the space that a voice signal takes up. Let's say the center of that line depicts your Fire dispatch frequency, or 155.000 Megahertz (MHz).
Prior to narrowbanding, basically, each radio transmission was allowed a 000.025 MHz (25 Kilohertz/KHz) total space around it's frequency that it could operate in. Without going into specifics, that means that your frequency of 155.000 was "allowed" from 154.9875 MHz to 155.0125 Mhz to operate. That is a 25 Khz space. We call this 25 KHz channel spacing.
After narrowbanding, all VHF (for the purposes of public safety, 150-174MHz), and UHF (421-512MHz), transmissions were only allowed to take up half that space, or 12.5 KHz (000.0125 MHz) instead of 25 KHz. Your fire dispatch frequency of 155.000 now could only take up from 154.99375-155.00625 (compared to 154.9875-155.0125). When you consider the frequency range of 150-174 MHz as a "spectrum", your transmissions after narrowbanding were only allowed to take up half the space in the spectrum as they did before. The wavy line you drew earlier is now half as wide.
Alright, cool, but why? To make room for more available licensed frequencies. That's it. When all the existing frequencies now take up half as much space in the spectrum as they did before, we've now got room in that same spectrum for TWICE as many licensed frequencies to give out to the end user like PD's, FD's, EMS, etc...
"So why did we HAVE to go digital?" You didn't. But, Many agencies took advantage of the narrowband requirement to purchase new and update radios and radio systems. Many OLD radios didn't even have the capability to DO narrowband, so they were not compliant with the narrowband mandate and could not be used, forcing agencies to purchase new radios. Some agencies with equipment like this said "OK, we have to buy new radios and possibly new equipment for our radio system to narrowband. Do we want to spend a bunch of money only to keep the junky same old type of system that we had, or do we want to spend money and update our radio system to something more effecient? We have to spend the money either way, we either stay with what we have, or upgrade and prepare for the future".
In the case of local agencies where I live, that's exactly what happened. They had some narrowbandable equipment, and some that wasn't narrowband compliant. They also were having trouble communicating between agencies on different types of radio systems. So all the agencies said "Well, we HAVE to spend the money to narrowband, lets use this opportunity to upgrade to P25 trunking and get everybody in the county on the same system at the same time". Still, many agencies across the country chose only to narrowband their current system and be done with it. In many cases, all that took (with modern enough equipment) was a contractor or city/county radio tech to come in, change all the current frequencies from 25khz spacing to 12.5khz spacing in your portables and mobiles, push the "program" button on the programming software, and be done with it. (There was also work at tower sites to narrowband the tower site equipment as well). Some agencies also took this time to license new frequencies for additional use, or update their current analog systems, or kept their existing conventional (non trunking) radio systems from analog voice to digital voice. The ONLY requirement was narrowbanding. Any upgrades past that were left entirely up to agencies.
I see a lot of people saying "Yeah we did the digital requirement". Not true. there was no requirement to go digital. Some agencies just knew they were going to have to spend money anyway, and took the opportunity to do massive upgrades instead of merely complying with the narrow band requirement.
Narrowband does not mean digital.
