If I open up this transmitter, or connect to the RS-232 serial port on the back, would there be a way to disable the built in pre-emphasis? I'm struggling with muddy, lackluster sound and believe this is the cause since I am also de-emphasizing in my audio chain.
I lowered my reference level and the audio seemed to come alive with great stereo separation and clarity. Having anything other than 75 micro seconds and de-emphasis checked in the audio chain results in severe over modulation. I've tried leaving de-emphasis off and setting the audio chain to 15 or 25 µs with the thinking that the transmitter may be set to 50, but it causes severe distortion. Therefore, unlike other Chinese transmitters, I believe this is actually set to 75 µs, but still would like to disable it anyway.
I would imagine that using two different modes of emphasis added up to achieve what you want would just add to complexity and potential audio issues down the road. So many of those transmitters are sold to buyers outside of china now it wouldn't surprise me if they switched to 75µs. If there is no internal setting or menu setting it still may be possible to bypass it by taking a good look at the internal circuitry and bypass the pre-emphasis by jumpering a few connections. Many of those designs use a simple capacitor and resistor combination on the left/right inputs as a passive filter. If it uses active filter components it may not be so easy of a modification.
Thanks for the advice Kage, but I cracked open the unit and didn't see any capacitors or resistors on the inputs. Therefore, I believe my transmitter uses an active filter. To combat this problem, I completely removed pre-emphasis from my audio processing and am just using what's built into the transmitter. This, at least, removes one of the variables, although I have no way of knowing if my transmitter is set up to 50 or 75 microseconds.
Unfortunately, I still can't seem to achieve peak modulation. When I look at a stereo with VU meters, the needles only peak about halfway or slightly above. Commercial radio stations peak higher. If I try to turn up the input, it results in a loss of stereo separation, loudness and quality. As a result, I turned the hard limiter way up to compensate for the quietness. Do you think the highs and the pre-emphasis could still be a problem after I removed it from my audio processor or could something else be going on here?
Do you think the highs and the pre-emphasis could still be a problem after I removed it from my audio processor or could something else be going on here?
More than likely that is the issue. This is the exact same problem cheap FM wireless transmitters have that people use for MP3 playback on their car stereos. The problem is that if the pre-emphasis is added internally to the transmitter and is the last stage of audio manipulation before it goes direct to the transmitters modulation stage there will be no way to combat the rising pre-emphasis curve. That means that no matter what someone does, the higher audio frequencies will always be louder than the lower frequencies. Since peak modulation doesn't care what audio frequencies are used, that means that the maximum peak that can be had is that of the peak high frequencies, leaving the bass and midrange frequencies at lower volume, and overall lower peak modulation before it causes overmodulation.
This is why with commercial gear they run a specific chain of processing to get the highest amplitude of all audio frequencies, usually through multi-band compression with the pre-emphasis added *before* rather than after the compression or high frequency audio limiter. If pre-emphasis was ran after all of the audio processing you could never achieve full modulation because higher audio frequencies would max out the transmitters peak modulation capability before the lower audio frequencies even get a chance. That lowers peak modulation overall, and keeps you from ever getting the loudness a commercial station has. The only way around that is to defeat the transmitters built in pre-emphasis, or to add de-emphasis right before your transmitter to combat its built in usage, and then use your audio processor to do proper pre-emphasis where it belongs in the audio chain for a professional broadcast sound.
You can look at one positive thing though, with FM broadcasting loudness is not really important unlike with AM. Usually people find loudness on FM offensive to the hearing anyway since to win the loudness wars most stations overly compress/limit/clip their audio to the point of ear fatigue.
From the NAB Engineering Handbook:
In the Region 2 countries, 75μs pre-emphasis is used in FM and television sound transmission. This pre-emphasis is up 17dB at 15kHz and can cause severe over-modulation if its effects are not controlled. The obvious solution — placing a wide-band peak limiter after the pre-emphasis filter — proved unsatisfactory because high-frequency overloads would cause severe spectral gain intermodulation: cymbal crashes would caused the sound to literally collapse. The Fairchild “Conax” (originally designed for disk cutting) was often used to ameliorate the problem. This device divided the audio into two bands with a 1kHz crossover and applied pre-emphasis, clipping, and high-pass filtering to the upper band. The high-pass filter reduced the difference-frequency intermodulation caused by the clipper, yielding reasonably acceptable sound.
The point to take from this is that any form of final limiting/compression is to be added after pre-emphasis. There really is no exception unless you want to risk overmodulation, and eventually channel splatter and occasional loss of your stereo pilot.
If you are tech savvy with some basic circuitry you can search around to find a suitable resistor/capacitor combination to force de-emphasis at the audio going into your transmitter. It's a quick and dirty fix but it can work.
Something like this..
Part values would have to be found by trial and error unless you are good at electronics math. One of those circuits for each left and right audio going into the transmitter. With some luck it would reverse the changes your transmitter internally does for pre-emphasis, then you do all of the pre-emphasis in your outboard audio processing where it belongs. The other option is to use a stereo graphic equalizer right before the input to your transmitter and roll off the high frequencies following the 75us curve to combat the transmitters built in pre-emphasis.
This transmitter uses the Rohm BH1414K IC chip. The chip is located under that large metal shielding plate that is soldered to the main board. Your audio left input and right input travels on the bottom side of the board and passes through two holes to the top side below that metal plate. The circuit consists of a 1uF electrolytic capacitor with audio passing into the (-) Negative side and passing out on (+) side. This then feeds a resistor/capacitor network that creates your pre-emphasis circuit. The value of that capacitor determines your pre-emphasis curve, either 50us or 75us. This transmitter to my understanding is shipped out with 50us pre-emphasis, the capacitor is a 1000pF capacitor, to make it 75us, that capacitor needs to be replaced with a 1500pF capacitor.
I have a topic here on this board if you click this link below you will see I posted images of the BH1414K IC chip that is on YOUR transmitter's main circuit board. Please visit this thread you will find it most helpful. I have to update that thread as I have learned a few things since I originally posted that thread.
Thanks for the info Kage and mbruce. Instead of replacing the capacitors, can I just remove them and jumper them to complete get rid of pre-emphasis? Also, where are the capacitors located in relation to the chip. Are they on the underside of the board (opposite side of the metal plate covering the chip) or do I need to remove the metal plate?
Yes. the red arrows you added to my photo that you borrowed, points to the ones that determines your pre-emphasis curve. They are on the TOP side of the main board under that metal shielding plate. My photo shows the chip after I removed the plate on mine, that is why it is exposed.
If you choose to jumper over them, which you can, be VERY careful!!! It may look easy, but it really isn't!
Getting that metal plate up off of the board can be tricky, it has tabs that go into the board though slotted holes in 4 locations and getting those solder beads all around the plate removed so the plate just lifts off isn't easy.
You need a high heat solder iron to apply enough heat to remove that plate, but you also can not burn the heck out of the circuit board.
Try prying one side up a little at a time, apply heat, pry again until you have that plate fully up off of the board.
Now you can see the chip. Match your chip to the one in the photo you borrowed above.
You have your RED arrows pointed to the CORRECT capacitors.
CAUTION_CAUTION_CAUTION!!!
My photo is NOT showing how small these parts are!! They are VERY SMALL AND HARD TO SEE!!
You will need high powered magnifying lenses of some sort, 10X or better, so you can be sure you have no soldering errors such as shorts!
You also need to know that the part you are pointing to is tied to the part next to it, with the number 513 on it, NOT THE ONE WITH 511!!!! It is very hard to tell because the parts cover the circuit traces!
If you jumper those two capacitors, BE SURE NOT TO BRIDGE THE SOLDER OR ALLOW THE JUMPER TO TOUCH THAT 511 NUMBERED RESISTER!!
Sorry to yell, but I need to make it clear, one goof and you'll damage that chip and that would be a total nightmare to replace!
Best of luck to you and let us know how things sound afterwards.
Transmitting at very low power without that plate to run tests is ok, but if your unit is 30 or 50 watts, put that plate back in place before you go above 1 watt or you'll have more issues with RFI in the audio.
mbruce thank you for the instructions and words of caution. I should have asked to borrow your picture before posting it. I will try this and post back.
As a temporary work around, stereo tool 7.5 has a calibration setting under the FM Output option. I believe this can be used to add de-emphasis before all the processing and compression occurs. One can cross reference this information regarding the De-Emphasis Response Table with the db limits that can be set in the calibration screen. Then, turn pre-emphasis back on in the processing. I tried this with good results and can get louder audio without the distortion coming through the radio.
Should look something like this, but I need to re-do my calculations to get more accurate results as the table gives different setpoints than whats in this program.
It was OK for you to use my picture, I believe you deleted the picture, because it is not showing up anymore at my end. You can add it back, because, anyone following this topic will be lost without it.
Mine didn't even come with the metal plate. Do I need to remove those capacitors or can I just jumper over the top of them?
Jumper them, removing them would leave you with just the resisters still in circuit and could leave you with a bad frequency response.
Not sure why your transmitter came without the BH1414K IC chip shielding plate, that's a good way for RF to get into areas it isn't supposed to get into.
Was your transmitter new or used?
In another post, Kage said I should have left my caps alone and do the processing outside of the transmitter. I did damage to my PC board messing around changing caps, now I have to contact the seller asking if they can sell me another mainboard. Ruined some circuit traces and cracked a cap unrelated to the pre-emphasis. Parts are too small to work with.
Perhaps, it is best you leave your transmitter alone as well, and do the processing outside the box.
Bruce, I'm trying to contact the manufacturer to see about getting just the main board with pre-emphasis disabled. Either buying one new or sending mine in. I contacted Yue Lan at www.czhfmtransmitter.com, but she doesn't seem to understand what I'm talking about. Perhaps it's a language barrier thing. I'm going to keep working on this.
Regarding the metal plate. I'm not too happy that it's missing. I purchased the transmitter new on ebay.
Sorry that your board got ruined. I appreciate the words of caution. Are you going to repair your transmitter? If not, how much to buy the metal plate off of yours?