FM? AM? Does it matter?

[Slyster]

I am a bit new to scanning... it seems to me when I am programming (non-digital) frequencies... I have some mode choices (i.e. AM, FM, CT, DC).. but the database or web source doesn't usually specify what type of signal it is.

- Does it really matter?
- Is there a general frequency guideline I can use to know when to set the Rc mode to FM or AM?

I know what CT is... and if I leave it to 0 it does a CT scan that includes all communications right? 'DC' I have not idea what that mode is.... but I see it in WIN96 under the Rx mode box.

 

[JJones27320]

to answer your question. 99.9% of things you will listen to on your scanner will be FM with the exception of aircraft which is AM. Now CT mode is CTCSS tone squelch mode that will allow you to block all voice on the frequency unless the tone you have set is transmitted. DC basicly works the same as CT except it is called Digital Code Squelch. RR database tells the code to enter under the tone heading on the database.In the database, if the tone number has PL after it then it is CT mode, if it has DPL after it then it is DC mode.

 

[gmclam]

Scanner modes

Analog audio is only sent via AM or FM. Every setting on your scanner will receive in FM, except the AM setting. Generally you'll find that when you manually program a frequency the default the scanner selects is usually correct. For example, program 154.190 and the scanner defaults to FM. Program 122.975 and the scanner defaults to AM. I've found only rare cases where the default is not correct.

Regarding the difference; in a perfect design, if a channel is set to FM and is being broadcast in AM, you'll hear silence when a carrier is present. And when a channel is set to AM and is broadcast in FM you certainly won't hear voice. But the reality for most scanner designs is that you can make out something with the wrong mode selected, but it won't be very clear.

Hope this helps. Good luck.

 

[scannerrail]

funny I was going to ask this sometimes I can hear whatever it is I want to listen to on both FM and AM but on AM it's not as loud as FM.

 

[Lyman]

to answer your question. 99.9% of things you will listen to on your scanner will be FM with the exception of aircraft which is AM. Now CT mode is CTCSS tone squelch mode that will allow you to block all voice on the frequency unless the tone you have set is transmitted. DC basicly works the same as CT except it is called Digital Code Squelch. RR database tells the code to enter under the tone heading on the database.In the database, if the tone number has PL after it then it is CT mode, if it has DPL after it then it is DC mode.

Having said that about CT, then I do not understand why CT is dispayed on it's own line below the frequency. I.e., the Frequency may be 128.8 and beneath that on it's own line is "CT: 137.25". Can you shed some light?

Thanks,

Lyman

 

[Don_Burke]

Any tone can be used on any RF frequency, so displaying both makes sense.

Some systems use the same tone on all frequencies within the system, some do not.

Having said that about CT, then I do not understand why CT is dispayed on it's own line below the frequency. I.e., the Frequency may be 128.8 and beneath that on it's own line is "CT: 137.25". Can you shed some light?

Thanks,

Lyman

 

[Lyman]

Any tone can be used on any RF frequency, so displaying both makes sense.

Some systems use the same tone on all frequencies within the system, some do not.

Thanks Don. But what is the point in displaying the second (137.25) freqyency?

 

[Don_Burke]

Thanks Don. But what is the point in displaying the second (137.25) freqyency?

That lets you know which system you are listening to.

For example, several of the tunnels around here use the same RF frequency.

By putting each in a seperate channel with the tone entered, I can use the alpha tag function so the transmitting tunnel is instantly identified.

If you do not have an alpha function, you will quickly learn to associate the tone displayed with the transmitting tunnel.

 

[gmclam]

RF frequency and Continuous Tone frequency....

Having said that about CT, then I do not understand why CT is dispayed on it's own line below the frequency. I.e., the Frequency may be 128.8 and beneath that on it's own line is "CT: 137.25". Can you shed some light?

You do realize that the RF frequency (your example of 128.8) is in MHz? That value is similar to where you tune your FM radio - for example 107.9 would actually be 107.9 MHz.

One issue with 2-way communications is that there are many users using the same RF frequency. One way to "separate" these agencies from each other is to transmit a tone which is not heard by the user but decoded by their receiver. Originally only a small number of specific frequencies were used for these tones, but digital electronics has allowed more analog tones (CT) to be used as well as a set of "digital codes" (DC). The analog tone frequencies are all below 300 cycles (300 Hz).

Some scanners, such as the PRO-97, can be set so that the squelch is only opened when a specific tone is received. Alternately, the tone frequency can be displayed even if the squelch is opened by the RF carrier. I also note that your example of 137.25 for a CT can not be correct. There is a specific set of CTs and DCs used. You can find this and more in the Wiki here at RR.

 

[Lyman]

You do realize that the RF frequency (your example of 128.8) is in MHz? That value is similar to where you tune your FM radio - for example 107.9 would actually be 107.9 MHz.

One issue with 2-way communications is that there are many users using the same RF frequency. One way to "separate" these agencies from each other is to transmit a tone which is not heard by the user but decoded by their receiver. Originally only a small number of specific frequencies were used for these tones, but digital electronics has allowed more analog tones (CT) to be used as well as a set of "digital codes" (DC). The analog tone frequencies are all below 300 cycles (300 Hz).

Some scanners, such as the PRO-97, can be set so that the squelch is only opened when a specific tone is received. Alternately, the tone frequency can be displayed even if the squelch is opened by the RF carrier. I also note that your example of 137.25 for a CT can not be correct. There is a specific set of CTs and DCs used. You can find this and more in the Wiki here at RR.

Correct - 137.25 was just an example. A real example is in front of me now: frequency displayed is 482.46250, and the CT displayed is 146.2.

What is the practical application or purpose of the CT for the listener?

Thanks,

Lyman

 

[BaLa]

Correct - 137.25 was just an example. A real example is in front of me now: frequency displayed is 482.46250, and the CT displayed is 146.2.

What is the practical application or purpose of the CT for the listener?

Thanks,

Lyman

It can be useful in identifying frequencies.
Especially with ducting

 

[n2mdk]

It's also useful in a high RF environment where stray signals might open the squelch. If you live in a place like NYC it's very handy having the frequencies CTCSS /PL or DCS encoded to keep the radio quiet when not receiving a valid signal.

 

[gmclam]

Continuous Tones ....

A real example is in front of me now: frequency displayed is 482.46250, and the CT displayed is 146.2.

What is the practical application or purpose of the CT for the listener?

I am in California, so I am not able to listen to what you are. However, I entered 482.46250 into the db here at RR for Texas and it came back with the City of Everman in Tarrant county as the only listing. Note that it shows a tone of 100.0. What that tells me is you are most likely not hearing that agency (since you see a tone of 146.2).
Data here: http://www.radioreference.com/modules.php?name=RR&action=isf&stid=48&freq=482.46250

For a better example I plugged 460.52500 into the db search for Texas and it came back with data here: http://www.radioreference.com/modules.php?name=RR&action=isf&stid=48&freq=460.52500

Note that Bell County Fire Dispatch has a tone of 167.9 Windcrest Police has a tone of 173.8 Houston Police has a tone of 123.0 and Stratford County EMS has a tone of 136.5 ALL ON THE SAME FREQUENCY!!! I am not sure how close any of these places are from each other, but here in California it is very common to pick up several agencies on the same frequency (one might be to the west and the other to the southeast). How do I know who I am receiving other than by careful listening to what they say? By the tone frequency.

Note; you'll see that some places refer to the tone as "PL" and some "CT". It is the SAME thing; it is just that Motorola trademarked PL (which means Private Line) whereas the term CT is generic.

 

[Lyman]

That lets you know which system you are listening to.

For example, several of the tunnels around here use the same RF frequency.

By putting each in a seperate channel with the tone entered, I can use the alpha tag function so the transmitting tunnel is instantly identified.

If you do not have an alpha function, you will quickly learn to associate the tone displayed with the transmitting tunnel.

Ah, okay. The light finally came on. Thanks!

Lyman

 

[Lyman]

I am in California, so I am not able to listen to what you are. However, I entered 482.46250 into the db here at RR for Texas and it came back with the City of Everman in Tarrant county as the only listing. Note that it shows a tone of 100.0. What that tells me is you are most likely not hearing that agency (since you see a tone of 146.2).
Data here: http://www.radioreference.com/modules.php?name=RR&action=isf&stid=48&freq=482.46250

For a better example I plugged 460.52500 into the db search for Texas and it came back with data here: http://www.radioreference.com/modules.php?name=RR&action=isf&stid=48&freq=460.52500

Note that Bell County Fire Dispatch has a tone of 167.9 Windcrest Police has a tone of 173.8 Houston Police has a tone of 123.0 and Stratford County EMS has a tone of 136.5 ALL ON THE SAME FREQUENCY!!! I am not sure how close any of these places are from each other, but here in California it is very common to pick up several agencies on the same frequency (one might be to the west and the other to the southeast). How do I know who I am receiving other than by careful listening to what they say? By the tone frequency.

Note; you'll see that some places refer to the tone as "PL" and some "CT". It is the SAME thing; it is just that Motorola trademarked PL (which means Private Line) whereas the term CT is generic.

>I entered 482.46250 into the db here at RR for Texas and it came back with the City of Everman in Tarrant county as the only listing.

Interesting. I found two for here in Taxachusetts. I wonder why that is?
Thanks for all the information!
Lyman

 

[gmclam]

>I entered 482.46250 into the db here at RR for Texas and it came back with the City of Everman in Tarrant county as the only listing.

Interesting. I found two for here in Taxachusetts. I wonder why that is?
Thanks for all the information!
Lyman

LOL. I did the search for TEXAS and you did the search for MASSACHUSETTS; that's why.

 

[Lyman]

LOL. I did the search for TEXAS and you did the search for MASSACHUSETTS; that's why.

...it's been one of those weeks...