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in search of a FM STEREO transmitter


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Hi Guys,
I found what I think is the "South African" stereo FM transmitter kit on Google. Their description and spec's of this product is detailed (many pages) and GOOD. http://home.global.co.za/~edm1/

Rohm Corp., the manufacturer of the lousy BA1404 stereo FM transmitter IC has some new ones that correct problems with that old one. Their BH1417 has a Phase-Locked-Loop for solid frequency stability. Silicon Chip (Australian magazine for electronic hobbiests) has a project for it.The expert that I mentioned before tested it and reports that its stability is excellent BUT ITS DISTORTION IS VERY HIGH.
Silicon Chip's project:
http://www.siliconchip.com.au/cms/A_30378/article.html



Hi Ras,
I've put in caps the good and bad points of the two transmitters to help your choice. The "South African" kit is much better but costs more. There are two kits listed in Google in the Micromitter search, but I can't see how they can charge $50 and $64 for just a $3 IC but they have a very nice case.
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Hi Guys,
A position in the list of a Google search keeps changing by the number of hits it gets. Searching for the Silicon Chip project by stereo FM Transmitter was on page 6 yesterday, today it's on page 1 thanks to us guys.
EnigmaOne is correct, it is much easier to find by searching for stereo FM Micromitter and links to two kits for it are also there.

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the EDM transmitter seems very good.. BUT...
it have a Frequency Response of : 20Hz - 15kHz... thats no good
it means u lose the higher sounds.. it lacks 7kHz to fill up the range.
and..
its quite expensive...80$ are allot of bucks.

maybe someone will publish the schematic for this, so we can all DIY it.
;D ;D ;D

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Hi Ras,
Stereo FM and TV audio is limited to 15KHz (AM radios are lucky to get to only about 4KHz and few do). Any frequencies higher are supposed to be sharply rolled-off to avoid beating with the 19KHz pilot tone and avoid aliasing noises in the stereo multiplexing. The pre-emphasis (treble boost) that is applied to the audio prior to the multiplexing makes good filtering of high frequency audio even more necessary.

Look at the frequency response of the Silicon Chip project (or maybe I saw it on the datasheet for the BH1417). You can see audio frequencies rising above about 3KHz due to pre-emphasis, but the simple filter in the IC starts its roll-off at only about 10KHz. 15KHz from a radio listening to it would be down quite a few decibels. At 19KHz, where the response should be zero, the response is still fairly strong, about where it would be without pre-emphasis.
The poor performance of those filters and the lack of a high-pass-filter for the stereo subcarrier sidebands causes the project's high distortion.
BTW, the pre-emphasis on the datasheet and in the Silicon Chip project is for the Australian, New Zealand and European standard of 50us. In North America, the standard is 75us (big mistake, but hey, it came first!) so the treble boost is even more and the project will produce even more distortion. If you don't change it, a North American radio listening to it would sound rather muffled.

The EDM (South African?) kit uses a microprocessor that of course needs a program to run. Copying their program without paying for it would classed as theft of their copyright. So forget about DIY.
How good is its frequency response? Nobody knows. They only say the frequencies without mentioning how far down in level they are!

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in the datasheet of the BH1417F
(http://www.rohm.com/products/databook/audio/pdf/bh1417f.pdf)

there is a scheme on page 2.
could someone look at it and see if its any good?
maybe its better than Silicon Chip project ?

i would go fo the SiliconChip project if u were not talking about distortion all the time - it is frightening

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Hi Ras,
There is hardly any difference between the circuits. There is nothing changed that would affect distortion.
About the only differences I can see is that Silicon Chip couldn't find a Soshin FM bandpass filter so made their own with a capacitor and a couple of coils, and they changed a few parts in the PLL.
Since Rohm doesn't show any spec's (you have to request for them and maybe pay), I trust that the Silicon Chip circuit is better but will also have high distortion.

Earlier you were worried about frequency response and I replied that it starts rolling-off at 10KHz. I looked again and discovered that it is worse, the roll-off begins cutting only 6KHZ and 15KHz is down about 5dB, and using North America's pre-emphasis will be worse:

post-1706-1427914210151_thumb.jpg

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Hi Ras,
Please don't underline your shortcut or it doesn't link.
If you enter Minimitter in a Google search, the project's link is on the 1st page and you can get the whole article for free!

As you can see by its spec's, this project also has high distortion that might be more when it transmits a higher level (they don't say how much modulation makes 200mV and 3% distortion).
The BA1404 is Rohm's 1st transmitter, the one everybody is laughing about:
1) It doesn't have a crystal nor PPL for its radio part so drifts its radio frequency all over the place. If your receiver doesn't have AFC and good receivers don't because radio stations don't drift, you will have to keep re-tuning. If your receiver is digital, you can't re-tune it and the transmitter's signal will come and go.
2) The square waves of the multiplexing aren't filtered so they interfere with its radio part.
3) The input audio frequencies aren't filtered so the high ones interfere with the 19KHz pilot. If you say the letter "S" into a microphone and transmit it through this circuit, your receiver will probably drop into mono. With music, it will keep switching between mono and stereo.
4) High distortion.
5) Etc.
Luckily, it isn't made anymore but lots of kits still have it.

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  • 2 weeks later...

hello again,
i m about to build this circuit, and i live in ISRAEL !
in another topic u wrote :
"The Micromitter is designed for Australia's 10mW RF power limit and uses their and Europe's 50us pre-emphasis standard. Check to see what your country's standards are"

could those be "wrong" ? i mean, if those standarts here in Israel are different should i make any chages to the circuit ?

does the "power limit" determines the transmission range ? if 10mW gives 20 meters does 100mW give 200 meters ? if so ... 10mW is enough for me as i want this transmitter only for in-house use !

i asked an expert here and he dont know of any pre-emphasis standart ! what difference does this "pre-emphasis" do ?

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Hi Ras,
I hope that you like the Micromitter. Please let us lnow what it sounds like.

For the short distance between rooms of your house (you don't have a mansion?), its 10mW should be fine, it will probably go about 100m outside to a good FM receiver. 100mW will probably go 400m, 1W maybe 800m. Because the radiation is in a hemisphere, a single receiving direction gets very little and most of its power goes in the other directions.

I don't know Israel, you should find out its standards. Phone a radio station and ask their techie:
1) Is the FM broadcast band from 88MHz to 108MHz?
2) Do they use odd-numbered frequencies like 87.7-88.9MHz and 106.7-107.9MHz?
3) Do they use 50us pre-emphasis like Australia and Europe or 75us like North America?

If Israel uses different RF frequencies then the Micromitter won't work there, unless you have a 88MHz to 108MHz analog-tuned radio or a digital one where you can program the required frequencies.

Pre-emphasis is the treble boost that FM radio and TV stations use when transmitting. FM radios and TVs have the same exact treble cut to make the sound proper and reduce hiss. If Israel uses North America's 75us standard then the Micromitter's received sound will have too much treble. I think the article says what to change in the circuit to make it correct.

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Hiya Guys,
The forum won't let me upload the zip file for the micromitter project as it's over a meg in size. but if anybody wants a copy of the siliconchip article then pm me and I can send it to you. I think it's a ripoff to charge just to look at an article from years ago. The kit is available from www.jaycar.com.au for $49.95 OZ.


Cheers Bryan1 ;D

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hello again,
we (in Israel) working according to Europe standarts.
meaning i dont have to change anything... ;D :D ;D

my question today :
i want to build this on a protoboard (that board with holes and copper on on side that i solder thing to)...
1) whats the best way to solder the chip to this board ?? as the BH1417F is surface-mount only it doesnt have any "legs".
2) on the part list there r some part that are also surface-mount only, like the inductors, is there any replacements for them ?? same parts but with "legs" so i can solder them to the protoboard

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Hi Rs,
That's correct, the surface-mount IC is tiny and has no legs.
It will be difficult, but you could connect your own 21 wires to it, or solder it to an adapter pcb.
You can easily make your own inductors.

Why not just buy the kit from Australia or New Zealand? Don't you know anybody there? I think Dazza in this forum is in "down under".
Kits-R-us in Hong Kong were also talking about making this kit.

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(sorry for all the dumb questions :P)

could u explain how to DIY those inductors ? or point me to a place where i can learn how to do this, i need 2 diferent inductors.

wouldnt it be wizer to replace the 2xRCA with one female-headphone-jack ? i see no advantage of using RCA.

TNX FOR ALL THE HELP
:)

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Hi Ras,
The kit comes with a coil former and an adjustable ferrite core. I would just make a simple coil with 8 to 10 turns of 1mm enamelled wire tightly wound with a 3mm air core, and use an adjustable 5pF to 35pF trimmer capacitor for tuning. The 2 bigger inductors can be purchased.
You can use any kind of input jack. They used RCAs because most component CD players use them.

I know what! C'mon over here (Canada) and help me shovel the tonne of snow from my driveway, and do it again after the snowplow clears my street, and I'll build the Micromitter for ya. ;D

post-1706-14279142143597_thumb.gif

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  • 10 months later...

Hi!
Sorry for bringing up such an old post.
I am in the search of a schematic for a small portable FM-transmitter and the micromitter seems to solve my problem! Does anyone have a copy of the article, or do I need to buy the entire magazine? I checked their webpage, and you cant see the entire article anymore (like someone posted ...more than a year ago ;P).

Best regards,
Alexander

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  • 2 weeks later...

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