posted 05-01-2001 11:08 PM ET (US)            

Are you talking about the FM-Broadcast band at 88-108 MHz?

I don't think I've ever noticed a VHF Marine radio that has the FM-Broadcast band, too.
There really are such radios?

posted 05-01-2001 11:50 PM ET (US)            

john

posted 05-02-2001 12:12 AM ET (US)            

Maybe like the Cadilac that tried to run a V8 on 6 then 4 cy to satisfy users wants for economy. NOT!

Standard AM/FM radios (and speakers) are not that expensive. I would rather have a Marine radio that was specifically built to possibly save my life, rather than getting the latest tunes.

posted 05-02-2001 12:30 AM ET (US)            

Thanks Tsuriki.

While I won't speak for everyone, there are times on the water that I would like to hear a few tunes. I certainly appreciate the value of a quality VHF, but am a bit cramped for space and was hopefull that in this day and age someone could incorporate a good VHF and FM band into a single space efficient unit. Again, I'm not looking for something that has CD/tape capability, just FM. And yes, BOSE does manufacture speakers designed specifically for marine use....

posted 05-02-2001 12:35 AM ET (US)            

In VHF-Marine the modulation is 5F3, that is 5 KHz deviation frequency modulation telephony, while FM-Broadcast is 75F3, 75 KHz deviation frequency modulation telephony.

The two bands require quite different demodulation equipment to recover the baseband components of their modulated signals.

posted 05-02-2001 12:37 AM ET (US)            

Wouldn't it be nice if the GPS or Fish /Finder people incorporated an LED TV in their unit...

Progress/customer needs (prices we will pay) may come totether with technology.

In the mean time, the old transistor radio will have to do for me.

posted 05-02-2001 03:42 PM ET (US)         

The frequency deviation of the signal limits the bandwidth and the dynamic range of the audio that is modulated on the RF signal. Marine Band being for voice only, and get by with a lower deviation than Broadcast FM. That doesn't mean that a Marine VHF couldn't demodulate Broadcast FM signal; but it does mean that the fidelity may be pretty bad.

I saw short write up of a new marine electronics gadget in one of my latest boating magaizines. Sorry I can't be more specific, but I think it was a hand held VHF radio that could also receive Television using the LCD display for the picture.

posted 05-02-2001 03:54 PM ET (US)            

Tsuriki

posted 05-03-2001 03:46 PM ET (US)         

As most of you know FM stands for Frequency Modulation. The frequency of the transmitted transmitted RF signal is changed in a linear manner; following the source signal. The source signal could be music, voice, video, data, etc. The frequency of the RF signal, but the amplitude remains constant.

AM stands for Amplitude Modulation. This is just the opposite of FM. The amplitude of the RF signal is varied according to the source signal; but the frequency of the RF signal remains constant. Again the signal can be voice, video, music, data, etc.

Both of these systems have advantages and disadvantages. Which is why both schemes are used depending on the situation. Neither one is perfect.

Since we are discussing FM audio, lets take a closer look at an FM Audio Receiver.

Broadcast FM has the range of 88 to 108 MHz; with a deviation of +/- 75 KHz for a total swing of 150 KHz.

Marine FM has the range 156.050 to 157.425 MHz; with a deviation of +/- 5 KHz for a total frequency swing of 10 KHz.

Weather Broadcast has the range of 162.400 to 162.550; with a deviation of +/- 5 Khz for a total swing of 10 Khz.

There are 6 sections of a radio receiver:

1) The RF section which is responsible for selecting and amplifying the incoming RF signal.

2) The Oscillator section which provides the internal RF used to generate the IF signal.

3) The Mixer section which mixes the RF and the Oscillator signals together to generate the IF signal.

4) The IF section that amplifies the IF signal. Also filters out other signals as well.

5) The Detector section which demodulates the IF signal and produces the audio signal.

6) The audio section that amplifies the audio signal and drives the speaker.

One concept that must be discussed before we proceed. If we mix two RF signals together we get four signals. The two original, the sum, and the difference. The difference signal is the one that is important for our discussion.

Here are how it works:

The antenna converts the RF magnetic signal that is radiating in the air from the transmitter into an electrical signal. The RF signal goes down the antenna lead to the RF section where it is amplified. The filters in the RF section only allow the current channel to be amplified and passed on.

The Oscillator section produces a variable RF signal. The Oscillator signal is such that the difference between the RF and Oscillator is constant (it is the frequency of the IF signal). The frequency of the oscillator is changed when the user changes the channel of the VHF or the FM receiver. The oscillator frequency of marine VHF units used to be set by crystals. Modern units use ICs to accurately set the Oscillator frequency. Crystal are no longer needed. Plus the ICs are controlled by the CPU. This means that the Oscillator frequency can be changed to allow a single receiver to tune VHF, FM Broadcast, and Weather channels. The proper ICs would just need to be used.

So we have the RF signal and the Oscillator signal. The RF signal is FM modulated at the source. The Oscillator signal is fabricated within the receiver; it is varied so that the difference between it and the RF base signal is constant. If we want the difference signal to be say 10 MHz then the Oscillator frequency will be a 10 MHz above or below the RF signal. The frequency of the Oscillator signal is varied when the user changes the channel, so as to keep the difference to 10 MHz. The key to remember is the Oscillator signal is generated in the receiver, is unmodulated, and is only changed when the channel is changed.

The RF signal and the Oscillator signal are mixed in Mixer section. This gives us four signals. The two original, the sum, and the difference. The sum and difference signals are FM modulated because the RF signal is.

These signals are fed to the IF section; the Oscillator, RF, and sum signals are summarily filtered out. This leaves the difference signal which is also called the Intermediate Frequency. This scheme is used because the difference signal is constant and much lower in frequency than the RF, Oscillator, or Sum signals. Thus we have a signal that is much easier to handle and is constant in frequency (not considering the modulation).

The IF section amplifies the IF signal (difference signal) and passes it on to the detector. The detector converts the FM modulated IF signal to an audio signal. The audio signal is passed on to the audio section which amplifies it and drives the speaker. Viola sound comes out the speaker.

So, can a VHF marine band radio receive FM broadcast? Yes. Is this a simple task for a manufacture to accomplish. That is a matter of opinion.

The two major tasks to accomplish:

1) Changing the Oscillator frequency. With modern ICs this is not all that difficult. “World-band” receivers have Oscillators that can be used over a very wide range.

2) FM Broadcast has a much higher deviation than VHF Marine or Weather bands. So the RF, Mixer, IF, and Detector sections would all need to be modified to handle the extra deviation. Otherwise the fidelity of the FM Broadcast would be terrible. In fact the sound would be so distorted that it would be unusable. Again I don’t think that this would be to difficult to accomplish. I have a Philips radio that can get Weather, TV sound, and FM broadcast. I got it for around $25. Clearly this means the possibility exists.

Greg also mentioned needing to change filters. The filters are used in the RF section to limit the incoming RF signal to the desired frequency. This is also a function of the RF section. This is so that only the RF signal of the current channel gets amplified and mixed with the Oscillator signal. Again, modern ICs have made it possible to have filters that work over a large frequency range, yet work well.

So is it possible to make a marine VHF receive Broadcast FM. Yes, definitely. Would the extra cost be justifiable to a manufacture; probably not. Even thought it would not be very much there simply aren’t enough boaters that would consider this an advantage to make it profitable. Not when one can by a small portable FM radio for $10 or less.

posted 05-03-2001 03:57 PM ET (US)            

Excellent post. Thanks.

I bet one of these days we will see a combo radio on the market and at a reasonable price.

Rember, a computer for less than $1,000..a usefull PDA, etc. etc. yea right...

For right now where can I get a shower radio?

posted 07-15-2001 02:18 AM ET (US)            

Direct web site of the distributor is:

http://www.mfagifts.com/proddet.asp?id=46&offset=18

News, tunes, Mariner's games....

posted 07-16-2001 07:51 AM ET (US)            

In the U.S., VHF Marine Band radios must be type-accepted by the FCC. This means the manufacturer has to submit a sample radio for test and inspection prior to getting approval.